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Kp and Kc Relationship Questions in English

Class 11 Chemistry · 6-1.Equilibrium (Chemical Equilibrium) · Kp and Kc Relationship

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201
DifficultMCQ
For the reaction $NH_4HS_{(s)} \rightleftharpoons NH_{3(g)} + H_2S_{(g)}$,if the total pressure in the reaction vessel at $105 \ ^\circ C$ is $1.12 \ atm$,then the $K_p$ for this equilibrium will be .........
A
$0.56$
B
$1.25$
C
$0.31$
D
$0.63$

Solution

(C) The equilibrium reaction is $NH_4HS_{(s)} \rightleftharpoons NH_{3(g)} + H_2S_{(g)}$.
Let the partial pressures at equilibrium be $P_{NH_3} = P_{H_2S} = x$.
The total pressure $P_{total} = P_{NH_3} + P_{H_2S} = x + x = 2x$.
Given $P_{total} = 1.12 \ atm$,so $2x = 1.12$,which gives $x = 0.56 \ atm$.
The equilibrium constant $K_p$ is given by $K_p = P_{NH_3} \times P_{H_2S} = x \times x = x^2$.
$K_p = (0.56)^2 = 0.3136 \approx 0.31$.
202
DifficultMCQ
For the elementary reaction $A_{2(g)} + B_{2(g)} \rightleftharpoons 2AB_{(g)}$,the rate of the forward reaction is given by $r_f = 1.7 \times 10^{-18} [A_2][B_2]$. If the rate of decomposition of gaseous $AB$ into $A_2$ and $B_2$ is given by $r_r = 2.4 \times 10^{-21} [AB]^2$,then the equilibrium constant for the formation of $AB$ from $A_2$ and $B_2$ will be ...
A
$1.4 \times 10^{-3}$
B
$2.8 \times 10^{-3}$
C
$1.4 \times 10^{2}$
D
$0.7 \times 10^{3}$

Solution

(D) For an elementary reaction $A_{2(g)} + B_{2(g)} \rightleftharpoons 2AB_{(g)}$,the equilibrium constant $K_c$ is the ratio of the forward rate constant $k_f$ to the reverse rate constant $k_r$.
Given $r_f = k_f [A_2][B_2] = 1.7 \times 10^{-18} [A_2][B_2]$,so $k_f = 1.7 \times 10^{-18}$.
Given $r_r = k_r [AB]^2 = 2.4 \times 10^{-21} [AB]^2$,so $k_r = 2.4 \times 10^{-21}$.
$K_c = \frac{k_f}{k_r} = \frac{1.7 \times 10^{-18}}{2.4 \times 10^{-21}} = \frac{1.7}{2.4} \times 10^3 \approx 0.708 \times 10^3 \approx 0.7 \times 10^3$.
203
DifficultMCQ
For the reaction $A_{(g)} + 2B_{(g)} \rightleftharpoons 2C_{(g)}$,$1 \ mol$ of $A$ and $1.5 \ mol$ of $B$ are taken in a $2 \ L$ vessel. If the concentration of $C$ at equilibrium is $0.35 \ M$,then the equilibrium constant $K_c$ of the reaction will be ....... $M^{-1}$.
A
$0.295$
B
$2.36$
C
$1.178$
D
$0.673$

Solution

(B) Initial moles: $A = 1 \ mol$,$B = 1.5 \ mol$,$C = 0 \ mol$.
Volume $V = 2 \ L$.
Initial concentrations: $[A]_0 = 1/2 = 0.5 \ M$,$[B]_0 = 1.5/2 = 0.75 \ M$.
At equilibrium,$[C] = 0.35 \ M$.
From the stoichiometry $A + 2B \rightleftharpoons 2C$,if $2x = 0.35$,then $x = 0.175 \ M$.
Equilibrium concentrations: $[A] = 0.5 - x = 0.5 - 0.175 = 0.325 \ M$.
$[B] = 0.75 - 2x = 0.75 - 0.35 = 0.4 \ M$.
$[C] = 0.35 \ M$.
$K_c = \frac{[C]^2}{[A][B]^2} = \frac{(0.35)^2}{(0.325)(0.4)^2} = \frac{0.1225}{0.325 \times 0.16} = \frac{0.1225}{0.052} \approx 2.3557 \approx 2.36 \ M^{-1}$.
204
MediumMCQ
For the reaction $CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}$,the value of $K_p$ at $800 \ ^oC$ is $1.16 \ atm$. If $1 \ mol$ of $CaCO_{3(s)}$ is taken in a $1 \ L$ container to start the reaction,what will be the partial pressure of $CO_2$ at equilibrium in $atm$?
A
$0.29$
B
$0.58$
C
$0.73$
D
$1.16$

Solution

(D) The given reaction is a heterogeneous equilibrium: $CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}$.
For heterogeneous equilibria involving solids and gases,the equilibrium constant $K_p$ is defined only by the partial pressure of the gaseous species.
Thus,$K_p = P_{CO_2}$.
Given that $K_p = 1.16 \ atm$,the partial pressure of $CO_2$ at equilibrium is $P_{CO_2} = 1.16 \ atm$.
205
MediumMCQ
For the reaction $C_{(s)} + CO_{2(g)} \rightleftharpoons 2CO_{(g)}$,the partial pressures of $CO_2$ and $CO$ are $2.0 \ atm$ and $4.0 \ atm$ respectively. The value of $K_p$ for the reaction is ............ .
A
$0.5$
B
$4$
C
$8$
D
$32$

Solution

(C) The equilibrium constant expression for the reaction $C_{(s)} + CO_{2(g)} \rightleftharpoons 2CO_{(g)}$ is given by:
$K_p = \frac{(P_{CO})^2}{P_{CO_2}}$
Given that $P_{CO} = 4.0 \ atm$ and $P_{CO_2} = 2.0 \ atm$.
Substituting the values:
$K_p = \frac{(4.0)^2}{2.0} = \frac{16}{2.0} = 8$
Thus,the value of $K_p$ is $8$.
206
DifficultMCQ
For the reaction $A + B \rightleftharpoons C + D$,the equilibrium constant is $10$. If the rate constant for the forward reaction is $203$,what will be the rate constant for the backward reaction?
A
$20.3$
B
$10.3$
C
$2.03$
D
$203$

Solution

(A) The equilibrium constant $K_c$ is defined as the ratio of the forward rate constant $K_f$ to the backward rate constant $K_r$:
$K_c = \frac{K_f}{K_r}$
Given $K_c = 10$ and $K_f = 203$:
$10 = \frac{203}{K_r}$
$K_r = \frac{203}{10} = 20.3$
207
MediumMCQ
For the reaction $A + B \rightleftharpoons C + D$,the initial concentrations of $A$ and $B$ are equal. If the equilibrium concentration of $D$ is twice the equilibrium concentration of $A$,then the equilibrium constant $K_c$ is:
A
$4$
B
$9$
C
$0.25$
D
$1/9$
208
MediumMCQ
$8 \ mol$ of $AB_3$ is taken in a $1.0 \ dm^3$ container. It dissociates according to the reaction $2AB_{3(g)} \rightleftharpoons A_{2(g)} + 3B_{2(g)}$. If $2 \ mol$ of $A_2$ are present at equilibrium,the equilibrium constant $K_c$ of the reaction is ....... $mol^2 \ L^{-2}$.
A
$12$
B
$3$
C
$27$
D
$36$

Solution

(C) The reaction is: $2AB_{3(g)} \rightleftharpoons A_{2(g)} + 3B_{2(g)}$
Initial moles: $8, 0, 0$
At equilibrium,$n(A_2) = 2 \ mol$. Since the stoichiometry of $A_2$ is $1$,the amount of $AB_3$ reacted is $2 \times 2 = 4 \ mol$.
Remaining moles at equilibrium:
$n(AB_3) = 8 - 4 = 4 \ mol$
$n(A_2) = 2 \ mol$
$n(B_2) = 3 \times 2 = 6 \ mol$
Since the volume is $1.0 \ L$,the concentrations are $[AB_3] = 4 \ M, [A_2] = 2 \ M, [B_2] = 6 \ M$.
$K_c = \frac{[A_2][B_2]^3}{[AB_3]^2} = \frac{2 \times (6)^3}{(4)^2} = \frac{2 \times 216}{16} = \frac{432}{16} = 27$.
209
MediumMCQ
In the reaction $PCl_5 \rightleftharpoons PCl_3 + Cl_2$,if at equilibrium there are $2 \ mol$ each of $PCl_5$,$PCl_3$,and $Cl_2$,and the total pressure is $3 \ atm$,then the value of the equilibrium constant $K_p$ is ....... $atm$.
A
$1$
B
$2$
C
$3$
D
$6$

Solution

(A) The reaction is $PCl_5 \rightleftharpoons PCl_3 + Cl_2$.
At equilibrium,the number of moles of each species is $n(PCl_5) = 2$,$n(PCl_3) = 2$,and $n(Cl_2) = 2$.
Total moles $n_{total} = 2 + 2 + 2 = 6 \ mol$.
The mole fraction of each species is $x = \frac{2}{6} = \frac{1}{3}$.
The partial pressure of each species is $P_i = x_i \times P_{total} = \frac{1}{3} \times 3 \ atm = 1 \ atm$.
$K_p = \frac{P(PCl_3) \times P(Cl_2)}{P(PCl_5)} = \frac{1 \times 1}{1} = 1 \ atm$.
210
MediumMCQ
$9.2 \ g$ of $N_2O_{4(g)}$ is taken in a $1 \ L$ closed vessel and heated until the following equilibrium is attained:
${N_2}{O_{4(g)}} \rightleftharpoons 2N{O_{2(g)}}$
If $50\%$ of $N_2O_{4(g)}$ dissociates at equilibrium,what will be the equilibrium constant (in $mol \ L^{-1}$)? (Mol. wt. of $N_2O_4 = 92$)
A
$0.2$
B
$0.4$
C
$2$
D
$0.1$

Solution

(A) Initial moles of $N_2O_4 = \frac{9.2 \ g}{92 \ g \ mol^{-1}} = 0.1 \ mol$.
Given that $50\%$ of $N_2O_4$ dissociates,the moles reacted $= 0.1 \times 0.5 = 0.05 \ mol$.
$\begin{array}{lccc} & N_2O_{4(g)} & \rightleftharpoons & 2NO_{2(g)} \\ \text{Initial moles} & 0.1 & & 0 \\ \text{Change} & -0.05 & & +2(0.05) \\ \text{Equilibrium moles} & 0.05 & & 0.1 \end{array}$
Since the volume is $1 \ L$,the concentrations are $[N_2O_4] = 0.05 \ mol \ L^{-1}$ and $[NO_2] = 0.1 \ mol \ L^{-1}$.
$K_c = \frac{[NO_2]^2}{[N_2O_4]} = \frac{(0.1)^2}{0.05} = \frac{0.01}{0.05} = 0.2 \ mol \ L^{-1}$.
211
MediumMCQ
For the reaction $H_{2(g)} + CO_{2(g)} \rightleftharpoons CO_{(g)} + H_2O_{(g)}$,if the reaction is started with $1 \ mol$ of $H_2$ and $CO_2$ each,and $x \ mol/L$ of $H_2$ is consumed at equilibrium,then the correct expression for $K_p$ will be ........
A
$x^2 / (1-x^2)$
B
$(1+x^2) / (1-x)^2$
C
$x^2 / (1-x)^2$
D
$x^2 / (2+x)^2$

Solution

(C) The reaction is $H_{2(g)} + CO_{2(g)} \rightleftharpoons CO_{(g)} + H_2O_{(g)}$.
At equilibrium,the concentrations are:
$[H_2] = (1-x)$,$[CO_2] = (1-x)$,$[CO] = x$,$[H_2O] = x$.
Since the number of moles of gaseous reactants equals the number of moles of gaseous products $(\Delta n_g = 0)$,$K_p = K_c$.
$K_p = \frac{[CO][H_2O]}{[H_2][CO_2]} = \frac{x \times x}{(1-x) \times (1-x)} = \frac{x^2}{(1-x)^2}$.
212
MediumMCQ
For the reaction $2Ag_2O_{(s)} \rightleftharpoons 4Ag_{(s)} + O_{2(g)}$,the partial pressure of $O_2$ is given by:
A
$2K_p$
B
$(K_p)^{1/2}$
C
$K_p$
D
$(K_p)^{1/3}$

Solution

(C) For the given reaction $2Ag_2O_{(s)} \rightleftharpoons 4Ag_{(s)} + O_{2(g)}$,the equilibrium constant expression $K_p$ is defined as the product of the partial pressures of the gaseous products raised to their stoichiometric coefficients,divided by the partial pressures of the gaseous reactants.
Since $Ag_2O$ and $Ag$ are in the solid state,their activities are taken as $1$.
Therefore,$K_p = P_{O_2}$.
Thus,the partial pressure of $O_2$ is equal to $K_p$.
213
MediumMCQ
For the reaction $PCl_{3(g)} + Cl_{2(g)} \rightleftharpoons PCl_{5(g)}$,the value of $K_c$ is $26$ at $250 \ ^oC$. The value of $K_p$ at this temperature will be ...........
A
$0.61$
B
$0.57$
C
$0.83$
D
$0.46$

Solution

(A) The given reaction is $PCl_{3(g)} + Cl_{2(g)} \rightleftharpoons PCl_{5(g)}$.
Here,the change in the number of moles of gaseous species is $\Delta n_g = 1 - (1 + 1) = -1$.
The temperature $T = 250 + 273 = 523 \ K$.
The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$.
Substituting the values: $K_p = 26 \times (0.0821 \times 523)^{-1}$.
$K_p = \frac{26}{0.0821 \times 523} \approx \frac{26}{42.94} \approx 0.605 \approx 0.61$.
214
MediumMCQ
For the reaction $A_{(g)} + 2B_{(g)} \rightleftharpoons 3C_{(g)} + D_{(g)}$,the value of $K_p$ at $1000 \ K$ is $0.05 \ atm$. The value of $K_c$ in terms of $R$ will be:
A
$20000 \ R$
B
$0.02 \ R$
C
$5 \times 10^{-5} \ R$
D
$5 \times 10^{-5} / R$

Solution

(D) The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction $A_{(g)} + 2B_{(g)} \rightleftharpoons 3C_{(g)} + D_{(g)}$,the change in the number of moles of gaseous species is $\Delta n_g = (3 + 1) - (1 + 2) = 4 - 3 = 1$.
Given $K_p = 0.05 \ atm$ and $T = 1000 \ K$.
Substituting the values: $0.05 = K_c(R \times 1000)^1$.
Therefore,$K_c = \frac{0.05}{1000 \ R} = \frac{5 \times 10^{-2}}{10^3 \ R} = 5 \times 10^{-5} / R$.
215
MediumMCQ
For which of the following reactions will $K_p = K_c$?
A
$N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$
B
$2NOCl_{(g)} \rightleftharpoons 2NO_{(g)} + Cl_{2(g)}$
C
$H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$
D
$CO_{2(g)} + C_{(s)} \rightleftharpoons 2CO_{(g)}$

Solution

(C) The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$.
For $K_p = K_c$,the change in the number of moles of gaseous products and reactants must be zero,i.e.,$\Delta n_g = 0$.
In option $(C)$,the reaction is $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$.
Here,$\Delta n_g = (2) - (1 + 1) = 0$.
Therefore,$K_p = K_c$ for this reaction.
216
MediumMCQ
For which of the following reactions will ${K_p} > {K_c}$?
A
${N_{2(g)}} + 3{H_{2(g)}} \rightleftharpoons 2{NH_{3(g)}}$
B
${H_{2(g)}} + {I_{2(g)}} \rightleftharpoons 2{HI_{(g)}}$
C
${PCl_{5(g)}} \rightleftharpoons {PCl_{3(g)}} + {Cl_{2(g)}}$
D
${2SO_{2(g)}} + {O_{2(g)}} \rightleftharpoons {2SO_{3(g)}}$

Solution

(C) The relationship between ${K_p}$ and ${K_c}$ is given by the equation: ${K_p} = {K_c}{(RT)^{\Delta n_g}}$.
For ${K_p} > {K_c}$,the value of ${\Delta n_g}$ must be positive $({\Delta n_g} > 0)$.
${\Delta n_g}$ is the difference between the number of moles of gaseous products and gaseous reactants.
For option $A$: ${\Delta n_g} = 2 - (1 + 3) = -2$.
For option $B$: ${\Delta n_g} = 2 - (1 + 1) = 0$.
For option $C$: ${\Delta n_g} = (1 + 1) - 1 = +1$.
For option $D$: ${\Delta n_g} = 2 - (2 + 1) = -1$.
Since ${\Delta n_g} > 0$ only for option $C$,the correct answer is $C$.
217
MediumMCQ
For the reaction $2HI \rightleftharpoons H_2 + I_2$,which of the following is true?
A
$K_p > K_c$
B
$K_c > K_p$
C
$K_p = K_c$
D
None of these

Solution

(C) The relationship between $K_p$ and $K_c$ is given by the formula $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction $2HI(g) \rightleftharpoons H_2(g) + I_2(g)$,the change in the number of moles of gaseous products and reactants is $\Delta n_g = (1 + 1) - 2 = 0$.
Substituting $\Delta n_g = 0$ into the formula,we get $K_p = K_c(RT)^0 = K_c \times 1 = K_c$.
Therefore,$K_p = K_c$.
218
MediumMCQ
For the following three equilibria $X, Y$,and $Z$ in the gaseous state at $300 \ K$,the increasing order of the ratio of $K_p$ to $K_c$ is:
$X: 2SO_2 + O_2 \rightleftharpoons 2SO_3$
$Y: PCl_5 \rightleftharpoons PCl_3 + Cl_2$
$Z: 2HI \rightleftharpoons H_2 + I_2$
A
$X = Y = Z$
B
$X < Y < Z$
C
$X < Z < Y$
D
$Z < Y < X$

Solution

(C) The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$,which implies $\frac{K_p}{K_c} = (RT)^{\Delta n_g}$.
Since $RT = 300R$ is constant and greater than $1$,the ratio $\frac{K_p}{K_c}$ increases as $\Delta n_g$ increases.
Calculate $\Delta n_g$ (moles of gaseous products - moles of gaseous reactants) for each:
For $X: \Delta n_g = 2 - (2 + 1) = -1$
For $Y: \Delta n_g = (1 + 1) - 1 = 1$
For $Z: \Delta n_g = (1 + 1) - 2 = 0$
Comparing the values of $\Delta n_g$: $-1 < 0 < 1$.
Therefore,the increasing order of the ratio is $X < Z < Y$.
219
MediumMCQ
For the reaction ${N_{2(g)}} + {O_{2(g)}} \rightleftharpoons 2NO_{(g)}$,the value of $K_c$ at $400 \ K$ is $4.0 \times 10^{-6}$. The value of $K_p$ for this reaction is .....
A
$2.4 \times 10^{-3}$
B
$4.0 \times 10^{-6}$
C
$4.0 \times 10^{-6} \times (RT)^2$
D
None of these

Solution

(B) The relationship between $K_p$ and $K_c$ is given by the formula $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction ${N_{2(g)}} + {O_{2(g)}} \rightleftharpoons 2NO_{(g)}$,the change in the number of moles of gaseous products and reactants is $\Delta n_g = 2 - (1 + 1) = 0$.
Since $\Delta n_g = 0$,we have $K_p = K_c(RT)^0 = K_c$.
Therefore,$K_p = 4.0 \times 10^{-6}$.
220
MediumMCQ
For the reaction $PCl_{3(g)} + Cl_{2(g)} \rightleftharpoons PCl_{5(g)}$,the value of $K_p$ at $250 \ ^oC$ is $0.61 \ atm^{-1}$. The value of $K_c$ at this temperature will be .... $(mol \ L^{-1})$.
A
$15$
B
$26.19$
C
$35$
D
$52$

Solution

(B) The given reaction is $PCl_{3(g)} + Cl_{2(g)} \rightleftharpoons PCl_{5(g)}$.
For this reaction,the change in the number of moles of gaseous species is $\Delta n_g = 1 - (1 + 1) = -1$.
The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$,which implies $K_c = K_p(RT)^{-\Delta n_g}$.
Given $K_p = 0.61 \ atm^{-1}$,$T = 250 + 273 = 523 \ K$,and $R = 0.0821 \ L \ atm \ K^{-1} \ mol^{-1}$.
$K_c = 0.61 \times (0.0821 \times 523)^{-( -1 )} = 0.61 \times (42.9383) = 26.19 \ mol \ L^{-1}$.
221
MediumMCQ
For the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)} + \text{heat}$,which of the following relations is correct?
A
$K_p = K_c$
B
$K_p = K_c (RT)^{-1}$
C
$K_p = K_c (RT)^{-2}$
D
$K_p = K_c (RT)$

Solution

(C) The relationship between $K_p$ and $K_c$ is given by the formula $K_p = K_c(RT)^{\Delta n}$.
For the given reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the change in the number of moles of gaseous products and reactants is $\Delta n = n_p - n_r = 2 - (1 + 3) = 2 - 4 = -2$.
Substituting $\Delta n = -2$ into the formula,we get $K_p = K_c(RT)^{-2}$.
222
MediumMCQ
For the reactions $N_{2(g)} + O_{2(g)} \rightleftharpoons 2NO_{(g)}$ and $NO_{(g)} \rightleftharpoons 1/2N_{2(g)} + 1/2O_{2(g)}$,the equilibrium constants are $K_1$ and $K_2$ respectively. What is the relationship between $K_1$ and $K_2$?
A
$K_1 = (1/K_2)^2$
B
$K_1 = K_2^2$
C
$K_1 = 1/K_2$
D
$K_1 = K_2$

Solution

(A) The first reaction is: $N_{2(g)} + O_{2(g)} \rightleftharpoons 2NO_{(g)}$ with equilibrium constant $K_1 = \frac{[NO]^2}{[N_2][O_2]}$.
The second reaction is: $NO_{(g)} \rightleftharpoons 1/2N_{2(g)} + 1/2O_{2(g)}$ with equilibrium constant $K_2 = \frac{[N_2]^{1/2}[O_2]^{1/2}}{[NO]}$.
Observe that the first reaction is the reverse of the second reaction multiplied by $2$.
If a reaction is reversed,the equilibrium constant becomes $1/K$. If a reaction is multiplied by a factor $n$,the equilibrium constant becomes $K^n$.
Therefore,$K_1 = (1/K_2)^2$.
223
MediumMCQ
For the reactions $NO_{(g)} + 1/2 O_{2(g)} \rightleftharpoons NO_{2(g)}$ and $2NO_{2(g)} \rightleftharpoons 2NO_{(g)} + O_{2(g)}$,the equilibrium constants at a given temperature are $K_1$ and $K_2$ respectively. If the value of $K_1$ is $4 \times 10^{-3}$,then the value of $K_2$ will be ....
A
$8 \times 10^{-3}$
B
$16 \times 10^{-3}$
C
$6.25 \times 10^{4}$
D
$6.25 \times 10^{6}$

Solution

(C) The first reaction is: $NO_{(g)} + 1/2 O_{2(g)} \rightleftharpoons NO_{2(g)}$ with equilibrium constant $K_1 = 4 \times 10^{-3}$.
The second reaction is: $2NO_{2(g)} \rightleftharpoons 2NO_{(g)} + O_{2(g)}$ with equilibrium constant $K_2$.
Notice that the second reaction is the reverse of the first reaction multiplied by $2$.
Therefore,the relationship between the equilibrium constants is $K_2 = (1/K_1)^2$.
Substituting the value of $K_1$: $K_2 = 1 / (4 \times 10^{-3})^2 = 1 / (16 \times 10^{-6}) = 10^6 / 16 = 0.0625 \times 10^6 = 6.25 \times 10^4$.
224
MediumMCQ
$CoO_{2(g)} + H_{2(g)} \rightleftharpoons CoO_{(s)} + H_2O_{(g)} \,;\, K_1 = 67$
$CoO_{2(g)} + CO_{(g)} \rightleftharpoons CoO_{(s)} + CO_{2(g)} \,;\, K_2 = 490$
Then the equilibrium constant for the following reaction is ....
$CO_{2(g)} + H_{2(g)} \rightleftharpoons CO_{(g)} + H_2O_{(g)}$
A
$0.137$
B
$423$
C
$7.31$
D
$557$

Solution

(A) Given reactions:
$(i) \, CoO_{2(g)} + H_{2(g)} \rightleftharpoons CoO_{(s)} + H_2O_{(g)} \quad K_1 = 67$
$(ii) \, CoO_{2(g)} + CO_{(g)} \rightleftharpoons CoO_{(s)} + CO_{2(g)} \quad K_2 = 490$
Target reaction:
$(iii) \, CO_{2(g)} + H_{2(g)} \rightleftharpoons CO_{(g)} + H_2O_{(g)} \quad K = ?$
To obtain reaction $(iii)$,subtract reaction $(ii)$ from reaction $(i)$:
$(iii) = (i) - (ii)$
Therefore,the equilibrium constant $K$ is given by:
$K = \frac{K_1}{K_2} = \frac{67}{490} \approx 0.137$
225
MediumMCQ
In the equilibrium system $AB_{(s)} \rightleftharpoons A_{(g)} + B_{(g)}$,if the equilibrium concentration of $A$ is doubled,the equilibrium concentration of $B$ will be .....
A
doubled.
B
unpredictable.
C
halved.
D
remain constant.

Solution

(C) For the reaction $AB_{(s)} \rightleftharpoons A_{(g)} + B_{(g)}$,the equilibrium constant expression is $K_c = [A][B]$.
Since $K_c$ is constant at a given temperature,if the concentration of $A$ is doubled (i.e.,$[A]' = 2[A]$),the new concentration of $B$ (let it be $[B]'$) must satisfy the same $K_c$ value.
$K_c = [A][B] = [A]'[B]'$
$K_c = [A][B] = (2[A]) \times [B]'$
$[B]' = \frac{[A][B]}{2[A]} = \frac{[B]}{2}$.
Therefore,the equilibrium concentration of $B$ will be halved.
226
MediumMCQ
For the reaction system $A_{(g)} \rightleftharpoons B_{(g)} + 2C_{(g)}$,if the equilibrium concentration of $B$ is doubled,the equilibrium concentration of $C$ will become ....
A
Double its original concentration
B
Half of its original concentration
C
$1 / \sqrt{2}$ times its original concentration
D
$\sqrt{2}$ times its original concentration

Solution

(C) The equilibrium constant expression for the reaction $A_{(g)} \rightleftharpoons B_{(g)} + 2C_{(g)}$ is given by: $K_c = \frac{[B][C]^2}{[A]}$.
Assuming the temperature remains constant,$K_c$ remains constant.
Let the initial concentrations be $[A]_1, [B]_1, [C]_1$ and the new concentrations be $[A]_2, [B]_2, [C]_2$.
Since $K_c = \frac{[B]_1[C]_1^2}{[A]_1} = \frac{[B]_2[C]_2^2}{[A]_2}$,and given $[B]_2 = 2[B]_1$,we have $\frac{[B]_1[C]_1^2}{[A]_1} = \frac{2[B]_1[C]_2^2}{[A]_2}$.
Assuming the volume of the system is constant and the reaction reaches a new equilibrium,the relationship simplifies to $[C]_2^2 = \frac{[C]_1^2}{2}$ (assuming $[A]$ remains relatively constant or considering the ratio change).
Therefore,$[C]_2 = \frac{[C]_1}{\sqrt{2}}$.
227
MediumMCQ
For the following reaction in gaseous phase $CO_{(g)} + \frac{1}{2} O_{2(g)} \to CO_{2(g)}$,$K_p/K_c$ is
A
$(RT)^{1/2}$
B
$(RT)^{-1/2}$
C
$(RT)$
D
$(RT)^{-1}$

Solution

(B) The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction $CO_{(g)} + \frac{1}{2} O_{2(g)} \to CO_{2(g)}$,the change in the number of moles of gaseous species is $\Delta n_g = n_{p(g)} - n_{r(g)} = 1 - (1 + 0.5) = -0.5$ or $-\frac{1}{2}$.
Substituting this into the equation,we get $K_p = K_c(RT)^{-\frac{1}{2}}$.
Therefore,the ratio $\frac{K_p}{K_c} = (RT)^{-\frac{1}{2}}$.
228
MediumMCQ
Assertion : For reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the unit of $K_C$ is $L^2 \, mol^{-2}$.
Reason : For the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the equilibrium constant $K_C = \frac{[NH_3]^2}{[N_2][H_2]^3}$.
A
If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
B
If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
C
If the Assertion is correct but Reason is incorrect.
D
If both the Assertion and Reason are incorrect.

Solution

(A) For the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the equilibrium constant expression is $K_C = \frac{[NH_3]^2}{[N_2][H_2]^3}$.
The units for concentration are $mol \, L^{-1}$.
Substituting these units into the expression: $K_C = \frac{(mol \, L^{-1})^2}{(mol \, L^{-1}) \times (mol \, L^{-1})^3} = \frac{mol^2 \, L^{-2}}{mol^4 \, L^{-4}} = mol^{-2} \, L^2$.
Thus,the unit of $K_C$ is $L^2 \, mol^{-2}$.
Both the Assertion and Reason are correct,and the Reason provides the correct explanation for the Assertion.
229
Easy
For the equilibrium,$2 \ NOCl \ (g) \rightleftharpoons 2 \ NO \ (g) + Cl_{2} \ (g)$,the value of the equilibrium constant,$K_{c}$ is $3.75 \times 10^{-6}$ at $1069 \ K$. Calculate the $K_{p}$ for the reaction at this temperature?

Solution

(N/A) The relationship between $K_{p}$ and $K_{c}$ is given by the equation: $K_{p} = K_{c}(RT)^{\Delta n}$.
For the given reaction: $2 \ NOCl \ (g) \rightleftharpoons 2 \ NO \ (g) + Cl_{2} \ (g)$.
The change in the number of moles of gaseous products and reactants is $\Delta n = (2 + 1) - 2 = 1$.
Given $K_{c} = 3.75 \times 10^{-6}$,$R = 0.0831 \ \text{L bar K}^{-1} \text{mol}^{-1}$,and $T = 1069 \ K$.
Substituting the values: $K_{p} = 3.75 \times 10^{-6} \times (0.0831 \times 1069)^{1}$.
$K_{p} = 3.75 \times 10^{-6} \times 88.8339$.
$K_{p} \approx 3.33 \times 10^{-4}$.
230
Medium
The value of $K_{p}$ for the reaction,$CO_{2(g)} + C_{(s)} \rightleftharpoons 2CO_{(g)}$ is $3.0$ at $1000 \ K$. If initially $P_{CO_{2}} = 0.48 \ bar$ and $P_{CO} = 0 \ bar$ and pure graphite is present,calculate the equilibrium partial pressures of $CO$ and $CO_{2}$.

Solution

(A) For the reaction,let $x$ be the decrease in pressure of $CO_{2}$.
$CO_{2(g)} + C_{(s)} \rightleftharpoons 2CO_{(g)}$
Initial pressure: $0.48 \ bar$ and $0 \ bar$.
At equilibrium: $(0.48 - x) \ bar$ and $2x \ bar$.
$K_{p} = \frac{p_{CO}^{2}}{p_{CO_{2}}} = 3.0$.
Substituting the values: $\frac{(2x)^{2}}{0.48 - x} = 3.0$.
$4x^{2} = 3(0.48 - x) \implies 4x^{2} + 3x - 1.44 = 0$.
Using the quadratic formula $x = \frac{-b \pm \sqrt{b^{2} - 4ac}}{2a}$:
$x = \frac{-3 \pm \sqrt{9 - 4(4)(-1.44)}}{8} = \frac{-3 \pm \sqrt{9 + 23.04}}{8} = \frac{-3 \pm 5.66}{8}$.
Taking the positive root: $x = \frac{2.66}{8} = 0.3325 \approx 0.33$.
Equilibrium partial pressures are:
$p_{CO} = 2x = 2 \times 0.33 = 0.66 \ bar$.
$p_{CO_{2}} = 0.48 - 0.33 = 0.15 \ bar$.
231
Medium
Find out the value of $K_{c}$ for each of the following equilibria from the value of $K_{p}$:
$(i)$ $2 NOCl_{(g)} \longleftrightarrow 2 NO_{(g)} + Cl_{2(g)}$; $K_{p} = 1.8 \times 10^{-2}$ at $500 \ K$
$(ii)$ $CaCO_{3(s)} \longleftrightarrow CaO_{(s)} + CO_{2(g)}$; $K_{p} = 167$ at $1073 \ K$

Solution

(N/A) The relation between $K_{p}$ and $K_{c}$ is given by the formula:
$K_{p} = K_{c}(RT)^{\Delta n}$
$(i)$ For the reaction $2 NOCl_{(g)} \longleftrightarrow 2 NO_{(g)} + Cl_{2(g)}$:
$\Delta n = (2 + 1) - 2 = 1$
$R = 0.0831 \ \text{bar L K}^{-1} \text{mol}^{-1}$
$T = 500 \ K$
$K_{p} = 1.8 \times 10^{-2}$
$1.8 \times 10^{-2} = K_{c}(0.0831 \times 500)^{1}$
$K_{c} = \frac{1.8 \times 10^{-2}}{41.55} = 4.33 \times 10^{-4}$
$(ii)$ For the reaction $CaCO_{3(s)} \longleftrightarrow CaO_{(s)} + CO_{2(g)}$:
$\Delta n = 1 - 0 = 1$
$R = 0.0831 \ \text{bar L K}^{-1} \text{mol}^{-1}$
$T = 1073 \ K$
$K_{p} = 167$
$167 = K_{c}(0.0831 \times 1073)^{1}$
$K_{c} = \frac{167}{89.1663} \approx 1.87$
232
MediumMCQ
At $450 \, K$,$K_{p} = 2.0 \times 10^{10} \, bar^{-1}$ for the given reaction at equilibrium.
$2 \, SO_{2(g)} + O_{2(g)} \longleftrightarrow 2 \, SO_{3(g)}$
What is $K_{c}$ at this temperature?
A
$7.48 \times 10^{11} \, M^{-1}$
B
$2.0 \times 10^{10} \, M^{-1}$
C
$5.35 \times 10^{8} \, M^{-1}$
D
$7.48 \times 10^{10} \, M^{-1}$

Solution

(A) For the given reaction,$\Delta n = 2 - 3 = -1$.
Given: $T = 450 \, K$,$R = 0.0831 \, L \, bar \, K^{-1} \, mol^{-1}$,$K_{p} = 2.0 \times 10^{10} \, bar^{-1}$.
The relationship between $K_{p}$ and $K_{c}$ is $K_{p} = K_{c}(RT)^{\Delta n}$.
Substituting the values: $2.0 \times 10^{10} = K_{c} (0.0831 \times 450)^{-1}$.
$K_{c} = 2.0 \times 10^{10} \times (0.0831 \times 450) = 2.0 \times 10^{10} \times 37.395$.
$K_{c} = 74.79 \times 10^{10} = 7.48 \times 10^{11} \, M^{-1}$.
233
Medium
$A$ sample of $HI_{(g)}$ is placed in a flask at a pressure of $0.2 \ atm$. At equilibrium,the partial pressure of $HI_{(g)}$ is $0.04 \ atm$. What is $K_{p}$ for the given equilibrium?
$2 HI_{(g)} \longleftrightarrow H_{2_{(g)}} + I_{2_{(g)}}$

Solution

(4.0) The initial pressure of $HI$ is $0.2 \ atm$. At equilibrium,it has a partial pressure of $0.04 \ atm$. The decrease in the pressure of $HI$ is $0.2 - 0.04 = 0.16 \ atm$.
The reaction is: $2HI_{(g)} \longleftrightarrow H_{2_{(g)}} + I_{2_{(g)}}$
Initial pressure: $0.2 \ atm, 0, 0$
At equilibrium: $0.04 \ atm, \frac{0.16}{2} \ atm, \frac{0.16}{2} \ atm$
At equilibrium: $0.04 \ atm, 0.08 \ atm, 0.08 \ atm$
$K_{p} = \frac{p_{H_{2}} \times p_{I_{2}}}{p_{HI}^2}$
$K_{p} = \frac{0.08 \times 0.08}{(0.04)^{2}} = \frac{0.0064}{0.0016} = 4.0$
Hence,the value of $K_{p}$ is $4.0$.
234
Medium
$K_{p} = 0.04 \ atm$ at $899 \ K$ for the equilibrium shown below. What is the equilibrium pressure of $C_{2}H_{6}$ when it is placed in a flask at $4.0 \ atm$ pressure and allowed to come to equilibrium?
$C_{2}H_{6(g)} \longleftrightarrow C_{2}H_{4(g)} + H_{2(g)}$

Solution

(D) Let $p$ be the pressure exerted by ethene and hydrogen gas (each) at equilibrium.
According to the reaction:
$C_{2}H_{6(g)} \longleftrightarrow C_{2}H_{4(g)} + H_{2(g)}$
Initial pressure: $4.0 \ atm$,$0$,$0$
At equilibrium: $(4.0 - p) \ atm$,$p \ atm$,$p \ atm$
We can write the expression for $K_{p}$ as:
$K_{p} = \frac{p_{C_{2}H_{4}} \times p_{H_{2}}}{p_{C_{2}H_{6}}}$
Substituting the values:
$\frac{p \times p}{4.0 - p} = 0.04$
$p^{2} = 0.16 - 0.04p$
$p^{2} + 0.04p - 0.16 = 0$
Using the quadratic formula $p = \frac{-b \pm \sqrt{b^{2} - 4ac}}{2a}$:
$p = \frac{-0.04 \pm \sqrt{(0.04)^{2} - 4 \times 1 \times (-0.16)}}{2 \times 1}$
$p = \frac{-0.04 \pm \sqrt{0.0016 + 0.64}}{2}$
$p = \frac{-0.04 \pm \sqrt{0.6416}}{2}$
$p \approx \frac{-0.04 + 0.801}{2} \approx 0.3805 \ atm$
Hence,at equilibrium,the pressure of $C_{2}H_{6}$ is:
$p_{C_{2}H_{6}} = 4.0 - 0.3805 = 3.6195 \ atm \approx 3.62 \ atm$
235
Medium
One of the reactions that takes place in producing steel from iron ore is the reduction of iron $(II)$ oxide by carbon monoxide to give iron metal and $CO_{2}$.
$FeO(s) + CO(g) \longleftrightarrow Fe(s) + CO_{2}(g);$ $K_{p} = 0.265$ at $1050 \, K$
What are the equilibrium partial pressures of $CO$ and $CO_{2}$ at $1050 \, K$ if the initial partial pressures are: $p_{CO} = 1.4 \, atm$ and $p_{CO_{2}} = 0.80 \, atm$?

Solution

(N/A) For the given reaction,
$FeO(s) + CO(g) \longleftrightarrow Fe(s) + CO_{2}(g)$
Initially: $p_{CO} = 1.4 \, atm$,$p_{CO_{2}} = 0.80 \, atm$
$Q_{p} = \frac{p_{CO_{2}}}{p_{CO}} = \frac{0.80}{1.4} \approx 0.571$
Since $Q_{p} > K_{p}$ $(0.571 > 0.265)$,the reaction proceeds in the backward direction.
Let $x$ be the change in pressure at equilibrium.
$K_{p} = \frac{p_{CO_{2}}}{p_{CO}} = \frac{0.80 - x}{1.4 + x} = 0.265$
$0.80 - x = 0.265(1.4 + x)$
$0.80 - x = 0.371 + 0.265x$
$1.265x = 0.429$
$x = 0.339 \, atm$
Equilibrium partial pressures:
$p_{CO_{2}} = 0.80 - 0.339 = 0.461 \, atm$
$p_{CO} = 1.4 + 0.339 = 1.739 \, atm$
236
Difficult
At $1127 \, K$ and $1 \, atm$ pressure,a gaseous mixture of $CO$ and $CO_2$ in equilibrium with solid carbon has $90.55 \%$ $CO$ by mass.
$C_{(s)} + CO_{2(g)} \longleftrightarrow 2CO_{(g)}$
Calculate $K_c$ for this reaction at the above temperature.

Solution

(N/A) Let the total mass of the gaseous mixture be $100 \, g$.
Mass of $CO = 90.55 \, g$
Mass of $CO_2 = 100 - 90.55 = 9.45 \, g$
Moles of $CO$,$n_{CO} = \frac{90.55}{28} = 3.234 \, mol$
Moles of $CO_2$,$n_{CO_2} = \frac{9.45}{44} = 0.215 \, mol$
Total moles = $3.234 + 0.215 = 3.449 \, mol$
Partial pressure of $CO$,$p_{CO} = \frac{3.234}{3.449} \times 1 = 0.938 \, atm$
Partial pressure of $CO_2$,$p_{CO_2} = \frac{0.215}{3.449} \times 1 = 0.062 \, atm$
$K_P = \frac{(p_{CO})^2}{p_{CO_2}} = \frac{(0.938)^2}{0.062} = 14.19$
For the reaction,$\Delta n = 2 - 1 = 1$
Using $K_P = K_C(RT)^{\Delta n}$:
$14.19 = K_C(0.0821 \times 1127)^1$
$K_C = \frac{14.19}{92.5257} \approx 0.153$.
237
Medium
Dihydrogen gas used in Haber's process is produced by reacting methane from natural gas with high temperature steam. The first stage of two stage reaction involves the formation of $CO$ and $H_2$. In second stage,$CO$ formed in first stage is reacted with more steam in water gas shift reaction,
$CO_{(g)} + H_2O_{(g)} \longleftrightarrow CO_{2(g)} + H_{2(g)}$
If a reaction vessel at $400^{\circ}C$ is charged with an equimolar mixture of $CO$ and steam such that $P_{CO} = P_{H_2O} = 4.0 \ bar,$ what will be the partial pressure of $H_2$ at equilibrium? $K_p = 10.1$ at $400^{\circ}C$

Solution

(D) Let the partial pressure of both carbon dioxide and hydrogen gas be $p$. The given reaction is:
$CO_{(g)} + H_2O_{(g)} \longleftrightarrow CO_{2(g)} + H_{2(g)}$
Initial: $4.0 \ bar \quad 4.0 \ bar \quad 0 \quad 0$
At equilibrium: $(4.0 - p) \ bar \quad (4.0 - p) \ bar \quad p \ bar \quad p \ bar$
It is given that $K_P = 10.1$
Now,
$\frac{P_{CO_2} \times P_{H_2}}{P_{CO} \times P_{H_2O}} = K_P$
$\frac{p \times p}{(4.0 - p)(4.0 - p)} = 10.1$
$\frac{p}{4.0 - p} = \sqrt{10.1} = 3.178$
$p = 12.712 - 3.178p$
$4.178p = 12.712$
$p = 3.04 \ bar$
Hence,at equilibrium,the partial pressure of $H_2$ will be $3.04 \ bar$.
238
Medium
Obtain the relation between equilibrium constant $K$ and $K'$ for forward and reverse reactions.

Solution

(N/A) Consider the synthesis of $HI$ as the forward reaction:
$H_{2(g)} + I_{2(g)} \rightleftharpoons 2 HI_{(g)} \quad (Eq.-i)$
According to the law of chemical equilibrium,the equilibrium constant $K$ is:
$K = \frac{[HI]^2}{[H_2][I_2]} = X \quad (Eq.-ii)$
For the reverse reaction (decomposition of $HI$):
$2 HI_{(g)} \rightleftharpoons H_{2(g)} + I_{2(g)} \quad (Eq.-iii)$
The equilibrium constant $K'$ for this reverse reaction is:
$K' = \frac{[H_2][I_2]}{[HI]^2} = \frac{1}{X} \quad (Eq.-iv)$
Comparing $(Eq.-ii)$ and $(Eq.-iv)$,we get:
$K = \frac{1}{K'} \quad \text{or} \quad K \times K' = 1$
Thus,the equilibrium constant for the reverse reaction is the inverse of the equilibrium constant for the forward reaction.
239
MediumMCQ
In the synthesis of $HI$,the amounts of $H_{2(g)}$,$I_{2(g)}$,and $HI_{(g)}$ at equilibrium were found to be $0.8 \ mol$,$0.8 \ mol$,and $2.4 \ mol$ respectively in a $10 \ L$ vessel. Calculate the equilibrium constant $(K_c)$ for the reaction $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$ and the equilibrium constant for the reverse reaction.
A
$K_c = 9, K_c' = 0.11$
B
$K_c = 0.11, K_c' = 9$
C
$K_c = 9, K_c' = 0.09$
D
$K_c = 0.09, K_c' = 9$

Solution

(A) The reaction is $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$.
Concentrations at equilibrium are:
$[H_2] = \frac{0.8 \ mol}{10 \ L} = 0.08 \ M$
$[I_2] = \frac{0.8 \ mol}{10 \ L} = 0.08 \ M$
$[HI] = \frac{2.4 \ mol}{10 \ L} = 0.24 \ M$
The equilibrium constant $K_c$ is given by:
$K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{(0.24)^2}{(0.08)(0.08)} = \frac{0.0576}{0.0064} = 9$.
For the reverse reaction $2HI_{(g)} \rightleftharpoons H_{2(g)} + I_{2(g)}$,the equilibrium constant $K_c'$ is:
$K_c' = \frac{1}{K_c} = \frac{1}{9} \approx 0.111$.
240
DifficultMCQ
For the reactions $(i) \ N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$ and $(ii) \ \frac{1}{2}N_{2(g)} + \frac{3}{2}H_{2(g)} \rightleftharpoons NH_{3(g)}$,if their equilibrium constants are $K_c(1)$ and $K_c(2)$ respectively,what is the relation between them?
A
$K_c(1) = K_c(2)$
B
$K_c(1) = (K_c(2))^2$
C
$K_c(2) = (K_c(1))^2$
D
$K_c(1) = \sqrt{K_c(2)}$

Solution

(B) For reaction $(i)$: $K_c(1) = \frac{[NH_3]^2}{[N_2][H_2]^3}$
For reaction $(ii)$: $K_c(2) = \frac{[NH_3]}{[N_2]^{1/2}[H_2]^{3/2}}$
Squaring the expression for $K_c(2)$: $(K_c(2))^2 = \left( \frac{[NH_3]}{[N_2]^{1/2}[H_2]^{3/2}} \right)^2 = \frac{[NH_3]^2}{[N_2][H_2]^3} = K_c(1)$
Therefore,the relation is $K_c(1) = (K_c(2))^2$ or $K_c(2) = \sqrt{K_c(1)}$.
241
Medium
Explain why the partial pressure $p$ of a gas is proportional to its concentration $c$.

Solution

(N/A) For reactions involving gases,it is often convenient to express the equilibrium constant in terms of partial pressure.
The ideal gas equation is given by $pV = nRT$ (Eq. $i$).
Rearranging for pressure,we get $p = (n/V)RT$ (Eq. $ii$).
Since concentration $c$ is defined as $n/V$ (in $mol \ L^{-1}$ or $mol \ dm^{-3}$),we can substitute this into the equation:
$p = cRT$ (Eq. $iii$).
Here,$R$ is the universal gas constant,$T$ is the temperature in Kelvin,and $c$ is the molar concentration of the gas.
At a constant temperature $(T)$,the product $RT$ is constant.
Therefore,the partial pressure $p$ of the gas is directly proportional to its concentration $c$,expressed as $p \propto c$ or $p \propto [\text{gas}]$.
242
MediumMCQ
For the gaseous reaction $aA + bB \rightleftharpoons cC + dD$,the relation between $K_p$ and $K_c$ is:
A
$K_p = K_c(RT)^{\Delta n}$
B
$K_c = K_p(RT)^{\Delta n}$
C
$K_p = K_c(RT)^{-\Delta n}$
D
$K_p = K_c + (RT)^{\Delta n}$

Solution

(A) For the gaseous reaction $aA_{(g)} + bB_{(g)} \rightleftharpoons cC_{(g)} + dD_{(g)}$,the partial pressure of each component is given by $P_i = [i]RT$.
The equilibrium constant $K_p$ is defined as:
$K_p = \frac{P_C^c \cdot P_D^d}{P_A^a \cdot P_B^b}$
Substituting $P_i = [i]RT$ into the expression:
$K_p = \frac{([C]RT)^c \cdot ([D]RT)^d}{([A]RT)^a \cdot ([B]RT)^b}$
$K_p = \frac{[C]^c \cdot [D]^d}{[A]^a \cdot [B]^b} \cdot (RT)^{(c+d)-(a+b)}$
Since $K_c = \frac{[C]^c \cdot [D]^d}{[A]^a \cdot [B]^b}$ and $\Delta n = (c+d) - (a+b)$,we get:
$K_p = K_c(RT)^{\Delta n}$
243
Difficult
What is the relationship between $K_{p}$ and $K_{c}$ when $\Delta n = 0$,$\Delta n > 0$,and $\Delta n < 0$?

Solution

The relationship is given by the equation $K_{p} = K_{c}(RT)^{\Delta n}$.
$1$. If $\Delta n = 0$,then $K_{p} = K_{c}(RT)^{0} = K_{c}$.
Example: $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$,where $\Delta n = 2 - (1+1) = 0$.
$2$. If $\Delta n > 0$,then $K_{p} = K_{c}(RT)^{\text{positive value}}$,which implies $K_{p} > K_{c}$.
Example: $PCl_{5(g)} \rightleftharpoons PCl_{3(g)} + Cl_{2(g)}$,where $\Delta n = (1+1) - 1 = +1$.
$3$. If $\Delta n < 0$,then $K_{p} = K_{c}(RT)^{\text{negative value}}$,which implies $K_{p} < K_{c}$.
Example: $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,where $\Delta n = 2 - (1+3) = -2$.
244
MediumMCQ
For the reaction $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$,$0.4 \ mol$ of $H_2$ and $I_2$ each are taken in a $2 \ L$ vessel. If $0.5 \ mol$ of $HI$ is formed at equilibrium,calculate the equilibrium constant $K_c$ and $K_p$.
A
$11.11$
B
$22.22$
C
$5.55$
D
$44.44$

Solution

(A) The reaction is $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$.
Initial moles: $H_2 = 0.4$,$I_2 = 0.4$,$HI = 0$.
At equilibrium,$n(HI) = 0.5 \ mol$.
From stoichiometry,$2 \ mol$ of $HI$ are formed from $1 \ mol$ of $H_2$ and $1 \ mol$ of $I_2$.
So,$0.5 \ mol$ of $HI$ is formed from $0.25 \ mol$ of $H_2$ and $0.25 \ mol$ of $I_2$.
Equilibrium moles: $n(H_2) = 0.4 - 0.25 = 0.15 \ mol$,$n(I_2) = 0.4 - 0.25 = 0.15 \ mol$,$n(HI) = 0.5 \ mol$.
Concentrations in $2 \ L$ vessel: $[H_2] = 0.15/2 = 0.075 \ M$,$[I_2] = 0.075 \ M$,$[HI] = 0.5/2 = 0.25 \ M$.
$K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{(0.25)^2}{(0.075)(0.075)} = \frac{0.0625}{0.005625} = 11.11$.
Since $\Delta n_g = 2 - (1+1) = 0$,$K_p = K_c(RT)^{\Delta n_g} = K_c = 11.11$.
245
MediumMCQ
$1 \ mol$ $N_2$ and $3 \ mol$ $H_2$ are heated at $473 \ K$ and $100 \ atm$ pressure. At equilibrium,the number of moles of $NH_3$ is $0.5 \ mol$. Calculate the equilibrium constant $K_p$ of the given reaction: $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$
A
$7.5 \times 10^{-6} \ atm^{-2}$
B
$1.5 \times 10^{-5} \ atm^{-2}$
C
$5.0 \times 10^{-6} \ atm^{-2}$
D
$2.5 \times 10^{-6} \ atm^{-2}$

Solution

(A) The reaction is: $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$
Initial moles: $N_2 = 1, H_2 = 3, NH_3 = 0$
At equilibrium: $N_2 = 1 - x, H_2 = 3 - 3x, NH_3 = 2x$
Given $2x = 0.5 \ mol$,so $x = 0.25 \ mol$.
Equilibrium moles: $N_2 = 0.75, H_2 = 2.25, NH_3 = 0.5$.
Total moles at equilibrium = $0.75 + 2.25 + 0.5 = 3.5 \ mol$.
Partial pressures: $P_{N_2} = (0.75/3.5) \times 100 = 21.43 \ atm$,$P_{H_2} = (2.25/3.5) \times 100 = 64.29 \ atm$,$P_{NH_3} = (0.5/3.5) \times 100 = 14.28 \ atm$.
$K_p = \frac{(P_{NH_3})^2}{(P_{N_2})(P_{H_2})^3} = \frac{(14.28)^2}{(21.43)(64.29)^3} \approx 7.5 \times 10^{-6} \ atm^{-2}$.
246
MediumMCQ
At $673 \ K$,for the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the equilibrium constant $K_c = 0.50$. Calculate $K_p$ at this temperature. (Given $R = 0.082 \ L \ atm \ K^{-1} \ mol^{-1}$)
A
$1.64 \times 10^{-4}$
B
$1.64 \times 10^{-3}$
C
$1.64 \times 10^{-2}$
D
$1.64 \times 10^{-5}$

Solution

(A) The relationship between $K_p$ and $K_c$ is given by the formula: $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the change in the number of moles of gaseous species is $\Delta n_g = n_p - n_r = 2 - (1 + 3) = 2 - 4 = -2$.
Given $K_c = 0.50$,$R = 0.082 \ L \ atm \ K^{-1} \ mol^{-1}$,and $T = 673 \ K$.
Substituting these values into the formula: $K_p = 0.50 \times (0.082 \times 673)^{-2}$.
$K_p = 0.50 \times (55.186)^{-2}$.
$K_p = 0.50 \times \frac{1}{(55.186)^2} = 0.50 \times \frac{1}{3045.49} \approx 1.64 \times 10^{-4}$.
247
MediumMCQ
For the reaction $N_2O_{4_{(g)}} \rightleftharpoons 2NO_{2_{(g)}}$ at $298 \ K$,the equilibrium constant $K_p$ is $0.14 \ atm$. Calculate the value of $K_c$. $(R = 0.082 \ L \ atm \ K^{-1} \ mol^{-1})$
A
$5.73 \times 10^{-3} \ M$
B
$1.74 \times 10^{-2} \ M$
C
$0.14 \ M$
D
$0.057 \ M$

Solution

(A) The relationship between $K_p$ and $K_c$ is given by the equation: $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction $N_2O_{4_{(g)}} \rightleftharpoons 2NO_{2_{(g)}}$,the change in the number of moles of gaseous products and reactants is $\Delta n_g = 2 - 1 = 1$.
Given $K_p = 0.14 \ atm$,$R = 0.082 \ L \ atm \ K^{-1} \ mol^{-1}$,and $T = 298 \ K$.
Substituting these values into the equation: $0.14 = K_c \times (0.082 \times 298)^1$.
$K_c = \frac{0.14}{0.082 \times 298}$.
$K_c = \frac{0.14}{24.436} \approx 5.73 \times 10^{-3} \ M$.
248
Difficult
For the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,the equilibrium constant $K_p = 41$ at $400 \ K$. Calculate $K_c$ for the following reactions at $400 \ K$:
$(a)$ $2N_{2(g)} + 6H_{2(g)} \rightleftharpoons 4NH_{3(g)}$
$(b)$ $2NH_{3(g)} \rightleftharpoons N_{2(g)} + 3H_{2(g)}$
$(c)$ $\frac{1}{2}N_{2(g)} + \frac{3}{2}H_{2(g)} \rightleftharpoons NH_{3(g)}$

Solution

(A) The relationship between $K_p$ and $K_c$ is given by $K_p = K_c(RT)^{\Delta n_g}$.
For the reaction $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$,$\Delta n_g = 2 - (1 + 3) = -2$.
Given $K_p = 41$,$T = 400 \ K$,and $R = 0.0821 \ L \ atm \ K^{-1} \ mol^{-1}$.
$41 = K_c(0.0821 \times 400)^{-2} \implies K_c = 41 \times (32.84)^2 \approx 44216.7$.
$(a)$ For $2N_{2(g)} + 6H_{2(g)} \rightleftharpoons 4NH_{3(g)}$,the reaction is multiplied by $2$. Thus,$K_c' = (K_c)^2 = (44216.7)^2 \approx 1.95 \times 10^9$.
$(b)$ For $2NH_{3(g)} \rightleftharpoons N_{2(g)} + 3H_{2(g)}$,the reaction is reversed and multiplied by $2$. Thus,$K_c'' = (1/K_c)^2 = (1/44216.7)^2 \approx 5.11 \times 10^{-10}$.
$(c)$ For $\frac{1}{2}N_{2(g)} + \frac{3}{2}H_{2(g)} \rightleftharpoons NH_{3(g)}$,the reaction is multiplied by $1/2$. Thus,$K_c''' = (K_c)^{1/2} = \sqrt{44216.7} \approx 210.28$.
249
MediumMCQ
For the reaction $2SO_{2(g)} + O_{2(g)} \rightleftharpoons 2SO_{3(g)}$ at $298 \ K$,the $K_c$ is $7 \times 10^{25}$. Calculate $K_c$ for the reaction $SO_{3(g)} \rightleftharpoons SO_{2(g)} + \frac{1}{2}O_{2(g)}$.
A
$1.195 \times 10^{-13}$
B
$1.428 \times 10^{-26}$
C
$3.78 \times 10^{-13}$
D
$7.0 \times 10^{-25}$

Solution

(C) Given the reaction: $2SO_{2(g)} + O_{2(g)} \rightleftharpoons 2SO_{3(g)}$ with equilibrium constant $K_{c1} = 7 \times 10^{25}$.
For the target reaction: $SO_{3(g)} \rightleftharpoons SO_{2(g)} + \frac{1}{2}O_{2(g)}$,we observe that the reaction is reversed and multiplied by a factor of $\frac{1}{2}$.
Therefore,the new equilibrium constant $K_{c2}$ is given by $K_{c2} = (\frac{1}{K_{c1}})^{1/2}$.
$K_{c2} = (\frac{1}{7 \times 10^{25}})^{1/2} = \frac{1}{\sqrt{7} \times 10^{12.5}} = \frac{1}{2.6457 \times 10^{12.5}} \approx 0.378 \times 10^{-12.5} = 3.78 \times 10^{-13}$.
250
MediumMCQ
For the reaction $2NOCl_{(g)} \rightleftharpoons 2NO_{(g)} + Cl_{2(g)}$,the value of equilibrium constant $K_p$ is $0.033 \ bar$ at $1060 \ K$. Calculate the value of $K_c$.
A
$3.7 \times 10^{-4}$
B
$3.1 \times 10^{-5}$
C
$4.2 \times 10^{-4}$
D
$2.8 \times 10^{-5}$

Solution

(A) The relationship between $K_p$ and $K_c$ is given by the formula: $K_p = K_c(RT)^{\Delta n}$.
For the reaction $2NOCl_{(g)} \rightleftharpoons 2NO_{(g)} + Cl_{2(g)}$,the change in the number of moles of gaseous species is $\Delta n = (2 + 1) - 2 = 1$.
Given $K_p = 0.033 \ bar$,$T = 1060 \ K$,and $R = 0.08314 \ L \ bar \ K^{-1} \ mol^{-1}$.
Substituting the values: $0.033 = K_c \times (0.08314 \times 1060)^1$.
$K_c = \frac{0.033}{0.08314 \times 1060} = \frac{0.033}{88.1284} \approx 3.74 \times 10^{-4} \ mol \ L^{-1}$.

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