Mix Examples - Chemical Reactions and Equations Questions in English

(N/A) An example of an exothermic combination reaction is the formation of water from hydrogen and oxygen: $2H_{2}(g) + O_{2}(g) \rightarrow 2H_{2}O(l) + \text{Heat}$.
$(b)$ In the reaction $H_{2}S + Cl_{2} \rightarrow 2HCl + S$:
- The oxidizing agent is $Cl_{2}$ because it gains electrons (or helps in the removal of hydrogen).
- The reducing agent is $H_{2}S$ because it loses hydrogen (or provides electrons).
$(c)$ The phenomenon is known as Rancidity. It can be prevented by storing food in airtight containers to prevent oxidation.

(N/A) $(i)$ The metal is copper $(Cu)$.
The black coloured substance formed is copper$(II)$ oxide $(CuO)$.
$(ii)$ The balanced chemical equations are:
Reaction $1$ (Oxidation of copper): $2Cu(s) + O_2(g) \xrightarrow{\Delta} 2CuO(s)$
Reaction $2$ (Reduction of copper oxide): $CuO(s) + H_2(g) \xrightarrow{\Delta} Cu(s) + H_2O(g)$

(N/A) Rancidity is the process of oxidation of fats and oils in food materials,which leads to an unpleasant smell and taste.
It can be prevented by the following methods:
$(i)$ Adding antioxidants to foods containing fats and oils.
$(ii)$ Flushing food containers with nitrogen gas to remove oxygen.

(N/A) It is a photochemical decomposition reaction. It can be defined as the decomposition of a substance in the presence of sunlight in which a single reactant breaks down to give simpler products.
$(b)$ The white silver chloride changes to grey,as it decomposes into silver and chlorine in the presence of sunlight.
The chemical equation is:
$2 AgCl (s) \xrightarrow{\text{Sunlight}} 2 Ag (s) + Cl_2 (g)$
$(c)$ This salt is used in black and white photography.

(N/A) This is a combination reaction. It is defined as a reaction in which a single product is formed from two or more reactants.
$(b)$ The balanced chemical equation is: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$. The chemical name of the product obtained is calcium hydroxide.
$(c)$ Two observations made during the reaction are:
$(i)$ The reaction occurs vigorously with the evolution of heat (exothermic reaction).
$(ii)$ The beaker becomes hot to the touch due to the release of thermal energy.

(N/A) $(i)$ The metal $X$ is Copper $(Cu)$. The process responsible for this change is known as corrosion.
$(ii)$ The green coating is Basic Copper Carbonate. Its chemical formula is $CuCO_{3} \cdot Cu(OH)_{2}$.
$(iii)$ Two important methods to prevent corrosion are:
$1$. Painting or oiling/greasing the surface.
$2$. Galvanisation or alloying.

(N/A) $(i)$ Take $2 \, g$ of silver chloride in a china dish.
(ii) Place this china dish in sunlight for some time.
(iii) Observe the colour of silver chloride after some time. Silver chloride turns grey in sunlight due to the decomposition of $AgCl$ into $Ag$ and $Cl_2$.
Chemical equation:
$2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g)$
Use: This reaction is used in black and white photography.

(N/A) $(i)$ Before heating: Pale green. After heating: Brown or reddish-brown.
$(ii)$ The gases produced are sulphur dioxide $(SO_{2})$ and sulphur trioxide $(SO_{3})$.
$(iii)$ The chemical equation for the thermal decomposition of ferrous sulphate is:
$2FeSO_{4}(s) \xrightarrow{\text{Heat}} Fe_{2}O_{3}(s) + SO_{2}(g) + SO_{3}(g)$

(N/A) $(i)$ Decomposition reactions require energy in the form of heat,light,or electricity to break down the reactants,meaning energy is absorbed.
$(ii)$ Iron is more reactive than copper. When an iron nail is dipped in a copper sulphate solution,iron displaces copper from the solution to form iron sulphate,which is light green in colour.
$(iii)$ During respiration,glucose $(C_6H_{12}O_6)$ combines with oxygen in the cells of our body to produce carbon dioxide,water,and energy. Since energy is released during this process,it is considered an exothermic reaction.

(N/A) The balanced chemical equation for photosynthesis is:
$6CO_{2}(g) + 12H_{2}O(l) \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} C_{6}H_{12}O_{6}(aq) + 6O_{2}(g) + 6H_{2}O(l)$
Explanation:
$1$. $6$ molecules of carbon dioxide $(CO_{2})$ in gaseous state react with $12$ molecules of water $(H_{2}O)$ in liquid state.
$2$. The reaction occurs in the presence of sunlight and chlorophyll (the green pigment in plants).
$3$. The products formed are $1$ molecule of glucose $(C_{6}H_{12}O_{6})$ in aqueous state,$6$ molecules of oxygen $(O_{2})$ in gaseous state,and $6$ molecules of water $(H_{2}O)$ in liquid state.

(N/A) $(i)$ Electrolysis of water is an endothermic reaction because it requires electrical energy to break the bonds of water molecules.
$(ii)$ Burning of natural gas (methane) is an exothermic reaction as it releases a large amount of heat energy during combustion.
$(iii)$ Decomposition of calcium carbonate is an endothermic reaction because it requires heat energy to decompose into calcium oxide and carbon dioxide.
$(iv)$ Burning of magnesium ribbon in air is an exothermic reaction as it releases heat and light energy during the formation of magnesium oxide.

(N/A) Corrosion: The process in which metals are eaten away gradually by the action of air,moisture,or a chemical on their surface.
$(b)$ The corrosion of iron is called rusting.
$(c)$ Corrosion of silver is recognised by the development of a black coating on the surface of the metal.

(N/A) Corrosion of iron is a serious problem because it causes significant structural damage to bridges,iron railings,ships,and all objects made of metals,especially those made of iron. Every year,an enormous amount of money is spent to replace damaged iron structures.
$(b)$ Corrosion can be prevented by several methods:
$1$. Painting: Applying a coat of paint on the surface of the iron object.
$2$. Galvanization: Coating iron with a thin layer of zinc.
$3$. Electroplating: Depositing a layer of a non-corrosive metal over the iron surface.
$4$. Greasing or Oiling: Applying oil or grease to moving parts of machines.

(N/A) To balance the equation $Fe(s) + H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)$:
$1$. Balance $Fe$: $3Fe(s) + H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)$
$2$. Balance $O$: $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)$
$3$. Balance $H$: $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$
The balanced equation is $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$.
$(b)$ The reaction $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + NaCl(aq)$ is a double displacement reaction because there is an exchange of ions between the reactants to form new products.

(N/A) $(i)$ The black coloured substance is formed because copper reacts with atmospheric oxygen upon heating to form copper $(II)$ oxide.
$(ii)$ This black substance is copper $(II)$ oxide $(CuO)$.
$(iii)$ The chemical equation for the reaction is: $2Cu(s) + O_2(g) \xrightarrow{\text{Heat}} 2CuO(s)$

(N/A) The chemical equation for the reaction between hydrogen gas and nitrogen gas to form ammonia is:
$3H_{2}(g) + N_{2}(g) \rightarrow 2NH_{3}(g)$
$(b)$ The chemical equation for the combustion of hydrogen sulphide gas in air (oxygen) to produce water and sulphur dioxide is:
$2H_{2}S(g) + 3O_{2}(g) \rightarrow 2H_{2}O(g) + 2SO_{2}(g)$

(N/A) The unbalanced equation is: $BaCl_{2} + Al_{2}(SO_{4})_{3} \longrightarrow AlCl_{3} + BaSO_{4}$.
To balance,we ensure the number of atoms of each element is equal on both sides: $3BaCl_{2}(aq) + Al_{2}(SO_{4})_{3}(aq) \longrightarrow 2AlCl_{3}(aq) + 3BaSO_{4}(s)$.
$(b)$ The unbalanced equation is: $K + H_{2}O \longrightarrow KOH + H_{2}$.
To balance,we ensure the number of atoms of each element is equal on both sides: $2K(s) + 2H_{2}O(l) \longrightarrow 2KOH(aq) + H_{2}(g)$.

(A) The chemical equation for the reaction is:
$AlCl_{3}(aq) + 3NH_{4}OH(aq) \longrightarrow Al(OH)_{3}(s) \downarrow + 3NH_{4}Cl(aq)$
Explanation:
$1$. Aluminium chloride $(AlCl_{3})$ reacts with ammonium hydroxide $(NH_{4}OH)$.
$2$. This is a double displacement reaction where aluminium hydroxide $(Al(OH)_{3})$ is formed as a gelatinous white precipitate,and ammonium chloride $(NH_{4}Cl)$ remains in the solution.
$3$. To balance the equation,we ensure the number of atoms of each element is equal on both sides: there is $1$ $Al$ atom,$3$ $Cl$ atoms,$3$ $N$ atoms,$15$ $H$ atoms,and $3$ $O$ atoms on both sides.

(N/A) $(i)$ The balanced chemical equation is:
$Na_{2}SO_{4}(aq) + BaCl_{2}(aq) \longrightarrow BaSO_{4}(s) \downarrow + 2NaCl(aq)$
$(ii)$ This is a double displacement reaction (specifically a precipitation reaction).
$(iii)$ Yes,a precipitate is formed.
$(iv)$ The precipitate formed is barium sulphate $(BaSO_{4})$,which is white in colour.

(N/A) The chemical reaction involves phosphorus $(P_4)$ reacting with chlorine gas $(Cl_2)$ to produce phosphorus pentachloride $(PCl_5)$.
The unbalanced equation is: $P_4 + Cl_2 \longrightarrow PCl_5$.
To balance the phosphorus atoms,we place a coefficient of $4$ before $PCl_5$: $P_4 + Cl_2 \longrightarrow 4PCl_5$.
To balance the chlorine atoms,we need $20$ chlorine atoms on the reactant side,so we place a coefficient of $10$ before $Cl_2$: $P_4(s) + 10Cl_2(g) \longrightarrow 4PCl_5(g)$.

(N/A) $(i)$ Dazzling white light is emitted during the burning of magnesium.
$(ii)$ $A$ white powdery substance (magnesium oxide) is formed after the burning process.

(N/A) $(i)$ This is a displacement reaction because zinc $(Zn)$ displaces hydrogen $(H_{2})$ from sulfuric acid $(H_{2}SO_{4})$.
$(ii)$ This is a combination reaction because calcium oxide $(CaO)$ and water $(H_{2}O)$ combine to form a single product,calcium hydroxide $(Ca(OH)_{2})$.

(N/A) $(i)$ This is a decomposition reaction because a single reactant breaks down into two or more simpler products upon heating.
$(ii)$ This is a double displacement reaction because there is an exchange of ions between the two reactants to form new compounds.

(N/A) $(i)$ The reaction between manganese dioxide and aluminium is a thermite reaction: $3 MnO_{2} + 4 Al \longrightarrow 2 Al_{2}O_{3} + 3 Mn$
$(ii)$ The reaction of iron with steam produces iron($II$,$III$) oxide and hydrogen gas: $3 Fe(s) + 4 H_{2}O(g) \longrightarrow Fe_{3}O_{4}(s) + 4 H_{2}(g)$

(N/A) chemical equation is a symbolic representation of a chemical reaction,where the reactants and products are expressed using their respective chemical symbols and formulae.
An unbalanced chemical equation is known as a skeletal chemical equation.
It provides information about the reactants and products involved in the reaction but does not represent the actual number of atoms of each element on both sides of the equation,thus failing to satisfy the law of conservation of mass.

(A) To balance a chemical equation,the number of atoms of each element must be equal on both the reactant and product sides.
$(a)$ For $KClO_{3} \longrightarrow KCl + O_{2}$:
There is $1$ $K$,$1$ $Cl$,and $3$ $O$ on the left,and $1$ $K$,$1$ $Cl$,and $2$ $O$ on the right. To balance oxygen,we multiply $KClO_{3}$ by $2$ and $O_{2}$ by $3$. Then,to balance $K$ and $Cl$,we multiply $KCl$ by $2$. The balanced equation is: $2KClO_{3} \longrightarrow 2KCl + 3O_{2}$.
$(b)$ For $Na_{2}CO_{3} + HCl \longrightarrow NaCl + H_{2}O + CO_{2}$:
There are $2$ $Na$ atoms on the left,so we multiply $NaCl$ by $2$. This gives $2$ $Cl$ atoms on the right,so we multiply $HCl$ by $2$. Checking the atoms: $2$ $Na$,$1$ $C$,$3$ $O$,$2$ $H$,and $2$ $Cl$ on both sides. The balanced equation is: $Na_{2}CO_{3} + 2HCl \longrightarrow 2NaCl + H_{2}O + CO_{2}$.

(N/A) $(i)$ $CaCO_{3} + 2HCl \longrightarrow CaCl_{2} + H_{2}O + CO_{2}$
$(ii)$ $N_{2} + 3H_{2} \xrightarrow{\text{pressure}} 2NH_{3}$
$(iii)$ $CS_{2} + 3O_{2} \longrightarrow CO_{2} + 2SO_{2}$

(N/A) The chemical reaction is: $Zn(s) + H_2SO_4(aq) \longrightarrow ZnSO_4(aq) + H_2(g) \uparrow$
Observations:
$(i)$ Evolution of hydrogen gas bubbles is observed.
$(ii)$ The reaction is exothermic,so the temperature of the reaction vessel increases.

(N/A) The chemical reaction is: $2KI(aq) + Pb(NO_3)_2(aq) \to PbI_2(s) \downarrow + 2KNO_3(aq)$
$(i)$ $A$ yellow precipitate of lead iodide $(PbI_2)$ is formed.
$(ii)$ The reaction is a double displacement reaction where the solution changes appearance due to the formation of an insoluble solid.

(N/A) The chemical equation for the thermal decomposition of lead nitrate is:
$2Pb(NO_{3})_{2}(s) \xrightarrow{\Delta} 2PbO(s) + 4NO_{2}(g) + O_{2}(g)$
Observations:
$(i)$ Evolution of reddish-brown fumes of nitrogen dioxide $(NO_{2})$ gas.
$(ii)$ Formation of a yellow-colored residue of lead$(II)$ oxide $(PbO)$ in the test tube.

(N/A) Applications of a decomposition reaction:
$(a)$ White silver chloride turns grey in sunlight due to the decomposition of silver chloride into silver and chlorine.
$2 AgCl (s) \xrightarrow{\text{sunlight}} 2 Ag (s) + Cl_2 (g)$
This reaction is used in black and white photography.
$(b)$ Potassium chlorate $(KClO_3)$ decomposes on heating to produce oxygen gas.
$2 KClO_3 (s) \xrightarrow{\text{heat}} 2 KCl (s) + 3 O_2 (g)$
This is a common laboratory method for the preparation of oxygen gas.
$(c)$ Quicklime $(CaO)$ is prepared by the thermal decomposition of calcium carbonate ($CaCO_3$,limestone).
$CaCO_3 (s) \xrightarrow{\text{heat}} CaO (s) + CO_2 (g)$
Quicklime is used in the manufacture of cement and for preparing slaked lime used in whitewashing.

(N/A) Observations:
$(a)$ The reaction occurs vigorously.
$(b)$ $A$ large amount of heat is released (exothermic reaction).
Uses of the product $Ca(OH)_2$ (Calcium hydroxide):
$(a)$ It is used for whitewashing walls. It reacts slowly with $CO_2$ in the air to form a thin layer of calcium carbonate $(CaCO_3)$ on the walls,providing a shiny finish.
$(b)$ It is used as a laboratory reagent for testing the presence of $CO_2$ gas,as it turns milky upon reaction with it.

(N/A) $(i)$ The metal has undergone corrosion.
$(ii)$ The metal is copper $(Cu)$.
$(iii)$ The green coating is basic copper carbonate,with the chemical formula $CuCO_{3} \cdot Cu(OH)_{2}$.
$(iv)$ Two ways to prevent this process are:
$1.$ Coating the metal surface with paint or oil/grease to prevent contact with moist air.
$2.$ Alloying the metal to increase its resistance to corrosion.

(N/A) When a piece of aluminium metal is added to dilute hydrochloric acid,a displacement reaction occurs. Aluminium is more reactive than hydrogen and lies above it in the reactivity series,so it displaces hydrogen from the dilute $HCl$ solution,resulting in the evolution of hydrogen gas.
The balanced chemical equation for this reaction is:
$2Al(s) + 6HCl(aq) \longrightarrow 2AlCl_3(aq) + 3H_2(g)$

(A) When a piece of $Zn$ metal is added to $CuSO_4$ solution, a displacement reaction occurs because $Zn$ is more reactive than $Cu$.
$Zn$ displaces $Cu$ from its solution.
$Zn(s) + CuSO_4(aq) \text{ (Blue colour)} \longrightarrow ZnSO_4(aq) \text{ (Colourless)} + Cu(s)$

(C) Silver $(Ag)$ is less reactive than copper $(Cu)$ according to the reactivity series of metals.
Therefore,silver cannot displace copper from its salt solution $(CuSO_4)$.
Since no reaction occurs,there is no chemical equation to write.

(N/A) $(i)$ The reaction between calcium carbonate and hydrochloric acid produces calcium chloride,water,and carbon dioxide: $CaCO_{3} + 2HCl \longrightarrow CaCl_{2} + H_{2}O + CO_{2}$
$(ii)$ The reaction between aluminium and hydrochloric acid produces aluminium chloride and hydrogen gas: $2Al + 6HCl \longrightarrow 2AlCl_{3} + 3H_{2}$
$(iii)$ The reaction between manganese dioxide and hydrochloric acid produces manganese$(II)$ chloride,chlorine gas,and water: $MnO_{2} + 4HCl \longrightarrow MnCl_{2} + Cl_{2} + 2H_{2}O$

(N/A) $(i)$ Combination reaction: In this reaction,two or more reactants combine to form a single product. Here,calcium oxide $(CaO)$ reacts with carbon dioxide $(CO_2)$ to form a single product,calcium carbonate $(CaCO_3)$.
$(ii)$ Displacement reaction: In this reaction,a more reactive element displaces a less reactive element from its compound. Here,sodium $(Na)$ displaces hydrogen from water $(H_2O)$ to form sodium hydroxide $(NaOH)$ and releases hydrogen gas $(H_2)$.

(N/A) $(i)$ Displacement reaction: $A$ more reactive metal $(Mg)$ displaces a less reactive metal $(Cu)$ from its salt solution $(CuSO_4)$.
$(ii)$ Decomposition reaction: $A$ single compound $(NH_4NO_2)$ breaks down into two or more simpler substances ($N_2$ and $H_2O$).

(N/A) The given reaction is a $Double \ Displacement \ Reaction$.
In a double displacement reaction, two reactant compounds exchange their ions to form two new compounds.
In this specific reaction, the $Cu^{2+}$ ion from copper sulfate exchanges with the $Na^{+}$ ion from sodium hydroxide to form copper hydroxide $(Cu(OH)_{2})$ and sodium sulfate $(Na_{2}SO_{4})$.

(N/A) The chemical phenomenon responsible for these coatings is $Corrosion$.
Silver coins develop a black coating of $Silver \text{ } Sulphide$ $(Ag_2S)$ due to the reaction with sulphur compounds present in the air.
Copper coins develop a green coating of $Basic \text{ } Copper \text{ } Carbonate$ $(CuCO_3 \cdot Cu(OH)_2)$ due to the reaction with moist carbon dioxide and oxygen in the air.

(N/A) $(i)$ Airtight containers prevent air (oxygen) from coming into contact with the food,which inhibits the oxidation of fats and oils present in the food. Since oxidation is the primary cause of rancidity,preventing it ensures that the food does not become rancid,thereby preserving its original smell and taste.

(N/A) $(i)$ Moist air leads to the corrosion of metals like iron (rusting),while acidic gases react with metals like copper and silver,causing them to tarnish or corrode.
$(ii)$ Nitrogen is an inert gas. By flushing the bags with nitrogen,manufacturers displace oxygen,which prevents the oxidation of fats and oils in the chips,thereby keeping them fresh and preventing rancidity.

(N/A) In the reaction $2 Na (s) + Cl_2 (g) \rightarrow 2 NaCl (s)$:
$1$. Sodium $(Na)$ loses electrons to form sodium ions $(Na^+)$. The process of loss of electrons is called oxidation. Since sodium undergoes oxidation and facilitates the reduction of chlorine,it acts as a reducing agent.
$2$. Chlorine $(Cl_2)$ gains electrons to form chloride ions $(Cl^-)$. The process of gain of electrons is called reduction. Since chlorine undergoes reduction and facilitates the oxidation of sodium,it acts as an oxidising agent.

(N/A) When white silver chloride $(AgCl)$ is exposed to sunlight,it undergoes a photochemical decomposition reaction.
In this process,silver chloride decomposes into metallic silver $(Ag)$ and chlorine gas $(Cl_2)$.
The chemical equation for this reaction is: $2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$.
The grey colour is due to the formation of metallic silver,which is grey in appearance.

(N/A) In a decomposition reaction,a single substance [compound] breaks down to produce two or more simpler substances [elements or compounds].
Conversely,in a combination reaction,two or more substances [elements or compounds] combine to form a single new substance.
Because the processes involve opposite chemical changes—breaking down versus building up—a decomposition reaction is considered the opposite of a combination reaction.

(C) The reaction between calcium oxide $(CaO)$, also known as quicklime, and water $(H_2O)$ is a highly exothermic combination reaction.
The chemical equation for this reaction is: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$.
In this process, calcium oxide reacts with water to form calcium hydroxide $(Ca(OH)_2)$, also known as slaked lime.
Since this reaction releases a large amount of heat, it is classified as an exothermic reaction.

(D) $1$. In this reaction, Aluminum $(Al)$ displaces Manganese $(Mn)$ from Manganese dioxide $(MnO_{2})$ because Aluminum is more reactive than Manganese. This is a displacement reaction.
$2$. The reaction releases a significant amount of heat, as indicated by the '+ Heat' term in the product side. Reactions that release heat are called exothermic reactions.
$3$. Therefore, the reaction is both a displacement reaction and an exothermic reaction.

(A) In a redox reaction,the reducing agent is the substance that gets oxidized (loses electrons or gains oxygen/loses hydrogen).
In the given reaction: $MnO_{2}(s) + 4HCl(aq) \longrightarrow MnCl_{2}(s) + 2H_{2}O(l) + Cl_{2}(g)$
$1$. The oxidation state of $Cl$ in $HCl$ changes from $-1$ to $0$ in $Cl_{2}$. Since it loses electrons,$HCl$ is oxidized.
$2$. The oxidation state of $Mn$ in $MnO_{2}$ changes from $+4$ to $+2$ in $MnCl_{2}$. Since it gains electrons,$MnO_{2}$ is reduced.
$3$. Since $HCl$ undergoes oxidation,it acts as the reducing agent.
Therefore,the correct option is $A$.

(B) In the given reaction: $CuO(s) + H_2(g) \longrightarrow Cu(s) + H_2O(l)$
$1$. Copper oxide $(CuO)$ loses oxygen to form copper $(Cu)$. The loss of oxygen is defined as reduction.
$2$. Hydrogen $(H_2)$ gains oxygen to form water $(H_2O)$. The gain of oxygen is defined as oxidation.
$3$. An oxidising agent is a substance that provides oxygen or removes hydrogen,thereby undergoing reduction itself.
$4$. Since $CuO$ provides oxygen to $H_2$ and gets reduced to $Cu$,$CuO$ acts as the oxidising agent.
Therefore,the correct option is $B$.