किसी अभिक्रियक के लिए एक अभिक्रिया द्वितीय कोटि की है। अभिक्रिया का वेग  कैसे प्रभावित होगा; यदि अभिक्रियक की सांद्रता-

$(i)$ दुगुनी कर दी जाए $(ii)$ आधी कर दी जाए

Let the concentration of the reactant be $[ A ]=a$

Rate of reaction, $R=k[A]^{2}$

$=k a^{2}$

$(i)$ If the concentration of the reactant is doubled, i.e. $[ A ]=2 a$, then the rate of the reaction would be

$R ^{\prime}=k(2 a)^{2}$

$=4 ka ^{2}$

$=4 R$

Therefore, the rate of the reaction would increase by $4$ times.

$(ii)$ If the concentration of the reactant is reduced to half, i.e. $[ A ]=\frac{1}{2} a$
the reaction would be

$R ^{\prime \prime}=k\left(\frac{1}{2} a\right)^{2}$

$=\frac{1}{4} k a$

$=\frac{1}{4} R$

Therefore, the rate of the reaction would be reduced to ${\frac{1}{4}^{th}}$