CBSE Class 12 · 2003 · Foreign · Chemical KineticsShort Answer

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i) Write the rate law for a first order reaction.
ii) Justify the statement that the half-life of a first order reaction is independent of the initial concentration of the reactant.

Answer: (i) Rate = k[A]. (ii) t₁/₂ = 0.693/k — no concentration term appears.

Solution

(i) For a first order reaction:

Rate = k [A]

(ii) From the integrated equation, at

t = t_{1/2}

[A] = [A]_{0} /2

k = \frac{2.303}{t _{1/2}} lo g \frac{[ A ] _{0}}{[ A ] _{0} /2} = \frac{2.303 lo g 2}{t _{1/2}}

t_{1/2} = \frac{0.693}{k}

The expression contains no concentration term, so the half-life is independent of the initial concentration.

CBSE Class 12 Board · 2003 · Foreign · Subjective

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