Question

A) The reactant concentration in a zero-order reaction was 9.00×10−2 M after 155 s and 2.00×10−2 M after 365 s . What is the rate constant for this reaction? B) What was the initial reactant concentration for the reaction described in Part A? C) The reactant concentration in a first-order reaction was 9.80×10−2 M after 15.0 s and 6.30×10−3 M after 100 s . What is the rate constant for this reaction? D) The reactant concentration in a second-order reaction was 0.630 M after 120 s and 3.70×10−2 M after 850 s . What is the rate constant for this reaction?

Answer 1

A) In a zero order reaction rate constant is defined by

[A]_t1 - [A]_t2 = k(t₂ - t₁) --------------- (i)

where k = rate constant , t₂ - t₁= time interval

Putting the values in equation i we get

9*10^-2 - 2*10^-2 = k (365 - 155)

=> 0.07 = k* 210

=> k = 0.07 /210 = 3*10^-4 M/s (approx.)

B) Let the initial concentration be [A]₀.

Again applying equation i we get,

[A]₀ - [A]_t=155 = k(155)

=> [A]₀ - 0.09 = 3*10^-4(155)

=> [A]₀ = 465*10^-4 + 9*10^-2

^=> [A]₀ = 13.65*10^-2 M

C) Rate constant eqaution for first order reaction is given by

k = 1/t [ ln { [A]_t1 / [A]_t2}] ------------------(ii)

t₁ = 15 s and t₂ = 100s

Putting the given values in the equation we get

k = 1/(100-15) [ln { 9.8*10^-2 / 6.3*10^-3}]

=> k = 1/85 [ ln { 15.55}]

=> k = 1/85 [2.74 ]

=> k = 0.032 s^-1

D) The rate constant equation of second order reaction is given by

1/[A]_t2 = 1/[A]_t1 + k(t₂ - t₁) ---------------(iii)

Putting the values in the reaction we get,

1/[3.7*10^-2] = 1/ [0.63] + k(850 - 120)

=> 25.44 = k( 730)

=> k = 0.0348 M^-1 s^-1

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