# Rates Solution Page 5

(d) Here is the experimental data for this reaction

 Experiment # Concentration[A] mol/L Concentration[B] mol/L Ratemol/litre-min 1 Constant 1 4 2 Constant 3 12 3 Constant 5 20 4 2 Constant 3 5 4 Constant 12 6 6 Constant 27

Using the rate law data write the rate equation:Rate = k [A]2 [B]1

What is the order? 3 . Why is the order different from the overall order?(From LMA)

The reaction does not occur in a single step. The reaction has a mechanism. The rate determining step shows that the reaction is a 3rd order reaction.

What is the rate of the reaction in terms of B?

(e) When do the coefficients of the balanced overall equation equal to the exponents of the rate equation. When the exponents of LMA = the exponents of the RDS or if m and n are the exponents of the LMA and a and b are the exponents of the RDS When LMA = RDS or m = a and n = b

(f) The following mechanisms are proposed as possible mechanisms for the overall equation/ reaction. Show your evaluation of each mechanism briefly and summarize by acceptance or rejection of each mechanism with reasoning.

 Mechanism Evaluation – Your Reasoning (I) (1) A + B $\rightarrow$ C (slow)(2) C + 2A $\rightarrow$ D + E(fast) (3) E $\rightarrow$ F (fast) (1) 1 + 2 + 3 does not add up to give overall equation(2) RDS does not agree with the experiment (3) Rate law, Rate = k[A]2[B]1 REJECT the suggested mechanism (II) (1) 2A + B $\rightarrow$ 2E (slow)(2) 2E + A $\rightarrow$ D + S (fast) (3) S $\rightarrow$ F (fast) (1) Elementary steps add up(2) RDS does agree with the experimentally determined rate law ACCEPT the suggested mechanism (III) (1) 2A + B $\rightarrow$ P (slow)(2) P + A = S + D (Fast) (3) 2S $\rightarrow$ F (fast) (1) The elementary steps does not add up to the overall equation(2) RDS does not agree with the experimentally determined rate law REJECT the suggested mechanism (IV) (1) 2A + B $\rightarrow$ D + G (slow)(2) G + A $\rightarrow$ H (fast) (3) H $\rightarrow$ F (fast) (1) The elementary steps does add up(2) RDS does agree with the experimental evidence ACCEPT the suggested mechanism (V) (1) 2A + B $\rightarrow$ H + F (fast)(2) H + A $\rightarrow$ D (slow) (1) The elementary step does add up(2) The RDS does not agree with observed rate law REJECT

RDS = Rate determining step, LMA = Law of mass action

1. (a) Heterogeneous catalysts is When the reactants and the catalyst are in different phases (states) then we have a heterogeneous catalyst. For example when Iron is used as a catalyst in the manufacture of ammonia in the Haber process. The promoter could be K2O, CaO or SiO2

(a) Give examples with explanations of the catalysis of H2 + O2 to form H2O or the decomposition of H2O2 into water and oxygen. You have to find the catalysts

H2 + O2 $\rightarrow$ H2O . The catalyst for this reaction is Platinum or Pt

Decomposition of hydrogen peroxide is accelerated by MnO2. Manganese dioxide is the catalyst.

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