Answer these questions pertaining to chemical kinetics. — Kinetics Chemistry Question
Problem Context
Answer these questions pertaining to chemical kinetics.
Determine the reaction rate at 10 seconds from the graph. Show your work.
Model Answer
Tangent to curve at 10 seconds:
∆M / ∆T = –0.39M / 32 s = –0.012 M ⋅ s–1
Using the same units for the reaction rate as in Part a, and assuming concentrations in mol·L–1, give the units for the rate constant of a reaction with an order of:
i. zero ii. one iii. two
Model Answer
i. rate = k units are M.s–1
ii. rate = k [ ] units are s–1
iii. rate = k [ ]2 units are M–1.s–1
Consider this reaction: 4HBr(g) + O2(g) → 2H2O(l) + 2Br2(g)
i. Express the reaction rates for HBr and Br2 in this reaction relative to that of O2.
ii. Explain why this reaction is unlikely to occur by direct collision of four HBr molecules with one O2 molecule.
Model Answer
i. Expressed in symbols:
This shows that the rate of disappearance of HBr is 4 times that of O2 and the rate of production of Br2 is twice the rate of disappearance of O2.
ii. More than mono- or bi-molecular steps improbable.
This mechanism has been suggested for the reaction in Part c:
HBr(g) + O2(g) → HOOBr(g) Step 1
HOOBr(g) + HBr(g) → 2HOBr(g) Step 2
HOBr(g) + HBr(g) → H2O(g) + Br2(g) Step 3
Give the rate equation in terms of reactants expected for this reaction if the rate-determining step is:
i. Step 1 ii. Step 2 iii. Step 3
Assume in each case that the steps before the rate-determining step are in rapid equilibrium. Outline your reasoning in each case.
Model Answer
i. rate = k [HBr] [O2] Rate is proportional to reactants in the rate-limiting step.
ii. rate = k [HBr]2 [O2] [HOOBr] in the rate equation must be stated in terms of the previous equilibrium.
iii. rate = k [HBr]2 [O2] 1/2 [HOBr] in the rate equation must be stated in terms of the previous equilibria.