# 12.E: Chemical Kinetics

### Q12.1

The following data were obtained for the reaction of methane with oxygen:

$$\ce{CH4(g) + 2 O2(g) \rightarrow CO2(g) + 2 H2O(l)}$$

time(min)

$$\ce{[CH4]}$$ (mol/L)

$$\ce{[CO2]}$$ (mol/L)

0

0.050

0

10

0.030

0.020

20

0.020

?

30

0.015

?

1. How many moles of $$\ce{CO2}$$ are produced for each mole of $$\ce{CH4}$$ that is used up?
2. What concentration of $$\ce{CH4}$$ is used up after 10 minutes?
3. What is the concentration of carbon dioxide produced after 20 minutes?
4. Write an equation for reaction rate in terms of $$\mathrm{\Delta [CO_2]}$$ over a time interval.
5. What is the reaction rate for the formation of carbon dioxide between 10 and 20 minutes?
6. What is the average reaction rate between 0 and 30 minutes?
7. Write an expression for reaction rate relating $$\mathrm{\Delta [O_2]}$$ to $$\mathrm{\Delta [CO_2]}$$.
8. At what rate is $$\ce{O2}$$ used up between 10 and 20 minutes?

### Q12.2

Nitric oxide ($$\ce{NO}$$) reacts with oxygen to form nitrogen dioxide:

$$\ce{2 NO(g) + O2(g) \rightarrow 2 NO2(g)}$$

1. How could you tell how many steps are in this reaction?
2. The following two step mechanism has been proposed for this reaction:

$$\ce{NO + O2 \rightarrow NO3}$$ (fast)

$$\ce{NO3 + NO \rightarrow 2 NO2}$$ (slow)

Which step is the rate determining step?

1. Write a rate equation for the rate determining step of this reaction, assuming it occurs as a single step that depends only on the collision between reactants.
2. What happens to the rate in (c) if the concentration of $$\ce{NO3}$$ is halved and the concentration of $$\ce{NO}$$ is tripled?
3. What is the order of the rate determining step with respect to $$\ce{NO}$$? What is the overall order of this step?

### Q12.3

Draw an energy diagram for a reaction where $$\mathrm{\Delta H = -40\, kJ}$$, the activation energy of the uncatalyzed reaction is +120 kJ, and the activation energy for a catalyzed reaction is + 80 kJ. Indicate the position of the activated complex for both catalyzed and uncatalyzed reactions.

### Q12.4

List three factors that affect reaction rate and briefly explain the basis for their effects.

### Q12.5

The breakdown of nitrous oxide gas ($$\ce{N2O}$$) to nitrogen gas and oxygen gas is believed to occur in two steps. In the first step, nitrous oxide breaks down to form nitrogen gas and a free oxygen atom.

1. Write a balanced equation for the overall reaction (do not use fractional coefficients).
2. Write balanced equations for each of the two steps.
3. Which substance could be considered a "reaction intermediate?"

### Q12.6

The rate of the reaction,

$\ce{HgCl2(aq) + \dfrac{1}{2} C2O4^2- (aq) \rightarrow Cl- (aq) + CO2(g) + \dfrac{1}{2} Hg2Cl2(s)}$

is followed by measuring the number of moles of Hg2Cl2 that precipitate per liter per second. The following data are obtained:

$$\ce{[HgCl2]}$$

$$\ce{[C2O4^{2-}]}$$

Initial Rate (mol/L·s)

0.10

0.10

1.3 × 10-7

0.10

0.20

5.2 × 10-7

0.20

0.20

1.0 × 10-6

0.20

0.10

2.6 × 10-7

1. What is the order of the reaction with respect to $$\ce{HgCl2}$$, with respect to $$\ce{C2O4^2-}$$, and overall?
2. Write the rate equation for the reaction.
3. Calculate $$\ce{k}$$ for the reaction.
4. When the concentrations of both mercury(II) chloride and oxalate ion are 0.30 M, what is the rate of the reaction?