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Chemistry LibreTexts

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?