Chapter 14 - Chemical Kinetics

Question 1

Define “chemical kinetics”.

Answer 1

The area of chemistry that’s concerned with the sppeds, or rates, of reactions.

Question 2

14.1

What four factors affect reaction rates?

Answer 2

1. Concentration of reactants
2. Physical state of reactants
3. Temperature at which the reaction occurs
4. The presence of a catalyst

Question 3

14.1

Why do the physical states of reactants affect the reaction rate?

Answer 3

When reactants are of different physical states, the rate of the reaction is dependent upon how much the substances touch.

So, for example, a solid that is ground up into a powder is going to react faster than a solid in the shape of a cube because the powder has more surface area to react with.

Question 4

14.1

Why do the concentrations of the reactants affect reaction rate?

Answer 4

The more concentrated a reactant is, the more molecules/atoms/whatever there are, and the more likely they are to collide with each other or other reactants. Increases frequency of collisions, which in turn increases rates.

Question 5

14.1

Why does the temperature at which a reaction occurs matter?

Answer 5

The hotter the temperature, the faster the molecules are moving (and the more energy they collide with), leading to more collisions, and thus a faster reaction.

Question 6

14.1

Why does the presence of a catalyst matter?

Answer 6

Catalysts increase reaction rates without getting used up. They affect the kinds of collisions (the mechanism) that lead to reaction.

Question 7

14.1

Describe reactions on a molecular level.

Answer 7

Reaction rates depend on the frequency of collisions between molcules. For a collision to lead to a reaction, the collision must occur with enough energy to stretch bonds to a critical length and with appropriate orientation for new bonds to form in the right locations.

Question 8

14.1 - Give It Some Thought (p. 575)

How does increasing the partial pressures of the reactive components of a gaseous mxture affect the rate at which the components react with one another?

Answer 8

Increasing the pressure means decreasing the space between molecules, increasing collisions, thus increasing the reaction rate.

Question 9

14.2

Define “reaction rate”.

Answer 9

The change in the concentration of reactants or products per unit of time. Usually molarity per second (M/s).

Question 10

14.2

Rates are always expressed as ___ quantities.

Answer 10

Positive

Question 11

14.2 (Exercise 14.15)

Consider the following hypothetical aqueous raction A (aq) –> B (aq). A flask is charged with 0.065 mol of A in a total volume of 100.0 mL. The following data are collected:

Time (min): 0, 10, 20, 30, 40

Moles of A: 0.065, 0.051, 0.042, 0.036, 0.031

(a) Calculate the number of moles of B at each time in the table, assuming that there are none at time 0.
(b) Calculate the average rate of disappearnce of A for each 10-minute interval, in units of M/s.
(c) Between t = 10 min and t = 30 min, what is the average rate of appearance of B in units of M/s? Assume that the volume of the solution is constant.

Answer 11

(a) 0, .014, .023, .029, .034
(b) (.051 - .065) = -.014 over 100 mL * 1000 mL = -.14
- .14/(10 - 0) = .014 M/m / 60 sec= 2.3 x 10^-4

(.042 - .051) = -.023 over 100 mL * 1000 mL = -.09

• .09/(20-10) = .009 M/m / 60 = 1.5 x 10^-4 M/s
  (c) (.029 - .014) = .015 over 100 mL * 1000 mL = .15/(30 - 10) = .0075 M/m / 60 sec = 1.25 x 10-4