Page 249 - Theory and Problems of BEGINNING CHEMISTRY

238 RATES AND EQUILIBRIUM [CHAP. 16

Two solutions for x, one positive and one negative, can be obtained from this equation, but only one will have physical
meaning. There cannot be any negative concentrations.
x = 0.329 M = [C] = [D]
The concentrations of A and B are then
[A] = 1.25 − 0.329 = 0.92 M
[B] = 1.50 − 0.329 = 1.17 M
The value from the equilibrium constant expression is therefore equal to the value of K given in the problem:
(0.329) 2
K = = 0.10
(0.92)(1.17)

Solved Problems

RATES OF CHEMICAL REACTION
16.1. Does sugar dissolve faster in hot coffee or in iced coffee? Explain.
Ans. In hot coffee. The higher the temperature, the faster the process.

16.2. Does lump sugar or granular sugar dissolve faster in water, all other factors being equal? Explain.
Ans. Granular sugar dissolves faster; it has more surface area in contact with the liquid.

16.3. Does a glowing lump of charcoal react faster in air or in pure oxygen? Explain.

Ans. It will react faster in pure oxygen; the concentration of oxygen is greater under those conditions.
16.4. At the same temperature, which sample of a gas has more molecules per unit volume—one at high
pressure or one at low pressure? Which sample would react faster with a solid substance, all other factors
being equal?
Ans. The high-pressure gas has more molecules per unit volume and therefore reacts faster.

16.5. The activation energy of a certain reaction is 15 kJ/mol. The reaction is exothermic, yielding 19 kJ/mol.
What is the activation energy of the reverse reaction?
Ans. 34 kJ/mol

16.6. Using the equation for the average kinetic energy of the molecules in a gaseous sample (Chap. 13)
◦
KE = 1.5kT , show that a 10 rise in temperature from 273 K to 283 K does not double the average
kinetic energy of the molecules.
Ans. KE 283 = 1.5k(283 K)
KE 273 = 1.5k(273 K)
Dividing gives
283
KE 283
= = 1.04
KE 273 273
The average energy increases about 4%; it does not double. Note that what doubles with a 10 C increase in
◦
temperature is the number of molecules with sufficiently high energy to react.

CHEMICAL EQUILIBRIUM
◦
16.7. Write an “equation” for the addition of heat to a water-ice mixture at 0 C to produce more liquid water
at 0 C. Which way does the equilibrium shift when you try to lower the temperature?
◦
−→
Ans. H 2 O(s) + heat ←− H 2 O(l)