5 Steps to a 5 AP Chemistry 2019

216 ❯ STEP 4. Review the Knowledge You Need to Score High

Note that there are 4 mol of gas (1 of nitrogen and 3 of hydrogen) on the left side

and 2 mol on the right. If the container is made smaller, the pressure will increase and

the equilibrium will shift to the right because 4 mol would be converted to 2 mol. The

concentrations of nitrogen and hydrogen gases would decrease, and the concentration of

ammonia would increase.

Remember: Pressure effects are only important for gases.

Changes in Temperature

Changing the temperature changes the value of the equilibrium constant. It also changes

the amount of heat in the system and can be treated as a concentration effect. To treat it

this way, one must know which reaction, forward or reverse, is exothermic (releasing heat).

One last time, let’s consider the Haber reaction:

N 22 (g)H+ (g)2 NH 3 (g)

The formation of ammonia is exothermic (liberating heat), so the reaction could be written as:

N( 22 g)++3H(g)2 NH 3 (g)heat

If the temperature of the reaction mixture were increased, the amount of heat would be

increased and the equilibrium would shift to the left to consume the added heat. In doing

so, the concentration of nitrogen and hydrogen gases would increase and the concentration

of ammonia gas would decrease. If you were in the business of selling ammonia, you would

probably want to operate at a reduced temperature, in order to shift the reaction to the right.

Consider the following equilibrium (endothermic as written), and predict what changes,

if any, would occur if the following stresses were applied after equilibrium was established:

CaCO 32 (s)C aO(s)C+ O(g)

a. add CO 2

b. remove CO 2

c. add CaO

d. increase T

e. decrease V

f. add a catalyst

Answers:

a. Left—the equilibrium shifts to remove some of the excess CO 2.

b. Right—the equilibrium shifts to replace some of the CO 2.

c. No change—solids do not shift equilibria unless they are totally removed.

d. Right—endothermic reactions shift to the right when heated.

e. Left—a decrease in volume, or an increase in pressure, will shift the equilibrium toward

the side with less gas.

f. No change—catalysts do not affect the position of an equilibrium.

Acid–Base Equilibrium

In the Reactions and Periodicity chapter, we introduced the concept of acids and bases.

Recall that acids are proton (H+) donors and bases are proton acceptors. Also, recall that

KEY IDEA