Page 250 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 16] RATES AND EQUILIBRIUM 239

The equilibrium shifts toward ice as you remove heat in an attempt to lower the temperature. (However, the
temperature does not change until all the water freezes and the equilibrium system is destroyed.)

16.8. What does Le Chˆatelier’s principle state about the effect of an addition of NH 3 on a mixture of N 2 and
H 2 before it attains equilibrium?
Ans. Nothing. Le Chˆatelier’s principle applies only to a system already at equilibrium.

◦
16.9. What effect would an increase in volume have on the following system at equilibrium at 500 C?
−→
2C(s) + O 2 (g) ←− 2CO(g)
Ans. The equilibrium would shift to the right. The increase in volume would decrease the pressure of each of the
gases, but not of the carbon, which is a solid. The number of moles of gas would be increased by the shift
to the right.

16.10. Hydrogen gas is added to an equilibrium system of hydrogen, nitrogen, and ammonia. The equilibrium
shifts to reduce the stress of the added hydrogen. Will there be more, less, or the same concentration of
hydrogen present at the new equilibrium compared with the old equilibrium?
−→
Ans. N 2 + 3H 2 ←− 2NH 3 + heat
There will be a greater concentration of hydrogen and of ammonia at the new equilibrium, and a lower
concentration of nitrogen. The concentration of hydrogen is not as great as the original concentration plus
that which would have resulted from the addition of more hydrogen, however. Some of the total has been
used up in the equilibrium shift.

16.11. For the reaction: heat + A + B ←− C, does a rise in temperature increase or decrease the rate of (a) the
−→
forward reaction? (b) the reverse reaction? (c) Which effect is greater?
Ans. (a) and (b) Increase. (An increase in temperature increases all rates.) (c) The added heat shifts the equilib-
rium to the right. That means that the forward reaction was speeded up more than the reverse reaction was
speeded up.

16.12. The balanced chemical equation
−→
3H 2 + N 2 ←− 2NH 3
means which one(s) of the following? (a) One can place H 2 and N 2 in a reaction vessel only in the ratio
3 mol to 1 mol. (b) If one places NH 3 in a reaction vessel, one cannot put any N 2 and/or H 2 in it. (c)If
one puts 3 mol of H 2 and 1 mol of N 2 into a reaction vessel, it will produce 2 mol of NH 3 .(d) For every
1 mol of N 2 that reacts, 3 mol of H 2 will also react and 2 mol of NH 3 will be produced. (e) For every 2
mol of NH 3 that decomposes, 3 mol of H 2 and 1 mol of N 2 are produced.

Ans. Only parts d and e are correct. The balanced equation governs the reacting ratios only. It cannot determine
how much of any chemical may be placed in a vessel—(a) and (b)—or if a reaction will go to completion—(c).

EQUILIBRIUM CONSTANTS
16.13. Write equilibrium constant expressions for the following equations. Tell how they are related.

−→ −→
(a)N 2 O 4 ←− NO 2 + NO 2 (b)N 2 O 4 ←− 2NO 2

[NO 2 ][NO 2 ] [NO 2 ] 2
Ans. (a) K = (b) K =
[N 2 O 4 ] [N 2 O 4 ]
The equilibrium constant expressions are the same because the chemical equations are the same. It is easy to
see why the coefficients of the chemical equation are used as exponents in the equilibrium constant expression
by writing out the equation and expression as in part a.