Unit 39 An introduction to chemical equilibrium
Fill in the blanks
1 irreversible
2 reversible
3 dynamic
4 a) yellow; orange; Cr2O72–(aq); CrO42–(aq)
b) orange; yellow; CrO42–(aq); Cr2O72–(aq)
5 a) forward
b) bac
...
Unit 39 An introduction to chemical equilibrium
Fill in the blanks
1 irreversible
2 reversible
3 dynamic
4 a) yellow; orange; Cr2O72–(aq); CrO42–(aq)
b) orange; yellow; CrO42–(aq); Cr2O72–(aq)
5 a) forward
b) backward
True or false
6 T
7 F The composition of a system at equilibrium remains constant.
Although the rates of the forward and backward reactions are the same, it does NOT mean that the
concentrations of the reactants and the products are the same at equilibrium.
8 F Equilibrium can only be established in a closed system.
9 F The value of Kc for a reaction can be used to judge the extent of a reaction, but NOT the rate at
which equilibrium is attained.
10 T If Qc > Kc, the system is not at equilibrium. Concentrations of the products must decrease and that of
the reactants must increase until Qc = Kc. Thus, a net backward reaction must occur until equilibrium is
reached.
Multiple choice questions
11 C Option A — For a reaction system at equilibrium, the forward and backward reactions still continue all
the time.
Option D — The composition of a system at equilibrium remains constant. However, the amount of
products may NOT be equal to the amount of reactants.2
12 D
Rate of forward reaction
2SO3(g) 2SO2(g) + O2(g)
Rate of backward reaction
2SO2(g) + O2(g) 2SO3(g)
At start high zero
As reaction proceeds
(before equilibrium)
decreasing increasing
but rate of forward reaction > rate of backward reaction
At equilibrium rate of forward reaction = rate of backward reaction
GPSXBSESFBDUJPO
40 H
40 H
0 H
CBDLXBSESFBDUJPO
40 H
0 H
40 H
TZTUFNJT
OPXBU
FRVJMJCSJVN
SBUFPG
GPSXBSE
SFBDUJPO SBUF
PGCBDLXBSE
SFBDUJPO
UJNFUPSFBDI
FRVJMJCSJVN
5JNF
3FBDUJPOSBUF
13 B Option B — After equilibrium has been established, the composition of the system remains constant.
Thus, [H2(g)] remains constant.
14 C Option C — Equilibrium can be reached from either direction, starting with the substances on the left
of the equation (i.e. H2(g) and I2(g)), or starting with the substances on the right (i.e.
HI(g)).
Thus, [HI(g)] in experiment A equals [HI(g)] in exp
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