7.1 Concept Check – Equilibrium 1
4(b) The following graph represents the change of concentration of reactant and product during a
reaction.
0.7
0.6
0.5
Product
0.4
[reactant] or
[product] / 0.3
mol dm –3
Reactant
0.2
0.1
0.0
0
(i)
10
20
30
Time / s
40
50
60
Calculate the average rate of reaction over the first 15 s, stating the units.
(3)
(ii)
After 19 s the concentrations of the reactant and product do not change. State what this
indicates about the reaction.
(1)
(Total 4 marks)
5.
Which statement(s) is/are true for a mixture of ice and water at equilibrium?
I.
The rates of melting and freezing are equal.
II.
The amounts of ice and water are equal.
III.
The same position of equilibrium can be reached by cooling water and heating ice.
A.
I only
B.
I and III only
C.
II only
D.
III only
(Total 1 mark)
7.
(a)
The following equilibrium is established at 1700°C.
CO2(g) + H2(g)
H2O(g) CO(g)
If only carbon dioxide gas and hydrogen gas are present initially, sketch on a graph a
line representing rate against time for (i) the forward reaction and (ii) the reverse reaction
until shortly after equilibrium is established. Explain the shape of each line.
(7)
10.
Which statement concerning a chemical reaction at equilibrium is not correct?
A.
The concentrations of reactants and products remain constant.
B.
Equilibrium can be approached from both directions.
C.
The rate of the forward reaction equals the rate of the reverse reaction.
D.
All reaction stops.
(Total 1 mark)
18.
Which statements are correct for a reaction at equilibrium?
I.
The forward and reverse reactions both continue.
II.
The rates of the forward and reverse reactions are equal.
III.
The concentrations of reactants and products are equal.
A.
I and II only
B.
I and III only
C.
II and III only
D.
I, II and III
(Total 1 mark)
22.
Which statement is always true for a chemical reaction that has reached equilibrium?
A.
The yield of product(s) is greater than 50.
B.
The rate of the forward reaction is greater than the rate of the reverse reaction.
C.
The amounts of reactants and products do not change.
D.
Both forward and reverse reactions have stopped.
(Total 1 mark)
ANSWER KEY
4(b)
(i)
after 15s (product) = 0.37 (mol dm–3);
rate = = 0.025;
2 sig figs
mol dm–3s–1/Ms–1/ ;
(ii)
3
at equilibrium/rates of forward and reverse reactions are equal/∆G = 0;
1
7 (a)
forward
Rate
reverse
Time
two curves – one labelled “forward” starting up high up y-axis and one labeled “reverse”
starting from zero;
curves merge and become horizontal;
No penalty for failing to label axes.
forward reaction:
highest concentration, thus rate high to begin with;
as reaction proceeds, concentrations decrease, so does rate;
reverse reaction:
zero rate initially/at t = 0 (since no products present);
rate increases as concentration of products increases;
equilibrium established when rate of forward reaction = rate of reverse reaction; 7
10
D
18
A
22
C