1.7- Simple Equilibria and Acid-Base Reactions

Question 1

(a)

reversible reactions

Answer 1

A reversible reaction is a reaction in which the products can react together to form the original reactants.

Question 2

(a)

dynamic equilibrium

Answer 2

In a dynamic equilibrium the rate of the forward reaction and backward reaction is the same; there is no further change in the concentrations of reactants and products.

Question 3

(b)

Le Chatelier’s principle

Answer 3

If a change is made to the conditions of a reversible reaction, the position of equilibrium will move to counteract the change.

Question 4

(b)

effect of changes in temperature on equilibrium position

Answer 4

Increasing the temperature will favour the endothermic reaction, so the equilibrium will shift towards the products of the endothermic reaction.

Decreasing the temperature will favour the exothermic reaction, so the equilibrium will shift towards the products of the exothermic reaction.

Question 5

(b)

effect of changes in pressure on equilibrium position

Answer 5

Increasing the pressure favours the side of the reaction with fewer moles of gas, so the position of the equilibrium will move to that side.

Decreasing the pressure favours the side of the reaction with more moles of gas, so the position of the equilibrium will move to that side.

Question 6

(b)

effect of changes in concentration on equilibrium position

Answer 6

Increasing the concentration will shift the equilibrium to the right to increase the concentration of the products.

Decreasing the concentration will shift the equilibrium towards the same side.

Question 7

(c)

Kc

Answer 7

Kc is the equilibrium constant in terms of concentration.

Question 8

(c)

Calculating Kc

Answer 8

aA + bB ⇌ cC + dD
Kc = [C]c[D]d/[A]a[B]b

Question 9

(c)

At 500 K, the following equilibrium is established in a sealed container:
N2 (g)+3H2 (g)⇌2NH3(g)
The equilibrium concentrations are:
[N2]=0.30moldm−3
[H2]=0.60moldm−3
[NH3]=0.40moldm−3
Calculate the value of the equilibrium constant Kc at this temperature.
Give your answer to 2 significant figures and include the units.

Answer 9

Kc = 2.5mol−2 dm6

Question 10

(d)

Brønsted-Lowry acid

Answer 10

A proton (H+) donor.

Question 11

(d)

Brønsted-Lowry base

Answer 11

A proton (H+) acceptor.

Question 12

(e)

PH of a strong acid

Answer 12

HA → H+ + A-
For a strong acid the concentration of acid = concentration of H+ ions.
pH = -log[H+ ]

Question 13

(e)

Expression used to calculate the hydrogen ion concentration of a strong acid

Answer 13

[H+] = 10-pH

Question 14

(g)

Strong acid

Answer 14

A strong acid is fully dissociated (or ionised) in aqueous solution.
E.g. HCl(aq) H+(aq) + Cl–(aq)

Question 15

(g)

Strong base

Answer 15

A strong base is a base that completely dissociates in solution to form hydroxide ions:
NaOH(aq) → Na+ (aq) + OH- (aq)

Question 16

(g)

Weak acid

Answer 16

A weak acid is an acid that only partially dissociates in solution: CH3COOH(aq) ⇌ CH3COO- (aq) + H+ (aq)

Question 17

(g)

Weak base

Answer 17

A weak base is a base that only partially dissociates in solution:
NH3 + H2O ⇌ NH4 + + OH-