Acids and Bases: The Ionic Product of Water, KW

Question 1

How much does water dissociate?

Answer 1

• Dissociates partially

Question 2

Give the equation that shows the dissociation of water

Answer 2

• H2O(l) ⇌ H+ (aq) + OH- (aq)

Question 3

Give the expression for the equilibrium constant (Kc) of water

Answer 3

• Kc = [H+(aq)] [OH-(aq)] / [H2O(l)]

Question 4

Why is the concentration of water considered to be constant?

Answer 4

• H2O(l) ⇌ H+ (aq) + OH- (aq)
• In the dissociation of water, the equilibrium lies far to left so there’s a lot of water compared to H+ and OH- ions
• Concentration of water isn’t affected, so remains constant

Question 5

If concentration of water is constant, how can the Kc expression for water be changed?

Answer 5

• Kc = [H+] [OH-] / [H2O]

- Can remove the [H2O] as it’s effectively 1

Question 6

Give the Kw expression

Answer 6

Kw = [H+(aq)] [OH-(aq)]

Question 7

What is Kw?

Answer 7

• Constant for the ionic product of water

Question 8

What is the value of Kw at 298K?

Answer 8

• 1.00 x 10–14 mol2 dm–6

Question 9

Is the dissociation of water an endothermic or exothermic process?

Answer 9

• Endothermic as bonds are broken

Question 10

What physical factors affect the value of Kw?

Answer 10

• Temperature

Question 11

How does temperature affect Kw?

Answer 11

• If temperature is increased, the equilibrium moves to the right
• More H+ and OH- ions are produced
• Kw increases

Question 12

In pure water, what is the ratio of H+ ions to OH- ions? What does this say about the concentrations of H+ and OH- ions?

Answer 12

• 1:1

- [H+] = [OH-]

Question 13

When dealing with pure water, how else can you express Kw?

Answer 13

• Kw = [H+]2