The Acid Dissociation Constant, Ka

Question 1

what is Ka used for?

Answer 1

weak acid dissociations

Question 2

how would you write the Ka for this reaction?
HA(aq) <–> H+(aq) + A-(aq)

Answer 2

Ka = [H+(aq)][A-(aq)] divided by [HA(aq)]

Question 3

does the value of Ka change with temperature?

Answer 3

yes, Ka is only fixed at a given temperature

Question 4

what happens to Ka when the dissociation is endothermic?

Answer 4

• warming the solution shifts equilibrium to the right
• so dissociates more
• value of Ka increases

Question 5

what happens to Ka when the dissociation is exothermic?

Answer 5

• warming the solution shifts equilibrium to the left
• so dissociates less
• value of Ka decreases

Question 6

when are Ka values standardised?

Answer 6

298K

Question 7

the larger the value of Ka…

Answer 7

the larger the value of Ka, the further the equilibrium shifts right
- greater dissociation so the acids has greater strength

Question 8

when equilibrium shifts right is dissociation more or less?

Answer 8

more

Question 9

who came up wth Ka and pKa?

Answer 9

Soren Sorenson

Question 10

what is the equation to find pKa?

Answer 10

pKa = -logKa

Question 11

what is the equation to find Ka?

Answer 11

Ka = 10 to the power of -pKa

Question 12

Calculate the pH of 0.050moldm-3 methanoic acid. Methanoic acid has a pKa of 3.75 at 298K.

Answer 12

• Ka = 1.78 x10-4
• write the acid dissociation equation
  HCOOH(aq) <–> HCOO-(aq) + H+(aq)
  -write the Ka equation
  Ka = [H+(aq)][HCOO-(aq)] divided by [HCOOH(aq)]
• assume that [H+] = [HCOO-]
• so Ka = [H+] squared divided by [HCOO-]
• pH = 2.53

Question 13

what are the assumptions we make when working out the value of Ka?

Answer 13

• when a weak acid dissociates, we can assume that dissociation of the acid is much greater than the dissociation of the water
• this means that we can assume that all H+ ions come from the weak acid only and neglect any small amounts of H+ from H2O

Question 14

compare the values of Ka and pKa.

Answer 14

• the stronger the acid, the larger the value of Ka and the smalller the value of pKa
• the weaker the acid, the smaller the value of Ka an the larger the value of pKa

Question 15

how can we check that our Ka value is correct?

Answer 15

• weak acids have a pH of above 2 and around 6
• strong acids have a pH of below 1

Question 16

when can the approximations not be applied when doing Ka?

Answer 16

• for ‘stronger’ weak acids when the concentration of the acid at equilibrium being equal to the concentration of acid undissociated may no longer be valid