3.1.12 - Acids and bases

Question 1

Kw expression (ionic product of water)

Answer 1

[H+]^2

Question 2

Weak acid and base reactions are…

Answer 2

Reversible

Question 3

pH equation

Answer 3

-log10[H+]

Question 4

Why was [H2O] not in the Kw expression?

Answer 4

[H2O] is constant

Question 5

[H+] equation

Answer 5

10^-pH

Question 6

How do you work out the pH of strong acids?

Answer 6

The concentration of acid = concentration of H+
Plug into pH expression

Question 7

Strong diprotic acid concentration =

Answer 7

[H+] x 2

Question 8

How do you work out the pH of strong bases?

Answer 8

[OH-] = [base conc]
Rearrange Kw expression

Question 9

Ka expressions (2)

Answer 9

[H+] [OH-] / [HA]

OR

[H+]^2 / [HA]

Question 10

pKa expression

Answer 10

-log10(Ka)

Question 11

Finding Ka using pKa

Answer 11

10^-pKa

Question 12

Where is the half equivalence point?

Answer 12

Mid-point of the horizontal section of the graph

Question 13

Titration calculation (concentration) steps

Answer 13

1. Form a balanced equation
2. Find moles
3. Find concentrations using ratios

Question 14

An acidic/basic buffer is comprised of…

Answer 14

Weak acid/base + its salt (strong acid/base)

Question 15

Finding pH of buffer (3)

Answer 15

• Use expression - Ka = [H+][Base conc] / [Acid conc]
• Salt concentration = base concentration
• Make sure all values are for equilibrium

Question 16

Secondary way of making buffers

Answer 16

Add a small amount of alkali to weak acid to get weak acid + salt

Question 17

Finding pH of buffer solution using alternate method

Answer 17

1. Write the balanced equation
2. Calculate moles of acid and base before reaction
3. Calculate moles after reaction using molar ratios
4. Calculate concentration using total solution volume
5. Plug into Ka expression

Question 18

At half equivalence point pKa = …(equation)…

Answer 18

pH=pKa
Ka = [H+]

Question 19

Why do data booklets not have a value for Ka?

Answer 19

Cannot have a value for Ka for non-reversible reactions

Question 20

Why is ethanoic acid weaker than ethanedioic acid?

Answer 20

1. 2 COOH groups of ethanedioic acid contain 2 very electronegative oxygen atoms
2. This has an electron withdrawing effect
3. Further polarising the O-H bonds
4. Causing more dissociation

Ethanoic acid has CH3 group, inductive effect, less polarsed O-H bond

Question 21

What determines dissassociaton potential?

Answer 21

Polarisation of the O-H bond

Question 22

What substance in air can change the pH of a solution? (3)

Answer 22

CO2 decreases pH, as it is an acidic gas