3.1.12 - Acids and bases Flashcards
Kw expression (ionic product of water)
[H+]^2
Weak acid and base reactions are…
Reversible
pH equation
-log10[H+]
Why was [H2O] not in the Kw expression?
[H2O] is constant
[H+] equation
10^-pH
How do you work out the pH of strong acids?
The concentration of acid = concentration of H+
Plug into pH expression
Strong diprotic acid concentration =
[H+] x 2
How do you work out the pH of strong bases?
[OH-] = [base conc]
Rearrange Kw expression
Ka expressions (2)
[H+] [OH-] / [HA]
OR
[H+]^2 / [HA]
pKa expression
-log10(Ka)
Finding Ka using pKa
10^-pKa
Where is the half equivalence point?
Mid-point of the horizontal section of the graph
Titration calculation (concentration) steps
- Form a balanced equation
- Find moles
- Find concentrations using ratios
An acidic/basic buffer is comprised of…
Weak acid/base + its salt (strong acid/base)
Finding pH of buffer (3)
- Use expression - Ka = [H+][Base conc] / [Acid conc]
- Salt concentration = base concentration
- Make sure all values are for equilibrium
Secondary way of making buffers
Add a small amount of alkali to weak acid to get weak acid + salt
Finding pH of buffer solution using alternate method
- Write the balanced equation
- Calculate moles of acid and base before reaction
- Calculate moles after reaction using molar ratios
- Calculate concentration using total solution volume
- Plug into Ka expression
At half equivalence point pKa = …(equation)…
pH=pKa
Ka = [H+]
Why do data booklets not have a value for Ka?
Cannot have a value for Ka for non-reversible reactions
Why is ethanoic acid weaker than ethanedioic acid?
- 2 COOH groups of ethanedioic acid contain 2 very electronegative oxygen atoms
- This has an electron withdrawing effect
- Further polarising the O-H bonds
- Causing more dissociation
Ethanoic acid has CH3 group, inductive effect, less polarsed O-H bond
What determines dissassociaton potential?
Polarisation of the O-H bond
What substance in air can change the pH of a solution? (3)
CO2 decreases pH, as it is an acidic gas
