AQA A Level Chemistry 3.1.12 - Acids and Bases
Calculating [H⁺]
-pH [H⁺] = 10
Calculating Ka from pKa
-pKa Ka = 10
How is a pH titration carried out?
1. Calibration curve of pH meter and electrode using buffer solutions. 2. Prepare graph of pH (y-axis) against volume of alkali added (x-axis). 3. Pipette 25cm³ 0.1M acid into 250cm³ beaker. Measure pH and record as initial pH. 4. Add 0.1M alkali from burette in appropriate portions, measuring and plotting pH after each addition. 5. Repeat up to 50cm³. 6. Plot curve of best fit.
Low pH means
Acidity
High pH means
Alkalinity
Buffer region
Around pa where pH changes very little, despite acid/base being constantly added.
An indicator colour change must...
Be rapid Occur at exactly the same time as the equivalence point.
Strong acid
Fully dissociates in water.
Dissociation of water
H₂O ⇌ H⁺/H₃O⁺ + OH⁻
Oxonium ion
H₃O⁺
Ka weak acid dissociation constant
Ka = [H⁺] [A⁻] [HA]
The ionic product of water (at 298K)
Kw = [H⁺] [OH⁻] = 1.0 x 10⁻¹⁴
pH
Measure of acidity by measuring concentration of H⁺ ions.
Equivalence volume/endpoint
Mi-point of "vertical section".
How can an acidic buffer be made?
Mix HA with Na⁺A⁻. Part-neutralise HA with NaOH.
How can a basic buffer be made?
Mix weak base with its salt (e.g. NH₃ with NH₄Cl). Part-neutralise weak base with HCl.
When H⁺ added to basic buffer...
OH⁻ + H⁺ → H₂O Unbalances equilibrium. Higher concentration of H₂O. Le Chatelier's principle. NH₃ + H₂O → NH₄⁺ + OH⁻
When OH⁻ added to acidic buffer...
OH⁻ + H⁺ → H₂O Unbalances equilibrium. Lower concentration of H⁺. Le Chatelier's principle. HA → H⁺ + A⁻
Weak acid
Only very slightly dissociates in water.
What colour changes does phenolphthalein show?
Pink - pH10 Pale pink - pH9 Colourless - pH8
Bronsted-Lowry base
Proton acceptor
Bronsted-Lowry acid
Proton donor
Buffer
Solution that maintains an almost constant pH on addition of small amounts of strong acid or strong base.
The more a weak acid dissociates...
The more H⁺ are formed. The stronger the acid. The larger the Ka value.
What does an acid/base reaction involve?
Transfer of the H⁺ from the acid to the base.
When H⁺ added to acidic buffer...
Unbalances equilibrium. Higher concentration of H⁺. Le Chatelier's principle. H⁺ + A⁻ → HA
When OH⁻ added to basic buffer...
Unbalances equilibrium. Higher concentration of OH⁻. Le Chatelier's principle. NH₄⁺ + OH⁻ → NH₃ + H₂O1.
When water boils what happens to it's pH?
Water dissociating is endothermic, and the reverse reaction is exothermic. Le Chatelier's principle. Equilibrium shifts to right to lower temperature. More H⁺ so pH lower (more acidic).
What does an acidic buffer consist of?
Weak acid (HA) with its salt (Na⁺A⁻)
Acid-base indicators
Weak acids. Conjugate acid and base forms have different colours.
What does a basic buffer consist of?
Weak base with its salt.
When can you NOT use methyl orange?
With a weak acid.
When can you NOT use phenolphthalein?
With a weak base.
What colour changes does methyl orange show?
Yellow - pH5 Orange - pH4 Red - pH3
At half equivalence...
[A⁻] = [HA] Ka = [H⁺] pKa = pH
Calculating pH of a base
[H⁺] = Kw [OH⁻]
Calculating pH
pH = -log [H⁺]
Calculating pKa
pKa = -log (Ka)
