Acids, Bases, and Titrations
Acids and Alkalis
pH Scale - Universal Indicator:
- Made from a mixture of dyes and tells you strength, it cannot be used in titrations as colour change is too gradual.
- pH 0-3 = Strongly acidic (red)
- pH 4-6 = Weakly acidic (orange → yellow)
- pH 7 = Neutral (green)
- pH 8-10 = Weakly alkaline (blue → purple)
- pH 11-14 = Strongly alkaline (purple)
Indicators:
An indicator is a chemical that changes colour when the pH changes.
1. Methyl orange:
Acid: Red
Neutral/Alkali: Yellow
Has 2 different colours to detect endpoint in titrations.
2. Phenolphthalein:
Acid/Neutral: Colourless
Alkali = Pale pink
3. Red litmus paper:
It stays red unless in the presence of alkali and will turn blue.
It can be used to test the presence of gases and solutions. Pepar must be dampened with distilled water and held inside the gas.
4. Blue litmus paper:
It stays blue unless in the presence of acid and will turn red.
It can be used to test the presence of gases and solutions. Pepar must be dampened with distilled water and held inside the gas.
5. Litmus solution - Contains a compound extracted from lichen.
Acid = Red
Neutral = Purple
Alkali = Blue
Acids in aq = H⁺ source
Bases in aq = OH⁻ source
| Soluble | Insoluble | |
|---|---|---|
| Acids | All common acids | - |
| Bases | All group 1 oxides, all hydroxides and ammonia | All other hydroxides + oxides |
| All nitrates |
- | |
| All chlorides except → |
lead/silver chloride | |
| All sulphates except → | barium, lead and calcium sulfate | |
| All ammonium salts |
- | |
| Sodium + Potassium carbonates | ← All carbonates except |
Neutralisation:
- An acid is a source of H⁺ ions; they are proton donors.
- A base is a substance that can neutralise an acid; they are proton acceptors.
→ An alkali is a source of OH⁻ ions and is a soluble base.
- The reaction between an acid and a base = neutralisation
→ H⁺ + OH- → H₂O (neutral product)
→ The acid donates protons accepted by the base.
Acid Reactions:
1. Metal + Acid → Salt + Hydrogen
2. Metal oxide + Acid → Salt + Water
3. Metal hydroxide + Acid → Salt + Water
4. Metal carbonate + Acid → Aalt + Water + Carbon dioxide
5. Ammonia + Acid → Ammonium salt
e.g., 2NH3 + H2SO4 → (NH4)2SO4
e.g. NH₃ + HNO₃ → NH₄NO₃
- Alkalis are soluble bases.
Titration Calculations:
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
2 : 1
0.0025 : 0.00125
Titrations:
- Titrations allow you to find out exactly how much acid is needed to neutralise an alkali (+ vice versa).
Method:
1. Measure out a set amount of acid using a volumetric pipette and add a few drops of indicator.
2. Fill a burette with alkali using a small funnel and record the starting volume.
3. Place the conical flask on a white tile under the burette tip.
4. Slowly add the alkali to the acid, continuously swirling to make sure the solutions are evenly combined at all times till the indicator colour suddenly changes.
5. Read the burette off and record the amount of alkali required to neutralise the acid.
6. Repeat 3x to get concordant results → ± 0.2cm3.
To make salt crystals:
7. Add the same volumes of acid and alkali to a clean flask without an indicator.
8. Evaporate ⅔ of the water from the solution using a water bath so the solution becomes saturated.
9. Leave the solution to cool so the salt can crystallise out.
10. Dry crystals between filter paper.
To make an anhydrous salt, all the water must be evaporated to obtain a dry powder with no water.
