Acids, pH and Making Salts
Acids, Bases and the pH Scale
An acid is a substance that produces hydrogen ions (\(H^+\)) when dissolved in water. A base is a substance that neutralises an acid. An alkali is a soluble base — it produces hydroxide ions (\(OH^-\)) when dissolved in water.
The pH scale runs from 0 (most acidic) to 14 (most alkaline), with 7 being exactly neutral. pH is measured using universal indicator (which changes colour) or a pH probe (which gives a precise digital reading).
Acids: produce \(H^+\) ions, pH 0–6. Neutral: pH 7. Alkalis: produce \(OH^-\) ions, pH 8–14. The neutralisation reaction: \(H^+ + OH^- \rightarrow H_2O\).
Neutralisation Reactions
Neutralisation occurs when an acid reacts with a base, producing a salt and water. The key reaction types are:
- Acid + metal → salt + hydrogen gas. E.g. \(Mg + H_2SO_4 \rightarrow MgSO_4 + H_2\)
- Acid + metal oxide → salt + water. E.g. \(CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O\)
- Acid + metal hydroxide → salt + water. E.g. \(NaOH + HCl \rightarrow NaCl + H_2O\)
- Acid + metal carbonate → salt + water + carbon dioxide. E.g. \(CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\)
The name of the salt produced depends on the acid used: hydrochloric acid (HCl) makes chlorides, sulfuric acid (\(H_2SO_4\)) makes sulfates, and nitric acid (\(HNO_3\)) makes nitrates.
HCl → chlorides. \(H_2SO_4\) → sulfates. \(HNO_3\) → nitrates. Salt name = metal name + acid ending. Example: sodium + sulfuric acid → sodium sulfate.
Making Pure Dry Salts
To make copper(II) sulfate crystals from copper(II) oxide and sulfuric acid (a core practical):
- Warm dilute sulfuric acid in a beaker.
- Add excess copper(II) oxide (black powder) and stir until no more dissolves — the excess ensures all acid is used up.
- Filter the mixture to remove the excess copper(II) oxide.
- Evaporate the blue filtrate gently until crystals begin to form, then leave to crystallise fully.
- Pat the blue crystals dry with filter paper.
Evaporate the filtrate over a water bath rather than heating it directly. The gentle heat stops the crystals spitting or losing their water of crystallisation. Adding the copper(II) oxide in excess is what guarantees that no acid is left in the filtrate.
Some salts are insoluble and cannot be made by neutralisation. They are made by precipitation: mix two solutions whose ions react to form the insoluble salt. For example, to make barium sulfate: mix barium chloride solution with sodium sulfate solution — barium sulfate precipitates immediately. Filter, wash with distilled water and dry.
$$BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$$
Titration
Titration is used to find the exact volume of acid needed to neutralise a known volume of alkali (or vice versa). The apparatus required includes a burette (to deliver the acid) and a pipette (to measure the exact volume of alkali). An indicator such as phenolphthalein or methyl orange shows when the endpoint is reached.
1. Rinse the burette with the acid solution, then fill it and record the initial reading to 0.05 cm³. 2. Use the pipette to measure an exact volume of alkali into a conical flask; add a few drops of indicator. 3. Add the acid from the burette drop by drop, swirling the flask. As the endpoint approaches, add half-drops. 4. Stop when the indicator changes colour permanently. Record the final burette reading. 5. Calculate the volume added (titre = final − initial). Repeat until two titres agree within 0.10 cm³; these are concordant results. Average the concordant results.
From a titration, use: \(n = c \times V\) where \(n\) = moles, \(c\) = concentration (mol/dm³), \(V\) = volume (dm³). Convert cm³ to dm³ by dividing by 1000.
Example: 25.0 cm³ of NaOH solution of concentration 0.10 mol/dm³ required 20.0 cm³ of HCl solution. Find the concentration of the HCl.
Moles NaOH \(= 0.10 \times 0.025 = 0.0025\) mol. From NaOH + HCl → NaCl + H₂O, the ratio is 1:1, so moles HCl = 0.0025 mol. Concentration HCl \(= \frac{0.0025}{0.020} = 0.125\) mol/dm³.
A key distinction for the exam is between strong and weak acids: strength refers to the degree of ionisation in water, not concentration. A concentrated weak acid like ethanoic acid has a lower pH than a dilute weak acid, but both ionise only partially. When making soluble salts by neutralisation, use an indicator to find the exact equivalence point; when using an insoluble base, add excess and then filter to remove it — no indicator is needed because the excess base is simply removed by filtration.