Revision notes · Chemical changes
Reactions of acids
Reactions of acids with metals4.4.2.1
Definition: Metals above hydrogen in the reactivity series react with acids in a redox reaction, producing a salt and hydrogen gas.
Acids react with metals above hydrogen in the reactivity series to produce a salt and hydrogen gas.
- •Acid + metal → salt + hydrogen.
- •This is a redox reaction: the metal is oxidised by losing electrons, while hydrogen ions are reduced by gaining electrons.
- •Unreactive metals below hydrogen in the reactivity series, such as copper, will not react with dilute acids.
| Acid | Salt produced |
|---|---|
| Hydrochloric acid | Chloride |
| Sulfuric acid | Sulfate |
| Nitric acid | Nitrate |
⚠️ Common mistake: Thinking that all metals react with acids, forgetting that unreactive metals like copper do not react.
🧠 Remember: Remember MASH: Metal + Acid → Salt + Hydrogen.
Neutralisation of acids and salt production4.4.2.2
Definition: A neutralisation reaction is a chemical reaction in which an acid reacts with a base or alkali to form a salt and water.
Acids are neutralised by metal oxides, metal hydroxides, and metal carbonates to produce salts.
- •Acid + metal oxide (or hydroxide) → salt + water.
- •Acid + metal carbonate → salt + water + carbon dioxide.
- •In aqueous solutions, neutralisation between an acid and an alkali is represented by the ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l).
- •The specific salt produced depends on the acid used and the positive ions in the base or alkali.
⚠️ Common mistake: Forgetting that metal carbonates produce carbon dioxide gas in addition to a salt and water.
🧠 Remember: Remember BAWS (Base + Acid → Water + Salt) and CAWCS (Carbonate + Acid → Water + Carbon Dioxide + Salt).
Soluble salts4.4.2.3
Definition: A soluble salt is an ionic compound that completely dissolves in water to form an aqueous solution.
Soluble salts are prepared by reacting an acid with an insoluble base, such as a metal oxide or carbonate.
- 1Add the insoluble reactant (metal, metal oxide, hydroxide or carbonate) to the acid a little at a time, stirring, until it is in excess — some solid remains undissolved, showing all the acid has reacted.
- 2Filter the mixture using filter paper and a funnel, collecting the salt solution as the filtrate and discarding the unreacted excess solid.
- 3Gently heat the filtrate in an evaporating basin over a water bath to evaporate off some of the water and concentrate the solution.
- 4Leave the concentrated solution to cool slowly so pure, dry salt crystals form, then filter and dry the crystals.
⚠️ Common mistake: Heating the salt solution until completely dry instead of heating gently to the point of crystallisation.
🧠 Remember: Follow the sequence: Add excess → Filter → Evaporate → Crystallise.
The pH scale and neutralisation4.4.2.4
Definition: The pH scale is a continuous scale from 0 to 14 that measures the acidity or alkalinity of an aqueous solution.
The pH of a solution measures its hydrogen ion concentration using universal indicator or a digital pH probe.
- •Acids produce hydrogen ions (H⁺) in aqueous solutions and have a pH value less than 7.
- •Alkalis produce hydroxide ions (OH⁻) in aqueous solutions and have a pH value greater than 7.
- •A neutral solution contains equal concentrations of H⁺ and OH⁻ ions and has a pH of 7.
- •A decrease of 1 unit on the pH scale represents a ten-fold increase in hydrogen ion concentration.
⚠️ Common mistake: Thinking a decrease in pH means fewer hydrogen ions, when a lower pH actually indicates a higher concentration of H⁺ ions.
🧠 Remember: Each drop of 1 pH unit multiplies H⁺ ion concentration by 10.
Titrations4.4.2.5
Definition: A titration is a quantitative technique used to calculate the exact concentration of an acid or an alkali.
Titrations use calibrated glassware to measure the precise volume of acid required to neutralise an alkali.
| Equipment | Role |
|---|---|
| Pipette | Measures an exact, fixed volume of alkali into the conical flask |
| Burette | Adds acid dropwise until the end point is reached |
- 1Use a pipette and pipette filler to measure a fixed volume (e.g. 25 cm³) of alkali into a clean conical flask, then add a few drops of a single indicator such as phenolphthalein.
- 2Fill the burette with acid, below eye level, and record the starting volume reading.
- 3Add the acid from the burette a little at a time, swirling the flask constantly, until the indicator just changes colour at the end point.
- 4Record the final burette reading and calculate the titre (volume of acid added).
- 5Repeat the titration until you get concordant results (titres within 0.10 cm³ of each other), then calculate the mean titre.
⚠️ Common mistake: Using universal indicator for a titration instead of a single indicator like phenolphthalein, leading to a gradual colour change rather than a sharp end point.
🧠 Remember: Pipette for fixed volume, burette for variable volume, concordant within 0.10 cm³.
Strong and weak acids4.4.2.6
Definition: A strong acid is an acid that completely ionises in aqueous solution to release hydrogen ions.
Acid strength depends on how fully the acid molecules dissociate into ions when dissolved in water.
| Term | Meaning |
|---|---|
| Strong acid | Fully ionises in water (e.g. hydrochloric acid) |
| Weak acid | Only partially ionises in water, a reversible reaction (e.g. ethanoic acid) |
| Concentration | How much acid is dissolved per unit volume — separate from strength |
- •At the same concentration, a strong acid has a higher concentration of H⁺ ions and a lower pH than a weak acid.
⚠️ Common mistake: Confusing acid strength (how fully it ionises) with acid concentration (how much acid solute is dissolved per dm³).
🧠 Remember: Strong = fully ionised; Weak = partially ionised; Concentrated = more acid particles per unit volume.
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