Chemical Reactions, Acids, Bases and Salts: notes

Physical and Chemical Changes

• Physical change: a change in form or state where no new substance is made. Melting ice and dissolving sugar are examples. The change can usually be reversed.
• Chemical change: a change where one or more new substances are formed. Burning magnesium and rusting iron are examples. It is usually hard to reverse.
• Signs of a chemical change: colour change, gas bubbles, a temperature change, or a solid forming from solutions (a precipitate).

Rates and Catalysts

• Rate of reaction: how fast reactants are used up or products are formed. Rate = amount of product ÷ time taken.
• Factors that speed up reactions: higher temperature, higher concentration, larger surface area (powder instead of lumps), and higher pressure for gases.
• Catalyst: a substance that speeds up a reaction and is not used up at the end. Manganese(IV) oxide speeds up the breakdown of hydrogen peroxide.

Energy Changes and Reversible Reactions

• Exothermic reaction: a reaction that gives out heat to the surroundings, so the temperature rises. Combustion is an example.
• Endothermic reaction: a reaction that takes in heat from the surroundings, so the temperature falls. Dissolving ammonium nitrate in water is an example.
• Reversible reaction: a reaction that can go both ways. The symbol ⇌ shows this.

Oxidation, Reduction and Electrolysis

• Oxidation: gain of oxygen, or loss of electrons.
• Reduction: loss of oxygen, or gain of electrons.
• Electrolysis: splitting a compound using electricity. Ions move to the electrodes. Positive ions (cations) go to the cathode (negative electrode). Negative ions (anions) go to the anode (positive electrode).
• Electroplating: coating an object with a thin layer of metal using electrolysis. The object to be plated is the cathode.

Acids, Bases, pH and Salts

• Acid: a substance with pH below 7. Acids produce H⁺ ions in water.
• Alkali: a soluble base with pH above 7. Alkalis produce OH⁻ ions in water.
• pH scale and indicators: universal indicator shows red for strong acids, green for neutral, and purple for strong alkalis.
• Neutralisation: acid + base → salt + water. Example: HCl + NaOH → NaCl + H₂O.
• Preparing a salt: add excess solid base to warm acid, filter off the leftover solid, then evaporate and crystallise the filtrate.
• Treating acid soil: farmers add lime (calcium hydroxide) to neutralise acidic soil.

Worked Calculation: Rate

A student collects 30 cm³ of gas in 60 seconds.

• Rate = 30 ÷ 60 = 0.5 cm³ per second

Tests for Ions and Gases

• Carbon dioxide: turns limewater milky.
• Hydrogen: makes a "squeaky pop" with a lighted splint.
• Chloride ions (Cl⁻): white precipitate with silver nitrate, dissolves in dilute ammonia? No: stays, so use dilute nitric acid to confirm.

Common slips

• Mixing up exothermic (heat out) and endothermic (heat in).
• Saying a catalyst is "used up". It is not.
• Writing that acids have pH above 7.
• Forgetting that the cathode is negative in electrolysis.
• Confusing oxidation with reduction.