OCR Gateway GCSE Chemistry A (J248) · C3 — Chemical reactions
Mini-Lesson
Chemical reactions
This mini-lesson walks you through the whole of OCR Gateway Topic C3 — Chemical reactions: writing & balancing equations, the mole, energetics, acids & bases, redox and electrolysis.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Items marked Higher are Higher-tier only. Press Start when you're ready.
C3.1 · Symbol equations
Writing & balancing equations
In a reaction, atoms are rearranged — never created or destroyed. So a symbol equation must have the same number of each type of atom on both sides. We balance by putting numbers in front of formulae (never changing a formula).
2H2 + O2 → 2H2O4 H and 2 O on the left = 4 H and 2 O on the right ✓
Add state symbols to show the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water (aqueous).
Watch out: you can only change the big numbers in front (the coefficients). Changing a small subscript — e.g. writing H₂O₂ instead of H₂O — makes a different substance.
Quick check
Balance the equation
?Balance: __ Mg + O2 → __ MgO. Which set of numbers makes the atoms balance?
C3.1 · Conservation of mass
Mass is conserved
The law of conservation of mass: no atoms are made or lost in a reaction, so the total mass of the products equals the total mass of the reactants.
Sometimes the mass in an open container seems to change — this is explained by the particle model:
A reaction that gives off a gas (e.g. a carbonate + acid) appears to lose mass, because the gas escapes into the air.
A metal that reacts with oxygen appears to gain mass, because oxygen atoms from the air join the solid.
Watch out: mass is always conserved. In a sealed container the reading never changes. Any apparent change just means a gas entered or left the open vessel.
Calculate
Your turn — conservation of mass
1In a sealed flask, 24 g of magnesium reacts completely with 16 g of oxygen. What mass of magnesium oxide is produced?
g
Hint: mass of products = mass of reactants = 24 + 16.
C3.1 · Relative formula mass
Relative formula mass, Mr
The relative formula mass (Mr) of a compound is found by adding up the relative atomic masses (Ar) of all the atoms in its formula.
Mr of CO2 = 12 + (2 × 16) = 44Ar(C) = 12, Ar(O) = 16
Worked example — H₂O
Mr(H₂O) = (2 × 1) + 16 = 18
Worked example — CaCO₃: 40 + 12 + (3 × 16) = 100
Because mass is conserved, the total Mr of the reactants equals the total Mr of the products once an equation is balanced.
Calculate
Your turn — relative formula mass
2Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3. Use Ar: Ca = 40, C = 12, O = 16.
Hint: 40 + 12 + (3 × 16).
C3.1 · The mole
The mole & the Avogadro constant
Chemists count particles in moles. One mole of any substance contains the Avogadro constant of particles:
NA = 6.02 × 10²³ per molethe number of atoms, molecules or ions in one mole
The mass of one mole equals the Mr in grams, so you can convert between mass and moles:
moles = mass ÷ Mramount (mol) = mass (g) ÷ relative formula mass
Worked example
How many moles in 88 g of CO₂ (Mr = 44)?
moles = 88 ÷ 44 = 2 mol
Calculate
Your turn — moles
3How many moles are there in 36 g of water, H2O? (Mr of H2O = 18.)
mol
Hint: moles = mass ÷ Mr = 36 ÷ 18.
C3.1 · Higher
Ionic & half equations (HT)
Many reactions happen between ions in solution. An ionic equation shows only the ions that actually change — spectator ions are left out. Neutralisation always comes down to:
H⁺(aq) + OH⁻(aq) → H2O(l)the ionic equation for any acid + alkali neutralisation
A half equation shows what happens to one ion, including the electrons (e⁻) gained or lost — used a lot in electrolysis:
Na⁺ + e⁻ → Na 2Cl⁻ → Cl2 + 2e⁻both charges AND atoms must balance on each side
Check: in a half equation the total charge must be equal on both sides once the electrons are counted.
Quick check (HT)
The neutralisation ionic equation
?Which ionic equation correctly represents the neutralisation of any acid by any alkali?
C3.2 · Energetics
Exothermic & endothermic
Every reaction involves an energy change with the surroundings:
An exothermic reaction releases energy to the surroundings — the temperature rises. Examples: combustion, neutralisation, many displacements.
An endothermic reaction takes in energy from the surroundings — the temperature falls. Examples: thermal decomposition, electrolysis, photosynthesis.
Reaction profiles. Both rise by the activation energy Eₐ first. Exothermic: products are lower than reactants. Endothermic: products are higher.
Activation energy (Eₐ): the minimum energy the reactants need to collide successfully and start reacting — the "hump" at the start of every profile.
Quick check
Exothermic or endothermic?
?A student adds two solutions together and the temperature of the mixture drops from 21 °C to 8 °C. What does this tell you?
C3.2 · Higher
Bond energy calculations (HT)
Breaking bonds takes in energy (endothermic); making bonds releases energy (exothermic). The overall change is:
ΔH = energy to break bonds − energy released making bondsΔH = (bonds broken) − (bonds made)
If more energy is released making bonds than is used breaking them, ΔH is negative → exothermic. If less, ΔH is positive → endothermic.
Worked example — H₂ + Cl₂ → 2HCl
Break: H–H (436) + Cl–Cl (243) = 679 kJ in
Make: 2 × H–Cl (2 × 432) = 864 kJ out
ΔH = 679 − 864 = −185 kJ/mol → exothermic ✓
Watch out: ΔH = broken − made. Get the order the wrong way round and your sign (and so exo/endo) flips.
Calculate (HT)
Your turn — bond energies
4For a reaction, the energy to break all the reactant bonds is 1500 kJ and the energy released making the product bonds is 1800 kJ. Calculate ΔH (kJ).
kJ
Hint: ΔH = broken − made = 1500 − 1800. (A negative answer = exothermic.)
C3.3 · Acids & bases
The pH scale & indicators
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. Acids release H⁺ ions in water; alkalis (soluble bases) release OH⁻ ions.
Universal indicator colours across the pH scale. pH 7 is neutral; below 7 acidic, above 7 alkaline.
Watch out:pH 7 means neutral (e.g. pure water), not "no solution" or "weak". Lower pH = more acidic = higher H⁺ concentration. (HT) a fall of 1 pH unit means H⁺ concentration ×10.
C3.3 · Reactions of acids
Neutralisation & making salts
A base neutralises an acid. Bases include metal oxides, metal hydroxides (alkalis) and carbonates. The general patterns:
acid + base → salt + wateracid + metal → salt + hydrogen · acid + carbonate → salt + water + carbon dioxide
The salt's name comes from the acid used:
Hydrochloric acid → a chloride (e.g. sodium chloride)
Watch out: with electrons, losing electrons is oxidation. It feels backwards, so chant OIL RIG every time.
Quick check
Oxidation or reduction?
?During electrolysis a sodium ion changes: Na⁺ + e⁻ → Na. The sodium ion has...
C3.4 · Electrolysis
Electrolysis splits compounds
Electrolysis uses electricity to break down an ionic compound that is molten or dissolved (so the ions are free to move). Ions are attracted to the oppositely charged electrode:
Cathode (−): attracts positive ions (cations) → metal (or hydrogen) forms here. Cations gain electrons (reduction).
Molten PbBr₂: lead forms at the cathode, bromine gas at the anode. Half equations (HT): Pb²⁺ + 2e⁻ → Pb · 2Br⁻ → Br₂ + 2e⁻
Aqueous solutions: at the cathode you get hydrogen instead of the metal if the metal is more reactive than hydrogen (e.g. Na). At the anode you get a halogen if a halide is present, otherwise oxygen (from OH⁻). Used to extract aluminium (from molten Al₂O₃) and to purify copper.
Quick check
Products of electrolysis
?Molten zinc chloride (ZnCl2) is electrolysed with inert electrodes. What forms at the negative cathode?
Quick check (HT)
Write the half equation
?At the anode, chloride ions form chlorine gas. Which is the correct, balanced half equation?
Sort it
Cathode or anode?
Tap a product, then the electrode it forms at during electrolysis.
⊖ Cathode (−) metals & hydrogen
⊕ Anode (+) non-metals
Recap
What to remember
C3.1: balance equations (numbers in front + state symbols); mass conserved; Mr = Σ Ar; moles = mass ÷ Mr; ionic & half equations (HT).
C3.2: exothermic (releases, ΔH −) vs endothermic (takes in, ΔH +); reaction profiles & activation energy; ΔH = bonds broken − bonds made (HT).
C3.3: pH scale & indicators; neutralisation; acid + metal/base/carbonate; salt named from the acid; strong vs weak (HT); redox = OIL RIG / gain–loss of oxygen.