OCR Gateway GCSE Chemistry A (J248) · C3 — Chemical reactions
Mini-Lesson
Chemical reactions
This mini-lesson walks you through OCR Gateway Topic C3 — Chemical reactions: writing & balancing equations, the mole, energetics, acids & bases, redox and electrolysis.
Make a guess: where does the extra mass come from?
Mission 1Where does the extra mass come from?
Mission 2Why does a burning match need a push?
Mission 3Why do only some acid reactions fizz?
Mission 4Why does water change the product?
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.
Mission 1 check
Where does the extra mass come from?
?Two open dishes sit on balances. In dish A, iron wool is heated in air and its mass goes up. In dish B, a metal carbonate reacts and its mass goes down. Which statement explains both readings?
C2.1 · 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 · Higher
The mole & the Avogadro constant (HT)
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 (HT)
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?
Mission 2 of 4
Why does a burning match need a push?
Make a guess: why does a match need a strike to start burning?
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, citric acid + sodium hydrogencarbonate, 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" in 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?
Mission 2 check
Why does a burning match need a push?
?A match head burns in an exothermic reaction. Yet a match left in its box never lights by itself. Why must it be struck first?
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.)
Mission 3 of 4
Why do only some acid reactions fizz?
Make a guess: why do only some acid reactions fizz?
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 and metal hydroxides (soluble bases are alkalis); carbonates also neutralise acids. 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: the first half of the salt name comes from the metal/base; the second half from the acid. Sulfuric → sulfate, not "sulfuride".
Quick check
Name the salt
?Zinc oxide reacts with nitric acid. What salt is formed?
Match it
What do acids produce?
Tap a reaction on the left, then its products on the right.
Mission 3 check
Why do only some acid reactions fizz?
?Dilute sulfuric acid is added to three tubes. One holds zinc, one copper(II) oxide and one copper(II) carbonate. Which statement explains which tubes fizz?
C3.3 · Higher
Strong vs weak acids (HT)
"Strong/weak" is about how much an acid ionises (dissociates) in water — it is not the same as dilute/concentrated:
A strong acidfully ionises in water (e.g. hydrochloric, sulfuric, nitric). All its molecules release their H⁺.
A weak acid only partially ionises (e.g. ethanoic acid, citric acid). Only a small fraction release H⁺.
At the same concentration, the strong acid has more H⁺ ions, so a lower pH.
Watch out:strong ≠ concentrated. Strong/weak is about degree of ionisation; dilute/concentrated is about how much acid is dissolved in the volume.
Mission 4 of 4
Why does water change the product?
Make a guess: why does water stop sodium forming at the cathode?
C3.3 · Oxidation & reduction
Redox: OIL RIG
Reduction and oxidation always happen together — a redox reaction. There are two ways to spot them:
In terms of oxygen:oxidation = gain of oxygen; reduction = loss of oxygen.
In terms of electrons (HT): remember OIL RIG — Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
Watch out: with electrons, losing electrons is oxidation. It feels backwards, so chant OIL RIG every time.
Quick check (HT)
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 aluminium oxide dissolved in molten cryolite) and to purify copper.
Quick check
Products of electrolysis
?Molten zinc chloride (ZnCl2) is electrolysed with inert electrodes. What forms at the negative cathode?
Mission 4 check
Why does water change the product?
?Molten sodium bromide is electrolysed and sodium forms at the cathode. When a solution of sodium bromide in water is electrolysed, hydrogen forms there instead. Why?
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 (C2.1); moles = mass ÷ Mr(HT); 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 = gain–loss of oxygen / OIL RIG (HT).