A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Basic concepts and calculations
CSCA Chemistry · Lesson 5 of 21Chemical reactions
Réactions chimiques · التفاعلات الكيميائية
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In a reaction atoms are rearranged, never created or destroyed. A balanced equation shows the same number of each atom on both sides.
Key ideas
- Balance by changing coefficients, never the formulas.
- Mass is conserved: total mass of reactants = total mass of products.
- Combustion of hydrocarbons gives CO₂ and H₂O.
- Oxidation is loss of electrons; reduction is gain of electrons.
Formulas to know
- 2H₂ + O₂ → 2H₂O
- CH₄ + 2O₂ → CO₂ + 2H₂O
- Mole ratios come from the coefficients
Worked example
Balance: _ Al + _ O₂ → _ Al₂O₃
- Oxygen: 2 on the left, 3 on the right. The common multiple is 6: 3 O₂ and 2 Al₂O₃.
- Now there are 4 Al on the right, so put 4 Al on the left.
Answer: 4Al + 3O₂ → 2Al₂O₃
Practice questions
1. In 2H₂ + O₂ → 2H₂O, how many moles of water come from 4 mol of H₂?
2 mol · 4 mol ✓ · 8 mol · 1 mol
The ratio H₂ : H₂O is 2 : 2, so 4 mol.
2. Which coefficients balance N₂ + _H₂ → _NH₃?
3 and 2 ✓ · 2 and 3 · 1 and 2 · 3 and 3
N₂ + 3H₂ → 2NH₃: 2 N and 6 H on each side.
3. In Zn → Zn²⁺ + 2e⁻, zinc is…
Reduced · Oxidised ✓ · Neutralised · Unchanged
Zinc loses electrons: that is oxidation.