Formulae and Chemical Equations
Chemistry· 3.1· 25 min read
1. Writing Chemical Formulae of Common Compounds (Core)★★☆☆☆⏱ 8 min
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Valency
The combining power of an atom or ion, equal to the charge of the ion for ionic compounds, or the number of covalent bonds formed for covalent compounds.
Example:
Sodium (Group 1) has valency +1, oxygen (Group 16) has valency -2
For all neutral compounds, the sum of positive valencies equals the sum of negative valencies. For ionic compounds, swap the valencies of the two ions to get the subscript count for each ion, and bracket polyatomic ions if you have more than one copy of the ion.
Write the correct chemical formula of calcium nitrate.
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Step 1: Identify valencies of the ions: Ca²⁺ = +2, NO₃⁻ = -1
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Step 2: Find the lowest common multiple of 2 and 1 = 2
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Step 3: Calculate number of each ion needed: 1 Ca²⁺ (2/2 =1), 2 NO₃⁻ (2/1=2)
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Step 4: Write the formula, bracketing the polyatomic nitrate ion: Ca(NO₃)₂
2. Balancing Chemical Equations (Core)★★★☆☆⏱ 7 min
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Balanced Chemical Equation
An equation where the number of atoms of every element is identical on the reactant (left) and product (right) sides, complying with the law of conservation of mass.
Example:
2H₂(g) + O₂(g) → 2H₂O(l): 4 H atoms and 2 O atoms on both sides
First write the unbalanced equation with correct formulae for all reactants and products. Balance elements that appear in only one reactant and one product first, then balance elements that appear in multiple substances. Only adjust whole number coefficients in front of formulae, never change subscripts (this changes the identity of the compound). Add state symbols last.
Balance the equation for the reaction of magnesium with hydrochloric acid, including state symbols: Mg + HCl → MgCl₂ + H₂
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Step 1: Count atoms on each side: Left = 1 Mg, 1 H, 1 Cl; Right = 1 Mg, 2 H, 2 Cl
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Step 2: Balance Cl first: add coefficient 2 in front of HCl: left Cl = 2, left H = 2
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Step 3: Verify H is now balanced (2 on both sides), Mg is already balanced
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Step 4: Add state symbols: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
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Step 5: Final check: all atom counts are equal on both sides, equation is correct
3. Extended Only: Deducing Balanced Equations from Mass Data★★★★☆Extended only⏱ 7 min
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For Extended tier candidates, you may be given masses of reactants and products to deduce the balanced equation. Use the mole formula to calculate the moles of each substance, then find the simplest whole number ratio of moles to get the coefficients for the balanced equation.
8.0g of copper(II) oxide reacts with 0.2g of hydrogen to form 6.4g of copper and 1.8g of water. Deduce the balanced equation for this reaction. A_r values: Cu=64, O=16, H=1.
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Step 1: Calculate M_r values for compounds: CuO = 64+16=80, H₂=2, H₂O=18
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Step 2: Calculate moles of each substance:
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Step 3: Divide all mole values by the smallest value (0.1) to get a 1:1:1:1 ratio
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Step 4: Write the balanced equation with state symbols: CuO(s) + H₂(g) → Cu(s) + H₂O(l)
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Step 5: Verify atom counts are equal on both sides: 1 Cu, 1 O, 2 H each side, correct
4. IUPAC Naming Rules for Common Inorganic Compounds★★☆☆☆⏱ 3 min
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Ionic compounds: Name the positive ion first, then the negative ion. Negative monatomic ions end in -ide, e.g. sodium chloride, magnesium oxide
Polyatomic ions with oxygen end in -ate (more oxygen atoms) or -ite (fewer oxygen atoms), e.g. potassium sulfate, sodium sulfite
Binary covalent compounds: Use prefixes (di-, tri-, tetra-) to show the number of each atom, omit mono- for the first element, e.g. carbon dioxide, dinitrogen trioxide
5. Common Pitfalls
Wrong move:
Writing OH₂ instead of (OH)₂ for two hydroxide ions in calcium hydroxide formula
Why:
Incorrect notation implies 2 hydrogen atoms attached to one oxygen, rather than two separate hydroxide groups, and will lose marks in exams
Correct move:
Always bracket polyatomic ions if you have more than one copy, so write Ca(OH)₂
Wrong move:
Changing subscripts in formulae when balancing equations, e.g. changing H₂O to H₂O₂ to balance oxygen
Why:
Subscripts define the identity of the compound, changing them creates a different substance that is not part of the reaction
Correct move:
Only adjust whole number coefficients in front of formulae to balance atom counts
Wrong move:
Forgetting to add state symbols to equations even when not explicitly asked
Why:
CIE IGCSE 0620 awards 1 mark for correct state symbols in nearly all equation questions, regardless of explicit instruction
Correct move:
Add state symbols (s), (l), (g), (aq) to every balanced equation unless told otherwise
Wrong move:
Writing the formula of sodium sulfate as NaSO₄
Why:
Sodium has valency +1, sulfate has valency -2, so the sum of charges is -1, making the formula not neutral
Correct move:
Use 2 sodium ions per sulfate ion to get neutral formula Na₂SO₄
Wrong move:
Using A_r of O=16 instead of M_r of O₂=32 when calculating moles of oxygen gas (Extended only)
Why:
Oxygen exists as diatomic molecules in its standard state, so 1 mole of O₂ has a mass of 32g, not 16g
Correct move:
Use relative molecular mass for all diatomic elements (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂) when calculating moles of the standard state element
6. Quick Reference Cheatsheet
Task | Rule | Example |
|---|---|---|
Write ionic formula | Sum of positive valencies = sum of negative valencies; bracket polyatomic ions if >1 | Aluminium sulfate: Al₂(SO₄)₃ |
Balance equation | Only change coefficients, never subscripts; add state symbols last | Combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) |
Extended: Deduce equation from mass | Calculate moles of all substances, divide by smallest mole value to get coefficient ratio | Mole ratio 1:2:1:2 → Zn + 2HCl → ZnCl₂ + H₂ |
Name ionic compound | Positive ion first, negative ion ends in -ide/-ate/-ite | Fe(NO₃)₃ = iron(III) nitrate |
7. Frequently Asked
Do I need to include state symbols in all equations?
Yes, unless the question explicitly states otherwise. CIE IGCSE 0620 exams award 1 mark for correct state symbols in nearly all equation questions, even if not explicitly requested.
How do I remember valencies of common ions?
Use group numbers for main group elements: Group 1 = +1, Group 2 = +2, Group 17 = -1, Group 16 = -2. Memorise common polyatomic ions: nitrate (NO₃⁻, -1), sulfate (SO₄²⁻, -2), hydroxide (OH⁻, -1), carbonate (CO₃²⁻, -2).
Going deeper
What's Next
Now that you can write and balance chemical equations, you have the foundational skill required for all stoichiometry calculations in CIE IGCSE Chemistry 0620 Unit 3. Incorrect formulae or unbalanced equations will make all subsequent calculation answers invalid, so mastering this subtopic eliminates a major source of mark loss in both Core and Extended tier papers. Next, you will learn to use balanced equations to calculate reacting masses, gas volumes, and solution concentrations, which make up ~15-20% of the total exam marks. Extended tier candidates will also use these equation writing skills for ionic half equations in redox reactions later in the course.