Formulae, Functional Groups and TerminologyCambridge IGCSE Chemistry: Revision notes
Section 1
What are molecular and empirical formulae?
Molecular formula shows the actual number and type of atoms in one molecule of a compound. Empirical formula shows the simplest whole number ratio of atoms in a compound.
For example:
- Glucose has molecular formula C₆H₁₂O₆
- The empirical formula of glucose is CH₂O (dividing all subscripts by 6)
To deduce empirical formula:
- Find the mole ratio of each element
- Divide by the smallest number to get whole number ratios
- Write the simplest ratio
Key relationship: The molecular formula is always a whole number multiple of the empirical formula.
| Feature | Molecular | Empirical |
|---|---|---|
| Shows | Actual atoms in one molecule | Simplest whole number ratio |
| Example (ethene) | C₂H₄ | CH₂ |
| Determined by | Mass spectrometry + empirical formula data | Combustion analysis |
| Can be same | Sometimes | Always the simplest possible |
Examiners often ask you to find empirical formula from a mole ratio—always divide by the smallest number first, then multiply if needed to get all whole numbers.
A compound contains C: 36 g, H: 6 g, O: 32 g. Find empirical formula. Step 1: Divide by atomic mass (C: 36/12 = 3 mol, H: 6/1 = 6 mol, O: 32/16 = 2 mol). Step 2: Divide by smallest (3 mol): C: 1, H: 2, O: 0.67. Step 3: Multiply by 3 to get whole numbers: C₃H₆O₂.
Section 2
How do you construct and interpret word and symbol equations?
Word equations describe chemical reactions using names: Reactants → Products
Symbol equations use chemical formulae and must be balanced so atoms are equal on both sides.
Balancing equations:
- Write the unbalanced equation with correct formulae
- Count atoms on each side
- Add coefficients (numbers before formulae) to balance
- Never change subscripts—only use coefficients
State symbols show the physical state of substances:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous solution (dissolved in water)
Example: Word equation: Magnesium + Oxygen → Magnesium oxide Symbol equation: 2Mg(s) + O₂(g) → 2MgO(s)
The coefficient 2 before Mg and MgO means there are 2 magnesium atoms and 2 magnesium oxide units.
Students often try to balance equations by changing subscripts (e.g., H₂O → H₃O). Never do this—only use coefficients outside the brackets.
Always include state symbols when asked for a symbol equation at IGCSE—examiners check these carefully. Remember (aq) for solutions and (g) for gases produced.
Balance: Fe + Cl₂ → FeCl₃. Left: 1 Fe, 2 Cl. Right: 1 Fe, 3 Cl. Multiply Fe by 2 and FeCl₃ by 2: 2Fe + 3Cl₂ → 2FeCl₃. Check: Left 2 Fe and 6 Cl; Right 2 Fe and 6 Cl. ✓
Section 3
How do you determine the formula of ionic compounds?
Ionic compounds form when electrons transfer from metal to non-metal atoms. The charges on ions determine the formula.
Method to deduce ionic formula:
- Identify the cation (positive ion) and anion (negative ion)
- Write the charge of each ion
- Use the criss-cross method: swap charges (as whole numbers) to become subscripts
- Simplify if both subscripts share a common factor
Common ions and charges:
| Cation | Charge | Anion | Charge |
|---|---|---|---|
| Na⁺ | +1 | Cl⁻ | −1 |
| Ca²⁺ | +2 | O²⁻ | −2 |
| Al³⁺ | +3 | NO₃⁻ | −1 |
| NH₄⁺ | +1 | CO₃²⁻ | −2 |
| Mg²⁺ | +2 | SO₄²⁻ | −2 |
Examples using criss-cross:
- Na⁺ and Cl⁻ → NaCl (1:1 ratio)
- Ca²⁺ and O²⁻ → CaO (1:1 ratio, charges swap and simplify)
- Al³⁺ and Cl⁻ → AlCl₃ (1:3 ratio, Al takes subscript from Cl's charge)
- Ca²⁺ and NO₃⁻ → Ca(NO₃)₂ (1:2 ratio; brackets used because NO₃⁻ is polyatomic)
When writing formulas with polyatomic ions (e.g., NO₃⁻, CO₃²⁻, OH⁻), use brackets if the subscript is greater than 1: Ca(NO₃)₂ not CaNO₃₂.
Deduce the formula of the ionic compound formed between Al³⁺ and SO₄²⁻. Criss-cross: Al takes the 2 from SO₄²⁻, and SO₄ takes the 3 from Al³⁺. Formula: Al₂(SO₄)₃. Check: Total charge = (2 × +3) + (3 × −2) = +6 − 6 = 0. ✓
Section 4
What are functional groups and organic terminology?
Functional groups are groups of atoms in organic molecules that determine the chemical properties of the compound. The same functional group behaves similarly regardless of the rest of the molecule.
Key functional groups:
| Functional Group | Formula | Example | Name Suffix/Prefix |
|---|---|---|---|
| Alkane (C-C single bonds only) | CₙH₂ₙ₊₂ | CH₄ methane | −ane |
| Alkene (C=C double bond) | CₙH₂ₙ | C₂H₄ ethene | −ene |
| Alcohol (hydroxyl group) | −OH | CH₃OH methanol | −ol |
| Carboxylic acid | −COOH | CH₃COOH ethanoic acid | −oic acid |
| Aldehyde | −CHO | HCHO methanal | −al |
| Ketone | −CO− | CH₃COCH₃ propanone | −one |
Important organic chemistry terms:
- Saturated = contains only single C−C bonds
- Unsaturated = contains C=C or C≡C bonds
- Homologous series = group of organic compounds with similar properties, differing by CH₂, following the same general formula
- Isomers = compounds with the same molecular formula but different structural arrangements
Example: Butane (C₄H₁₀) and isobutane are isomers with the same molecular formula but different structures.
Think of functional groups as 'LEGO bricks'—the same −OH group acts like a brick that can be clicked onto different structures, and each time it gives similar chemical properties.
Examiners test naming of organic compounds extensively—learn the prefixes (meth−, eth−, prop−, but−) and suffixes (−ane, −ene, −ol, −al, −one, −oic acid) as these determine the correct IUPAC name.
Section 5
How do you construct ionic equations from symbol equations?
Ionic equations show only the ions and molecules that actually change during a reaction, omitting spectator ions (ions present but unchanged).
Steps to write ionic equations:
- Write the full balanced symbol equation with state symbols
- Write the complete ionic equation: break all soluble ionic compounds into their ions, keep insoluble compounds and covalent compounds as whole formulas
- Identify spectator ions (appear on both sides unchanged)
- Remove spectator ions to get the net ionic equation
Example: Full equation: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Complete ionic: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
Spectator ions: NO₃⁻ and Na⁺ (appear unchanged on both sides)
Net ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Key rules for complete ionic equations:
- Soluble salts: break into ions
- Insoluble compounds (s): keep as whole formula
- Gases (g): keep as whole formula
- Covalent compounds (like H₂O): keep as whole formula
To identify spectator ions easily, write the complete ionic equation and look for any ion appearing on both the left and right sides—that's a spectator ion. Remove it.
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). Complete ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l). Spectator ions: Na⁺ and Cl⁻. Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l).
Must Know
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Molecular formula = actual number of atoms in one molecule; empirical formula = simplest whole number ratio. Find empirical formula by dividing mole ratios by the smallest number.
-
Balanced symbol equations must have equal atoms on both sides; use coefficients only (never change subscripts), and always include state symbols (s), (l), (g), (aq).
-
Ionic compound formulae are deduced using the criss-cross method: swap the charges (ignoring signs) to become subscripts. Remember to use brackets for polyatomic ions when subscript > 1.
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Functional groups determine chemical properties: alcohols have −OH, carboxylic acids have −COOH, alkenes have C=C. Homologous series differ by CH₂ and have similar properties.
-
Net ionic equations show only the particles that change; remove spectator ions (ions unchanged on both sides) from the complete ionic equation.
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The same functional group in different molecules always behaves similarly—this is why classification by functional group is central to organic chemistry.
That's the notes covered.
Carry on to the next subtopic.