5.1.1Additional lessons
Balancing Equations — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 59 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Why Equations Balance
A balanced equation shows the ratio in which substances react.
Conservation of mass
- Conservation of mass Atoms are rearranged but never created or destroyed in a chemical reaction, so each element has the same count on both sides.
Reactants and products
- Reactants and products Formulae on the left are the starting substances and those on the right are the new substances formed.
Coefficients
- Coefficients Whole numbers placed in front of formulae change the amount of a substance, whereas subscripts inside formulae must never be altered.
State symbols
- State symbols (s), (l), (g) and (aq) show the physical state of each species, and these are part of a complete equation.
Part 2 of 6A Systematic Method
1
Write the correct formula for every reactant and product before attempting to balance anything.
2
Count the atoms of each element on both sides, treating a polyatomic ion such as SO₄²⁻ as a unit if it appears unchanged on both sides.
3
Change the coefficient of one substance at a time, starting with the element that appears in the fewest species.
4
Leave elements, particularly O₂ and H₂, until last, and use a fraction temporarily if needed, then double everything.
5
Recount every element, add state symbols, and confirm that the charges balance in ionic equations.
Part 3 of 6Types of Equation
Types of Equation
Word equation
- Word equation Uses the names of substances, such as magnesium + hydrochloric acid → magnesium chloride + hydrogen.
Symbol equation
- Symbol equation Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g), showing the exact amounts and states.
Ionic equation
- Ionic equation Leaves out spectator ions so only the species that change are shown, for example Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g).
Combustion
- Combustion Balance carbon, then hydrogen, then oxygen, as in C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O, or doubled to avoid fractions.
| Check | Question |
|---|---|
| Atoms | Is each element equal on both sides? |
| Charge | Is the total charge equal on both sides? |
| States | Is every species given a state symbol? |
Part 4 of 6Typical Errors to Avoid
Typical Errors to Avoid
Changing formulae
- Changing formulae Writing MgO₂ to balance oxygen changes the substance and is not allowed;
- use a coefficient instead.
Splitting ions
- Splitting ions Do not break a polyatomic ion apart when it is unchanged on both sides, because it is easier to balance as a unit.
Diatomic elements
- Diatomic elements H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ exist as molecules, so single atoms must not be used as formulae.
Final check
- Final check Counting each element at the end catches errors that occur when one change affects another substance.
Part 5 of 6Worked Example
Question Balance: Al(s) + H₂SO₄(aq) → Al₂(SO₄)₃(aq) + H₂(g).
- Balance Al first2Al on the left gives Al₂ in the product.
- Treat sulfate as a unit3 sulfate in Al₂(SO₄)₃ needs 3H₂SO₄.
- Hydrogen3H₂SO₄ contains 6 H, giving 3H₂.
- RecountAl 2 = 2, S 3 = 3, O 12 = 12, H 6 = 6.
Answer 2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)
Part 6 of 6Exam Checklist & Tips
5.1.1 – Key Points for the Exam
- Balanced equation An equation with equal numbers of each type of atom on both sides.
- Coefficient The number in front of a formula showing how many moles of that substance take part.
- State symbol A symbol (s), (l), (g) or (aq) showing the physical state of a species.
- Spectator ion An ion present but not involved in the reaction, which is left out of an ionic equation.
Common Mistakes
- Changing subscripts in a formula to balance an equation.
- Forgetting to multiply every atom in a formula by its coefficient, including atoms inside brackets.
- Omitting state symbols or using (aq) for an insoluble solid.
- Leaving a fractional coefficient in an equation that is meant to show whole numbers of particles.
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