1.5.1Principles of Chemistry
Word and Chemical Equations — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 22 of 50
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.
Section 1 of 12Conservation of mass
1Atoms are rearranged
- Atoms are rearranged In a chemical reaction the atoms in the reactants join up in new ways to make the products.
2None created or destroyed
- None created or destroyed The same atoms are present before and after the reaction.
3Mass stays constant
- Mass stays constant The total mass of the reactants equals the total mass of the products, which is the law of conservation of mass.
4Equations must balance
- Equations must balance A symbol equation therefore needs the same number of each type of atom on both sides of the arrow.
Section 2 of 12Word equations
Layout
- Layout Reactants are written on the left and products on the right, separated by an arrow that means 'reacts to form': reactants → products.
Conditions and catalysts
- Conditions and catalysts A catalyst or a condition such as heat can be written above the arrow.
Example
- Example sodium hydroxide + hydrochloric acid → sodium chloride + water.
Reading the question
- Reading the question Sometimes the products are named first in the question,
- so read carefully to decide which substances are reactants and which are products.
Section 3 of 12Balancing a symbol equation
1
Write the word equation, then replace each name with the correct formula. Never change a formula to help balance.
2
Count the atoms of each element on both sides of the arrow.
3
Add large numbers (coefficients) in front of formulae to make the atom counts equal, adjusting one element at a time.
4
Recount all the elements, leaving hydrogen or oxygen until last if they appear in several substances.
5
Add state symbols where required: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
Section 4 of 12Worked example: magnesium and oxygen
| Stage | Working |
|---|---|
| Word equation | magnesium + oxygen → magnesium oxide |
| Unbalanced symbols | Mg + O2 → MgO |
| Count | Left has 2 O atoms, right has 1 O atom |
| Balance oxygen | Put 2 in front of MgO: Mg + O2 → 2MgO |
| Balance magnesium | Put 2 in front of Mg: 2Mg + O2 → 2MgO |
| With states | 2Mg(s) + O2(g) → 2MgO(s) |
Section 5 of 12Common reactions to be able to write
Typical equations
Neutralisation
- Neutralisation NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Burning carbon
- Burning carbon C(s) + O2(g) → CO2(g)
Ammonia and nitric acid
- Ammonia and nitric acid NH3 + HNO3 → NH4NO3, making the fertiliser ammonium nitrate.
Section 6 of 12Conservation of Mass
Chemical reactions rearrange atoms, so the same atoms exist before and after the reaction.
Conservation of mass
- Conservation of mass No atoms are created or destroyed in a reaction, so the total mass of the reactants always equals the total mass of the products.
Reactants
- Reactants The substances on the left of the arrow that are used up during the reaction, which are the chemical ingredients.
Products
- Products The substances on the right of the arrow, which contain the same atoms as the reactants joined together in a new way.
Word equation
- Word equation A statement of a reaction using full names, such as hydrogen + oxygen → water, with conditions or a catalyst written above the arrow.
Section 7 of 12Writing Balanced Equations
1
Write the word equation first, listing the reactants on the left and the products on the right using their correct names.
2
Replace each name with its formula, remembering that hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as diatomic molecules.
3
Balance one element at a time from left to right by changing only the numbers in front of formulae, never the subscripts.
4
Balance elements that appear on their own last, then check that every element has the same number of atoms on each side.
5
Add state symbols (s), (l), (g) or (aq) when the question asks for them.
Section 8 of 12Common Reaction Types
Common Reaction Types
Combustion
- Combustion A substance burns in oxygen to form an oxide, such as sulfur + oxygen → sulfur dioxide, written S(s) + O₂(g) → SO₂(g).
Neutralisation
- Neutralisation An acid and a base form a salt and water, such as NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
Metal and acid
- Metal and acid A metal reacts with an acid to form a salt and hydrogen, such as Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Metal and oxygen
- Metal and oxygen A metal burns in oxygen to form a metal oxide, which is the basis of the worked example below.
Section 9 of 12Worked Example
Question
- Iron burns in oxygen to form iron(III) oxide.
- Write a balanced symbol equation with state symbols for this reaction.
- Unbalanced equationFe(s) + O₂(g) → Fe₂O₃(s).
- Balance irontwo Fe in Fe₂O₃ requires 4 Fe on the left, so 4Fe(s) + O₂(g) → 2Fe₂O₃(s).
- Balance oxygentwo Fe₂O₃ contain 6 O atoms, so 3 O₂ are needed: 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s).
Answer 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)
Section 10 of 12Exam Checklist & Tips
1.5.1 – Key Points for the Exam
- Reactant A substance that is used up in a chemical reaction.
- Product A substance formed in a chemical reaction.
- Conservation of mass The principle that the total mass of the reactants equals the total mass of the products.
- Balanced equation An equation with the same number of atoms of each element on both sides.
- State symbol A letter in brackets showing whether a substance is solid, liquid, gas or aqueous.
- Coefficient The number placed in front of a formula to balance an equation.
Common Mistakes
- Changing a subscript to balance an equation, which changes the substance itself rather than the number of atoms.
- Writing H rather than H₂ for hydrogen, or O rather than O₂ for oxygen, since these are diatomic molecules.
- Balancing elements that appear alone first, which makes the remaining elements harder to balance.
- Adding state symbols to every equation when the question does not ask for them.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Reactant | A substance that is used up in a chemical reaction. |
| Product | A substance that is made in a chemical reaction. |
| Law of conservation of mass | In a chemical reaction the total mass of the products equals the total mass of the reactants. |
| Balanced equation | An equation with the same number of each type of atom on both sides. |
| State symbol | A symbol (s), (l), (g) or (aq) showing the physical state of a substance. |
| Catalyst | A substance that speeds up a reaction without being used up. |
Section 12 of 12Exam technique
1
Changing the small subscript numbers in a formula to balance an equation. Only the large numbers in front may be changed.
2
Writing the word equation with products first, because the question mentioned them first.
3
Forgetting that diatomic elements such as O2, H2 and Cl2 exist as pairs of atoms.
4
Leaving out state symbols when asked, or using (aq) for a pure liquid.
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