Symbols, Formula and Equations II – SS1 Chemistry Lesson Note (First Term)

A chemical equation is a short way of showing a chemical reaction using symbols and formulae.

Behavioural objectives

By the end of the lesson, students should be able to:

1. State the law of conservation of mass.
2. Balance chemical equations using the inspection and algebraic methods.
3. Write balanced chemical equations from word equations.
4. Identify the types of chemical reactions.
5. Apply the mole concept to chemical equations.
6. Solve simple stoichiometric problems from balanced equations.

What is a chemical equation?

A chemical equation represents a chemical reaction using symbols and formulae.

What are the parts of a chemical equation?

• Reactants: the substances that react, written on the left.
• Products: the substances formed, written on the right.
• Arrow (→): means “yields” or “produces”.
• Coefficient: the number in front of a formula, showing how many units take part.

Example: 2H₂ + O₂ → 2H₂O (reactants → products)

What is the law of conservation of mass?

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction.

• Mass: the total mass of the reactants equals the total mass of the products.
• Atoms: in a balanced equation, each element has the same number of atoms on both sides.

What is the difference between a coefficient and a subscript?

A coefficient is the large number written before a formula. It shows how many units of that substance take part.

A subscript is the small number written below and after a symbol. It shows how many atoms of that element are in one unit of the formula.

How do you balance chemical equations?

You balance an equation by adding coefficients until each element has equal atoms on both sides.

Method 1: Inspection method (trial and error)

1. Write the unbalanced equation.
2. Count the atoms of each element on both sides.
3. Add coefficients to balance each element. Never change subscripts.
4. Check that every element is balanced.
5. Make sure the coefficients are in the smallest whole-number ratio.

Example 1: Balance H₂ + O₂ → H₂O

1. Count atoms. Left: H = 2, O = 2. Right: H = 2, O = 1.
2. Balance oxygen by putting 2 before H₂O: H₂ + O₂ → 2H₂O.
3. Recount. Left: H = 2, O = 2. Right: H = 4, O = 2.
4. Balance hydrogen by putting 2 before H₂: 2H₂ + O₂ → 2H₂O.
5. Check. Left: 4H, 2O. Right: 4H, 2O. Balanced.

Example 2: Balance Fe + O₂ → Fe₂O₃

1. Count atoms. Left: Fe = 1, O = 2. Right: Fe = 2, O = 3.
2. Balance Fe by putting 2 before Fe: 2Fe + O₂ → Fe₂O₃.
3. Count oxygen. Left: O = 2. Right: O = 3.
4. The common multiple of 2 and 3 is 6. Put 3 before O₂ and 2 before Fe₂O₃: 2Fe + 3O₂ → 2Fe₂O₃.
5. Recount Fe. Left: Fe = 2. Right: Fe = 4.
6. Put 4 before Fe: 4Fe + 3O₂ → 2Fe₂O₃.
7. Check. Left: 4Fe, 6O. Right: 4Fe, 6O. Balanced.

Method 2: Algebraic method

The algebraic method uses letters for the coefficients and solves them, which helps with longer equations.

Example: Balance C₃H₈ + O₂ → CO₂ + H₂O

1. Assign letters: aC₃H₈ + bO₂ → cCO₂ + dH₂O.
2. Write one equation for each element. C: 3a = c. H: 8a = 2d. O: 2b = 2c + d.
3. Let a = 1. Then c = 3 and d = 4.
4. Find b: 2b = 2(3) + 4 = 10, so b = 5.
5. Write the result: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

What are the types of chemical reactions?

There are five types of chemical reaction: synthesis, decomposition, single displacement, double displacement and combustion.

What are state symbols in equations?

State symbols are letters in brackets that show the physical state of each substance.

• (s): solid.
• (l): liquid.
• (g): gas.
• (aq): aqueous solution, which means dissolved in water.

Examples:

• Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• CaCO₃(s) → CaO(s) + CO₂(g)
• 2H₂(g) + O₂(g) → 2H₂O(l)

What information does a balanced equation give?

A balanced equation shows which substances react, what is formed and in what ratio.

• Qualitative information: which substances react and what is formed.
• Quantitative information: the ratio of moles of reactants and products.
• Physical states: the state of each substance, if state symbols are shown.

From 2H₂(g) + O₂(g) → 2H₂O(l), we learn:

• 2 molecules of hydrogen react with 1 molecule of oxygen.
• 2 moles of hydrogen react with 1 mole of oxygen.
• The mole ratio is 2 : 1 : 2.
• Hydrogen and oxygen are gases and water is a liquid.

What is stoichiometry?

Stoichiometry is the calculation of quantities in chemical reactions from balanced equations.

What are the key ideas?

• Coefficients: they give the mole ratio of the substances.
• Molar mass: it converts between mass and moles, using moles = mass ÷ molar mass.
• Avogadro’s number: it converts between moles and number of particles.

Example: moles of oxygen needed for 4 moles of hydrogen

Equation: 2H₂ + O₂ → 2H₂O

1. Read the mole ratio: 2 moles H₂ : 1 mole O₂.
2. Moles of O₂ = 4 × (1 ÷ 2) = 2 moles.
3. Answer: 2 moles of O₂ are needed.

How do you write equations from word equations?

You write an equation from words by naming the substances, writing their formulae and then balancing.

1. Identify the reactants and products.
2. Write the correct chemical formulae.
3. Balance the equation.
4. Add state symbols if required.

Example 1: Sodium reacts with chlorine to form sodium chloride

1. Reactants: sodium (Na) and chlorine (Cl₂). Product: sodium chloride (NaCl).
2. Unbalanced: Na + Cl₂ → NaCl.
3. Balanced: 2Na + Cl₂ → 2NaCl.

Example 2: Calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide

1. Reactant: calcium carbonate (CaCO₃). Products: calcium oxide (CaO) and carbon dioxide (CO₂).
2. Write: CaCO₃ → CaO + CO₂.
3. It is already balanced (1 : 1 : 1).
4. Add state symbols: CaCO₃(s) → CaO(s) + CO₂(g).

What are some worked examples?

Example 1: Balance Al + O₂ → Al₂O₃

1. Count atoms. Left: Al = 1, O = 2. Right: Al = 2, O = 3.
2. Balance Al: 2Al + O₂ → Al₂O₃.
3. Balance O using 6 as the common multiple: 2Al + 3O₂ → 2Al₂O₃.
4. Al is now unbalanced. Left: Al = 2. Right: Al = 4.
5. Put 4 before Al: 4Al + 3O₂ → 2Al₂O₃.
6. Check. Left: 4Al, 6O. Right: 4Al, 6O. Balanced.

Example 2: Mass of oxygen needed to react with 6 g of magnesium

Equation: 2Mg + O₂ → 2MgO. (Mg = 24, O = 16)

1. Moles of Mg = mass ÷ molar mass = 6 ÷ 24 = 0.25 mol.
2. Mole ratio: 2 mol Mg : 1 mol O₂.
3. Moles of O₂ = 0.25 × ½ = 0.125 mol.
4. Mass of O₂ = moles × molar mass = 0.125 × 32 = 4 g.
5. Answer: 4 g of oxygen is required.

Class activities

Activity 1: Balance these equations

1. Na + H₂O → NaOH + H₂
2. Fe + HCl → FeCl₂ + H₂
3. N₂ + H₂ → NH₃
4. C₄H₁₀ + O₂ → CO₂ + H₂O
5. Ca + O₂ → CaO

Activity 2: Classify each reaction

1. 2KClO₃ → 2KCl + 3O₂
2. Fe + CuSO₄ → FeSO₄ + Cu
3. 2Mg + O₂ → 2MgO
4. AgNO₃ + NaCl → AgCl + NaNO₃
5. CH₄ + 2O₂ → CO₂ + 2H₂O

Activity 3: Write balanced equations

1. Hydrogen reacts with nitrogen to form ammonia.
2. Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas.
3. Methane burns in oxygen to form carbon dioxide and water.

Evaluation

1. State the law of conservation of mass.
2. Distinguish between a coefficient and a subscript.
3. Balance Fe + O₂ → Fe₃O₄ and Al + HCl → AlCl₃ + H₂.
4. Name the type of reaction in 2H₂O → 2H₂ + O₂ and in Zn + CuSO₄ → ZnSO₄ + Cu.
5. Calculate the mass of oxygen needed to react completely with 4.8 g of magnesium in 2Mg + O₂ → 2MgO. (Mg = 24, O = 16)

Assignment

1. Write the five types of chemical reaction, the general form of each, and one balanced example of each.