Representing Chemical Change

Introduction

Chemical changes occur when elements or compounds react to form new substances. These changes can be represented in three common ways:

• Sentence form: Iron reacts with sulphur to form iron sulphide.
• Word equation: Iron + sulphur → iron sulphide.
• Chemical equation using symbols: Fe + S → FeS.

Another example:

• Sentence: Ammonia reacts with oxygen to form nitrogen monoxide and water.
• Word equation: Ammonia + oxygen → nitrogen monoxide + water.
• Chemical equation: 4NH₃ + 5O₂ → 4NO + 6H₂O.

In a chemical equation, the substances written on the left of the arrow are the reactants and those on the right are the products.

To write balanced chemical equations you must:

1. Know the chemical symbols for the elements involved.
2. Be able to write correct chemical formulae for reactants and products.
3. Balance chemical equations using the appropriate laws (conservation of mass and atoms).
4. Include state (phase) symbols where required.

Chemical symbols

Chemical symbols (one- or two-letter shorthand for elements) are used in chemical equations. You should learn the symbols for common elements, especially the first 36 elements of the periodic table, and other frequently encountered elements such as Fe, Cu, Ag, Pb, Zn, Al, etc.

Activity suggestions:

• Write down the symbols and names of elements you know; compare with a peer and add any you missed.
• Test yourself and a classmate by making short naming/recognition quizzes.

Writing chemical formulae

A chemical formula shows which elements are present in a compound and the number of atoms of each element. The number of atoms of an element (if greater than one) is shown as a subscript. For example, calcium carbonate is CaCO₃, and water is H₂O.

If you need more practice with writing formulae, review earlier chapters on bonding and ion formation (ionic and covalent formula writing).

Exercise 14-1

Write down the chemical formula for each of the following compounds:

1. iron (III) chloride
2. zinc nitrate
3. aluminium sulphate
4. calcium hydroxide
5. magnesium carbonate
6. product when carbon reacts with oxygen
7. product when hydrogen reacts with nitrogen
8. potassium oxide
9. copper (II) bromide
10. potassium dichromate

Write down the name for each of the following compounds:

1. SO₂
2. KMnO₄
3. (NH₄)₂SO₄
4. BaF₂
5. Cr(HSO₄)₃
6. CH₄

Balancing chemical equations

The law of conservation of mass

Definition: The mass of a closed system of substances remains constant regardless of the processes acting inside the system. Matter can change form but cannot be created or destroyed.

Because of this law, in any chemical reaction the total mass (and hence the total number of atoms of each element) of the reactants must equal the total mass (and total number of atoms of each element) of the products.

Example (simple): Fe + S → FeS.

Relative atomic masses: Fe ≈ 55.8, S ≈ 32.1, so one Fe atom plus one S atom gives a formula unit FeS with relative mass ≈ 87.9. The mass of reactants equals the mass of product when the equation is balanced.

Note: Some elements exist as molecules in their elemental form (e.g., S₈, O₂). For writing empirical or formula-unit equations we often use the simplest unit (S, O, etc.) when forming formulae, but when balancing equations their molecular forms must be taken into account as appropriate.

Practical balancing activities

Using coloured balls, jelly sweets, marbles or paper cut-outs to represent atoms and molecules helps visualise balancing. Build reactant and product models and adjust coefficients until the number of each type of atom is equal on both sides.

Inspection method - steps to balance an equation

1. Identify and write correct chemical formulae for the reactants and products.
2. Write the unbalanced equation with reactants on the left and products on the right separated by an arrow (→).
3. Count the number of atoms of each element in reactants and in products.
4. Adjust coefficients (whole numbers placed before formulae) to obtain the same number of atoms of each element on both sides.
5. Check finally that all atoms balance and that coefficients are in the simplest whole-number ratio.
6. Add any required additional information such as state symbols (s), (l), (g), (aq).

Worked examples

Practice exercises: balancing equations

Exercise 14-2

Balance the following equations (show balanced equations only):

1. Mg + O₂ → MgO
2. Ca + H₂O → Ca(OH)₂ + H₂
3. CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂
4. CaCl₂ + Na₂CO₃ → CaCO₃ + NaCl
5. C₁₂H₂₂O₁₁ + O₂ → H₂O + CO₂
6. Barium chloride reacts with sulphuric acid to produce barium sulphate and hydrochloric acid.
7. Ethane (C₂H₆) reacts with oxygen to form carbon dioxide and steam (water vapour).
8. Ammonium carbonate decomposes to gaseous products: (NH₄)₂CO₃ (s) → NH₃ (aq) + CO₂ (g) + H₂O (l)
9. Hydrogen fuel cells reaction: H₂ (g) + O₂ (g) → H₂O (l)
10. Synthesis of ammonia (Haber process): N₂ (g) + H₂ (g) → NH₃ (g)

State (phase) symbols and other information

The physical state of each substance in a chemical equation is indicated in brackets immediately after its formula:

• (g) - gas
• (l) - liquid
• (s) - solid
• (aq) - aqueous solution (dissolved in water)

Heat required for a reaction is commonly indicated by placing the delta symbol (Δ) above the arrow or by writing "Δ" before the arrow. For example:

NH₄Cl → Δ → NH₃ (g) + HCl (g)

Further practice and examples with state symbols

General experiment: relationship between product amount and reactant amount

Aim: To investigate how the amount of product formed changes with the amount of reactant used.

Apparatus:

• Flask
• Measuring cylinder
• Water bath or bowl
• Delivery tube
• Funnel with stopcock
• Stopper
• Sodium hydrogen carbonate (NaHCO₃) powder
• Dilute sulphuric acid (H₂SO₄)

Method (outline):

1. Weigh about 20 g of NaHCO₃ and place it into a flask.
2. Set up the apparatus to collect gas generated into a measuring cylinder over water.
3. Measure 5 mL of dilute H₂SO₄ in a funnel with a stopcock and add it to the NaHCO₃ by opening the stopcock slowly.
4. Observe and record the volume of gas collected (CO₂).
5. Repeat using 10 mL of acid and compare the volume of gas collected.
6. Write a balanced equation for the reaction. (Hint: carbon dioxide, water and sodium sulphate are formed.)

Expected observation: More gas is collected when a larger volume (or amount) of acid is used, provided the solid is in excess or not limiting.

Balanced equation for the reaction between sodium hydrogen carbonate and sulphuric acid (one possible balanced form):

2NaHCO₃ (s) + H₂SO₄ (aq) → Na₂SO₄ (aq) + 2CO₂ (g) + 2H₂O (l)

Summary

• A chemical equation uses element symbols and formulae to describe a chemical reaction.
• Reactants are written on the left and products on the right of the arrow (→).
• Writing correct chemical formulae is essential for representing chemical change accurately.
• The law of conservation of mass requires that the total mass (and the number of each type of atom) of reactants equals that of products in a closed system.
• An equation is balanced when the number of atoms of each element is the same on both sides; coefficients are used to balance equations.
• State symbols (s), (l), (g) and (aq) indicate the physical state of each substance in an equation.

End of chapter exercises

1. Propane combustion: Balance the following equation:

   C₃H₈ (l) + O₂ (g) → CO₂ (g) + H₂O (l)

2. Methane burning in oxygen:

   Balance the following reaction for methane (CH₄) burning in oxygen.