Word Equations, Exothermic and Endothermic Reactions

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1. Key words

2. Writing a word equation

In a chemical reaction new substances are made. A word equation shows this using names only.

1. Write the reactants on the left, joined by +.
2. Draw an arrow (→). It means "react to make". Never use an equals sign (=).
3. Write the products on the right, joined by +.
4. Check: every element in the reactants must also appear somewhere in the products — atoms are not made or destroyed, just rearranged.

3. Naming the products

Two elements joined — the metal keeps its name and the non-metal ends in -ide: oxygen → oxide, chlorine → chloride, sulfur → sulfide. So iron + sulfur → iron sulfide; sodium + chlorine → sodium chloride.

Names ending in -ate contain oxygen as well: copper carbonate contains copper, carbon and oxygen; zinc sulfate contains zinc, sulfur and oxygen.

Salts from acids. When an acid reacts, the product called a salt takes its first name from the metal and its second name from the acid:

4. Reaction patterns to learn

Most reactions you meet at Stage 8 follow a small number of patterns. Learn the pattern and you can write the word equation for any metal or acid.

From a description to an equation. Read the question and underline the substances. Those you start with (or that are added or used up) are reactants; those that are made, formed or given off are products. Air or burning means oxygen is a reactant even if the question does not say it.

5. Exothermic and endothermic changes

Every chemical reaction involves an energy change. Energy (usually as heat) moves between the chemicals and their surroundings. The surroundings are everything around the reacting particles — the water they are dissolved in, the container, the thermometer, the air, your hand.

6. Identifying the type from a temperature change

1. Measure the starting temperature of the reactants with a thermometer.
2. Mix the reactants (stir) and watch the thermometer.
3. Record the highest temperature (if it rises) or the lowest temperature (if it falls).
4. Work out temperature change = final temperature − starting temperature.
5. A rise means exothermic; a fall means endothermic.

Making it accurate and fair: use a polystyrene cup with a lid — polystyrene is a good thermal insulator, so less energy escapes to or comes in from the room. Use the same volume of liquid and the same mass of solid each time, start at the same temperature, and repeat to check your results. Wear eye protection when using acids.

Reading a thermometer: read at eye level and work out what each small division is worth before you read it (for example, if there are 10 divisions between 20 °C and 30 °C, each is 1 °C).

7. Worked examples

Worked example 1 — When sodium burns in chlorine, sodium chloride is made. Write the word equation. [1]

1. Reactants (start): sodium and chlorine. Product (made): sodium chloride.
2. sodium + chlorine → sodium chloride
3. Element check: sodium ✓, chlorine ✓ on both sides.

Worked example 2 — Complete the word equation: magnesium + sulfuric acid → __________ + __________ [2]

1. Pattern: metal + acid → salt + hydrogen.
2. Salt: metal name magnesium + sulfuric → sulfate = magnesium sulfate (1).
3. Other product: hydrogen (1).
4. Common mistake: "magnesium sulfuric" or "magnesium sulfide". Sulfuric acid always gives a sulfate.

Worked example 3 — Copper carbonate is a green powder. When it is heated it turns black (copper oxide) and a gas that turns limewater milky is given off. Write the word equation. [2]

1. The gas that turns limewater milky is carbon dioxide.
2. copper carbonate → copper oxide + carbon dioxide (1 for the reactant, 1 for both products).
3. Only one reactant here: heat is the condition, not a substance, so it is not written as a reactant (you may write "heat" above the arrow).

Worked example 4 — A student adds sodium hydroxide solution to hydrochloric acid. The temperature goes from 22 °C to 31 °C. (a) Calculate the temperature change. [1] (b) Is the reaction exothermic or endothermic? Explain. [2]

1. (a) 31 − 22 = 9 °C (a rise).
2. (b) Exothermic (1) — the temperature rose, so energy was given out to the surroundings (1).
3. Common mistake: writing "the reaction took in heat so it got hotter". Getting hotter means energy came out of the reacting chemicals.

Worked example 5 — Ammonium nitrate dissolves in water. The temperature falls from 20 °C to 13 °C. (a) Is dissolving ammonium nitrate exothermic or endothermic? [1] (b) Suggest a use for this. [1] (c) Why is it called a process here, not a reaction? [1]

1. (a) Endothermic — the temperature fell by 7 °C.
2. (b) An instant cold pack for sports injuries.
3. (c) Dissolving is a physical change: no new substance is made (the solid can be got back by evaporating the water).

Worked example 6 — Mariam tests four solids dissolving in water. Her results are in the table. Which solids dissolve exothermically? [2]

1. Work out each change: A +7, B −4, C 0, D +13.
2. Rises = exothermic: A and D (1 mark each).
3. C shows no change — neither exothermic nor endothermic (or too small to measure).

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3