Chemical Reactions
Senior 1 · Chemistry
Chemical Reactions
Lesson 2 of 5
20 min
On a cold night in the hills of Musanze, it is tempting to carry a glowing imbabura charcoal stove into a closed bedroom for warmth. Every year, around the world, people die this way. Charcoal is mostly carbon. With plenty of air it burns to carbon dioxide; but in a closed room the oxygen runs low, and it burns to carbon monoxide, a poisonous gas that has no colour and no smell.
Chemists sum up these two reactions in two short lines:
carbon + oxygen → carbon dioxide (plenty of air)
carbon + oxygen → carbon monoxide (too little air)
Each line is a word equation. It names every substance that reacts and every substance that is made.
Definition
Word equation: a summary of a chemical reaction that uses the names of the reactants and products. The reactants are written on the left and the products on the right, joined by an arrow. Example: iron + sulfur → iron(II) sulfide.
Figure 2.1: The parts of a word equation.
Table 2.1: Word equations for reactions you meet every day
| Where it happens | Word equation |
|---|---|
| Charcoal burning in plenty of air | carbon + oxygen → carbon dioxide |
| Biogas or Lake Kivu methane burning in a cooker | methane + oxygen → carbon dioxide + water |
| An iron roof rusting | iron + oxygen + water → hydrated iron(III) oxide (rust) |
| Respiration in your cells | glucose + oxygen → carbon dioxide + water |
| Photosynthesis in a leaf (light is needed) | carbon dioxide + water → glucose + oxygen |
| Heating limestone in a lime kiln | calcium carbonate → calcium oxide + carbon dioxide |
| An antacid relieving indigestion | hydrochloric acid + magnesium hydroxide → magnesium chloride + water |
Worked example
Problem. Write a word equation for this reaction, and name the reactants and the products: "When granules of zinc are dropped into dilute sulfuric acid, bubbles of hydrogen gas are given off and zinc sulfate forms in the solution."
Solution. Pick out the substances at the start and at the end.
zinc + sulfuric acid → zinc sulfate + hydrogen
Water is present only as the solvent; it is not a reactant here, so it is not written.
Worked example
Problem. Green copper(II) carbonate powder is heated strongly in a test tube. It turns black, and the gas given off turns limewater milky. The black solid is an oxide. Write the word equation.
Solution. The only reactant is copper(II) carbonate. A gas that turns limewater milky is carbon dioxide. The black oxide must contain copper, so it is copper(II) oxide. Heat is the condition:
copper(II) carbonate copper(II) oxide + carbon dioxide
Check: copper, carbon and oxygen appear on both sides, as they must, because atoms are not created or destroyed.
Worked example
Problem. Which of these is a correctly written word equation? Explain what is wrong with the others. (a) iron + sulfur = iron sulfide; (b) sodium + chlorine → sodium chloride; (c) water → hydrogen + oxygen + electricity.
Solution. (b) is correct. (a) uses an equals sign; a reaction goes from reactants to products, so an arrow is needed, and iron here forms iron(II) sulfide. (c) lists electricity as a product; electricity is a condition, not a substance, so it belongs above the arrow: water hydrogen + oxygen.
Warning
Carbon monoxide kills. Never burn charcoal, a kerosene stove or a generator in a closed room, a closed kitchen or a garage. The gas has no colour or smell. Headache, dizziness and sleepiness are warning signs: get into fresh air at once.
Tip
Read a word equation aloud as a sentence: "Zinc and sulfuric acid react to form zinc sulfate and hydrogen." If it does not make a sensible sentence, check your arrow and your names.
FuseSchool: Chemical Equations (4 min). Watch how the video compares an equation to a recipe, and notice which substances are the reactants and which are the products.
Practice
Sources and credits
