Week 8 · Lesson 2 Chemical Equations: symbolic equations, conservation of mass, balancing step by step · 3 lessons
1
Writing a reaction as a symbolic equation
Science Prep 2 Term 1, Unit 2: pp. 49–56
GoalRead and write a symbolic equation with formulas, an arrow, state symbols and conditions.
New words
symbolic equation · a short way to show a reaction using formulas and symbolsstate symbols · letters in brackets that show the state: (s), (l), (g), (aq)catalyst · a substance that speeds a reaction and is not used up
Explanation
A chemical equation is a short code for a reaction. Reactants go on the left, products on the right, and an arrow means “gives”. Each substance is written with its formula, such as H₂O for water, CO₂ for carbon dioxide and O₂ for oxygen gas. Letters in brackets tell the state: (s) solid, (l) liquid, (g) gas, and (aq) dissolved in water, an aqueous solution. Conditions are written on the arrow, for example “heat” or “catalyst”. A catalyst speeds a reaction up but is not used up, so it stays the same at the end. Look at the hydrogen peroxide equation in the picture: two molecules of hydrogen peroxide give two molecules of water and one of oxygen, and the catalyst on the arrow makes it fast. The numbers in front of formulas, such as the 2 in 2H₂O₂, tell us how many molecules or units take part. Symbols and formulas are the same all over the world, in every language.
2H2O2catalyst→2H2O + O2
Worked examples
CH4(g) + 2O2(g)→CO2(g) + 2H2O(l)
Burning methane (natural gas): methane gas reacts with oxygen gas to give carbon dioxide gas and liquid water. The equation with states is shown. The (g) and (l) tell us which substance is a gas and which a liquid.
NaHCO3(s) + HCl(aq)→NaCl(aq) + H2O(l) + CO2(g)
Baking soda (a solid) dropped into dilute hydrochloric acid, which we only describe in words, releases a gas. The (aq) symbol shows that the acid and the salt formed are dissolved in water, and (g) shows that carbon dioxide leaves as a gas.
Read the equation: 2H₂ + O₂ → 2H₂O. In words: two molecules of hydrogen react with one molecule of oxygen to give two molecules of water.
Class activity
Equation cards. Write each formula of the equation 2H₂ + O₂ → 2H₂O on its own card, and put an arrow card between the sides. Read it aloud in words. Then do the same for CH₄ + 2O₂ → CO₂ + 2H₂O. For each card say the substance and its state (gas, liquid). Check with a grown-up that the formulas are copied exactly.
