Chemical Reactions, Reactants, and Products
A chemical reaction is a process in which one or more substances, the reactants, are converted into different substances, the products, and it is represented by a chemical equation that uses formulas, state symbols and coefficients. In brief: the equation 2H2(g) + O2(g) → 2H2O(l) says that two molecules (or moles) of hydrogen gas react with one molecule (or mole) of oxygen gas to produce two molecules (or moles) of liquid water.
Chemical equations are the language of chemical change. Reading and writing them correctly is the basis of stoichiometry, thermochemistry and nearly every other topic in chemistry. This article explains the parts of an equation, the meaning of coefficients and state symbols, how conditions and catalysts are shown, how reactions are classified, how to read an equation in terms of moles and mass, and gives worked examples and some history.
What are the parts of a chemical equation?
- Reactants: the formulas of the starting substances, on the left side of the arrow, separated by plus signs.
- Products: the formulas of the substances formed, on the right side.
- Arrow: shows the direction of change and is read “yields”, “produces” or “forms”. A double arrow (⇌) shows a reversible reaction that reaches equilibrium.
- Coefficients: whole numbers in front of formulas that show how many units of each substance take part. They are chosen so that the equation is balanced.
- State symbols: letters in parentheses after the formula.
- Conditions: written above or below the arrow, such as heat (Δ), a catalyst, light (hν) or a temperature.
What do the symbols mean?
| Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | Fe(s) |
| (l) | Liquid | H2O(l) |
| (g) | Gas | CO2(g) |
| (aq) | Aqueous solution (dissolved in water) | NaCl(aq) |
| → | Yields (reaction goes to completion) | 2Mg + O2 → 2MgO |
| ⇌ | Reversible reaction | N2 + 3H2 ⇌ 2NH3 |
| Δ above the arrow | Heat is applied | CaCO3 → CaO + CO2 (Δ) |
| Formula over the arrow | Catalyst | H2O2 → H2O + O2 (MnO2) |
| ↑ or ↓ after a formula | Gas escapes or precipitate forms (older style) | CO2↑, AgCl↓ |
How do you read an equation?
The coefficients tell the ratio of particles and moles. The equation 2H2 + O2 → 2H2O can be read in four equivalent ways:
- 2 molecules of hydrogen + 1 molecule of oxygen → 2 molecules of water.
- 2 mol of H2 + 1 mol of O2 → 2 mol of H2O.
- 4.03 g of H2 + 32.00 g of O2 → 36.03 g of H2O (mass is conserved).
- For gases at the same temperature and pressure, 2 volumes of H2 + 1 volume of O2 → 2 volumes of steam.
The coefficients do not give the ratio of masses or volumes of solids and liquids directly: for these, convert to moles using molar masses.
How are reactions classified?
| Type | General pattern | Example |
|---|---|---|
| Synthesis (combination) | A + B → AB | 2Na + Cl2 → 2NaCl |
| Decomposition | AB → A + B | 2H2O → 2H2 + O2 |
| Single replacement | A + BC → AC + B | Zn + CuSO4 → ZnSO4 + Cu |
| Double replacement | AB + CD → AD + CB | AgNO3 + NaCl → AgCl + NaNO3 |
| Combustion | Fuel + O2 → oxides | CH4 + 2O2 → CO2 + 2H2O |
Reactions can also be classified as acid-base, precipitation, redox, exothermic or endothermic, and many reactions belong to more than one class.
How do you know a reaction has occurred?
Signs of a reaction include a color change, formation of a gas or a precipitate, release or absorption of heat or light, and a change in smell. The most reliable evidence is the identification of new substances with different properties. A reaction needs particles to collide with enough energy and in the right orientation. The minimum energy is the activation energy. Catalysts lower the activation energy and speed up a reaction without being used up.
Worked examples
- Identify the parts. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g). Reactants: solid zinc and aqueous hydrochloric acid. Products: aqueous zinc chloride and hydrogen gas. The coefficient 2 in front of HCl means two formula units per atom of zinc.
- Moles and mass. 6.54 g of zinc is 6.54 ÷ 65.38 = 0.100 mol. The equation shows 1 mol of Zn gives 1 mol of H2, so 0.100 mol of H2 forms, with mass 0.100 × 2.016 = 0.202 g.
- Write an equation from a description. “Solid iron reacts with oxygen gas to form solid iron(III) oxide.” Reactants Fe(s) and O2(g), product Fe2O3(s). Skeleton: Fe + O2 → Fe2O3. Balanced: 4Fe(s) + 3O2(g) → 2Fe2O3(s).
- Classify. 2KClO3 → 2KCl + 3O2 has one reactant and two products, so it is a decomposition. Mg + 2HCl → MgCl2 + H2 has an element replacing hydrogen in a compound, so it is a single replacement.
- Mass check. In 2H2 + O2 → 2H2O: reactants 2 × 2.016 + 32.00 = 36.03 g; products 2 × 18.015 = 36.03 g. The masses agree.
- Particles. How many water molecules form from 0.500 mol of O2? The ratio is 1 : 2, so 1.00 mol of H2O forms, which is 1.00 × 6.022 × 1023 = 6.02 × 1023 molecules.
How did chemical equations develop?
Alchemists described changes in words and symbols. Antoine Lavoisier in the 1780s wrote “equations” with an equals sign to express that the masses of the substances are conserved. John Dalton introduced atomic symbols in 1808, and Jöns Jacob Berzelius proposed letter symbols in 1813, which led to the modern form. Over the 19th century, chemists such as Alexander Williamson and Stanislao Cannizzaro refined atomic masses and formulas, so that equations could be written correctly. The arrow replaced the equals sign, and the double arrow for reversible reactions was proposed by Jacobus van ‘t Hoff in 1884.
Which mistakes are common?
Mistake 1: Treating the arrow as an equals sign for the substances
The arrow shows that reactants change into different products. The numbers of atoms are equal on both sides, but the substances are not.
Mistake 2: Reading coefficients as mass ratios
Coefficients give mole ratios. Convert to mass with molar masses.
Mistake 3: Leaving out state symbols when they matter
(aq) and (s) show whether a substance is dissolved or a solid, which is important for net ionic equations.
Mistake 4: Writing the formula of a diatomic element as a single atom
Hydrogen, oxygen, nitrogen and the halogens are written as H2, O2, N2, F2, Cl2, Br2 and I2 when they are free elements.