Chemical Equations and Conservation of Mass
Chemical Formulae: Reading the Language of Chemistry
Chemical formulae represent substances using element symbols and subscript numbers. The subscript number applies to the element immediately before it: \(H_2O\) contains 2 hydrogen atoms and 1 oxygen atom; \(CaCO_3\) contains 1 calcium, 1 carbon and 3 oxygen atoms; \(CO_2\) contains 1 carbon and 2 oxygen atoms. A coefficient (a large number written in front of a formula) multiplies every atom in that formula: \(2H_2O\) means two water molecules, so 4 hydrogen atoms and 2 oxygen atoms in total. Working out formulae from names requires knowing the charges of common ions: \(Na^+\) and \(Cl^-\) combine in a 1:1 ratio to give NaCl; \(Mg^{2+}\) and \(O^{2-}\) combine in a 1:1 ratio to give MgO; \(Al^{3+}\) and \(O^{2-}\) must combine in a 2:3 ratio to give \(Al_2O_3\).
Writing Chemical Equations
A chemical equation shows what happens in a chemical reaction: reactants on the left of the arrow (\(\rightarrow\)) and products on the right. A word equation uses the names of substances: magnesium + oxygen \(\rightarrow\) magnesium oxide. A symbol equation uses formulae: \(2Mg + O_2 \rightarrow 2MgO\). Symbol equations carry more information — they show the ratio of atoms reacting — and must be balanced.
Conservation of Mass and Balancing Equations
The conservation of mass is a fundamental principle: in a chemical reaction, atoms are rearranged but never created or destroyed. Therefore, the number of each type of atom must be the same on both sides of the equation. Balancing involves adding coefficients (never changing subscript numbers in formulae) until both sides match.
Balance the combustion of methane: \(CH_4 + O_2 \rightarrow CO_2 + H_2O\)
Step 1 — Count atoms on each side: Left: 1 C, 4 H, 2 O. Right: 1 C, 2 H, 2 O. Carbon is balanced. Hydrogen is not (4 vs 2). Oxygen may need revisiting.
Step 2 — Balance hydrogen: Put a 2 in front of \(H_2O\): \(CH_4 + O_2 \rightarrow CO_2 + 2H_2O\). Now: Left: 1 C, 4 H, 2 O. Right: 1 C, 4 H, 4 O. Oxygen is now unbalanced (2 vs 4).
Step 3 — Balance oxygen: Put a 2 in front of \(O_2\): \(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\). Now: Left: 1 C, 4 H, 4 O. Right: 1 C, 4 H, 4 O. Balanced.
Check: count every atom type on both sides. The equation is correct when every count matches. Never change the subscript numbers — that would change the substance itself.
Equations must be balanced because mass is conserved — atoms are not created or destroyed. To balance: count atoms on each side, add coefficients in front of formulae until both sides match, and never change the subscript numbers in a formula.
State Symbols
State symbols provide additional information about the physical form of each substance at the conditions of the reaction. They are written in brackets after each formula:
- (s) — solid
- (l) — liquid
- (g) — gas
- (aq) — aqueous (dissolved in water)
For example, the reaction of sodium with water: \(2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)\). State symbols help you understand reaction conditions: an (aq) label tells you the substance is in solution; (g) tells you a gas is released.
Conservation of Mass: Experimental Evidence
Conservation of mass can be verified experimentally. If a reaction is carried out in a sealed container and the total mass is measured before and after, it remains the same. However, in reactions carried out in open containers, the mass may appear to change: if a gas is produced and escapes, the total mass of the remaining substances decreases; if a substance from the air (such as oxygen) is incorporated, the remaining solid may appear heavier. These observations are fully consistent with conservation of mass — the ‘missing’ or ‘extra’ mass is simply the gas that left or entered the system. A classic example is the thermal decomposition of calcium carbonate: \(CaCO_3(s) \rightarrow CaO(s) + CO_2(g)\). The solid left behind weighs less than the original carbonate because \(CO_2\) gas escapes. The total mass (solid + escaping gas) is unchanged.
Conservation of mass: the total mass of reactants always equals the total mass of products. In open systems, gas escaping the container makes the remaining mass appear less — but the gas is still part of the products. In sealed systems, mass is unchanged.
Some reactions are better described using half-equations that show electron transfer explicitly. These are used for electrolysis (and redox reactions at Higher tier). In electrolysis of molten sodium chloride, at the cathode (negative electrode), sodium ions gain electrons: \(Na^+ + e^- \rightarrow Na\). At the anode (positive electrode), chloride ions lose electrons: \(2Cl^- \rightarrow Cl_2 + 2e^-\). Each half-equation must balance for both atoms and charge. These show clearly that electrolysis involves forced transfer of electrons by an external electrical supply.
Equations are the universal language of chemistry. Whether describing a simple neutralisation (\(HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)\)), a displacement reaction (\(Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)\)), or the combustion of a fuel, the same rules apply: balance atoms, include state symbols, and check that mass is conserved. Learning to write and interpret equations fluently is one of the most transferable skills in chemistry.