Atoms are not created or destroyed in a reaction, so each element must have the same number of atoms on both sides of an equation. Balancing changes coefficients, never formulae.
The law of conservation of mass states that in a chemical reaction, the total mass of the products equals the total mass of the reactants.
The atomic explanation is that a reaction rearranges atoms into new combinations. It does not create atoms or destroy them. So the number of atoms of each element is the same before and after the reaction. Since each atom has a fixed mass, the total mass is also unchanged.
A balanced equation is that statement written in symbols. For every element, the atom count on the left equals the atom count on the right.
In the skeletal equation Mg + O₂ → MgO, the right side has one oxygen atom fewer than the left. Where has that atom gone?
Nowhere. The real reaction loses no atoms. The skeletal equation is simply wrong about the numbers of each substance taking part.
Balancing corrects those numbers so that every atom on the left also appears on the right.
A coefficient is a number written in front of a formula. It multiplies every atom in that formula. 2 H₂O means two water molecules: 4 hydrogen atoms and 2 oxygen atoms.
The rule for balancing is: change coefficients only; never change a formula.
To get two oxygen atoms on the water side, you write 2 H₂O. You may not write H₂O₂. H₂O is water. H₂O₂ is hydrogen peroxide, a different compound with different properties. Changing a subscript changes which substance the equation is about.
Only the coefficient changes. A coefficient changes how many molecules of a substance take part. A subscript is part of the formula, and changing it changes the substance.
The usual method is to adjust coefficients one at a time and recount, aiming for the smallest whole numbers. This is called the hit-and-trial method. Take iron reacting with steam: Fe + H₂O → Fe₃O₄ + H₂.
Balance H₂ + O₂ → H₂O.
Oxygen first: 2 on the left, 1 on the right. Write 2 in front of H₂O, giving 2 oxygen atoms on the right.
2 H₂O contains 4 hydrogen atoms. The left has 2. Write 2 in front of H₂.
Result: 2 H₂ + O₂ → 2 H₂O. Hydrogen: 4 and 4. Oxygen: 2 and 2.
A balanced equation can carry two more pieces of information.
The physical state of each substance is written in brackets after its formula: (s) solid, (l) liquid, (g) gas, and (aq) aqueous, meaning dissolved in water. The iron reaction becomes:
3 Fe(s) + 4 H₂O(g) → Fe₃O₄(s) + 4 H₂(g)
The (g) on water shows that the reactant is steam, not liquid water. Iron reacts with steam in this way; with liquid water at room temperature the reaction is far slower.
The conditions needed for a reaction are written above or below the arrow. These can be a temperature, a pressure, light, or a catalyst, which is a substance that speeds up a reaction without being used up.
1Why must a chemical equation be balanced?
2When balancing an equation, you may…
3The balanced equation for burning magnesium is 2 Mg + O₂ → ____ MgO.
4Why is 2 H₂O used instead of H₂O₂?
5Match each state symbol to its meaning.
6In 3 Fe(s) + 4 H₂O(g) → Fe₃O₄(s) + 4 H₂(g), why is water marked (g)?
7Which uses the smallest whole-number coefficients to balance H₂ + O₂ → H₂O?
Next: two types of reaction — combination and decomposition.
Noescio