Chemistry · General chemistry I · Concept
Balancing chemical equations
To balance a chemical equation, change only the coefficients, never the subscripts, until every element has the same number of atoms on both sides. Balance the most complex substance first, leave free elements such as O₂ for last, and recount at the end.
What balancing means
A chemical equation must conserve atoms: every atom in the reactants appears in the products. A coefficient in front of a formula says how many units react; a subscript inside the formula says what the substance is. Changing a subscript changes the substance, so balancing changes coefficients only.
A reliable order of work
Start with an element that appears in only one reactant and one product, usually in the most complex compound. Balance elements that appear on their own, such as O₂, H₂ or a metal, last, because changing their coefficient changes nothing else. Recount every element after each change.
A fraction is allowed, then cleared
If the last element needs a half, as when O₂ must supply an odd number of oxygen atoms, use the fraction to finish, then multiply every coefficient by 2 to get whole numbers.
Then clear the fraction
Doubling every coefficient gives 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Both forms describe the same ratios; whole numbers are the usual convention for a reported equation.
Ions: charge balances too
In an equation with ions, the total charge on each side must also match. Redox reactions in solution are balanced as two half-reactions: balance every element except oxygen and hydrogen, add H₂O for oxygen and H⁺ for hydrogen (in acidic solution), add electrons for charge, then scale so the electrons cancel.
Check with an atom count
List each element with its count on both sides. Multiply each subscript by its coefficient: 4H₂O holds 8 hydrogen and 4 oxygen atoms. Every row must match.
Common mistakes
- Changing a subscript, as in writing H₂O₂ for 2H₂O: hydrogen peroxide is a different substance.
- Multiplying a coefficient into only one atom of a formula: 3H₂O contains 6 hydrogen and 3 oxygen atoms.
- Leaving a fraction in a reported equation when whole-number coefficients are asked for.
- Balancing the atoms of an ionic equation but not its charge.
Key terms
- Chemical equation
- A symbolic statement of a reaction: reactant formulas, an arrow and product formulas, each with a coefficient. It is balanced when every element and the net charge total the same on both sides.
- Law of conservation of mass
- In a chemical reaction in a closed system, matter is neither created nor destroyed: the products’ total mass equals the reactants’ total mass, because atoms are only rearranged.
- Combustion reaction
- A reaction of a substance with oxygen, usually rapid and releasing heat and light. Complete combustion of a hydrocarbon gives CO₂ and H₂O.
- Half-reaction
- The oxidation half or the reduction half of a redox reaction, written with its electrons shown. To combine two half-reactions, multiply them so the electrons lost equal the electrons gained.
- Redox reaction
- A reaction in which electrons move from one substance to another, changing oxidation states. Oxidation (losing electrons) and reduction (gaining electrons) always happen together.
- Oxidation state
- A number that tracks electrons by treating every bond as if it were ionic, following set rules. An increase means oxidation and a decrease means reduction; it is usually not the atom’s real charge.
Work through an example
Balance the combustion of propane: C₃H₈(g) + O₂(g) → CO₂(g) + H₂O(g).
Balance the combustion of propane →Sources and scope
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