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Balancing Chemical Equations
Conservation of Mass, Coefficients, and Step-by-Step Method
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Balancing chemical equations is the process of making sure the number of each type of atom is the same on both sides of a chemical reaction. This matters because atoms are conserved in ordinary chemical reactions, so matter is not created or destroyed. A balanced equation gives the correct ratios of reactants and products. Those ratios are essential for predicting how much product will form and how much reactant is needed.
To balance an equation, you change coefficients, which are the numbers placed in front of chemical formulas, rather than changing subscripts inside formulas. Changing a subscript would change the identity of the substance, but changing a coefficient only changes the amount of that substance. A good strategy is to count atoms on both sides, balance one element at a time, and check your work at the end. Many equations can be balanced by inspection, while more complex ones may require a more systematic method.
Key Facts
- Law of conservation of mass: total atoms of each element must be equal on both sides of the equation.
- Only coefficients can be changed when balancing, not subscripts in chemical formulas.
- Example: H2 + O2 -> H2O becomes 2H2 + O2 -> 2H2O.
- Example: Fe + O2 -> Fe2O3 becomes 4Fe + 3O2 -> 2Fe2O3.
- Atom count rule: coefficient x subscript = total atoms of that element in one formula unit.
- Use the smallest whole-number coefficients possible in the final balanced equation.
Vocabulary
- Coefficient
- A number placed in front of a chemical formula that tells how many units of that substance are present.
- Subscript
- A small number within a chemical formula that shows how many atoms of an element are in one molecule or formula unit.
- Reactant
- A starting substance that is consumed during a chemical reaction.
- Product
- A substance formed as the result of a chemical reaction.
- Conservation of mass
- The principle that matter is not created or destroyed, so the total number of each type of atom stays the same in a reaction.
Common Mistakes to Avoid
- Changing subscripts to make atom counts match, which is wrong because it changes the substance itself instead of just the amount present.
- Forgetting to multiply all atoms in a formula by the coefficient, which leads to incorrect atom counts on that side of the equation.
- Balancing one element and not rechecking earlier elements, which is wrong because a later coefficient can unbalance atoms you already matched.
- Leaving fractional or non-simplified coefficients in the final answer, which is wrong because balanced chemical equations are usually written with the smallest whole-number coefficients.
Practice Questions
- 1 Balance the equation: N2 + H2 -> NH3.
- 2 Balance the equation: C3H8 + O2 -> CO2 + H2O.
- 3 A student changes H2O into H2O2 to balance an equation. Explain why this is not allowed and describe what should be changed instead.