Reading a chemical equation
A chemical equation describes a reaction. The starting substances, called reactants, are written on the left; the substances formed, called products, are on the right; and an arrow means “react to form”. In 2 H₂ + O₂ → 2 H₂O, two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.
Conservation of mass
In the 1780s, Antoine Lavoisier carefully weighed reactions in sealed containers and showed that the total mass does not change. We now know why: in a chemical reaction, atoms are rearranged into new combinations, but none are created or destroyed. A balanced equation shows the same number of each kind of atom on both sides.
Coefficients vs. subscripts
- A subscript (the small number in H₂O) tells you how many atoms are in one molecule. It is part of the substance’s identity and must never be changed to balance an equation.
- A coefficient (the big number in 2 H₂O) tells you how many molecules there are. It multiplies every atom in the formula.
A step-by-step method
- Write the correct formulas for all reactants and products.
- Count the atoms of each element on both sides.
- Balance elements that appear in only one substance on each side first — often metals, then carbon, then hydrogen.
- Leave elements that appear on their own (such as O₂ or H₂) until last, since changing them affects nothing else.
- Check every element again, and reduce the coefficients to the smallest whole numbers.
Worked example: burning propane
Start with C₃H₈ + O₂ → CO₂ + H₂O.
- Carbon: 3 on the left, so put 3 in front of CO₂.
- Hydrogen: 8 on the left, so put 4 in front of H₂O (4 × 2 = 8).
- Oxygen: on the right there are 3 × 2 + 4 × 1 = 10 atoms, so put 5 in front of O₂.
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂OHistory and interesting facts
- In the 1770s and 1780s Antoine Lavoisier weighed substances before and after reactions in sealed vessels and showed that the total mass does not change. This law of conservation of mass is the reason every equation must balance.
- When natural gas burns on a stove, the balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O: one methane molecule needs two oxygen molecules, which is why burners need a good air supply.
- Car airbags inflate in a fraction of a second using a balanced reaction: 2NaN₃ → 2Na + 3N₂. Engineers use the coefficients to calculate exactly how much solid is needed to fill the bag with nitrogen gas.
- Coefficients count particles, so they also count moles. A balanced equation is therefore the starting point for every calculation of how much product a reaction can make.
Key terms
- Reactant and product
- A starting substance in a reaction, and a substance that is formed.
- Coefficient
- The number in front of a formula showing how many molecules or formula units take part.
- Subscript
- The small number in a formula showing how many atoms of an element are in one unit.
- Stoichiometry
- Using the ratios in a balanced equation to calculate the amounts of substances that react and form.
Why it matters
- Industry: balanced equations tell chemists exactly how much of each raw material is needed, such as nitrogen and hydrogen for ammonia fertilizer.
- Engines and stoves: the right fuel-to-air ratio for complete combustion comes straight from the balanced equation.
- Medicine and pharmacy: stoichiometry is used to make and dose medicines accurately.
- Environment: balanced equations let scientists calculate how much CO₂ is released by burning a given amount of fuel.
Common misconceptions
- “The number of molecules must be the same on both sides.” Only the number of each type of atom must match; molecules can change in number.
- “You can fix an equation by changing H₂O to H₂O₂.” That creates a different substance.
- “Mass is lost when things burn.” The products include gases that escape; in a sealed container, the mass stays the same.
Key takeaways
- Atoms are conserved in chemical reactions.
- Balance with coefficients only, never subscripts.
- Balance unique elements first and lone elements last.
- Use the smallest whole-number coefficients.