Chemical Reactions: How to Balance an Equation

Nothing is lost

In a chemical reaction atoms are not created or destroyed; they only change partners. In 1789 the French chemist Antoine Lavoisier, weighing everything carefully, gases included, showed that the total mass before and after a reaction is the same. This is the law of conservation of mass. It is why chemists write equations with the same atoms on both sides: reactants on the left, an arrow, products on the right. When a candle burns and seems to vanish, the wax has turned into gases that drift away. The picture follows single atoms as they leave their old molecules and join new ones.

Burning methane, counted

Natural gas is mostly methane. Burning it with oxygen gives carbon dioxide and water: CH₄ + 2 O₂ → CO₂ + 2 H₂O. Count the atoms. Left side: 1 carbon, 4 hydrogen, and 2 × 2 = 4 oxygen. Right side: 1 carbon, 2 × 2 = 4 hydrogen, and 2 + 2 = 4 oxygen. Every column matches, so the equation is balanced. The big number in front of a formula, its coefficient, multiplies every atom in it, while the small number below an element belongs to that element alone. Gas burners need fresh air: with too little oxygen they make poisonous carbon monoxide, so keep appliances serviced and rooms ventilated.

Why heat and catalysts speed things up

For two particles to react they must collide, hard enough and the right way round. Heat makes particles move faster, so there are more collisions and more of them are energetic enough. As a rough rule, many reactions run about twice as fast for every 10 °C of warming, which is one reason we keep food in a fridge. A catalyst offers an easier route by lowering the energy needed to get going, and it is not used up. Car exhausts use metals such as platinum as catalysts, and the enzymes in your body are natural ones. Raise the temperature in the picture and watch more successful collisions flash.

Heat in, heat out

Reactions move energy around. An exothermic reaction gives energy to its surroundings, usually as heat: burning methane warms a kitchen, because the products hold less chemical energy than the reactants. An endothermic process takes energy in, so the surroundings cool. The instant cold packs sold for sports injuries use an endothermic change, a salt dissolving in water. Photosynthesis is endothermic too, because it stores light energy in sugar. The picture plots energy as a hill-shaped curve: the products end lower for exothermic and higher for endothermic, and the hill between is the energy needed to get started.

Rust and leaves

Chemistry is outdoors too. Iron rusts when it meets oxygen in the presence of water: 4 Fe + 3 O₂ → 2 Fe₂O₃, with water joining in to make the orange, flaking hydrated form. Check the atoms: 4 Fe on the left, 2 × 2 = 4 on the right; 3 × 2 = 6 O on the left, 2 × 3 = 6 on the right. In leaves, photosynthesis uses light: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂. Carbon: 6 and 6. Hydrogen: 12 and 12. Oxygen: 12 + 6 = 18 on the left, 6 + 12 = 18 on the right. Plants build sugar from air and water and release the oxygen we breathe.

Balance an equation

Now balance one yourself. Under the picture you set the coefficient for each substance, and counters show the atoms on the left and right. Change only coefficients, never the small numbers inside a formula. Method: start with an atom that appears in just one substance on each side, and leave elements that stand alone for last. Worked example: H₂ + O₂ → H₂O. Oxygen is 2 on the left and 1 on the right, so write 2 H₂O. Now hydrogen is 2 on the left and 4 on the right, so write 2 H₂. The result 2 H₂ + O₂ → 2 H₂O has 4 H and 2 O on each side. Then try Fe + O₂ → Fe₂O₃ and check yourself against 4, 3 and 2.

Chemical reactions