Lesson 7.2: The masses of reactants and products are equal

Lesson Focus

This lesson explores the law of conservation of mass. You'll see why mass remains constant in chemical reactions and learn how balanced chemical equations represent this fundamental principle of chemistry.

Learning Objectives

• Explain why total mass does not change in a chemical reaction.
• Recognize how a chemical equation represents a chemical reaction.
• Outline how to balance a simple chemical equation.
• Measure the mass of reactants and products in an experiment.

Early scientists saw matter seemingly disappear, like wood burning to ash. Antoine Lavoisier showed this was an illusion by heating mercury in a sealed jar. The total mass never changed, proving that matter only rearranges. This established the law of conservation of mass: atoms are never created or destroyed.

A chemical equation shows how atoms rearrange in a reaction. It lists starting materials, called reactants, on the left and the resulting substances, or products, on the right. An arrow shows the direction of change. For example, C + O₂ → CO₂ means carbon and oxygen form carbon dioxide.

Because mass is conserved, an equation must have the same number of each type of atom on both sides. The equation CH₄ + O₂ → CO₂ + H₂O is initially unbalanced because the hydrogen and oxygen atoms don't match. An unbalanced equation incorrectly suggests atoms were created or destroyed during the reaction.

To fix an unbalanced equation, we use coefficients, which are numbers placed in front of chemical formulas. For ammonia synthesis, N₂ + H₂ → NH₃ becomes N₂ + 3H₂ → 2NH₃. This ensures atom counts are equal. Crucially, you can only change coefficients, never the subscripts within a formula.

Conservation of mass has practical uses, like in airbags. A small amount of solid sodium azide decomposes via the reaction 2NaN₃ → 2Na + 3N₂. The balanced equation shows this produces a large volume of nitrogen gas, which rapidly inflates the bag. This shows how equations predict real-world outcomes.