Rate of chemical reactions
The rate of a chemical reaction is the change in concentration of a reacting substance per unit of time: v = (c₁ – c₂) : t, in mol/(l·s). The higher the concentration, the more often particles collide and the faster the reaction: sulfur burns much faster in pure oxygen than in air. Each 10 °C of heating usually raises the rate 2–4 times (γ times): v₂ = v₁ · γ^((t₂ – t₁) : 10). For solids the rate depends on the exposed surface: a powder reacts faster than a lump. The nature of the substances matters too: potassium reacts with water faster than sodium, and copper does not react at all (these experiments only with the teacher). A catalyst speeds up a reaction without being used up (MnO₂ in the decomposition of H₂O₂, V₂O₅ in the oxidation of SO₂); an inhibitor slows a reaction down, for instance protecting metal from rusting. Sometimes a reaction must be speeded up (in industry), sometimes slowed down (rusting, food spoiling).
“Sugar and heat”: at home with an adult (no hot water, only lukewarm tap water): put the same amount of cube sugar and granulated sugar into identical glasses of cold and lukewarm water and note in how many seconds each dissolves. Fill a table and say which factor raised the rate. No chemicals are used; do not drink it.