reaction rate

Think of how quickly milk curdles, how fast iron rusts on a wet fence, or how soon a fizzy tablet stops bubbling in water. Some chemical changes finish in a flash; others creep along for years. The reaction rate is simply how fast a reaction goes — how much reactant disappears, or how much product appears, in a given slice of time.

More precisely, the reaction rate is the change in the concentration (or amount) of a substance divided by the time taken for that change, usually written in moles per litre per second. Because reactants vanish while products build up, we define a single rate for the whole reaction by dividing each substance's rate of change by its coefficient in the balanced equation, so everyone agrees on one number. Rate generally falls as the reaction proceeds, because reactants get used up, so chemists often quote the instantaneous rate at a particular moment rather than an average over a long span.

Why it matters: rate is what chemical kinetics is all about, and it is a completely separate question from whether a reaction can happen at all. Thermodynamics tells you whether a reaction is downhill in energy; kinetics tells you how long the trip takes. A reaction can be hugely favourable yet so slow it never seems to start — diamond turning into graphite is the classic example.