Strong acids dissociate fully in water to produce the maximum number of H+ ions. This means if you had one mole of hydrochloric acid (HCl) molecules, they would all ‘split’ to form one mole of H+ ions and one mole of Cl– ions.
Weak acids, such as ethanoic acid (CH3COOH), do not fully dissociate. In fact, about only one per cent of ethanoic acid molecules split up to form H+ ions and CH3COO– ions at any one time.
This means that the pH values of strong acids are lower than that of weak acids, which explains why the rate of reaction of strong acids with substances (such as metals, metal carbonates etc) is higher than that of weak acids.
This also explains why the temperature rise during a reaction with strong acids is higher than that of weak acids.
Weak and strong should not be mistaken for dilute and concentrated. A dilute acid has the acid molecules mixed with a large amount of water, so that there is only a low concentration of H+ ions. Concentrated acids have little to no water molecules mixed with the acid molecules, meaning the concentration of H+ ions is high.