Calculate average rates and recognise temperature, concentration, surface-area and catalyst effects.
Method and worked examples
Reaction rate describes how quickly reactants are used or products form. Average rate = measured change / elapsed time, using units such as millilitres per second. A steeper product–time graph means a greater rate; a plateau means no further product is being recorded. Rate and final amount are different: a faster reaction can reach the same final product quantity sooner.
Higher temperature usually increases rate because particles move faster and a larger fraction of collisions have sufficient energy. Greater concentration often increases collision frequency. Smaller solid pieces expose more surface area for reaction. A catalyst provides an alternative pathway with a lower activation-energy barrier and is not used up overall. These rules assume other relevant conditions and reactant amounts are comparable.
Example 1
Gas volume rises from 10mL at 5s to 34mL at 13s. Find the average rate over that interval.
Change =34−10=24mL; time =13−5=8s. Average rate =824=3mL/s.
Example 2
Two curves end at 60mL, but one reaches 60mL sooner. Compare rate and yield.
The earlier curve has a faster overall reaction. The final measured gas volume is the same, so it did not necessarily produce more product.
Watch out:A catalyst increases rate by changing the pathway. It does not supply extra reactant atoms.
Reaction rates and collision patterns | Year 10 Science | Ascent Arcade