Everything on the GCSE Chemistry Rate of Reaction poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
What rate of reaction is
Rate of reaction is how fast reactants are turned into products. It is measured as the amount of product formed (or reactant used up) per unit time. Slow reactions include rusting; fast reactions include explosions.
Measuring rate
- Plot amount of product formed against time. The curve rises steeply at first, then flattens as reactants are used up.
- The rate at any instant is the gradient of a tangent drawn at that point.
Mean rate = change in y ÷ change in x
Collision theory
For a reaction to happen, two things must be true:
- The particles must collide with each other.
- The collision must have at least the activation energy - the minimum energy needed to react.
On an energy profile diagram, reactants sit at a higher energy than the activation energy barrier only if they have enough energy; products are shown at a lower energy level on the other side.
The four factors that affect rate
Four things change the rate of reaction by changing how often or how hard particles collide.
- Concentration or pressure - more particles in the same volume means more frequent collisions.
- Surface area - more particles exposed means more frequent collisions.
- Temperature - particles move faster, so collisions are more frequent and more energetic.
- Catalyst - more collisions reach the activation energy (see below).
Catalysts
- A catalyst speeds up a reaction without being used up, so it is not in the overall equation.
- It provides an alternative reaction pathway with a lower activation energy (Eₐ), so a greater proportion of collisions are successful.
- Shown on a reaction profile as a lower peak between the same reactants and products - the curve for the catalysed pathway sits below the uncatalysed curve.