9701 · 8.1
Rate of reaction — FAQ
Frequently asked questions for 9701 Rate of reaction. Direct answers first, then deeper explanation — then practise with marking.
Why is the initial rate of a reaction the fastest?
The initial rate (at time t=0) is the maximum rate for a reaction because the concentrations of the reactants are at their highest. This results in the greatest frequency of collisions, and therefore the highest rate of successful collisions.
Does a catalyst get used up in a reaction?
No, a catalyst is chemically unchanged at the end of a reaction. While it may participate in intermediate steps of the reaction mechanism, it is always regenerated, so its overall quantity remains constant.
How do I accurately draw a tangent to a curve to find the rate?
Place a ruler on the graph so it touches the curve at the specific time point you are interested in. Adjust the ruler's angle until it appears to 'balance' on the curve at that single point, with the gaps between the ruler and the curve looking equal on either side of the point of contact. Draw a long line to make it easier to read coordinates for the gradient calculation (Δy/Δx).
Is rate of reaction always positive?
Yes, the rate of reaction is always quoted as a positive value. When calculating the rate from the change in concentration of a reactant, you will get a negative value because its concentration decreases. The formula for rate with respect to a reactant is Rate = -Δ[Reactant]/Δt, which makes the final rate positive.