Rate of reaction — common mistakes

When explaining the effect of temperature on rate, a top-level answer must mention both points: increased collision frequency AND a greater proportion of particles having energy ≥ $E_a$. The second point carries more marks as it's the more significant factor.

When calculating rate from a graph, always draw a large tangent to the curve. This minimises errors when reading the coordinates to calculate the gradient (Δy/Δx). Remember to state the units in your final answer, which are typically mol dm⁻³ s⁻¹ or cm³ s⁻¹.

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.

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.

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).

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.