Exam Style Question - 4

Question: A student reacted marble chips (calcium carbonate) with excess hydrochloric acid and collected the carbon dioxide in a gas syringe. After 40 s, 60 cm³ of gas had been collected. Calculate the mean rate of reaction over the first 40 s and explain how the initial rate could be found from a graph. (3 marks)Rate of reaction is the change in the amount or concentration of a reactant or product per unit time. Mean rate = volume of gas ÷ time = 60 ÷ 40 = 1.5 cm³ s^-1.To find the initial rate, plot the volume of CO₂ against time and draw a tangent to the curve at t = 0. The gradient of the tangent (change in volume ÷ change in time) is the initial rate.The graph is steepest at the start, when the concentration of acid is highest, and gradually levels off. It becomes horizontal when all the calcium carbonate has reacted, because the acid is in excess.Other ways to follow this reaction include measuring the loss in mass as carbon dioxide escapes, or removing samples at intervals and titrating the remaining acid.