A student investigated the effect of the size of marble chips on the rate of the reaction between marble chips and hydrochloric acid - AQA - GCSE Chemistry Combined Science - Question 5 - 2018 - Paper 2

As time progresses, the volume of gas produced increases initially and then levels off towards 60 cm³. This suggests that the reaction rate decreases as it approaches completion, indicating that the reactants are being consumed.

At 15 seconds, I would locate the corresponding volume of gas produced from the graph, then divide that volume by the time interval to calculate the rate.

The units for the rate of this reaction are in cubic centimeters per second (cm³/s).

Smaller marble chips increase the rate of the reaction as they have a larger surface area available for the acid to react with, leading to more frequent collisions.

All sizes of marble chips eventually reach the maximum volume of gas because the reaction is limited by the amount of hydrochloric acid available.

The surface area of the large cube is calculated as follows:

Each face of the cube has an area of $2 \times 2 = 4 \text{ cm}^2$, with 6 faces total. Therefore:

$\text{Surface Area} = 6 \times 4 = 24 \text{ cm}^2$

Smaller chips have a larger surface area for the same volume, allowing for more frequent collisions between the acid and marble chips, which increases the rate of reaction according to collision theory.

The sloping part for lower concentration is less steep because the reaction is slower due to fewer acid particles present, resulting in less frequent collisions.