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

The graph shows a general increasing trend in the volume of gas produced over time. Initially, the volume increases rapidly, but the rate of increase slows down as time progresses. Specifically, between 0 to 15 seconds, the volume increases significantly, indicating a fast reaction rate.

To find the rate at 15 seconds, I would locate the volume of gas produced at that specific time on the graph. Then, I would calculate the gradient between the 10 and 20 second marks or draw a tangent line to the curve at the 15-second point to determine the slope, which represents the rate.

The units for the rate of this reaction are cm³/s.

Smaller marble chips increase the rate of reaction because they have a larger surface area for the acid to act upon, leading to more frequent collisions.

All three sizes produce a maximum volume of 60 cm³ of gas because they all react completely with the same amount of hydrochloric acid, which limits the maximum volume of gas produced regardless of chip size.

The surface area of the large cube, which has a side length of 2 cm, is calculated as follows:

Surface Area = 6 × (side length)² = 6 × (2 cm)² = 6 × 4 cm² = 24 cm².

Smaller marble chips provide a larger surface area which allows for more frequent collisions with acid particles, leading to a higher reaction rate. This aligns with the collision theory which states that reaction rates depend on the frequency of collisions between reactant particles.

The results show that the reaction with lower concentration acid is slower because there are fewer acid particles available to collide with the marble chips, leading to less frequent collisions and thus a reduced rate of reaction compared to the higher concentration of acid.