Methods for following the rate of a reaction, calculating mean rate from quantity and time, drawing and interpreting rate graphs, and finding the rate at a given moment using a tangent.

A C5.1 calculation and method question. Reward: the mean rate is the quantity of product divided by the time: $\text{rate} = \dfrac{48}{30} = 1.6\ \text{cm}^3/\text{s}$. Two methods to follow a gas-producing reaction: (1) collect the gas in a gas syringe and measure the volume of gas at regular time intervals, or (2) stand the reaction flask on a balance and measure the loss in mass at regular intervals as the gas escapes. Markers credit the correct mean rate of 1.6 cm3 per second, and two valid methods (gas syringe for volume, or balance for mass loss). A third valid method is timing how long a fixed amount of product takes to form. A common slip is to divide time by volume instead of volume by time.

A Higher tier graph-interpretation question. Reward: the gradient is steepest at the start because the concentration of the reactants is highest then, so collisions are most frequent and the reaction is fastest. As the reaction proceeds, the reactants are used up, so their concentration falls, collisions become less frequent, and the rate (gradient) decreases. The gradient becomes zero when the line is flat (horizontal) because the reaction has finished (a reactant has run out), so no more gas is produced. To find the rate at a particular time, draw a tangent to the curve at that point and calculate its gradient (change in volume divided by change in time). Markers credit the highest concentration giving the steepest initial gradient, the falling concentration reducing the rate, the flat line meaning the reaction has stopped, and drawing a tangent to find the rate at a point. A common error is to read a single coordinate rather than a gradient.