The human digestive system: Students should be able to carry out rate calculations for chemical reactions.

The idea

Rate calculations quantify how quickly a reactant is used up or a product is formed. In the digestive system context, they can be applied to enzyme-controlled reactions, for example measuring the volume of gas produced or the loss in mass of a reaction mixture over time. The rate is found by dividing the amount changed by the time taken, giving units such as cm³/s or g/s. For a graph of amount against time, the rate at any point is the gradient of the tangent at that point; the average rate over a period is the total change divided by the total time.

What earns the marks

• State the rate equation as rate = amount of reactant used or product formed ÷ time taken, and include correct units such as g/s or cm³/s.
• Calculate an average rate from a table of measurements by dividing the total change in amount by the total time elapsed.
• Determine the rate at a specific time from a graph by drawing a tangent and calculating its gradient.
• Interpret a calculated rate in context, for example explaining that a higher rate means more product is formed per unit time.
• Use consistent units throughout a calculation and convert between units such as seconds and minutes where necessary.

Common mistakes and how to put them right

• Dividing time by amount instead of amount by time.

  Put it right by writing the equation as rate = change in amount ÷ time and checking the units.

• Reading the gradient from a curved graph without drawing a tangent.

  Put it right by drawing a straight tangent at the required point and using two points on that tangent.

• Mixing units, such as using cm³ and minutes to give cm³/min when the question asks for cm³/s.

  Put it right by converting all measurements to the required units before calculating.

Exam tips

• Write down the rate equation before substituting numbers so the examiner can see your method.
• When using a graph, draw the tangent clearly with a ruler and label the two points you use to find the gradient.
• Check that your final answer has the correct unit and a sensible size for the reaction being described.