Calculating rates of reactions: mean rate of reaction = quantity of reactant used / time taken

The idea

The mean rate of reaction is the average speed over a chosen time interval. Calculate it using: mean rate = quantity of reactant used ÷ time taken, or mean rate = quantity of product formed ÷ time taken. For example, if 2.4 g of reactant is used in 40 s, the mean rate is 2.4 g ÷ 40 s = 0.06 g/s. For a reactant, the quantity used is always the initial reading minus the final reading, regardless of when the interval starts. For a product, it is the final minus initial. Always include units: cm³/s, g/s, or mol/s (Higher Tier). A graph shows how rate varies; drawing a tangent allows finding the rate at a particular moment.

What earns the marks

• Mean rate is calculated using: mean rate = quantity of reactant used ÷ time taken, or quantity of product formed ÷ time taken.
• The quantity of reactant used is always found by subtracting the final reading from the initial reading, regardless of the start time.
• The time taken is the duration of the interval in seconds, minutes or another consistent unit.
• The units of mean rate are the units of quantity divided by the units of time, such as cm³/s, g/s or mol/s (Higher Tier only).
• Rate at a particular moment can be found by drawing a tangent to the curve and calculating its gradient.

Common mistakes and how to put them right

• Assuming the total amount of reactant at the end equals the amount used if the interval starts at zero.

  Put it right: Calculate the difference between initial and final readings.

• Forgetting to include units or writing the units incorrectly, for example writing g instead of g/s.

  Put it right: Divide the quantity unit by the time unit.

• Dividing time by quantity instead of quantity by time.

  Put it right: The equation is quantity ÷ time, so a larger quantity in the same time gives a larger rate.

Exam tips

• Write down the equation, substitute the numbers with units, and give the answer with the correct unit.
• If a graph is provided, read the quantity values from the graph at the start and end of the interval before calculating the mean rate.