Calculating rates of reactions Flashcards – AQA GCSE Combined Science: Trilogy (8464) — Chemistry — Foundation

What does a curved quantity–time graph show about the rate of reaction?

The rate is changing during the reaction.

What is a tangent to a curve at a chosen point?

A straight line that just touches the curve at that point and follows its direction there.

How do you calculate the rate of reaction at a specific time from a curved quantity–time graph?

Draw a tangent at the chosen time. Use two well-chosen points on the tangent to calculate its gradient: $\text{rate} = \frac{\text{change in quantity}}{\text{change in time}}$

How is the mean rate of a reaction calculated from the quantity of reactant or product?

$\text{mean rate} = \frac{\text{quantity of reactant used}}{\text{time taken}} = \frac{\text{quantity of product formed}}{\text{time taken}}$

How are the quantity of reactant used and the quantity of product formed calculated over a time interval?

• Reactant used: initial quantity − final quantity
• Product formed: final quantity − initial quantity

What does the mean rate describe if a reaction’s rate changes during a time interval?

The average rate over that interval.

What are the mean reaction rate units when time is measured in seconds and quantity is measured as mass in grams or volume in cubic centimetres?

• Mass: $\mathrm{g/s}$
• Volume: $\mathrm{cm^3/s}$