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A mean rate is a change in measured quantity divided by elapsed time. A gas syringe can measure gas production, a balance can measure mass loss when gas escapes, and an optical endpoint can measure how long a visible change takes. On a product–time graph, steeper gradient means faster rate; a plateau means no further net product is being measured. It does not mean the reaction was always slow.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Calculate mean and instantaneous rates
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which graph feature shows instantaneous rate?
Tangent gradient. It measures local change per time.
After watching 2: Why does powdered marble react faster than equal-mass large chips?
More exposed surface area. More reaction sites are available.
What you'll be able to do
A mean rate is a change in measured quantity divided by elapsed time. A gas syringe can measure gas production, a balance can measure mass loss when gas escapes, and an optical endpoint can measure how long a visible change takes. On a product–time graph, steeper gradient means faster rate; a plateau means no further net product is being measured. It does not mean the reaction was always slow.
Reacting particles must collide with sufficient energy. More concentrated solutions or higher gas pressures increase collision frequency. Powdering a solid increases exposed surface area. Heating makes particles move faster and increases the fraction with enough energy to react. A catalyst provides a lower-activation-energy pathway and remains chemically unchanged overall. It is not used up like a reactant, although a real catalyst can become contaminated or damaged.
To compare concentration, hold temperature, reactant amounts where appropriate, particle size and the measurement method constant. Start timing consistently and repeat trials. In a disappearing-cross experiment, the endpoint is subjective; compare the same fixed amount of obscuring product so inverse time is a relative-rate proxy. To find rate at one moment, draw a tangent at that time. Product volume at the end reflects total amount produced, not just rate.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Two curves reach the same final volume but one is initially steeper. Interpret them.
Rate differs while final product amount can be the same.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.