AQA · Chemistry · 8462
GCSE Chemistry (AQA): Measuring Rates of Reaction from a Graph
Topic 6 — The Rate & Extent of Chemical Change. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Measuring Rates of Reaction from Graphs - Mean Rate & Rate at a Specific Time"; each card deep-links to the moment in the video it came from. Lesson 6.04 of Cognito's AQA GCSE Combined Science (Trilogy) and Chemistry course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-c_6.04.
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Why is dividing total product formed by total time a limited measure of rate?
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It only gives the mean rate over that period, not how the rate changed during the reaction.
How do you find the rate of reaction at a specific time from a curved graph?
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Draw a tangent to the curve at that time and calculate the tangent's gradient.
Why does a volume-of-gas against time graph eventually plateau?
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A reactant has been completely used up, so no more product is formed.
A reaction produces 1,200 cm³ of hydrogen in 4 minutes. What is the mean rate in cm³/s?
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5 cm³/s (1200 ÷ 240 s).
What are the units of rate taken from a mass-of-reactant against time graph?
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Grams per second (g/s), e.g. 0.72 g ÷ 100 s = 0.0072 g/s.
At what point in a reaction is the rate normally fastest?
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At the very beginning, because the largest amounts of reactants are present.
How is the mean rate of a reaction calculated?
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Amount of reactant used (or product formed) ÷ time taken.
Why should a tangent be drawn as long as possible when finding a rate?
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Longer change-in-y and change-in-x values can be read off the axes more accurately.
What does a steep section of a volume-of-gas against time graph indicate?
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A high rate of reaction — lots of product is being formed per unit time.
Define a tangent to a curve.
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A straight line that just touches the curve and has the same gradient as the curve at that point.
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