Reading Orders and Half-Lives from Rate Graphs · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Reading order from rate graphs

Science · Chemistry · ages 17-18
Name ______________________   Date ____________

  1. How do you find the rate at 30 seconds from a concentration-time curve?

    • Read the concentration at 30 seconds
    • Divide the starting concentration by 30
    • Draw a tangent at 30 seconds and find its gradient

  2. Which shape on a rate-concentration plot means first order?

    • A flat horizontal line
    • A straight line through zero
    • An upward bending curve

  3. If every half-life of a reaction is the same, the reaction is first order.

    Circle one:   True   False

  4. A student plots rate against concentration and gets a flat line. What is the order?

    • Zero order
    • First order
    • Second order

  5. A first order reaction has a half-life of 10 s. Work out k in per second using k = 0.693 / half-life.

    Answer: ______________

  6. For a first order reaction, which gradient gives the rate constant k?

    • The intercept of the rate-concentration line
    • The gradient of the rate against concentration line
    • The area under the rate-concentration line

  7. A first order reaction starts at 0.8 mol/dm3 and its half-life is 50 s. What is the concentration after 100 s?

    Answer: ______________

  8. A student sees a rate-concentration curve bending upward through zero and claims first order. What is the mistake?

    • The plot proves nothing about order
    • The reaction is zero order because the line is curved
    • The reaction is second order, since first order gives a straight line
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading order from rate graphs W1-mt_gCToNUUYll-s1

  1. Draw a tangent at 30 seconds and find its gradient · The tangent gradient at that time is the rate. A bare reading gives a concentration, not a rate.
  2. A straight line through zero · Only the straight line through zero means first order. The other shapes mean different orders.
  3. True · A constant half-life is the signature of first order. Drifting half-lives would rule it out.
  4. Zero order · A flat line means the rate ignores concentration, which is zero order.
  5. 0.0693 · Divide 0.693 by 10 to get k = 0.0693 per second.
  6. The gradient of the rate against concentration line · For first order, rate against concentration is straight with gradient k.
  7. 0.2 · Two half-lives pass, so halve twice: 0.8 to 0.4 to 0.2 mol/dm3.
  8. The reaction is second order, since first order gives a straight line · An upward curve means second order. First order would be a straight line through zero.
Worksheet · LightMySky