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Reaction rates and enthalpy
5 ECAT Chemistry questions with answers and explanations.
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What are the units of the rate constant of a first order reaction?
- 1 mol dm^-3 s^-1
- 2 s^-1
- 3 dm^3 mol^-1 s^-1
- 4 mol s
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Correct answer: s^-1
Rate = k[A] gives k = rate / concentration, whose units are s^-1.
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According to the Arrhenius equation, raising the temperature:
- 1 Decreases the rate constant
- 2 Increases the rate constant
- 3 Has no effect on the rate constant
- 4 Makes the activation energy negative
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Correct answer: Increases the rate constant
A higher temperature puts more molecules above the activation energy, so k increases.
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The enthalpy of neutralisation of a strong acid with a strong base is about:
- 1 +57 kJ/mol
- 2 -57 kJ/mol
- 3 -570 kJ/mol
- 4 0 kJ/mol
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Correct answer: -57 kJ/mol
The reaction is H+ + OH- giving water, and it releases about 57 kJ per mole of water formed.
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The order of a reaction can be determined:
- 1 From the balanced equation alone
- 2 Only by experiment
- 3 From the colour of the products
- 4 From the molar mass
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Correct answer: Only by experiment
The order depends on how the rate changes with concentration, which has to be measured.
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For a reaction with rate = k[A]^2, doubling [A] makes the rate:
- 1 Double
- 2 Four times as large
- 3 Eight times as large
- 4 Unchanged
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Correct answer: Four times as large
The rate is proportional to [A] squared, so doubling [A] multiplies the rate by 4.