Physical Chemistry
8 marks · NormalThe reaction below is carried out in a laboratory and studied for both its energy profile and the effect of temperature on its rate:
Draw a labelled energy profile diagram for this reaction given that the enthalpy change, , is negative.
Describe what is meant by activation energy and indicate it on your diagram.
The experiment is repeated at a higher temperature. Explain using collision theory how this affects the rate of the reaction.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Correct exothermic energy profile diagram: reactants at higher energy than products. Axes labelled, curve showing activation energy peak, and delta H indicated as negative drop. · must show y-axis: Energy, x-axis: Reaction progress, curve with peak, reactants above products, downward arrow labelled ∆H, activation energy shown
(b) Activation energy is the minimum amount of energy reactant particles must have to react when they collide. It is shown as the energy barrier ('peak') that must be overcome for the reaction to proceed. · must show definition of activation energy, reference to diagram/energy barrier
(c) At a higher temperature, particles move faster so there are more collisions per second. Also, a higher proportion of particles have energy greater than or equal to the activation energy, leading to more successful collisions and a faster reaction rate. · must show more collisions per second, more particles have enough energy, explains greater rate
Must show: energy axis labelled, reaction progress/coordinate axis labelled, reactants and products marked, delta H and activation energy marked
Common mistakes: incorrect sign of delta H; omitting activation energy from diagram; describing all collisions as successful
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Axes correctly labelled: energy (y) and reaction progress (x).
A2 (a) · 1 mark · Curve shows reactants at higher energy than products; ∆H shown as negative (downward arrow).
A3 (a) · 1 mark · Activation energy (Ea) clearly marked on the diagram (height from reactants to peak).
B1 (b) · 1 mark · Correctly defines activation energy.
B2 (b) · 1 mark · Makes explicit reference to energy barrier or peak in diagram.
C1 (c) · 1 mark · States that increased temperature increases frequency of collisions.
C2 (c) · 1 mark · Explains more particles have enough energy to react (activation energy).
C3 (c) · 1 mark · Links this to greater rate/successful collisions.
Worked solution
a) Suitable diagram:
b) Activation energy is the minimum energy that the reacting particles must have for a reaction to occur when they collide. On the diagram, it is the energy difference from reactants to the top of the curve (the highest point).
c) Increasing temperature means particles move faster, resulting in more frequent collisions. More importantly, a greater proportion of reactant particles have energy equal to or greater than the activation energy, so more collisions are successful, increasing the rate of reaction.