Reactions involving iodine are commonly used to investigate rates of reaction - Scottish Highers Chemistry - Question 2 - 2017
Question 2
Reactions involving iodine are commonly used to investigate rates of reaction.
(a) One reaction involves hydrogen and iodine reacting together to form hydrogen iodi... show full transcript
Worked Solution & Example Answer:Reactions involving iodine are commonly used to investigate rates of reaction - Scottish Highers Chemistry - Question 2 - 2017
Step 1
Explain, using bond enthalpies, which bond is more likely to break first during this reaction.
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Answer
The I-I bond is more likely to break first because it has a lower bond enthalpy compared to the H-H bond. The I-I bond has a bond enthalpy around 151 kJ mol⁻¹, which is less than the H-H bond enthalpy of 436 kJ mol⁻¹. Thus, the I-I bond will break more easily.
Step 2
Add a second curve to the graph to show the distribution of kinetic energies at 400°C.
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The second curve should be shifted to the right of the original curve. At a higher temperature, the distribution of kinetic energies increases, indicating that a greater proportion of molecules have higher kinetic energy, resulting in more effective collisions.
Step 3
State the effect of increasing temperature on the position of equilibrium.
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Increasing the temperature shifts the equilibrium position to the left, favoring the reactants because the reaction is exothermic.
Step 4
State why changing the pressure has no effect on this equilibrium reaction.
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Changing the pressure has no effect because the number of moles of gaseous reactants and products is equal.
Step 5
State the term for the unstable arrangement of atoms.
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The term for the unstable arrangement of atoms is 'activated complex' or 'transition state.'
Step 6
Calculate the enthalpy change, in kJ, for the forward reaction.
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The enthalpy change for the forward reaction is calculated as:
ΔH = Eₐ (backward) - Eₐ (forward) = 182.8 kJ - 173.2 kJ = 9.6 kJ.
Thus, ΔH = - 9.6 kJ.
Step 7
State the effect that platinum would have on the activation energy for the reaction.
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Platinum will lower the activation energy for the reaction by providing an alternative reaction pathway.
Step 8
Suggest a reason why the beaker should be dry.
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The beaker should be dry to keep the concentration of the reactants constant, as the presence of water would dilute the reactants, affecting the rate of reaction.
Step 9
Calculate the time, in seconds, for the reaction in experiment 3.
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The time taken for the reaction in experiment 3 is 122 seconds.
Step 10
Explain why decreasing the concentration of iodide ions lowers the reaction rate.
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Decreasing the concentration of iodide ions lowers the reaction rate because it reduces the number of successful collisions between reactant particles, ultimately leading to fewer effective reaction events.
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