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
In the reaction between hydrogen ( ext{H}_2 ext{)} and iodine ( ext{I}_2 ext{)}, the bond that is more likely to break first is the I–I bond due to its lower bond enthalpy compared to the H–H bond. The bond enthalpy of H–H is approximately 436 kJ mol⁻¹, while that of I–I is about 151 kJ mol⁻¹. Therefore, the weaker 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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Answer
The second curve should peak further to the right of the original curve on the graph indicating an increase in the distribution of kinetic energies at a higher temperature (400 °C). This shift reflects the higher average kinetic energy of the reactant molecules due to increased thermal energy.
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 (toward the reactants) for the exothermic reaction.
Step 4
State why changing the pressure has no effect on this equilibrium reaction.
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Changing the pressure has no effect on this equilibrium reaction because the number of moles of gaseous reactants and products remains equal (2 moles of H₂ and I₂ yield 2 moles of HI).
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'.
Step 6
Calculate the enthalpy change, in kJ, for the forward reaction.
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The enthalpy change for the forward reaction can be calculated as the difference between the potential energies of the reactants and products. Based on the diagram, the enthalpy change is:
State the effect that platinum would have on the activation energy for the reaction.
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The presence of platinum as a catalyst would lower the activation energy for the reaction, making it proceed more quickly.
Step 8
Suggest a reason why the beaker should be dry.
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The beaker must be dry to ensure that the concentrations of the reactants are not diluted by any water present, which could affect the reaction rate.
Step 9
Calculate the time, in seconds, for the reaction in experiment 3.
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The time for the reaction in experiment 3 is given as 123 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 molecules. Lower concentration leads to fewer moles of reactant available, which decreases the frequency of collisions needed for the reaction to occur.
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