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 and iodine to form hydrogen iodide, the I–I bond is more likely to break first due to its weaker bond enthalpy compared to the H–H bond. The bond enthalpy of the I–I bond is approximately 151 kJ mol⁻¹, which is significantly lower than that of the H–H bond at around 436 kJ mol⁻¹. Therefore, the weaker bond will break more readily, facilitating the reaction to proceed.
Step 2
Add a second curve to the graph to show the distribution of kinetic energies at 400 °C.
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To draw the second curve for kinetic energies at 400 °C, it should peak further to the right than the original curve, indicating that higher temperatures increase the average kinetic energy of the reactant molecules. The new curve should also reflect an increase in the number of molecules with higher energy, displaying a broader distribution.
Step 3
State the effect of increasing temperature on the position of equilibrium.
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Increasing the temperature will shift the position of equilibrium to the left, towards the reactants, as the reaction is exothermic. This occurs in accordance with Le Chatelier's principle, where the system will adjust to absorb the added heat.
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 there are the same number of moles of gaseous reactants and products (2 moles of HI gas produced from 1 mole of H₂ and 1 mole of I₂). Therefore, altering the pressure does not influence the position of equilibrium.
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 reaction can be calculated using the formula:
during the forward reaction, the enthalpy change is calculated as:
9.6extkJ
Step 7
State the effect that platinum would have on the activation energy for the reaction.
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Platinum, as a catalyst, would lower the activation energy for the reaction, allowing it to proceed at a faster rate without being consumed in the process.
Step 8
Suggest a reason why the beaker should be dry.
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The beaker should be dry to ensure that there is no water present, as the presence of water can dilute the reactants and alter the concentrations, potentially leading to inaccuracies in the reaction rate measurement.
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 122 seconds, based directly from the experimental data provided.
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. As the concentration of I⁻ ions decreases, there are fewer reactants available to collide and react, leading to a decreased rate of reaction.
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