This question is about rates of reaction - AQA - A-Level Chemistry - Question 1 - 2020 - Paper 2
Question 1
This question is about rates of reaction.
Phosphinate ions (H₃PO₂⁻) react with hydroxide ions to produce hydrogen gas as shown.
H₃PO₂⁻ + OH⁻ → H₄PO₂⁻ + H₂
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Worked Solution & Example Answer:This question is about rates of reaction - AQA - A-Level Chemistry - Question 1 - 2020 - Paper 2
Step 1
Use the graph in Figure 1 to determine the initial rate of reaction for this experiment. State its units. Show your working on the graph.
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Answer
To find the initial rate of reaction, draw a tangent to the curve at time = 0 seconds. The slope of this tangent will give the initial rate of reaction. Measure the height of the graph at this point to determine the volume of gas produced, and divide that by the time interval to find the rate in units of cm³/s.
Step 2
State the relationship between the initial concentration of phosphinate and time (t).
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Answer
The relationship is inversely proportional, meaning as the initial concentration of phosphinate ions increases, the time (t) taken to collect a fixed amount of hydrogen gas decreases.
Step 3
Deduce the order of the reaction with respect to phosphinate.
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Answer
The order of the reaction with respect to phosphinate is 2, indicated by the inverse relationship between concentration and time.
Step 4
Complete the diagram in Figure 2 to show how the hydrogen gas could be collected and measured.
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Answer
In Figure 2, the conical flask should be connected to the gas syringe with a tube, allowing gas to escape into the syringe, with the syringe providing a way to measure the volume of gas produced accurately.
Step 5
Deduce the overall effect on the rate of reaction when the concentrations of both L and M are halved.
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Answer
When both concentrations L and M are halved, the rate of reaction is reduced by a factor of 8, as given by the rate equation, since rate ∝ [L][M]².
Step 6
Calculate the concentration of M if the rate constant is 21.3 mol⁻² dm⁵ s⁻¹.
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Answer
The overall order of reaction refers to the sum of the powers of the concentration terms in the rate equation, indicating how the rate of reaction is affected by changes in the concentrations of the reactants.
