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Hydrogen peroxide solution decomposes to form water and oxygen - AQA - A-Level Chemistry - Question 4 - 2022 - Paper 3
Question 4
Hydrogen peroxide solution decomposes to form water and oxygen. 2 H₂O₂(aq) → 2 H₂O(l) + O₂(g) The reaction is catalysed by manganese(IV) oxide. A student determin... show full transcript
Worked Solution & Example Answer:Hydrogen peroxide solution decomposes to form water and oxygen - AQA - A-Level Chemistry - Question 4 - 2022 - Paper 3
Step 1
Explain why the reaction is fastest at the start.
Answer
At the start of the reaction, the concentration of hydrogen peroxide is at its highest. This high concentration increases the frequency of effective collisions between reactant particles, leading to a higher rate of reaction. As the reaction proceeds, the concentration of hydrogen peroxide decreases, resulting in fewer successful collisions and a slower reaction rate.
Step 2
Draw a tangent to the curve when the concentration of hydrogen peroxide solution is 0.05 mol dm⁻³.
Answer
A suitable tangent should be drawn touch at the point on the curve corresponding to a concentration of 0.05 mol dm⁻³. The tangent indicates the rate of the reaction at this specific concentration.
Step 3
Use your tangent to calculate the gradient of the curve at this point.
Answer
The gradient can be calculated using the change in concentration over time as shown by the slope of the tangent. For example, if the change in y (concentration) is approximately 0.0015 mol dm⁻³ and the change in x (time) is 10 seconds, the gradient is calculated as follows:
Gradient = rac{0.0015 ext{ mol dm}^{-3}}{10 ext{ s}} = 0.00015 ext{ mol dm}^{-3} ext{s}^{-1}.
Step 4
Use Figure 1 and the expression to calculate [H₂O₂] when 20 cm³ of oxygen has been collected.
Answer
From Figure 1, at 20 cm³ of oxygen collected, the concentration of hydrogen peroxide can be found. Using the formula:
[H₂O₂] = [H₂O₂]_0 imes rac{(V_{max} - V_l)}{V_{max}}
Substituting in the known values, where [H₂O₂]₀ = 0.068 mol dm⁻³, Vₗ = 20 cm³, and Vₑₘₐₓ = 100 cm³:
[H₂O₂] = 0.068 imes rac{(100 - 20)}{100} = 0.068 imes 0.8 = 0.0544 ext{ mol dm}^{-3}.
Step 5
Step 6
State how the graph shows this order.
Answer
The graph shows first-order kinetics with respect to H₂O₂ if a straight line through the origin can be drawn, indicating that the rate is directly proportional to the concentration of hydrogen peroxide. If the rate doubles as the concentration doubles, this linear relationship confirms the first-order reaction.