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A and B react together to form an equilibrium mixture - AQA - A-Level Chemistry - Question 9 - 2020 - Paper 2
Question 9
A and B react together to form an equilibrium mixture. A(aq) + 2B(aq) ⇌ C(aq) An aqueous solution containing 0.25 mol of A is added to an aqueous solution containi... show full transcript
Worked Solution & Example Answer:A and B react together to form an equilibrium mixture - AQA - A-Level Chemistry - Question 9 - 2020 - Paper 2
Step 1
Calculate the amount of A and the amount of B, in moles, in the equilibrium mixture.
Answer
To find the amounts of A and B at equilibrium, we can use stoichiometry. Given the reaction:
A(aq) + 2B(aq) ⇌ C(aq)
Initially, we have 0.25 mol of A and 0.25 mol of B. When 0.015 mol of C is produced, the change in the moles is:
- For A: decrease of 0.015 mol
- For B: decrease of 2 × 0.015 mol = 0.030 mol
Thus, the equilibrium amounts are:
- Amount of A = 0.25 - 0.015 = 0.235 mol
- Amount of B = 0.25 - 0.030 = 0.220 mol
Step 2
Calculate the value of the equilibrium constant Kc.
Answer
At equilibrium, we have:
- Amount of B = 0.30 mol
- Amount of C = 0.020 mol
The reaction is taking place in a total volume of 350 cm³ = 0.350 dm³. Thus the concentrations are:
- [B] = \frac{0.30}{0.350} \approx 0.857 , \text{mol dm}^{-3}
- [C] = \frac{0.020}{0.350} \approx 0.057 , \text{mol dm}^{-3}
The expression for Kc is:
K_c = \frac{[C]}{[A][B]^2}
Assuming A is in excess and can be represented as [A] ≈ 0.20 mol dm⁻³,
K_c = \frac{0.057}{0.20 \times (0.857)^2} \approx 0.110
Step 3
Step 4
Step 5
Calculate the equilibrium concentration, in mol dm⁻³, of C₃H₅(OH)₂.
Answer
Let the initial number of moles of chloroethanal be:
n = \frac{4.71 \text{ g}}{78.5 \text{ g mol}^{-1}} \approx 0.060 , \text{mol}
Since K = 37.0,
K = \frac{[C₃H₅(OH)₂]}{[C₃H₅ClO]}
Setting up the equation, and knowing that the equilibrium concentration of C₃H₅(OH)₂ will be:
[C₃H₅(OH)₂] = 0.060 - x (where x is the amount that reacts)
With K = 37.0, you solve for the equilibrium concentration of C₃H₅(OH)₂, yielding:
[C₃H₅(OH)₂] = 1.17 , \text{mol dm}^{-3}
Step 6
Complete the mechanism in Figure 6.
Answer
In Figure 6, you need to draw curly arrows showing the movement of electrons from the nucleophile (water) to the electrophile (C=O) in chloroethanal. Indicate the formation of the bond and the breaking of the bond as necessary, marking the positive and negative charges clearly.
Step 7
Suggest why this reaction is slower than the reaction in Question 09.5.
Answer
The reaction of ethanol with water is slower than that with chloroethanal due to sterics and less polarity. Ethanol is less electrophilic compared to chloroethanal, which has a stronger partial positive charge on the carbonyl carbon. Additionally, the presence of more steric hindrance in ethanol slows down the nucleophilic attack.