An acidified solution of butanone reacts with iodine as shown - AQA - A-Level Chemistry - Question 1 - 2022 - Paper 2
Question 1
An acidified solution of butanone reacts with iodine as shown.
CH₃CH₂COCH₃ + I₂ → CH₃CH₂C(O)CHI + HI
Give the name of CH₃CH₂COCH₃I.
Displayed formula
The rate eq... show full transcript
Worked Solution & Example Answer:An acidified solution of butanone reacts with iodine as shown - AQA - A-Level Chemistry - Question 1 - 2022 - Paper 2
Step 1
Draw the displayed formula for CH₃CH₂COCH₃I
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Answer
The displayed formula for CH₃CH₂COCH₃I is:
H H H H O
| | | | ||
H - C - C - C - C - I
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H H H H
The name of CH₃CH₂COCH₃I is 1-iodobutan-2-one.
Step 2
Calculate the value of the rate constant, k, for the reaction
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Answer
Using the rate equation:
rate=k[CH3CH2COCH3][I2][H+]
We can rearrange to find k:
k=[CH3CH2COCH3][I2][H+]rate
Substituting the values into the equation:
k=4.35×10−3×0.00500×0.8251.45×10−4
Calculating k, we obtain:
k=277.59extdm3extmol−2exts−1
Step 3
Calculate the initial rate of reaction when all the initial concentrations are halved
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Answer
When concentrations are halved, the initial rate can be calculated as follows:
rate′=k[2CH3CH2COCH3][2I2][2H+]
Which can be simplified to:
rate′=81×rate
Thus, substituting in the original rate:
rate′=81.45×10−4=1.81×10−5extmoldm−3exts−1
Step 4
Suggest an observation used to judge when all the iodine had reacted
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Answer
A suitable observation would be to observe a change in color; the disappearance of the brown color of iodine indicates that the iodine has reacted completely.
Step 5
Describe and explain the shape of the graph in Figure 1
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Answer
The graph shows a decreasing trend, indicating that as temperature increases, the rate of reaction increases. This is due to the fact that higher temperatures provide more energy to the particles, resulting in more collisions with sufficient energy to overcome the activation energy barrier. This relationship is not linear and suggests an exponential trend.
Step 6
Deduce the time taken for the reaction at 35 °C
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Answer
From Figure 1, at approximately 35 °C, the reciprocal of the rate constant, 1/k, is about 33 s. Thus, the time taken for the reaction at this temperature is 33 s.
Step 7
Calculate the value of the activation energy, Eₐ
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Answer
Using the provided equation:
ln(k1k2)=REa(T11−T21)
Substituting in the values:
ln(1.55×10−51.70×10−4)=8.31Ea(3031−3331)
Solving for Eₐ gives:
Ea=64.6extkJmol−1
Step 8
Name and outline the mechanism for the reaction of butanone with KCN followed by dilute acid
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Answer
The mechanism is known as nucleophilic addition. In the first step, the cyanide ion (CN⁻) attacks the carbonyl carbon of butanone, forming an intermediate alkoxide ion. In the second step, the alkoxide ion is protonated by dilute acid to yield the final product, 2-hydroxybutanitrile.
