Ethane reacts with chlorine to form chloroethane - CIE - A-Level Chemistry - Question 3 - 2015 - Paper 1
Question 3
Ethane reacts with chlorine to form chloroethane.
C2H6(g) + Cl2(g) → C2H5Cl(g) + HCl(g)
a) Use bond energies from the Data Booklet to calculate the enthalpy change... show full transcript
Worked Solution & Example Answer:Ethane reacts with chlorine to form chloroethane - CIE - A-Level Chemistry - Question 3 - 2015 - Paper 1
Step 1
a) Use bond energies from the Data Booklet to calculate the enthalpy change for this reaction. Include a sign in your answer.
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Answer
To calculate the enthalpy change for the reaction, follow these steps:
Bond Breaking: Determine the energy required to break the bonds in the reactants:
C–H bonds in ethane: There are 6 C–H bonds, each requiring 410 kJ/mol to break, totaling:
6×410=2460 kJ/mol
Cl–Cl bond in Cl2: 1 bond requires 242 kJ/mol.
Total energy for breaking bonds:
2460+242=2702 kJ/mol
Bond Forming: Determine the energy released when forming the bonds in the products:
C–Cl bond in chloroethane: 1 C–Cl bond releases 340 kJ/mol.
H–Cl bond: 1 bond releases 431 kJ/mol.
Total energy for forming bonds:
340+431=771 kJ/mol
Enthalpy Change Calculation: Subtract the energy of the bonds formed from the energy of bonds broken:
Thus, the enthalpy change for the reaction is -119 kJ/mol.
Step 2
ii) State the conditions needed for this reaction to occur.
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Answer
The reaction between ethane and chlorine requires:
UV light or high temperature (sunlight is sufficient).
These conditions provide the energy needed for the initiation stage of the reaction.
Step 3
iii) Use a series of equations to describe the mechanism of this reaction including the names of each stage and an indication of how butane can be produced as a minor by-product.
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The mechanism involves three stages:
Initiation:
The chlorine molecules (Cl2) dissociate under UV light:
Cl2→2Cl∗
Propagation:
Ethane reacts with a chlorine radical:
C2H6+Cl∗→C2H5Cl+H∗
The hydrogen radical then reacts with another chlorine molecule:
H∗+Cl2→HCl+Cl∗
The cycle continues, allowing for the production of chloroethane.
If there is sufficient chlorine, further substitution can occur, leading to butane as a minor by-product:
C2H5Cl+Cl∗→C2H4Cl2+HCl
And still further substitutions yield butane:
C2H5Cl+Cl∗→C4H10+HCl
Termination:
Two radicals combine to stop the chain reaction:
Cl∗+H∗→ClH
Or between two carbon radicals:
C2H5∗+C2H5∗→C4H10
Thus, butane can be formed as a minor by-product during these radical reactions.
Step 4
b) Give the name of X.
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Answer
The intermediate compound, X, is ethene.
Step 5
ii) Suggest the reagent and conditions needed for reaction 1.
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For reaction 1, the reagent is KOH or NaOH, and the conditions required are ethanol and heat/reflux.
Step 6
iii) Suggest the reagent and conditions needed for reaction 2.
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For reaction 2, the reagent is H2 and the catalyst is Pt or Ni.
