AS Paper 2 Chemistry

Question 1

Halogens are used to make halogenated organic compounds.
(a) The refrigerant used in air conditioners is a mixture of fluorinated alkanes. These
compounds are made by fluorination reactions.
The mechanism for the reaction of fluorine with an alkane or with a fluoroalkane is a
free-radical substitution similar to the reaction of chlorine with methane.
(i) Write the overall equation for the reaction of fluorine with methane to form
trifluoromethane (CHF3). [1]

(ii) Write equations for the following steps in the mechanism for the reaction of
fluorine with trifluoromethane (CHF3) to form tetrafluoromethane (CF4).
Initiation step
……………………………………………………………………………………………….
First propagation step
……………………………………………………………………………………………….
Second propagation step
……………………………………………………………………………………………….
A termination step leading to the formation of hexafluoroethane.
……………………………………………………………………………………………….

Answer 1

(a) (i) CH4 + 3F2 –> CHF3 + 3HF

(ii) M1 Initiation
F2 → 2F*
M2 First propagation
F* + CHF3→ CF3 + HF
M3 Second propagation
F2 + CF3 →CF4 + F
M4 Termination (must make C2F6)
2CF3 → C2F6 or CF3CF3
Penalise absence of dot once only.
Radical dot on *CF3 can be anywhere but if the structure is
drawn out, the dot must be on the carbon atom. Penalise this
error once only.
Penalise once only for a line and two dots to show a bond.
Penalise each of “Fl” and lower case F, once only in this clip

Question 2

Which molecule is not produced when ethane reacts with bromine in the presence of
ultraviolet light? [1]

A C2H4Br2
B HBr
C H2
D C4H10

Answer 2

C

Question 3

Chloromethanes, such as dichloromethane and trichloromethane, are produced in industry
as they have many uses.
Trichloromethane has been used in the manufacture of the refrigerant
chlorodifluoromethane.
(a) Chlorine can react with dichloromethane (CH2Cl2) to form trichloromethane (CHCl3).
(i) Write an equation for each of the following steps in the mechanism for this
reaction. [3]
Initiation step

…………………………………………………………………………………………………
First propagation step

…………………………………………………………………………………………………
Second propagation step

…………………………………………………………………………………………………
(ii) Give one essential condition for this reaction and name the type of mechanism. [2]
Essential condition …………………………………………………………………….
Type of mechanism ……………………………………………………………………

Answer 3

(a) (i) M1 Initiation
Cl2 2Cl*
Penalise absence of dot once only.
Penalise + or – charges every time
M2 First propagation
Cl* + CH2Cl2 *CHCl2 + HCl
Accept dot anywhere on CHCl2 radical but if the structure is
drawn out, the dot must be on the carbon atom. Penalise this
error once only
Penalise once only for a line and two dots to show a bond.
M3 Second propagation
Cl2 + CHCl2 CHCl3 + Cl
Penalise once only for double headed curly arrows

(ii) M1 Condition
ultra-violet / uv / sun light
OR high temperature
OR 400°C ≤ T ≤ 900°C

M2 Type of mechanism
(free-) radical substitution (mechanism)

Question 4

A molecule of CFC-11 breaks down in the upper atmosphere to form a
chlorine free radical.
Give the equation for this reaction. [1 ]

Answer 4

CCl3F → *CCl2F + *Cl

Question 5

A typical refrigerator contained 0.50 kg of CFC-11 (Mr = 137.5).
One molecule of CFC-11 causes the destruction of approximately 100 000 molecules
of ozone.
Use these data to estimate the number of molecules of ozone that can be destroyed
by 0.50 kg of CFC-11
Give your answer in standard form.
The Avogadro constant, L = 6.022 × 1023 mol–1 [2]

Answer 5

M1 amount of CFC-11 = 500/137.5 ( = 3.64) mol
M2 molecules of O3 = 3.64 x 100,000 x 6.022 x 1023
= 2.19 x 1029

Question 6

State the benefit to life on Earth of ozone in the upper atmosphere [1]

Answer 6

Absorbs (harmful) ultraviolet / uv (light / radiation)

Question 7

Suggest one reason why the use of CFCs was not restricted until several years
after Rowland and Molina published their research. [1]

Answer 7

• lack of evidence that ozone was being depleted
• lack of alternatives to CFCs
• commercial interest to continue to use CFCs
• hard to obtain international agreement

Question 8

CFC-11 is a greenhouse gas that can contribute to global warming.
State and explain how CFC-11 is able to contribute to global warming [2]

Answer 8

M1 absorbs infrared radiation
M2 molecule has polar bonds