Download atmosphere test salters

A
End of module test
The Atmosphere end of module test
60 marks (1 hour)
1 Chlorofluorocarbons (CFCs) have been widely used as propellants, blowing agents and cleaning solvents, but their use leads to
environmental damage in the atmosphere. As a result, alternative compounds are now being used. Some alkanes have been used;
other common replacements are HFCs (hydrofluorocarbons) and HCFCs (hydrochlorofluorocarbons). Here are some data for three
compounds: a CFC, an alkane and an HCFC.
Compound
Formula
Boiling point/K
Flammable
ODP*
Price
A
CFCl3
297
no
1.0
medium
B
CH3CH2CH3
231
yes
0.0
low
C
CF3CCl2H
302
no
0.02
high
* ODP is the Ozone Depletion Potential
aSuggest why compound B cannot be used as cleaning solvent.
[1]
bSuggest a disadvantage of using compound C as a replacement for compound A.
[1]
cHFCs and HCFCs are more reactive than CFCs and are broken down in the troposphere. The first step in this process
involves reaction with OH radicals. Equation 1.1 shows the reaction of compound C with OH radicals.
CF3CCl2H + OH → CF3CCl2 + H2O
Equation 1.1
iDraw a dot–cross diagram (showing the outer electron shells only) for OH and explain why it is called a radical.
iiGive the formula of another radical in Equation 1.1.
iiiExplain why Equation 1.1 is called a propagation step.
[3]
[1]
[1]
dThe reaction in Equation 1.1 is important because it removes HCFC molecules from the troposphere before they can
reach the stratosphere. By contrast, CFCs are unreactive until they reach the stratosphere.
iExplain in detail the effect of CFCs on the ozone in the stratosphere, and how this affects human health.
iiExplain how the measurements showing a large decrease in the concentration of stratospheric ozone were
overlooked at first.
[5]
[2]
eThe bond enthalpy of the OH bond is +464 kJ mol–1.
iCalculate the minimum energy required to break a single OH bond. (Avogadro constant, NA = 6.02 × 1023 mol–1.) [2]
iiCalculate the minimum frequency of radiation needed to break the OH bond.
[3]
(Planck constant, h = 6.63 × 10–34 J Hz–1.) Give your answer to three significant figures.
iiiSuggest why OH radicals are not produced in the troposphere by the action of sunlight on water molecules,
but by another route.
[2]
[TOTAL: 21 MARKS]
(OCR Chemistry B (Salters) questions, adapted for the 2008 specification)
2 A student wrote the following piece about the effect of carbon dioxide on climate change.
It is partially correct.
Read it through, then answer the questions that follow.
The Sun radiates energy to the Earth in the form of visible radiation.
This is absorbed by the Earth and emitted as another type of radiation.
Carbon dioxide in the atmosphere absorbs this radiation and warms the atmosphere. Carbon dioxide is the third most common gas present in dry air. The thin layer of carbon dioxide in the troposphere resembles the glass of a greenhouse which reflects infrared
radiation back into the greenhouse.
Global warming can be shown to be occurring and the increase of carbon dioxide in the atmosphere can also be
demonstrated.
Therefore there must be a link between the two.
There are some ways of reducing carbon dioxide emissions and other ways of removing carbon dioxide from the
atmosphere.
One of the latter is dissolving carbon dioxide in the oceans; a dynamic equilibrium exists between CO2 in the air
and CO2 in the water.
1
2
3
4
5
6
7
8
9
aAdd to sentence 1 to make it fully correct.
[1]
bSentence 2 is correct but incomplete. What type of radiation is emitted?
[1]
ci How does carbon dioxide absorb the radiation? (sentence 3)
iiHow does this warm the atmosphere? (sentence 3)
Salters Advanced Chemistry, Pearson Education Ltd 2008. © University of York.
This document may have been altered from the original.
984_04_SAC SP_A.indd 111
[2]
[2]
111
16/4/08 16:19:35
A
End of module test
di Sentence 4 is incorrect. Give the correct gas with its percentage.
ii The percentage of carbon dioxide in an air sample is 0.033%. Express this in parts per million (ppm).
[2]
[1]
ePart of sentence 5 is incorrect. Rewrite the sentence as a correct version.
[2]
fGive evidence for each of the statements in sentence 6.
[2]
gComment on sentence 7.
[1]
hGive one way of reducing carbon dioxide emissions and one way of removing carbon dioxide from the atmosphere
(apart from dissolving in the oceans) (sentence 8).
[2]
ii Explain what is meant by dynamic equilibrium (sentence 9).
[2]
iiExplain why the process referred to in sentence 9 is not a true equilbrium.
[1]
iiiAn equilibrium does exist between dissolved carbon dioxide and hydrogencarbonate ions in solution:
CO2(aq) + H2O Y HCO3–(aq) + H+(aq)
Describe and explain what would happen to the concentration of CO2(aq) if some acid (H+) was added to this
equilibrium.
[2]
[TOTAL: 21 MARKS]
3 Ozone is beneficial to us in the stratosphere but harmful in the troposphere.
The following reactions take place in the troposphere:
O2 → 2O O + O3 → 2O2
O2 + O → O3 ai What sort of bond breaking occurs in equation 3.1?
iiDescribe in words how ozone is made starting from oxygen molecules and how it breaks down.
iiiDescribe one polluting effect of tropospheric ozone.
equation 3.1
equation 3.2
equation 3.3
[1]
[3]
[1]
bSome other reactions that occur in the troposphere are:
NO + O3 → NO2 + O2 NO2 + O → NO + O2 equation 3.4
equation 3.5
iGive a man-made source of NO.
Use equations 3.4 and 3.5 to answer the following:
iiExplain why NO might be a catalyst for the breakdown of ozone.
iiiName the type of catalysis shown by NO.
ivName the intermediate in the catalysis of the breakdown of ozone by NO.
[1]
[1]
[1]
[1]
cNO is only a catalyst for the breakdown of ozone if the activation enthalpy of equations 3.4 and 3.5 is less than that of
equation 3.2.
Complete the enthalpy profile to illustrate the statement in c. Indicate the activation enthalpies of reactions 3.2, 3.4
and 3.5 on the diagram.
[2]
Energy
O � O2 � NO2
O � O3 � NO
2O2 � NO
dExplain, in terms of activation enthalpy, how temperature affects the rate of a reaction.
[3]
eThe reaction of methane with chlorine is another radical reaction.
The first two steps are:
Cl2 → 2Cl Cl + CH4 → CH3 + HCl equation 3.6 initiation
equation 3.7 propagation
[1]
iWrite equation 3.8, which is another propagation step producing CH3Cl.
iiWrite a possible termination step for the reaction.
[1]
iiiExplain why the activation energy of the termination step is low, yet the reaction is slow in the troposphere.
[2]
[TOTAL: 18 MARKS]
112
984_04_SAC SP_A.indd 112
Salters Advanced Chemistry, Pearson Education Ltd 2008. © University of York.
This document may have been altered from the original.
16/4/08 16:19:35