CHM2 Green Chemistry Q

8/6/2019 CHM2 Green Chemistry Q

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1. The enthalpy change of combustion of two fuels is listed below.

fuel enthalpy of combustion/kJ mol 1

hydrogen, H 2 280

octane, C 8H18 5510

(a) Calculate the enthalpy change per unit mass for each of the fuels.

Hydrogen:

Octane:

(3)

(b) Suggest, giving two reasons, which substance is the more useful as a fuel for motor carsand give your reasoning.

Reason 1.................................................................................................................

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Reason 1.................................................................................................................

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(c) Suggest one disadvantage of using the fuel chosen in (b)

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(Total 6 marks)


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2. The following table shows some properties of two different fuels.

Fuel Hydrogen Ethanol

Formula H2 C2H5OH

Boiling temperature / C 252 78

Enthalpy of combustion per gram / kJ 143 30

Cost per tonne / 100 500

(a) Write equations to show the complete combustion of:

(i) hydrogen ..........................................................................................................

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(ii) ethanol .............................................................................................................

.......................................................................................................................... (4)

(b) Hydrogen and ethanol are used as motor car fuel. Suggest the advantages anddisadvantages of each of these fuels in this application. Use the table, and your answers to

part (a) above to help you.

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(Total 9 marks)


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3. Read the passage below on EMISSION C ON TR OL straight through and then more carefully,in order to answer the following questions.

EMISSION C ON TR OL

Photochemical smog, a brown haze just above the horizon, was first observed in Los Angeles inthe 1940s. Research showed that the smog was produced by the action of sunlight on air containing a variety of pollutants, many of them produced by burning fuels.

In a conventional petrol engine, a mixture of fuel and air is ignited by a spark. Petrol isessentially a hydrocarbon mixture so if combustion is complete the reaction products are carbondioxide and water. The proportions of air and fuel are vitally important; the most efficient is thestoichiometric ratio. This is the ratio of air to fuel which, in theory, leads to completecombustion of the fuel. In practice, incomplete combustion results in the formation of toxiccarbon monoxide and also leaves unburnt hydrocarbons which are the main cause of

photochemical smogs. There is also reaction between nitrogen and oxygen gases in the engine, producing oxides of nitrogen, usually referred to as NO x.

Lean-burn engines use a higher air:fuel ratio. They work at lower temperatures, producing less NO x but more unburnt hydrocarbons. Diesel engines also work at lower temperatures, producing less of all the gaseous pollutants but significantly more solid particles such as carbon.

Catalytic converters are fitted to vehicle exhaust systems to deal with gaseous pollutants. Theearliest catalysts tried were metals such as nickel, copper and cobalt. These were relativelycheap but were adversely affected by high temperatures and were poisoned by sulphur andlead compounds in the fuel. Platinum was found to be very much more satisfactory. Very small

particles of the metal were supported on a ceramic honeycomb to increase its surface area.

Getting the right conditions for combustion is not easy. Carbon monoxide and unburnthydrocarbons need to be oxidised but the nitrogen oxides have to be reduced to nitrogen gas. Aningenious solution is to use the nitrogen oxides to oxidise the other pollutants, but this stillrequires some oxygen for complete removal of the hydrocarbons and carbon monoxide. Modernthree-way catalysts (so-called because they deal with all three types of pollutants) only work effectively if the composition of the original air:petrol mixture is carefully controlled; too muchfuel will mean insufficient oxygen in the exhaust fumes. The exhaust system must therefore befitted with oxygen sensors linked back to an electronically controlled fuel injection system.Efficiency is also improved by incorporating other platinum group metals such as rhodium andsometimes palladium.

Catalysts for diesel-engined vehicles have to include an efficient filter to remove carbon particles. This leads to a further problem because the solid carbon (soot) needs to be burned toallow the filter to continue to work effectively. Soot combustion needs a temperature of 500 600C and the exothermic reaction with oxygen can raise it to as much as 1000 C,which candamage the filter. This problem has been tackled by first converting all nitrogen oxides tonitrogen dioxide (NO 2) using a platinum catalyst, and then using the nitrogen dioxide to oxidisethe carbon, a reaction which takes place at 200 C.


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The catalyst will only work if it is hot. Platinum needs a temperature of 240C but rhodiumhelps by reducing this to 150C. This means that the catalyst is ineffective when the engine isfirst started, a major problem under investigation. Unleaded fuel is essential and the introductionof very low sulphur fuel has helped to eliminate the bad-egg smell noticed if hydrogensulphide is produced.

(573 words)

Adapted from Emission control by B. Harrison, Education in Chemistry, September 2000.

(a) Why is the brown smog first seen in Los Angeles said to be photochemical?(1 )

(b) Write a balanced equation for the reaction between carbon monoxide and nitrogenmonoxide (NO), producing carbon dioxide and nitrogen gas.

(1 )

(c) Explain what is meant by the incomplete combustion of a fuel. (1 )

(d) Why do conventional petrol engines give higher NO x emissions than lean-burn or diesel engines?

(1 )

(e) Three platinum group metals used in catalyst systems are mentioned in the passage.Suggest another metal of this type which might be tried in the search for improvedcatalysts.

(1 )

(f) Why has it been found necessary to change the composition of motor fuel for enginesfitted with converters?

(1 )

(g) Why are catalysts least effective when the engine is first started?(1 )


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You are not asked to summarise the whole passage, nor to include equations in your summary.

At the end of your summary state the number of words you have used .

Credit will be given for answers written in good English, using complete sentences and using technical words correctly and chemical names rather than formulae. Avoid copying long

sections from the original text. Numbers count as one word, as do standard abbreviations, units and hyphenated words. Any title you give your passage does not count in your word total.

There are penalties for the use of words in excess of 120.(Total 7 marks)

4. Read the passage on BUILDING A BETTER BLEACH - A GREEN CHEMISTRYCHALLENGE straight through and then more carefully. Answer the following questions.

BU ILDING A B E TT E R B LE ACH A G R EEN CH EMIS TRY CHA LLENGE

Your dark red T-shirt has turned everyone elses white washing pink. The problem is how doyou remove the stain?

A stain is a colour where you dont want it, but stain removal is not actually removal at all.Instead the stain molecules are altered chemically so they no longer reflect light in the same wayas before. We call it bleaching and chemistry is behind the process.

The active ingredient in household bleach, sodium hypochlorite (NaOCl), keeps white clothingwhite and your toilet sparkling, but how does the bleach act on stain molecules? Bleaching is an

example of a redox reaction. The colour of the stain is a property caused by the chemicalstructure of its molecules. When the bleach removes electrons from a molecule, the chemicalstructure of the molecule is changed and properties like colour are altered. Any excess sodiumhypochlorite is washed away in the rinse water. However, oxidation with such chlorine-based

bleaches sometimes adds chlorine atoms to the stain molecules as well as removing electrons.This can lead to the formation of hazardous by-products such as dioxins, which can persist inthe environment and accumulate in the food chain.


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(a) Name sodium hypochlorite, NaOCl, using Stock notation.

............................................................................................................................. ........(1 )

(b) Explain what is meant by a free radical .

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(c) TAMLs can act as catalysts in the peroxide bleaching process. Explain how catalystsincrease the rate of a reaction.

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(d) Describe the THREE key features of an environmentally benign process.

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(e) Suggest why accumulation of dioxins in the food chain may be harmful to people.

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(Total 7 marks)


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5. This question is about some of the chemicals used in car engines and their reactions.

(a) Compound X , shown below, is one component of petrol.

(i) Name X .

...........................................................................................................................(1 )

(ii) Give the empirical formula of X .

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(iii) X can be made by cracking decane, C 10H22.

Assuming only one other product forms in a cracking reaction, deduce themolecular formula of this other product.

(1 )

(iv) What is the sign of the enthalpy change for the reaction in which decane iscracked? Give a reason for your answer.

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HCCCCCH

H CH H H H

H CH H HCH

3

3 3


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(v) If the air supply in a car engine is poor, there is not enough air for carbon dioxideto be produced.

Use this information to suggest ONE possible equation for the combustion of X inthis engine. Use the molecular formula of X in your equation.

(2)

(b) When air enters a car engine, as well as the fuel burning, nitrogen and oxygen can react toform nitrogen(II) oxide.

N2(g) + O 2(g) 2NO(g) H = + 180 kJ mol 1

(i) What, if any, is the effect on the percentage of nitrogen(II) oxide in an equilibriummixture of these three gases if the pressure and temperature are increased?Explain your answers.

Increase in pressure

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Increase in temperature

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(ii) In a car exhaust pipe, nitrogen(II) oxide passes over a catalytic converter.The following reaction occurs.

2NO(g) + 2CO(g) N 2(g) + 2CO 2(g) H = 746 kJ mol 1

Explain why this reaction speeds up when the car engine has been running for afew minutes.

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................................................................................................................. ..........(1 )

(iii) A textbook says The catalytic converter converts the gases coming out of theengine into less harmful ones.


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State, with a reason, which of the four gases in the equation in (ii) you consider to be least harmful.

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...........................................................................................................................(1 )

(iv) The diagram below shows the reaction profile for the change which occurs in thecatalytic converter.

On the diagram, show the activation energy, E A.

Add a line showing the reaction profile if no catalyst is present.(2)

(Total 1 2 marks)

Energy

2NO(g) + 2CO(g)

N (g) + 2CO (g)2 2

Progress of reaction


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Formula of substance Standard enthalpy of formation / kJ mol 1

CH 4 (g) 75

O2 (g) 0

CO 2 (g) 394

H2O (g) 242

The equation for the complete combustion of methane at 150 C is:

CH 4(g) + 2O 2(g) CO 2(g) + 2H 2O(g)

Use the given data to calculate the enthalpy of combustion of methane under theseconditions.

(2)

(ii) The standard enthalpy of combustion of methane is 891 kJ mol 1 .

Explain why this is very different from the value you have calculated in (a)(i).

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............................................................................................................................. ........(1 )


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(Total 7 marks)


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9. Which of the following fuels has the smallest carbon footprint?

A petrol made from crude oil

B hydrogen made from methane

C ethanol made from sugar

D coal(Total 1 mark)


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11. Chlorine was used in swimming pools as a bactericide.

The amount of chlorine present can be determined by adding excess potassium iodide solutionto a known volume of swimming pool water. This reacts to form iodine:

Cl2(aq) + 2I (aq) p I2(aq) + 2Cl

(aq)

The amount of iodine formed is then found by titration with sodium thiosulfate solution of known concentration.

The ionic equation for the reaction between iodine and sodium thiosulfate in aqueous solution is

I2(aq) + 2S 2O32 (aq) p S4O6

2 (aq) + 2I (aq)

A student carried out the determination of chlorine in a sample of swimming pool water.A record of the measurements obtained is given below:

Volume of water sample tested = 1000 cm 3

Final reading of burette = 16.3 cm 3

Initial reading of burette = 7 cm 3

Volume added from burette = 9.3 cm 3

Concentration of sodium thiosulfate solution = 0.00500 mol dm 1


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(ii) Calculate the number of moles of sodium thiosulfate used in the titration.

(1 )


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(ii) Suggest one other reason why the use of chlorine is undesirable in swimming pools.

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(iv) Chlorine dioxide, ClO 2, undergoes a disproportionation reaction when it reactswith water.

4ClO 2 + 2H 2O p HClO + 3HClO 3

Explain, in terms of oxidation numbers, why this is a disproportionation reaction.

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(c) Discuss and explain the science communitys advice that CFCs should no longer be usedin aerosols, foams and refrigerants. Support your answer with one or more equations.

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(Total 1 7 marks)

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CHM2 Green Chemistry Q

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