8 Reversible Rreactions

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Classified Chemistry IGCSE Paper_3_Topic 8

8 – Reversible Reactions Mostafa Barakat (0100 165 44 62)

129

Topic 8

Reversible reactions

Chemical equilibrium





130

1) Nov 2004 Q (3_a)

a) It is manufactured by the following reversible reaction.

(i) Reversible reactions can come to equilibrium. Explain the term equilibrium…………………………………………………………………………………………

………………………………………………………………………………………[2]

(ii) At 400 C, the percentage of methanol in the equilibrium mixture is lower

than at 300 C. Suggest an explanation…………………………………………………………………………………………

………………………………………………………………………………………[2] (iii) Suggest two advantages of using high pressure for this reaction. Give areason for each advantage

advantage ……………………………………………………………………..........

reason ……………………………………………………………………..........

advantage ……………………………………………………………………..........

reason ……………………………………………………………………..........

[5]

2) Nov 2005 Q (3)

Reversible reactions can come to equilibrium. They have both a forward and abackward reaction.

(a) When water is added to an acidic solution of bismuth (III) chloride, a whiteprecipitate forms and the mixture slowly goes cloudy.

(i) Explain why the rate of the forward reaction decreases with time.…………………………………………………………………………………………

………………………………………………………………………………………[2]

(ii) Why does the rate of the backward reaction increase with time?…………………………………………………………………………………………

………………………………………………………………………………………[1]

the rate of forward reaction equal the rate of backward reaction

the forward reaction, which is exothermic, is favoured by lower temperatures

increased rate

because molecules collide more frequently

increased yield

because high pressure favours side with few molecules

The rate of the forward reaction decreases with time because the concentration of

the reactants decreases

because The concentration of the products or the reactants on the right hand side

increases with time





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(iii) After some time why does the appearance of the mixture remainunchanged?

…………………………………………………………………………………………

………………………………………………………………………………………[2]

(iv) When a few drops of concentrated hydrochloric acid are added to thecloudy mixture, it changes to a colourless solution. Suggest an explanation.

…………………………………………………………………………………………

………………………………………………………………………………………[2]

(b) Both of the following reactions are reversible

(i) Suggest a reason why an increase in pressure does not affect the positionof equilibrium for reaction 1

…………………………………………………………………………………………

………………………………………………………………………………………[1]

(ii) What effect would an increase in pressure have on the position of equilibrium for reaction 2? Give a reason for your answer.

…………………………………………………………………………………………

………………………………………………………………………………………[2]

3) June 2006 Q (5_b)

b) The equation for a stage of the Contact process is

2( ) 2( ) 3( )2 2

g g g SO O SO

The percentage of sulphur trioxide in the equilibrium mixture varies withtemperature

(i) How does the percentage of sulphur trioxide in the equilibrium mixture varyas the temperature increases? Circle the correct answer.

Increases Stay the same Decrease [1]

because reaction has come to equilibrium

The addition of hydrochloric acid moves position of equilibrium to the left. The BiOCl,

a precipitate, reacts with the acid to form BiCl which is a colourless solution

Equal number of moles or volumes of reactants and products, so position of

equilibrium not affected

3

the equilibrium moves to the right as this side has fewer moles

or has the smaller volume.





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(ii) Is the forward reaction in the equilibrium 2SO2 + O2 2SO3

exothermic or endothermic? Give a reason for your choice.

…………………………………………………………………………………………

………………………………………………………………………………………[2]

4) June 2008 Q (5_a, b)

Carbonyl chloride, COCl2, is a colourless gas. It is made by the followingreaction.

a) When the pressure on the equilibrium mixture is decreased, the position of equilibrium moves to left.

(i) How does the concentration of each of the three chemicals change?

…………………………………………………………………………………………

………………………………………………………………………………………[2]

(ii) Explain why the position of equilibrium moves to left.…………………………………………………………………………………………

………………………………………………………………………………………[2]

b) Using the information given with the equation, is the forward reaction

exothermic or endothermic? Give a reason for your choice.…………………………………………………………………………………………

………………………………………………………………………………………[2]

5) June 2009 Q (6_b)

Ammonia is manufactured by the Haber process.

N2(g) + 3H2(g) 2NH3(g)

the forward reaction is exothermic

b) The following graph shows how the percentage of ammonia in theequilibrium mixture changes with temperature.

exothermic because the forward reaction is favoured by low temperatures so it

is exothermic.

the concentration of CO and Cl increase and the concentration of COCl decrease2 2

because ‘a decrease in pressure favours the side with more moles of gas

or the bigger volume’.

The forward reaction is exothermic. because the forward reaction is favoured

by low temperatures or by cooling





133

(i) Explain the term equilibrium.…………………………………………………………………………………………

…………………………………………………………………………………………

…………………………………………………………………………………………

………………………………………………………………………………………[2]

ii) How does the percentage of ammonia vary with temperature? …………………………………………………………………………………………

………………………………………………………………………………………[1]

c) (i) Sketch a graph which shows how the percentage of ammonia in theequilibrium mixture varies with pressure.

(ii) Explain why the graph has the shape shown.

…………………………………………………………………………………………

…………………………………………………………………………………………

…………………………………………………………………………………………

………………………………………………………………………………………[2]

6) Nov 2009 Q (6_a, iv, b, i)

6 (a) Sulfuric acid is made by the Contact process

2 2 32 2SO O SO

(iv) If the temperature is decreased to 300 C, the yield of sulfur trioxideincreases.Explain why this lower temperature is not used.…………………………………………………………………………………………

………………………………………………………………………………………[1]


decrease with increase temperature

increase pressure favours the side with lower volume or the smaller number

of molecules or moles that is the right hand side or products side

the rate would be too slow for the process to be economic





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(b) Sulfuric acid was first made in the Middle East by heating the mineral,green vitriol, FeSO4.7H2O. The gases formed were cooled.

(i) How could you show that the first reaction is reversible?…………………………………………………………………………………………

………………………………………………………………………………………[2]

7) June 2010 [31] Q (6)

6 Iodine reacts with chlorine to form dark brown iodine monochloride

2 22Cl ICl

This reacts with more chlorine to give yellow iodine trichloride.

There is an equilibrium between these iodine chlorides.

(a) Explain what is meant by equilibrium........................................................................................................................................

.......................................................................................................................................

.................................................................................................................................. [2]

(b) When the equilibrium mixture is heated it becomes a darker brown colour.Is the reverse reaction endothermic or exothermic? Give a reason for your choice.

.......................................................................................................................................

.......................................................................................................................................

.................................................................................................................................. [2]

(c) The pressure on the equilibrium mixture is decreased.

(i) How would this affect the position of equilibrium and why?

It would move to the ...................................................................................... [1]

reason ................................................................................................................

....................................................................................................................... [1]

(ii) Describe what you would observe.

.......................................................................................................................................

.................................................................................................................................. [1]

add water to the yellow powder, green crystals would form.


endothermic because it was favoured by an increase in temperature

left

because as a decrease in pressure favours the side with the greater volume

or with more molecules,

the solution becomes more dark brown





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8) June 2010 [32] Q (8)

(b) At most temperatures, samples of nitrogen dioxide are equilibrium mixtures

(i) At 25 C, the mixture contains 20 % of nitrogen dioxide. At 100 C this hasrisen to 90 %. Is the forward reaction exothermic or endothermic?

Give a reason for your choice.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

(ii) Explain why the colour of the equilibrium mixture becomes lighter whenthe pressure on the mixture is increased.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

9) Nov 2010 [31] Q (4_b)

(b) Ammonia is made by the Haber process.

N2(g) + 3H2(g) 2NH3(g) forward reaction is exothermic

The percentage of ammonia in the equilibrium mixture varies with conditions.

The conditions actually used are 200 atmospheres, 450°C and an iron catalyst.

(i) The original catalyst was platinum. Suggest a reason why it was changedto iron.

....................................................................................................................... [1]

(ii) Explain why the highest pressure gives the highest percentage of ammonia in the equilibrium mixture.

............................................................................................................................

....................................................................................................................... [2]

(iii) What happens to the unreacted nitrogen and hydrogen?

............................................................................................................................

....................................................................................................................... [1]

exothermic because an increase in temperature favoured the reverse reaction

or a decrease in temperature favoured the forward reaction

position of the equilibrium move to right because this side has smaller volume

or small number of molecules

iron was less expensive than platinum

high pressure favours the side which has the lower volume (of gases) or the smaller

number of molecules (of gas)

The unused nitrogen and hydrogen is recycled/sent back over the catalyst





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(iv) State one advantage and one disadvantage of using a lower temperature.

advantage ..........................................................................................................

....................................................................................................................... [1]

disadvantage ............................................................................................................................................................................................................................. [1] 10) June 2011 [31] Q (4_b)

4 (b) Sulfur dioxide is used to make sulfur trioxide in the contact process

2( ) 2( ) 3( )2 2

g g g SO O SO

The following reaction is exothermic. The conditions used are

Temperature: 450 C

Pressure: 2 atmospheresCatalyst: vanadium (V) oxide

Explain mentioning both position of equilibrium and rate, why these conditionsgive the most economic yield

............................................................................................................................

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

11) June 2011 [32] Q (6_a)6 (a) Methanol can be made from a mixture of carbon monoxide andhydrogen.

( ) 2( ) 3 ( )2

g g g CO H CH OH

The forward reaction is exothermic.

(i) Explain why the concentration of methanol at equilibrium does not change.

............................................................................................................................

....................................................................................................................... [2](ii) Suggest conditions, in terms of temperature and pressure, which wouldgive a high yield of methanol.

............................................................................................................................

....................................................................................................................... [2]

(iii) How would the conditions used in practice compare with those given in (ii)?Give an explanation of any differences.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

the yield is high (as the forward reaction is exothermic)

a slow (uneconomic) reaction rate

for a high yield need low temperature then rate would be too slow or uneconomic

so the optimum temperature is 450 C / presence of catalyst would increase

and does not alter the yield (at that temperature) higher pressure which would increase

yield but high pressure expensive

rate at which methanol formed by forward reaction equals rate it is reacting in

back reaction

low temperature

high pressure

high pressure can be used

cannot use a low temperature as rate would be too slow the rate would not

be economic





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12) Nov 2011 [31] Q (4_a, b)

4 Reversible reactions can come to equilibrium. The following are threeexamples of types of gaseous equilibria.

A2(g) + B2(g)

2AB(g) reaction 1 A2(g) + 3B2(g) 2AB3(g) reaction 2

2AB2(g) 2AB(g) + B2(g) reaction 3

(a) Explain the term equilibrium.

.......................................................................................................................................

.................................................................................................................................. [2]

(b) The following graphs show how the percentage of products of a reversiblereaction at equilibrium could vary with pressure.

For each graph, decide whether the percentage of products decreases,increases or stays the same when the pressure is increased, then matcheach graph to one of the above reactions and give a reason for your choice.

(i)

effect on percentage of products .......................................................................

reaction ..............................................................................................................

reason ................................................................................................................

....................................................................................................................... [3]

(ii)

effect on percentage of products .......................................................................reaction ..............................................................................................................

The rates of the forward reaction and reverse reaction are equal.

increase

reaction 2

because the volume of reactants is bigger than volume of products

the same

reaction 1





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reason ................................................................................................................

....................................................................................................................... [3]

(iii)

effect on percentage of products .......................................................................

reaction ..............................................................................................................reason ................................................................................................................

....................................................................................................................... [3]

13) Nov 2011 [32] Q (3_c)

(c) When antimony chloride is added to water, a faint white precipitate formsand the mixture slowly goes cloudy.

(i) Explain why after some time the appearance of the mixture remains unchanged.

.......................................................................................................................................

.................................................................................................................................. [2]

(ii) When a few drops of concentrated hydrochloric acid are added to themixture, it changes to a colourless solution. Suggest an explanation.

.......................................................................................................................................

.................................................................................................................................. [1]

(iii) Suggest how you could make the colourless solution go cloudy................................................................................................................................... [1]

14) Nov 2011 [33] Q (6_b)

(b) Nickel ores are converted into nickel(II) oxide. This can be reduced to impurenickel by heating with carbon. The nickel is purifi ed by the following reversiblereaction.

because the volume of the reactants equal the volume products

decrease

reaction 3because volume of products is bigger than volume of reactants

because at equilibrium rate of forward reaction equals rate of back reaction

and macroscopic properties do not change with time

equilibrium moves to left (SbOCl used up)

add water / add more SbCl3





139

(i) Impure nickel is heated at 60◦C. The forward reaction occurs

The nickel carbonyl, a gas, moves into a hotter chamber at 200 C. Thebackward reaction occurs and the nickel carbonyl decomposes.

Is the forward reaction exothermic or endothermic? Give a reason for your answer.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

(ii) Explain why the forward reaction is favoured by an increase in pressure.

............................................................................................................................

....................................................................................................................... [2]

(iii) Suggest what happens to the impurities........................................................................................................................ [1]

15) June 2012 [31] Q (4_c)

(c) Vanadium(V) oxide is used to catalyse the exothermic reaction betweensulfur dioxide and oxygen in the Contact Process.

2 2 32 2SO O SO

The rate of this reaction can be increased either by using a catalyst or byincreasing the temperature. Explain why a catalyst is used and not a higher

temperature.…………………………………………………………………………………………..

…………………………………………………………………………………………..

.................................................................................................................................... [2]

16) Nov 2012 [31] Q (5_b)

5 Carbonyl chloride, COCl 2, is widely used in industry to make polymers,dyes and pharmaceuticals.

(b) Carbonyl chloride is now made by the reversible reaction given below.

CO(g) + Cl 2(g) COCl 2(g)

exothermic because forward reaction favoured by low temperatures

because there are more molecules on the left hand side of the equation

or less molecules on the right hand side of the equation.

do not react or left behind

because catalyst would not affect yield but increase in temperature would decrease

the yield of sulfur trioxide





140


The reaction is catalysed by carbon within a temperature range of 50 to 150 °C.

(i) Predict the effect on the yield of carbonyl chloride of increasing thepressure.

Explain your answer.

............................................................................................................................

....................................................................................................................... [2]

(ii) If the temperature is allowed to increase to above 200 °C, very littlecarbonyl chloride is formed. Explain why.

............................................................................................................................

....................................................................................................................... [2]

(iii) Explain why a catalyst is used.

....................................................................................................................... [1]

17) Nov 2012 [32] Q (7_b)

(b) Methanol is manufactured using the following method.

CH4(g) + H2O(g) → CO(g) + 3H2(g) reaction 1

CO(g) + 2H2(g) CH3OH(g) reaction 2

The conditions for reaction 2 are:

pressure 100 atmospheres

catalyst a mixture of copper, zinc oxide and aluminium oxide

temperature 250 °C


(i) Why is high pressure used in reaction 2?

............................................................................................................................

....................................................................................................................... [2]

(ii) Explain why using a catalyst at 250 °C is preferred to using a higher temperature of 350 °C and no catalyst.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [3]

18) Nov 2012 [33] Q (7_c)

7 Ammonia is made by the Haber process.

N2(g) + 3H2(g) 2NH3(g)

(c) This reaction is carried out at a high pressure, 200 atmospheres.

State, with an explanation for each, two advantages of using a high pressure.

increase yield of COCl ; increase pressure favours side with less gas molecules2

increased temperature favoured the back reaction or reduced the yield

because it is endothermic.

increase rate at lower temperatures

to increase the yield of methanol because high pressure favours the side with the

fewer moles of gas

catalyst causes faster reaction at low temperature

at 250 C (lower temp) higher yield because forward reaction is exothermic

at 350 C (higher temp) lower yield because forward reaction is exothermic





141

............................................................................................................................

............................................................................................................................

............................................................................................................................

....................................................................................................................... [5]19) June 2013 [32] Q (4_a)

4 At present the most important method of manufacturing hydrogen is steamreforming of methane.

(a) In the first stage of the process, methane reacts with steam at 800 °C.

4( ) 2 ( ) 2( ) ( )3 g g g g CH H O H CO

In the second stage of the process, carbon monoxide reacts with steam at200 °C.

( ) 2 ( ) 2( ) 2( )

g g g g CO H O CO H

(i) Explain why the position of equilibrium in the first reaction is affected bypressure but the position of equilibrium in the second reaction is not.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

(ii) Suggest why a high temperature is needed in the first reaction to get a

high yield of products but in the second reaction a high yield is obtained at alow temperature.

............................................................................................................................

....................................................................................................................... [2]

20) June 2013 [33] Q (5 _ b)

(b) Almost all samples of nitrogen dioxide are an equilibrium mixture of nitrogen dioxide, NO2, and dinitrogen tetroxide, N2O4.

In the forward reaction, a bond forms between the two nitrogen dioxidemolecules.

NO2 + NO2 O2N – NO2

(i) Explain the term equilibrium mixture.

............................................................................................................................

....................................................................................................................... [1]

(ii) The syringe contains a sample of the equilibrium mixture. The plunger waspulled back reducing the pressure.

1) faster rate because more collisions between particles

2) higher yield of ammonia because high pressure favours the reaction with side of

less moles;

because in first reaction the number of moles of reactants and products are different

whereas in second reaction number of moles of reactants and products are equal

because first reaction is endothermic while second reaction is exothermic

at equilibrium, the rate of the forward reaction is equal to the rate of the

back reaction





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How would the colour of the gas inside the syringe change? Give anexplanation for your answer.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [3]

(iii) A sealed tube containing an equilibrium mixture of nitrogen dioxide anddinitrogen tetroxide was placed in a beaker of ice cold water.

The colour of the mixture changed from brown to pale yellow.

Is the forward reaction exothermic or endothermic? Give an explanation for your choice.

............................................................................................................................

....................................................................................................................... [2]

(iv) What other piece of information given in the equation supports your answer to (iii)?

NO2 + NO2 → O2N –NO2

....................................................................................................................... [1]

goes brown

because the position of equilibrium moving to the left since the reactant has

more moles of gas

exothermic

because low temperatures favour the exothermic reaction

forward reaction is bond forming

8 Reversible Rreactions

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