96 Theme 3 I Chemistry of Reoctions

For each of the questions, there are four possible choices.Choose the one you consider coruect and record your choice.

d Which of the following is true when an exothermic reaction

occurs?

A The surroundings gets cooler.

B The enthalpy change for the reaction is positive.

C The products are at a lower energy level than thereactants.

D The energy change for bond breaking is greater than theenergy change for bond forming. ( )

d2. The diagram below shows the conversions between the threestates of water.

2""'\r'\water i-+ ice

d

d

Which of the following conversions are endothermic?

I lce - Steam

ll Water * lce

lll Steam - WaterlV lce - Water

A I and ll only B I and lV only

C ll and lll only D lll and lV only (

3. Which of the following is an exothermic process?

A lVelting of rce

B Evaporation of ethanol

C Formation of iodine vapour from iodine crystalsD Condensation of water vapour t0 form liquid droplets

(

4. Dissolving solid calcium chloride in water is an exothermicprocess. Which of the following graphs shows the temperature

changes as the solid is added to water, stined and the solution

left to stand?

A remperature

temperatureroom

0

B

temperature

roomtemperature

Temperature

TemperatureC

Time

Time0

Temperature

roomtemperature

Time

5. Below is an energy profile diagram for a chemical reactionshowrng the energy changes l, ll, lll and lV

products

D

d

III

tI

IV

IIII

I

Time

0

reactants

Reaction pathway

Energy from Chemicols

()

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d

d

d

Which of the following energy changes indicates theactivation energy for the catalysed reaction?

AI B IIcilr Drv ( )

6. What effect does an increase in temperature have on theactivation energy and enthalpy change of a chemical reaction?

Activation energy Enthalpy change

A Unchanged U nchanged

B Decreases I ncreases

C Unchanged Decreases

D I ncreases Unchanged

1. Which of the following occurs in a hydrogen fuel cell?

A Hydrogen and oxygen react at the electrodes to formwater, generating electricrty.

B Water is split up at the electrodes to form hydrogen and

oxygen, generating electricity directly.C Hydrogen and oxygen react at the electrodes to form

water, generating heat energy which is then convertedto electricity.

D Water is split up at the electrodes to form hydrogen and

oxygen, releasing heat energy which is then convertedinto electricity (

)

B. Which of the following are ways of obtaining hydrogen foruse as fuel?

Electrolysis of waterCracking of petroleumFractional distillation of airBeaction of metals with dilute acids

Unit13 I tn*rgylrr:nCherricals 97

10. Which of the following is/are true for the reaction?

I The 0-H bond is weaker than the H-H or the 0 = 0 covalentbond.

ll Energy released in breaking the H-H and 0 = 0 bonds is

greater than the energy absorbed in forming the 0-H bonds.

lll Energy released in forming the 0-H bonds is greater thanthe energy absorbed in breaking the H-H and 0 = 0 bonds.

A ll only B lll onlyC I and ll only D I and lll only (

)

11. Which of the following is an exothermic reaction?

A Formation ofglucose and oxygen from carbon dioxide and

waterB Decomposition of calcium carbonate

C Heating of hydrated copper(ll) sulfate to form itsanhydrous form

D Beaction of aqueous sodium hydroxide and dilutehydrochloric acid ( )

12. Which of the following is an endothermic reaction?

A C{s)+ 0,(g) - C0lg)B H-(aq)+ 0H-(aq) - H,0(l)

C Ba2laq) + SOrF(aQ) - BaS0a(s)

D CH,C00H(aq)= CH,C00-(aq)+ H.(aq) (

13. Syngas is the name given t0 a gaseous fuel mixture containing

varying amounts of carbon monoxide and hydrogen. lt can be

made using the following reaction:

C(s)+ H,O(g) - C0(g)+ H,(g)

When steam is passed over white-hot coke, the coke willgradually cease t0 be white-hot and steam will begin tocondense on the coke. Before this happens, the steam is cutoff and hot air is sent in for a short interval. This cycle is thenrepeated.

Which of the f0llowing best explains the need for the hot air?

A To stop the exothermic reaction so as to prevent explosionB To burn up unnecessary impurities so as to avoid

contaminationC To constantly supply the endothermic reaction with

sufficient heat energyD To increase the temperature of the reactants so as t0

speed up the reaction ( )

14. When ice melts, the energy change of the process can beexpressed as follows:

H,O(s) + ll,g111 lH = +6.00 kJ/mol

Which of the following is true based on the above reaction?

A 6.00 kJ of heat is released when one mole of ice melts.B 6.00 kJ of heat is released when one mole of water

freezes.

C 6.00 kJ of heat is absorbed when one mole of steamcondenses.

D 6.00 kJ of heat is absorbed when one mole of liquid waterboils. ( )

\\,s

\\,g

M0

\\,g

\\,s

I

il

ilt

IV

AC

I and ll onlyll and lll only

B I and lV onlyD lll and lV only (

Refer to the following to answer questions 9 and 10.

Heat is given off when hydrogen burns in air according to the followingequation:

2H,(g) + 0,(g) - 2H,0(g) AH = -483.6 kJ

d9. Which of the following factors account for the enthalpychange observed in the above reactlon?

I The formation of 0-H bondsll The breaking of the 0-H bondslll The breaking of the H-H and 0 = 0 bondslV The formation of the H-H and 0 = 0 bonds

A I and lll only B I and lV onlyC ll and lll only D ll and lV onty (

)

()

)

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98 lheme 3 I Chemistry of Reoctions

15 Hydrogen combines through an endothermic reaction withiodine to form hydrogen iodide gas.

H,(g)+ l,(g) t 2Hl(g) AH = +26.5 kJ/mol

Which of the following is not true about the reaction?

A The energy content of the product is higher than that ofthe reactants.

B The activation energy needed for the reaction will be

greater than 26.5 kJ.

C The energy absorbed in bond-breaking is greater thanthat liberated in bond-forming.

D The combined bond energies oftwo H-l bonds are greater

than that of one H-H and one l-l bonds. ( )

U 16. Hydrogen reacts with chlorine to form hydrogen chloride gas

according to this equation:

H,(g)+ C/,(g) - ZHCtls) AH = -92.3 kJ/mol

Which of the following explains the heat change for thisreaction?

A The H-H and C/-Cl bonds are stronger than the H-C/bonds.

B [Volecules liberate energy when they combine to formnew compounds.

C The number of bonds formed is greater than the number

of bonds broken.

D The energy absorbed to break bonds is less than thatliberated to form bonds. (

)

\\,g 17. Ammonia reacts with oxygen to form nitrogen oxide and wateras follows:

+NH,(O) + 50,(u) - 4N0(s) + 6H,0(O) AH = -382 kJ/mol

Which of the following is not true?

A 382 kJ of heat will be evolved for each mole of NH,

reacted.

B The reverse reaction is endothermic with an enthalpychange of +382 kJ/mol.

C N0 and HrO have a total energy content greater than that

of NH, and 0, combined.

D The four N-0 and twelve 0-H bonds are stronger thanthe twelve N-H bonds and five 0 = 0 bonds. ( )

\V0 1 8. The energy profile diagram for a given catalysed reaction is

shown below:

Energy

Which of the following shows the correct energy profile

diagram for the uncatalysed reaction?

I Energy

products

reactants

Reactionpathway

B En".gy

reactants productsReactionpathway

f, Energy

reactants

I Energy

reactants

productsReactionpathway

Reactionpathway

reactants

Reaction pathway

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Refer to the following t0 answer questions 19 and 20.

The equation for the nickel-catalysed reaction of ethane and hydrogenis shown as follows:

NtiC,Ho(g)+ H,(g)-r5 C,Uo1g1 lH = -130 kJ/mol

The energy profile diagram for this reaction is shown below:

Energy

reactants

products

Extent of reaction

U 19. When could a reaction occur between a C,Ho molecule and

a H, molecule?

A Each time they collideB Only when they collide with energy exactly equal to YC 0nly when they collide with energy equal to or greater

than XD 0nly when they collide with energy equal to 0r greater

thanY ( )

\V0 20. Which of the following would be affected by the addition of

nickel to the above reaction?

A Xonly B YonlyC XandYonly D X,YandZ (

)

\Vg 21. Which of the following energy profile diagrams shows the

slowest endothermic reaction?

ABEnergy Energy

Reaction Reactionpathwaypathrvay

Energy Energy

Unil 13 I t.;;': ;:1.:: :.;;.1'.,'' | :.;'t-i ) i.'.. ;t:::': :,:'' /':' 99

22. Coal, petroleum and natural gas are known as fossil fuels. Thecombustion of an effective fossil fuel must have an activationenergy that is neither too high nor too low.

What happens if their combustions have very low activationenerg ies?

A They will ignite very easily and be hazardous.B They will evaporate too easily, causing high wastage.C They will produce too much greenhouse gases when

burnt.D They will not produce useful amounts of energy when

burnt. ( )

23. The following energy profile diagrams show the energychanges for the heating of one mole of four differentcompounds in air. Which energy profile diagram is for thecompound that shows the greatest potential for use as a fuel?

A Energy

reactants

products Reactionpathway

B Energy

reactantsproducts

Reactionpathrvay

(, Energy

products

reactantsReactionpathway

! Energy

Reactionpathway

1Y

Zi

x

\Vs

t\,0

C D

Reactionpathway

Reactionpathway

()

reactants

products

()

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1OO Theme 3 I Chemistrv of Reoctions

U 24. When the wick of a candle is touched by a lit match, thecandle begins to burn. When the match is removed, thecandle continues to burn. What is the role of the match in

the reaction involving the candle wax?

A lt acts as a catalyst.B lt supplies the activation energy.

C lt lowers the activation energy barrier.D lt increases the rate of the combustion. ( )

Refer to the following to answer questions 25 and26.

Hydrogen is increasingly being investigated as a fuel for the future.It burns in oxygen according to the following reaction:

2H,(g) + 0,(g) - 2H,0(l) AH = -571.8 kJ

\\,0 25. How much energy is released when 4.2 kg of hydrogen is

burnt in air?

A 3x105kJ B 6x105kJC 9x106kJ D 1.2x106kJ ( )

W,26. Which 0f the following describe(s)the advantages of carrying

out this reaction in a fuel cell rather than burning hydrogen

and converting the heat into electricity?

I lt reduces pollution to the environment.

ll Less energy is wasted or lost during conversion.

lll lt increases the ease of storage and handling of hydrogen.

A I only B ll only

C I and lll only D ll and lll only ( )

Refer to the following to answer questions 27 to30.

The following table compares the properties of four different fuels.

\Vg 27. Which of the following shows the correct order of fuelswhich produce decreasing amounts of energy when 1 g ofthe compound is completely burnt?

A Hydrogen, methane, petrol, ethanolB Hydrogen, methane, ethanol, petrol

C lVethane, hydrogen, petrol, ethanolD lVethane, hydrogen, ethanol, petrol ( )

28. Which of the following shows the conect order of fuels whichproduce decreasing amounts of energy when 1 litre of thecompound is completely burnt?

A Petrol, ethanol, hydrogen, methaneB Petrol, ethanol, methane, hydrogen

C Ethanol, petrol, hydrogen, methane

D Ethanol, petrol, methane, hydrogen ( )

29. Which 0f the following shows the correct order of fuelswhichproduce decreasing amounts of carbon dioxide per mole offuel used?

A Petrol, methane, ethanol, hydrogen

B Petrol, ethanol, methane, hydrogen

C [Vethane, ethanol, petrol, hydrogen

D Ethanol, petrol, methane, hydrogen ( )

30. lVethane in the form of compressed natural gas (CNG) is beingused as a fuel for cars.

Which of the following showthe advantages of using methaneas a car fuel compared to petrol?

I Produces more heat per unit of massll Enables cars to travel longer distances for each tank of

fuellll Produces lower levels of carbon dioxide per unit of energy

released

Mg

\Vg

w

U

A I and ll only

C lland lllonlyB I and lll onlyD l, ll and lll

31. Ethyne (CrHr) undergoes hydrogenatlon to form ethane as

follows:

C,H,(g)+ 2H,(g) - C,Hu(g)

The average energies of the bonds in the substances involved

are shown in the table below

Bond Bond energy/kJ mol-t

C_H 413

L-L 341

C=C 612

C=C 839

H-H 432

What is the enthalpy change for this reaction?

A -176 kJ/mol B -296 kJ/molC +176 kJ/mol D +296 kUmol

Property Ethanol Hydrogen Methane Petrol

lVolar mass/g 4b 2 tb 114

Density/kg l-1 0.79 6.4 x 10r 0.69

Entha I py

change of

combustion/

kJ molr

-1 360 -285 -891 -5460

()

B.4x 10 5

()

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Refer to the f0llowing t0 answer questions 32 and 33.

Hydrogen gas can be stored in metal hydride compounds undermoderate temperature and pressure, which gives them a safetyadvantage.

ln order to store hydrogen, the gas is passed sl ightly over atmosphericpressure over the metal alloy. Most metal alloys bind with hydrogenvery strongly to form the metal hydride. This reaction as shown belowis reversible and the gas can be released again.

lVetal(s) + Hydrogen(g) + [Vletal hydride(s)

W,, Which of the following energy prof ile diagrams bestrepresents the energy changes for the processes?

A Energy

metal hydride

metal + hydrogen

Reactionpathway

B Energy

metal

metal + hydrogen

Reactionpathway

Unit 13 I {"nergy fram Chery:iccls 101

33. Which of the following is required to release the hydrogenfrom the metal hydride for use in suitable applications?

A Cooling down the metal hydrideB Supplying heat energy t0 the metal hydrideC Beducing the pressure over the metal hydrideD lncreasing the pressure over the metal hydride ( )

34. Heat is given off when hydrogen burns in air according to theequation:

Equation l: 2Hlg)+ 0,(g) - 2H,0(g) AH = -483.6 kJ

The heat of reaction for the combustion of hydrogen in aircan also be expressed as follows:

Equation ll: 2H,(g)+ 0,(g) - 2H,0(l) AH = -571.8 kJ

ln this case, liquid water is formed instead of steam.

Which of the following explains why the value for AH inequation ll is different from that in equation l?

A Some heat energy is absorbed due to the condensationof steam to liquid water.

B Some heat energy is absorbed for the breaking of bonds

to form more liquid waterC lVore heat energy is released due to the condensation of

steam to liquid water.D lVore heat energy is released due to the formation of

more bonds to form more water. ( )

35. Which of the following statements is not true?

A Photosynthesis is an exothermic reaction.

B An exothermic reaction only needs heat to initiate.C An endothermic reaction requires a constant supply of

heat as long as the reaction proceeds.

D Beactions with high activation energies do not occurspontaneously and may require heat or the addition ofcatalyst to initiate the reaction. ( )

36. Which of the following only involves an exothermic process?

A C(s)+ 0,(s) - C0lg)g H,(s)+ C[(u) - 2HCl(s)

C Pb'?t(aq) +ZCl (aql - PbC/z(s)

D C/,(ao)+ 2Br(aq) - ZCt(aql + Br,(aQ) ( )

C Er".gy

U

W

w

W,metal + hydrogen

D Energy

metal hvdride

Reactionpathway

Reactionpathway

+ hydrogen

metal hydride

()

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1O2 Theme 3 I Chcrnistry r:f l?ecclions

37. The energy changes in the following reversible reaction is

shown in the energy profile diagram below.

2N,(g) + Hls) = zNH, (s)

Energy

w

w

What does X represent?

A Heat of reaction for the reverse reactionB Heat of reaction for the forward reaction

C The energy absorbed for the decomposition of NH,molecules

D The minimum energy needed to initiate the reaction ofthe N, and H, molecules (

)

x

Reaction progress

\[4838. The enthalpy changes for the reaction of hydrogen with the different halogens are shown below.

What deduction can be made based on the information in the table?

A Formation of H-l bond releases the greatest am0unt of energy.

B Compared to the other halogens, iodine forms the weakest bond with hydrogen

C Bond-breaking in the reaction of H, and

D Bond-breaking in the reaction of H, and

to form Hl requires the largest amount of energy.

to form HF requires the largest amount of energy

l2

F2

Nl4D39. Hydrochlorrc acid reacts with sodium hydroxide in an

exothermic reaction as shown:

HC/(aq)+ Na0H(aq) - NaC/(aq)+ H,0(l) AH, = -57 2 L..1

When ethanoic acid is used instead of hydrochloric acid, thereaction goes t0 completion as follows:

CH,CO0H(aq)+ Na0H(aq) - NaCHgCO0(aq)+ H,0(l)AH,=-554L''1

Why is the value of AH, different from that of AH,?

A Less heat energy is released due to less water formed.

B lVore heat energy is released due to more water formed.

C Some heat energy is absorbed for the dissociation ofethanoic acid molecules to form hydrogen ions.

D lVore heat energy is released due to the dissociation ofethanoic acid molecules to form hydrogen ions. ( )

40. Hydrogen undergoes combustion with oxygen according tothe following reaction:

H-H ,t0H-H +U=o ----------> Ht \H

0

H

AH = -488 kJ

The amount of energy needed to break the bonds in thereaction are shown below:

Bond Energy needed to break the bond/kJ

H-H 436

0=0 496

0-H X

What is the value of x?

H

A

C

?20

464

Halogen (X) Energy ol the X-X bond /kJ molr Beaction with hydrogen AH/kJ mol{

Fl uori ne F-F 158 Hr+ Fr+ fgp -273.3

Chlorine ct-ct242 Hr+ Clr- ZHCI -92.3

Bromine Br-Br 193 Hr+Brr- 2HBr -35.3

lodine l-l 1 51 Hr+lr+lX1 +25.4

B 222

D 710

()

()

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Wo Aqueous hydrochloric acid was added from a burette to25.0 cm3 of 1.50 mol/dm3 aqueous sodium hydroxide. The

temperature change was measured each time a portion ofthe acid was added.

The results of the experiment were plotted on a graph as

follows:

Temperature ('C)

Refer to the following t0 answer questions 42 and 43.

The table below gives information about some alkanes and alcohols.

U 42. What is the quantity of heat evolved from the combustionof 60 dm3 of propane, measured at room temperature and

pressure?

A s550 kJ B 5946 kJ

C 6660 kJ D 1.33 x 105 kJ ( )

Unit I3 I '.., .,,-),'i t'.;t. i-'-.; '::': :',t-..'-: 103

Based on the graph, which of the following statements is

true?

A Neutralisatron is complete at R.

B The sample of alkali has fully reacted at 0.C The reaction is exothermic from P to O, and becomes

endothermic when more acid is added.D The temperature increases from P to O because more heat

is absorbed in breaking of bonds than in the formation ofnew bonds. ( )

43. Which of the following deductions can be drawn from thedata given in the table?

I Each additional -CHr- group causes an increase ofapproximately 655 kJ of energy t0 the enthalpy change

of combustion of the alkanes.

ll The increase in enthalpy change of combustion due to

each additional *CH,- group for the alcohols is greater

than that observed for the alkanes.

lll Alcohols release less energy than the correspondingalkanes containing the same number of carbon atoms.

A I and ll only B I and lll only

C ll and lll only D l, ll and lll ( )

P

0

R

Total volume of acid added (cm3)

\V0

EXTENSION QUESTIONS

No. of carbon atoms Name of alkaneEnthalpy change of

combustion/kJ mol{ Name ol alcoholEnthalpy change of

combustion/kJ moli

1 It/etha ne -890 lt/ethanol -126

2 Ethane -1 560 Ethanol -1361

3 Propa ne -2220 Propanol -2021

4 Buta ne -2811 Butano I -2676

5 Pentane -3509 Pentanol -s329

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1O4 Theme 3 I Chenristry *f Reoctions

Refer to the following to answer questions 44 and 45.

Ethanoic acid reacts with sodium bicarbonate in an endothermicreaction.

Two experiments were conducted with a fixed volume of excess

aqueous ethanoic acid used in each case. The amount of sodium

carbonate used for the two experiments are as follows:

Experiment I 1 0.0 g of sodium bicarbonateExperiment ll 20.0 g of sodium bicarbonate

w44. Which of the following shows the graphs obtained for thetwo experiments?

A remperature

roomtemperature

Time

Temperature

temperature

Time

C Temperature

roomtemperature

I, II

Time

D Temperature

room

II

room

IItemperature

Time

U 45. ln the two experiments, which point on the graph shows thetime when the reaction first reaches completion?

Temperatul

Aroom

temperature

Time

The energy change for the reaction of magnesium withhydrochloric acid is shown by the following equation:

lVg(s) + 2HC/(aq) - I\4gC/,(aq) + HJg) AH = -460 kJ

Which of the following would result in the smallest rise intemperature?

A 24 g of magnesium added to 1.0 dm3 of 2.0 mol/dm3hydrochloric acid

B 48 g of magnesium added to 1.0 dm3 of 2.0 mol/dm3hydrochloric acid

C 48 g of magnesium added to 1.0 dm3 of 4.0 mol/dm3hydrochloric acid

D 144 g of magnesium added to 1.0 dm3 of 1.0 mol/dm3hydrochloric acid ( )

Befer to the following t0 answer questions 47 and 48.

The hydrated salt, sodium sulfate decahydrate (NarS0o .10H,0), wasamong the first to be studied as a material for the storage of solarenergy.

When sodium sulfate decahydrate is heated to 32'C and above,it releases its water of crystallisation and dissolves in it to forma solution. When the solution is cooled, the solid hydrated salt is

obtained again. This reversible reaction is as Jollows.

Na,SOo.1 0H,0(s) = Na,SOo(aq) + 1 0H,0(l)

NI4D47 What is true of this reversible reaction?

A The fonvard reaction releases more heatthan the reverse

reaction.

B The fonruard reaction absorbs less heat than the reverse

reaction.

C The forward reaction is endothermic and the reverse

reaction is exothermic.

D The forward reaction is exothermic and the reversereaction is endothermic { )

B

C

D

W*

B

I

)(

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NI4D48. The hydrated salt, when heated by solar energy, can thus be

used to store the energy for subsequent use.

Which of the following describe the storage of solar energyin the hydrated salt?

I There is a reversible change of state from solid to liquid.ll Temperature of the mixture rises as more and more solar

energy is absorbed.lll The solar energy absorbed is stored in the aqueous

solution and released to the surroundings when it cools.lV Temperature of the mixture remains the same as more and

more solar energy is absorbed until the fonvard reactionis complete.

A I and lV B ll and lllC l, lland lll D l,llland lV (

)

Unit l3 I tnerSy lrorrr Cherniccls 105

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