Energetics SQ

HKDSE Chemistry A Modern View Part VIII Chemical Reactions and Energy

©Aristo Educational Press Ltd. 2010 537

Structured Questions

Chemistry: Chapter 35 Energy changes in chemical reactions

Combined Science (Chemistry Part): Chapter 33 Energy changes in chemical

reactions

Section 35.1 (Section 33.1)

1

A reaction between zinc granules and hydrochloric acid is carried out in a stoppered

flask, i.e. the volume remains constant.

(a) (i) Write an equation for the reaction.

(ii) During the reaction, how does the pressure change? Explain briefly.

(iii) State the relationship between heat change at constant volume and change

in internal energy.

(b) If the reaction is carried out in an open system, i.e. open to the atmosphere,

(i) how does the pressure change?

(ii) the change in internal energy is not equal to heat change. Explain briefly.

(c) If the amount of zinc granules used is halved, what will be the effect on the heat

released?

[8M]


2

Neutralization is a common exothermic reaction.

(a) What is an exothermic reaction?

(b) State and explain the common feature of exothermic reactions.

(c) If 25.0 cm3 of 1 M HCl and 25.0 cm3 of 2 M HCl is added to 25.0 cm3 of 1 M

NaOH respectively, which reaction will give out more heat? Explain your answer

briefly.

[6M]

3

Natural gas is an energy source. The main component is methane.

(a) Write an equation for the combustion of methane.

(b) Is burning methane an exothermic or endothermic reaction?

(c) Explain your answer in part (b) in terms of forming and breaking of chemical

bonds.

[8M]



4

The combustion of hydrogen in a jar of oxygen is found to produce water only.

(a) State the types and number of bonds formed and bonds broken during the course

of the reaction respectively.

(b) Construct a labelled enthalpy level diagram for the reaction.

(c) The above reaction is applied to provide energy for launching of space shuttles.

State TWO advantages for the application.

[11M]

5

These days, hand-warmers are very popular and are easily available in stores. The

heat pad in the hand-warmer contains iron powder. The reaction involved is the

oxidation of iron powder. Generally, hand-warmers can remain warm for several

hours.

(a) State and explain whether the enthalpy change of the above reaction is positive

or negative.

(b) State whether the reaction is very rapid or not.

(c) Sam, an S4 student, claimed that the reaction of hand-warmers is similar to the

one of rusting. He tried to make some hand-warmers by using useless iron

metals.

Do you think Sam would succeed? Explain your answer.

(d) Another type of hand-warmer is made by the ‘supersaturated solution’ (the

crystallization-type). The reaction is triggered by snapping a small metal chip

buried inside the pad.

(i) From your knowledge, what is the benefit of this type of hand-warmer over

the iron-containing one?

(ii) Explain the above processes in terms of the Law of Conservation of Energy.

[9M]

6

Flameless ration heater (FRH) is a water-activated chemical heater which is used to

heat the food. It is commonly included with meals, Ready-to-Eat. The reaction

involved in FRH is similar to the process of rusting. In FRH, the metal used is

magnesium which is mixed with a small amount of iron.

(a) Is the reaction involved in FRH exothermic or endothermic?

(b) Why is magnesium used in FRH?

(c) What is the function of the small amount of iron?

(d) A small amount of water is added to activate the reaction. Write an equation for

the reaction involved.



(e) (i) Iron is initially oxidized, and then reduced back to its elemental form. Write

an equation for the reaction involved.

(ii) State the kind of reaction involved in (e)(i).

[9M]

7

The reaction between hydrogen sulphide and sulphur dioxide produces sulphur (S8)

and water. Sulphur (S8) is always found to deposit at the vents of volcanoes.

(a) Write an equation for the reaction between hydrogen sulphide and sulphur

dioxide.

(b) The enthalpy change of reaction is 1876 kJ mol1. Is the reaction endothermic

or exothermic?

(c) What kinds of bonds are broken during the reaction?

(d) What kinds of bonds are formed during the reaction?

(e) Construct an enthalpy level diagram for the reaction.

[10M]

8

Consider the following chemical equation:

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

(a) State whether the above reaction is endothermic or exothermic. Explain your

answer.

(b) Draw an enthalpy level diagram for the reaction.

(c) State whether the reverse reaction is endothermic or exothermic.

[6M]

9

Identify each of the following as an exothermic or endothermic process. Explain your

answer briefly.

(a) Evaporation of water

(b) Dissolving ammonium nitrate in water

(c) Dissociation of a hydrogen molecule: H2(g) 2H(g)

[6M]

10

A student made the following statement.

‘All products of chemical reactions have relatively lower enthalpies than the reactants.

Otherwise, the reaction cannot proceed.’

(a) Comment on the above statement.

(b) Give ONE example to support your answer in (a). Illustrate your answer with the

help of an enthalpy level diagram.

[8M]



11

(a) Give TWO sources of energy which do NOT depend directly or indirectly on

solar energy.

(b) A 2.0 cm long magnesium ribbon was added to 100.0 cm3 of 2.0 M hydrochloric

acid. All the magnesium reacted and the temperature of the acid increased by

20.0C.

(i) Predict the temperature rise if 1.0 cm long magnesium ribbon was used

instead.

(ii) What would be the volume of 2.0 M hydrochloric acid to which the 1.0 cm

long magnesium ribbon was added so as to produce a temperature rise of

20.0C?

(c) (i) Given that: Ca(s) + Cl2(g) CaCl2(s) ΔH = 800.0 kJ mol1

Calculate the amount of energy released when 4.0 g of calcium is burnt in

excess chlorine.

(ii) Hence, construct an enthalpy level diagram to show the enthalpy change of

the reaction in (c)(i) clearly.

[9M]

12

An instant cold pack is a device which is composed of two bags, one containing water

which is inside a second bag containing ammomium nitrate. This convenient instant

cold pack can replace crushed ice in treating sports injuries and reducing swelling or

pain.

(a) (i) Explain the working principle of instant cold pack.

(ii) Write a thermochemical equation for the related reaction.

(b) Suggest another salt which can also be employed as a chemical reagent in instant

cold pack.

(c) How can we obtain a larger cooling effect?

[7M]


13

Methane, butane, and hydrogen can all be used as rocket fuels. The standard enthalpy

changes of combustion of these fuels are given below:

ΔH c [Methane] = 890.0 kJ mol1

ΔH c [Butane] = 2877 kJ mol1

ΔH c [Hydrogen] = 286.0 kJ mol1

(a) Write balanced equations for the combustion of each fuel.

(b) Calculate the heat released of combustion for 1.0 g of each fuel.

(c) Hence, suggest which one is the best rocket fuel. Explain your answer.



[8M]

14

Given that:

H c [Methanol] = 843.0 kJ mol1

H c [Ethanol] = 1371 kJ mol1

H c [Propan-1-ol] = 2010 kJ mol1

(a) Which of the above alkanols releases the largest amount of energy per gram of

substance burnt? Show your calculation.

(b) Hence, which alkanol is the most appropriate for using as a fuel? Explain your

answer.

[6M]

15

People usually use instant hot packs for keeping warm and treating muscular fatigue.

(a) Suggest a common salt used as a chemical reagent in instant hot packs.

(b) Hence, explain the working principle of instant hot packs.

(c) Write a thermochemical equation for the standard enthalpy change of solution of

the common salt suggested in part (a). You are required to use x to represent the

value of the standard enthalpy change of solution.

[5M]


16

When 50.0 cm3 of 2.0 M hydrochloric acid was added to 50.0 cm3 of 2.0 M sodium

hydroxide solution in a simple calorimeter, there was a rise in temperature of 13.0C.

(Assume that the specific heat capacity and density of the solution are 4.2 J g1 K1

and 1.0 g cm3 respectively.)

(a) Calculate the enthalpy change of neutralization for one mole of water formed.

(b) What would be the temperature rise if 100.0 cm3 of 2.0 M hydrochloric acid

were added to 100.0 cm3 of 2.0 M sodium hydroxide solution? Explain your

answer with the help of calculations.

[9M]

17

The graph below shows how the temperature changes when 3.0 M NaOH was added

to 20.0 cm3 of 3.0 M HCl. The experiment was carried out in an expanded polystyrene

cup.



(a) Referring to the graph,

(i) what is the volume of NaOH needed to neutralize the acid completely?

(ii) what is the temperature change caused by the neutralization?

(b) Write an equation for the reaction. (State symbols should be clearly shown.)

(c) Define ‘standard enthalpy change of neutralization’.

(d) Calculate the standard enthalpy change of neutralization between sodium

hydroxide and hydrochloric acid.

(e) It was found that the experimental value calculated in (d) was less negative than

the theoretical value (57.1 kJ mol1). State TWO reasons for the difference.

(f) Suggest TWO improvements to obtain a more accurate result.

(Assume that the specific heat capacity and density of the reaction mixture are

4.2 J g1 K1 and 1.0 g cm3 respectively.)

[13M]

18

Ethanol is produced from glucose (C6H12O6) by fermentation. Carbon dioxide is the

only by-product. A fermentation reaction was carried out in a large plastic vessel

contained in a wooden box. The fermentation vessel was surrounded by insulating

material to reduce the heat loss generated by the fermentation reaction. 600.0 g of

glucose was added to water. The total volume of reaction mixture was 20.0 L. Some

yeast was added to start the fermentation.

(a) Write a balanced equation for this reaction.

(b) The temperature of reaction mixture in the fermentation vessel increased from

20.0C to 34.5C. Calculate the minimum amount of heat generated by the

reaction.

(c) Hence, determine the enthalpy change for the fermentation of glucose in kJ

mol1.


Tem

pera

ture

(C

)

Volume of alkali added (cm3)




[6M]

19

A 50.0 cm3 of 0.5 M potassium hydroxide was mixed with 50.0 cm3 of 0.5 M

hydrochloric acid in a glass beaker rapidly. The temperature of reaction mixture rose

from 19.6C to 23.1C.

(a) State whether the reaction is exothermic or endothermic.

(b) Write an equation for the reaction.

(c) Explain why the solutions were mixed rapidly.

(d) Calculate the enthalpy change of the reaction in kJ mol1.

(e) Identify the major source of error in the experimental procedure described above.

Suggest ONE way to minimize the error.

(f) The experiment was repeated with 0.51 M HCl instead of 0.5 M HCl. State and

explain what the temperature change would be.



[12M]

20

A student set up an experiment to determine the enthalpy change of solution of

ammonium nitrate. A known mass of ammonium nitrate was dissolved in a known

volume of water. The experimental set-up is shown below.

(a) The experimental value was not matched with the theoretical value of the

enthalpy change of solution of ammonium nitrate.

(i) Is the experimental value smaller or greater than the theoretical value?

Explain your answer briefly.

(ii) Give TWO suggestions to the experimental set-up to improve the

experimental value.

(b) The student carried out another experiment in an improved set-up. It was found

that an excess of zinc powder was added to 50.0 cm3 of 0.1 M AgNO3. The

temperature of the solution rose from 21.1C to 25.4C. Determine the enthalpy

thermometer

known mass of

ammonium nitrate

glass beaker

known volume of

water



change of the reaction: Zn(s) + 2Ag+(aq) Zn2+(aq) + 2Ag(s)

(Assume that the specific heat capacity and density of the solution are 4.2 J g1

K1 and 1.0 g cm3 respectively.)

[7M]

21

An experiment was carried to determine the standard enthalpy change of combustion

of ethanol using the apparatus shown in the diagram.

The following data was recorded.

Initial mass of spirit burner = 221.4 g Initial temperature of water = 19.0C

Final mass of spirit burner = 219.1 g Final temperature of water = 59.0C

Mass of water in the beaker = 250.0 g Specific heat capacity of water = 4.2 J g1

K1

(a) Determine the enthalpy change of combustion of ethanol using the above data.

(b) It is found that the calculated value of the enthalpy change of combustion of

ethanol does not match with the theoretical value found in data books. Give ONE

reason for this.

(c) Suggest TWO amendments for the above set-up so as to improve the accuracy of

the experiment results.

[6M]

22

A 40.0 cm3 sample of 0.6 M nitric acid at 18.0C was mixed with 40.0 cm3 of 0.6 M

barium hydroxide at 18.0C in a simple calorimeter. The highest temperature recorded

after mixing was 22.0C.

(a) Write an equation for the reaction.

(b) Determine the standard enthalpy change of neutralization.

(c) Hence, write a thermochemical equation for the standard enthalpy change of

neutralization.

(d) Would you expect the calculated value be less or more negative than the

theoretical value? Give TWO reasons for your answer.


ethanol

water

beaker




[10M]

23

A 30.0 cm3 sample of 1.0 M hydrochloric acid at 20.3C was mixed with 50.0 cm3 of

0.50 M sodium hydroxide solution at 20.3C in a simple calorimeter. The highest

temperature recorded after mixing was 24.4C.

(Assume that the specific heat capacity and density of the reaction mixture are the

same as those of water i.e. 4.2 J g1 K1 and 1.0 g cm3 respectively.)

(a) Write an equation for the reaction.

(b) Calculate the standard enthalpy change of neutralization between hydrochloric

acid and sodium hydroxide.

(c) The theoretical value of the standard enthalpy change of neutralization for the

above reaction is 57.1 kJ mol1. Is the calculated value obtained in (b) smaller

than, greater than or equal to the theoretical value? Explain your answer briefly.

(d) If hydrochloric acid in the above reaction is replaced with ethanoic acid with

equal volume and molarity, would you expect any differences in the value

obtained in (b)? Explain your answer briefly.

[10M]

24

Jason added some zinc granules to dilute hydrochloric acid in a beaker. The

temperature of the solution before and after adding zinc granules were 25.4C and

27.2C respectively.

(a) Write an equation for the reaction. (State symbols should be clearly shown.)

(b) State and explain whether the reaction is exothermic or endothermic.

(c) Predict the temperature change if zinc granules are replaced with magnesium

ribbon in the reaction. Explain your answer briefly.

[7M]

25

A student set up an experiment to determine the enthalpy change of neutralization

between sodium hydroxide and hydrochloric acid. The set-up is shown below.

The student found that 40.0 cm3 of 0.6 M sodium hydroxide required 30.0 cm3 of

hydrochloric acid to achieve the maximum temperature rise of the mixture. The

temperature change was 4.3C.



(a) Suggest TWO improvements for the above set-up to achieve more accurate

result.

(b) Calculate the enthalpy change of neutralization between HCl and NaOH with the

help of information given.

(Assume that the specific heat capacity and density of the solution are 4.2 J g1


(c) What would happen to the temperature of solution when the addition of HCl is

continued beyond 30.0 cm3?

[6M]

26

The table below shows data for the four organic compounds methanal, ethanal,

methanol and ethanol.

Organic compound ΔH /c kJ mol1

Methanal 517.5

Ethanal 1063.5

Methanol 715.0

Ethanol 1371.0

(a) Write a thermochemical equation for the standard enthalpy change of

combustion of methanal.

(b) The complete combustion of 2.0 g of one of the above organic compounds

releases nearly 60.0 kJ of heat energy. What is the organic compound?

(c) In terms of the energy released when burnt, which organic compound is the best

for use as fuel in motor cars? Explain your answer briefly.

[13M]

27

An experiment was carried out to determine the enthalpy change of combustion of

propan-1-ol. The experimental set-up was shown below.

HCl(aq) + NaOH(aq)

thermometer

beaker



(a) Write the equation for the complete combustion of propan-1-ol.

(b) Give a suggestion to the above set-up so as to minimize heat loss to the

surroundings.

(c) Burning 2.88 g of propan-1-ol caused the temperature of 500.0 cm3 of water to

rise by 46.0C. Calculate the enthalpy change of combustion of propan-1-ol.

(d) Some black substances were found under the beaker after the experiment.

(i) What were the black substances?

(ii) Suggest how they were formed.

(e) The enthalpy change obtained in (c) cannot be called as ‘standard enthalpy

change’. Explain why.

(Assume that the specific heat capacity and the density of water are 4.2 J g1 K1

and 1.0 g cm3 respectively.)

[7M]

28

In an experiment, 20.0 cm3 of 1.0 M HCl solution at 25C was added to 10.0 cm3 of

1.0 M Na2CO3 solution at 25C in an expanded polystyrene cup with stirring. The

highest temperature recorded after mixing was 40.0C.

(a) Write the equation for the reaction between HCl(aq) and Na2CO3(aq).

(b) State TWO observations during the experiment.

(c) Calculate the enthalpy change of the reaction.

(d) The theoretical value of the enthalpy change of the reaction is greater than the

calculated value. Give TWO reasons for the difference.

(Assume that the specific heat capacity and the density of the reaction mixture

are 4.2 J g1 K1 and 1.0 g cm3 respectively.)

[9M]

29

A spirit burner containing 5.0 g sample of propanol (C3H7OH) is used to heat up a

beaker of water (0.65 L) from 25.0C to 58.0C. The standard enthalpy change of

combustion of propanol is 2010 kJ mol1.

(Assume that the density and specific heat capacity of water are 1.0 g cm3 and 4.2 J

g1 K1 respectively.)

propan-1-ol

water (500.0 cm3)

a beaker



(a) State ONE assumption in the experiment.

(b) Calculate the actual mass of propanol used to heat the water.

(c) Calculate the percentage purity of propanol in the sample. Hence, comment on

the effect of the percentage purity on the experimental result.

[8M]

30

A 50.0 cm3 sample of 1.20 M sodium hydroxide at 25.0C was mixed with 40.0 cm3

of 0.55 M sulphuric acid at 25.0C in a simple calorimeter. The highest temperature

recorded after mixing was 50.0C.

(Assume that the specific heat capacity and the density of the resultant solution are


(a) Determine the type of reaction and give the definition of the standard enthalpy

change of the reaction.

(b) State ONE assumption in the experiment.

(c) Calculate the standard enthalpy change of the reaction.

[9M]

Sections 37.137.2 (Sections 35.135.2)

31

Given that:

ΔH c [H2(g)] = 286.0 kJ mol1

ΔH c [C(s)] = 394.0 kJ mol1

ΔH f [C6H12O6(s)] = 1286 kJ mol1

(a) Define ‘standard enthalpy change of combustion’.

(b) Write an equation for the standard enthalpy change of combustion of glucose.

(c) Construct an enthalpy change cycle to determine the enthalpy change involved in

the combustion of 1.0 g of glucose.

[10M]

32

Given that:

ΔH f [H2O(l)] = 286.0 kJ mol1

ΔH f [CO2(g)] = 395.0 kJ mol1

ΔH f [CH3COCH3(l)] = 250.0 kJ mol1

(a) Write an equation for the standard enthalpy change of combustion of

CH3COCH3(l).

(b) Construct an enthalpy change cycle to determine the standard enthalpy change of

combustion of CH3COCH3(l).

[6M]



33

Consider the following reactions:

I N2(g) + O2(g) 2NO(g) ΔH 1 = +180.0 kJ mol1

II 2NO(g) + O2(g) 2NO2(g) ΔH 2 = 114.0 kJ mol1

(a) Explain why only reaction I is endothermic in nature.


the formation of N2O5(g).

(c) It is known that N 2 O 5 (g) will decompose to other gases under

suitable conditions. Suggest a use for N 2 O 5 (g).

[9M]

34

Brian wants to determine the standard enthalpy change of combustion of butane. He

writes an equation for the combustion of butane as follows:

2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(l)

(a) Comment on the above equation. Write a correct equation for Brian.

(b) In order to determine the enthalpy change, a beaker containing 500.0 cm3 water

is placed under the combustion chamber using butane as the fuel.

Brian suggests that the beaker should be covered with a lid to obtain a more

accurate temperature change. Comment on his suggestion.

(c) The temperature difference was found to be 40.0C and the mass of butane used

was 4.1 g. Calculate the standard enthalpy change of combustion of butane.

(Assume that the specific heat capacity and density of water are 4.2 J g1 K1 and

1.0 g cm3 respectively.)

[7M]

35

Given that:

ΔH f [AgClO3(s)] = 30.0 kJ mol1

ΔH f [AgCl(s)] = 127.0 kJ mol1

ΔH f [ClO2(g)] = +102.0 kJ mol1

(a) Write a thermochemical equation for the formation of 1 mole of ClO2(g).

(b) Construct an enthalpy change cycle to determine the standard enthalpy change

for the following reaction:

2AgClO3(s) + Cl2(g) 2AgCl(s) + 2ClO2(g) + O2(g)

[7M]

2NO2(g) + 2

1O2(g) N2O5(g) ΔH 3 = 55.0 kJ mol1 III



36

Given that:

ΔH f [C8H18(l)] = 250.0 kJ mol1

ΔH f [H2O(l)] = 286.0 kJ mol1

ΔH f [CO2(g)] = 394.0 kJ mol1

(a) Write an equation for the complete combustion (only carbon dioxide and water

are formed) of 1 mole of C8H18(l).


combustion of C8H18(l).

(c) Determine the enthalpy change involved in the combustion of 0.2 kg of C8H18(l).

[9M]

37

The following steps show the process of manufacturing sulphuric acid.

Step 1: S(s) + O2(g) SO2(g)

Step 2: SO2(g) + O2(g) SO3(g)

Step 3: SO3(g) + H2O(l) H2SO4(l)

Given that:

ΔH f [H2O(l)] = 286.0 kJ mol1

ΔH f [SO2(g)] = 297.0 kJ mol1

ΔH f [SO3(g)] = 396.0 kJ mol1

ΔH f [H2SO4(l)] = 814.0 kJ mol1

(a) Balance the chemical equation in step 2.


reaction in step 2.

(c) Construct an enthalpy change cycle to determine the standard enthalpy change of

reaction in step 3.

[11M]

38

Iron can be extracted from its ore haematite by heating iron(III) oxide in haematite

with carbon.

Given that:

C(s) + O2(g) CO2(g) ΔH 1 = 393.5 kJ mol1

4Fe(s) + 3O2(g) 2Fe2O3(s) ΔH 2 = 1648 kJ mol1

(a) Write a balanced chemical equation for the reaction between iron(III) oxide and

carbon.


reaction in (a).



(c) State whether the reaction in (a) is an exothermic or endothermic reaction.

[7M]

39

A thermite reaction is a reaction between aluminium metal and iron(III) oxide. The

reaction is highly exothermic and is commonly applied in welding e.g. join rail tracks.

The equation for thermite reaction is shown below:

2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(s)

Given that:

ΔH f [Fe2O3(s)] = 824.0 kJ mol1

ΔH f [Al2O3(s)] = 1676 kJ mol1

The specific heat capacity of Al2O3(s) = 79.0 J mol1 K1

The specific heat capacity of Fe(s) = 25.1 J mol1 K1

(Assume that the specific heat capacities of Fe2O3(s) and Al(s) are negligible.)

(a) Construct an enthalpy change cycle to determine the standard enthalpy change of

the thermite reaction.

(b) Determine the energy required to raise the product mixture by 1 K if 1 mol of

Fe2O3 is reacted.

(c) What will be the final temperature of the product mixture if 1 mol of Fe2O3 is

reacted under standard conditions?

(d) Iron has a melting point of 1808 K. It is a major component of steel. Explain

why the thermite reaction is suitable for welding steel with reference to the

answer in (c).

[10M]

40

Given that:

CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l) ΔH 1 = 364.0 kJ mol1

CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l) ΔH 2 = 192.0 kJ mol1

(a) Write a chemical equation for decomposition of calcium carbonate.


decomposition of calcium carbonate by using the data given above.

[6M]

41

Carbon disulphide (CS2) is a highly flammable liquid. It burns in air, which is

analogous to the combustion of methane.

(a) What is the standard enthalpy change of formation of a substance?

(b) Write a chemical equation for the combustion of carbon disulphide.

(c) Given that:



ΔH f [CO2(g)] = 395.0 kJ mol1

ΔH f [SO2(g)] = 297.0 kJ mol1

ΔH c [CS2(l)] = 1076 kJ mol1

Construct an enthalpy change cycle to determine the standard enthalpy change of

formation of carbon disulphide.

[9M]

42

The enthalpy changes of formation of substances cannot be determined directly by the

simple calorimetric methods. However, we can determine them indirectly by applying

Hess’s Law.

(a) Define ‘standard enthalpy change of formation’.

(b) State Hess’s Law.

(c) The following equation shows the formation of ethylene, C2H4.

2C(s) + 2H2(g) C2H4(g)

(i) Construct an enthalpy change cycle that can determine the standard

enthalpy change of formation of ethylene.

(ii) Use the following data to calculate the standard enthalpy change of

formation of ethylene.

[9M]

43

experiment to determine the enthalpy change of combustion of sucrose

crose = 2.2 g

er = 25.5C

g1 K1

tion of sucrose.

An

(C12H22O11(s)) was carried out by using a bomb calorimeter. The following data was

recorded.

Mass of su

Mass of water = 550.0 g

Initial temperature of wat

Final temperature of water = 41.2C

Specific heat capacity of water = 4.2 J

(a) Define ‘standard enthalpy change of combustion’.

(b) Write a chemical equation for the complete combus

C(s) + O2(g) CO2(g) ΔH 1 = 395.0 kJ mol1

H2(g) + 2

1O2(g) H2O(l) ΔH 2 = 286.0 kJ mol1

C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(l) ΔH 3 = 1409 kJ mol1



(c) Determine the standard enthalpy change of combustion of sucrose using the

(d) ical equation for the formation of sucrose.

4.0 kJ mol1 and 286.0

[12M]

44

bardier beetles are beetles commonly found in North America and Europe. When

C6H4O2(aq) + H2(g) ΔH 1 = +177.0 kJ mol1

above data.

Write a chem

(e) Given that the ΔH f [CO2(g)] and ΔH f [H2O(l)] are 39

kJ mol1 respectively. Construct an enthalpy change cycle to determine the

standard enthalpy change of formation of sucrose.

Bom

being attacked, the beetle defends itself by ejecting a hot spray from its abdomen. The

beetle contains two chemicals, hydroquinone (C6H4(OH)2) and hydrogen peroxide

(H2O2) in separate reservoirs in its abdomen. The two chemicals react and produce

quinone (C6H4O2) and water.

Given that:

C6H4(OH)2(aq)

2H2O2(aq) 2H2O(l) + O2(g) ΔH 2 = 190.0 kJ mol1

(a) Write a chemical equation for the reaction between hydroquinone and hydrogen

(b) n enthalpy change cycle to determine the standard enthalpy change of

(c)

[10M]

5

Define the term ‘standard enthalpy change of formation’.

of all elements in their

(c) es, rhombic sulphur and monoclinic sulphur. Both of

) ΔH 1 = 297.16 kJ mol1

or endoth er.

nge for

peroxide.

Construct a

reaction between hydroquinone and hydrogen peroxide.

Hence, suggest the origin of hot spray ejected by the beetles.

4

(a)

(b) Explain why the standard enthalpy changes of formation

standard states are zero.

Sulphur has two allotrop

them burn to form sulphur dioxide.

S(s) (monoclinic) + O2(g) SO2(g

S(s) (rhombic) + O2(g) SO2(g) ΔH 2 = 296.83 kJ mol1

(i) Are the reactions exothermic ermic? Explain your answ

(ii) Construct an enthalpy change cycle to determine the enthalpy cha

reaction: S(rhombic) S(monoclinic)

H (g) + 22

1O2 2(g) H O(l) ΔH = 286.0 kJ mol3

1



[9M]

46

that: +(aq) + e ΔH 1 = +105.56 kJ mol1

(a) Given

Ag(s) Ag

Construct an enthalpy change cycle to determine the standard enthalpy change of +

(b) rmine the standard enthalpy change of 3 3

(c)

47

(s)] = 1669 kJ mol1 1

enthalpy change cycle to determine the

48

reaction: Ag (aq) + Cl (aq) AgCl(s).

An experiment was carried out to dete

reaction between AgNO3 and HCl. 50.0 cm of 0.1 M AgNO3 and 50.0 cm of

0.1 M HCl are mixed. The temperature rose by 2.8C.

(i) Write an equation for the reaction.

ic? (ii) Is the reaction endothermic or exotherm

(iii) Determine the standard enthalpy change of reaction.

(iv) Suggest an apparatus used to determine the standard enthalpy change of

reaction.

Why the enthalpy changes of formation cannot be determined by using simple

calorimetric method? Explain briefly with the aid of an example.

(Assume that the specific heat capacity and density of the reaction mixture are 1 1 34.2 J g K and 1.0 g cm respectively.)

[15M]

Given that:

ΔH f [Al2O3

ΔH f [Fe2O3(s)] = 822.0 kJ mol

(a) By using the given data, construct an

standard enthalpy change of the reaction: Fe2O3(s) + 2Al(s) 2Fe(s) +

Al2O3(s).

(b) State whether the reaction is exothermic or endothermic.

(c) Draw an enthalpy level diagram to represent the reaction.

[9M]

2

1Cl2(g) + e Cl(aq) ΔH 2 = 167.15 kJ mol1

Ag(s) + 2

1Cl2(g) AgCl(s) ΔH 3 = 127.07 kJ mol1



Burning hexane in excess oxygen can form carbon dioxide and water as the only

an equation for the complete combustion of hexane.

lpy change cycle to

6 14 2 2

products.

(a) Write

(b) Use the data in the table below to construct an entha

determine the standard enthalpy change of combustion of hexane.

Substance C H (g) CO (g) H O(l)

ΔH f/kJ mol1 167.0 394.0 286.0

[7M]

49

en that:

) + H2O(g) O2(g) + 2HCl(g) ΔH = 46.0 kJ mol1

1

e formation of OCl2 from O2 and Cl2. (State

(b) to determine the standard enthalpy change of

[7M]

50

uid hydrazine, N2H4, is sometimes used as a rocket propellant.

rom its elements.

Giv

(1) OCl2(g

(2) ΔH f [HCl(g)] = 92.3 kJ mol1

(3) ΔH f [H2O(g)] = 242.0 kJ mol

(a) Write an equation to represent th

symbols should be clearly shown.)

Construct an enthalpy change cycle

formation of OCl2(g).

Liq

(a) (i) Write an equation for the formation of liquid hydrazine f

(ii) Given that:

2

1N (g) + O2 2(g) NO2(g) ΔH1

H2(g) + 2

1O2(g) H2O(g) ΔH2

N2H4(l) + 3O2(g) 2NO2(g) + 2H2O(g) 3ΔH

Using the above data and the enthalpy change c o determine the ΔH f

(b) (i) ogen peroxide to

(ii) ata in the table below and the enthalpy change cycle to calculate

ycle t

[N2H4(l)] in terms of ΔH1, ΔH2 and ΔH3.

In a rocket, liquid hydrazine is reacted with liquid hydr

produce nitrogen and water vapour. Write a balanced equation for this

reaction.

Use the d

the standard enthalpy change of reaction in (b)(i).

Substance N2H4(l) H2O2(l) H2O(g)



ΔH f /kJ mol1 50.6 187.8 285.8

[11M]

51

en that:

)] = 74.0 kJ mol1

Giv

ΔH f [CH4(g

C(s) C(g) ΔH = +717.0 kJ mol1

(a) Write a thermochemical equation for the formation of CH4(g).

enthalpy change

CH4(g) C(g) + 4H(g)

[7M]

52

itrogen tetroxide (N2O4) is one of the most important rocket propellants. It is

en hydrazine and dinitrogen

(b) ΔH f of N2H4, N2O4, H2O and N2 are 50.6 kJ mol1, 9.16 kJ mol1, 1

(c) .08 moles of N2H4 is reacted with a stoichiometric

[5M]

53

pane is used as fuel for cooking. Burning propane is an exothermic reaction. The

energy. Calculate the

(c) a hazard warning label that should be put on the container carrying

(d) as is formed by reacting carbon with hydrogen.

(b) Construct an enthalpy change cycle to determine the standard

for the following reaction:

Din

commonly used in combination of a hydrazine(N2H4)-based rocket fuel. Hydrazine

reacts with dinitrogen tetroxide to produce N2 and H2O.

(a) Write a balanced equation for the reaction betwe

tetroxide.

Given that

241.8 kJ mol and 0 kJ mol1 respectively. Calculate the standard enthalpy

change for the reaction in (a).

Calculate the heat released if 0

amount of N2O4.

Pro

standard enthalpy change of combustion of propane is 2220 kJ mol1.

(a) Write an equation for the complete combustion of propane.

(b) Boiling 1 L of water from 20.0C requires 350.0 kJ of heat

mass of propane required for combustion so as to produce such amount of heat

energy.

Suggest

propane.

Propane g

2

1H2 (g) (g) H ΔH = +218.0 kJ mol1


©Aristo Educational Press Ltd. 2010

(i) Write an equation for formation of propane gas.

557

.0 kJ mol1 and 285.0 kJ

(iii) n process of propane exothermic or endothermic? Explain

the specific heat capacity and the density of water are 4.2 J g1

[9M]

54

thanol is an alternative fuel for internal combustion engines. It is used as fuels in

ion for the complete combustion of methanol.

7 kJ mol1,

(c) he combustion of 50.0 g of liquid

[6M]

(ii) Given that ΔH c of C(s) and H2(g) are 394

mol1 respectively. Determine the standard enthalpy change of formation of

propane gas.

Is the formatio

your answer.

(Assume that


Me

vehicles e.g. racing cars. It burns with sufficient oxygen in internal combustion

engines to provide energy.

(a) Write a balanced equat

(b) Given that ΔH f of methanol, carbon dioxide and water are 238.

393.5 kJ mol1 and 241.8 kJ mol1 respectively. Calculate the standard

enthalpy change of combustion of methanol.

Determine the enthalpy change involved in t

methanol.

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