## Energy changes in chemical and physical processes Questions

1. 6g of Potassium nitrate solid was added to 120cm3 of water in a plastic beaker.

The mixture was stirred gently and the following results were obtained.

Initial temperature = 21.5oC

Final temperature = 17.0 oC

(a) Calculate the enthalpy change for the reaction

(Density =1g/cm3, C= 4.2jg-1K-1)

(b) Calculate the molar enthalpy change for the dissolution of potassium nitrate

(K=39, N= 14, O =16)

2.  (a) The heat of combustion of ethanol, C2H5OH is 1370KJ/mole.

(i) What is meant by heat of combustion?

(ii) Calculate the heating value of ethanol

(H = 1.0, C = 12.0, O = 16.0)

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3.  Use the information below to answer the questions that follow:-

Ca(s) + ½ O2(g) CaO(s)
H =-635KJ/mol

C(s) + O2(g)   CO2(g)
H= -394KJ/mol

Ca(s) + C(s) + 3/2O2(g) CaCO3
H = -1207KJ/mol

Calculate the enthalpy change for the reaction:

Ca(s) + CO2(g) CaCO3(s)

4.  0.92g of ethanol were found to burn in excess air producing a temperature rise of 32.5ºC

in 200cm3 of water.

C=12.0 H=1.0 O=16.0

Density of water 1g/cm3

Specific heat capacity of water 4.2kj kg-1k-1

a) Write the equation for combustion of ethanol

b) Determine the molar heat of combustion of ethanol

5.  Study the information in the following table and answer the questions that follow. The letters

do not represent the actual chemical symbols of the elements.

 ELEMENT U V W X Y Z NUMBER OF PROTONS 18 20 6 16 19 17 NUMBER OF NEUTRONS 22 20 8 16 20 20

Which of the above elements are:

(i) Likely to be radioactive?

(ii) Able to form a compound with the highest ionic character?

6.  The diagram below shows energy levels for the reaction

½ H2(g) + ½ F2(g)  HF(g)

(a) Work out the activation energy for the reaction

(b) Calculate the heat of formation of HF

(c) Is the reaction endothermic or exothermic?

7.  Using the heats of combustion of the following substances, calculate the heat of formation

of ethanol

C(s) + O2 (g) CO2 (g) H = -393KJmol-1

H2 (g) + ½ O2 (g) H2O(l)H = -286KJmol-1

CH3CH2OH(l) + O2 (g)  2CO2 (g) + 3H2O (l) H = 1386KJmol-1

8.  Nitrogen and hydrogen react reversibly according to the equation:-

N2(g) + 3H2(g) 2NH3(g); H = -92kjmol-1

The energy level diagram for the above reaction is shown below:-

(a) How would the yield of ammonia be affected by:

(i) A decrease in temperature

(ii) An increase in pressure

(b) How does a catalyst affect reversible reaction already in equilibrium?

(c) On the above diagram, sketch the energy level diagram that would be obtained when

iron catalyst is added to the reaction

9.  Study the table below and answer the questions that follow

Bond type
bond energy kJmol
1

C-C 346

C = C 610

C-H 413

C-Br 280

Br-Br 193

a) Calculate the enthalpy change for the following reaction

C2H4(g) + Br2(g) C2H4Br2(g)

b) Name the type of reaction that took place in (a) above

1 mark

10.  Bond energies for some bonds are tabulated below:-

 BOND BOND ENERGY KJ/mol H – H 436 C = C 610 C- H 410 C – C 345

Use the bond energies to estimate the enthalpy for the reaction

C2H4(g) + H2(g)    C2H6(g)

11.  The able shows the results obtained when 20.2g of potassium nitrate was added in

50cm3 of water.

 Time in (min) 0 0.3 1 1.3 2 2.3 3 3.3 4 Temperature (oC ) 25 25 25 25 17 17 20 20 20

(i) Draw the graph of temperature against time

(ii) Using the graph, determine the temperature change

(iii) Calculate the heat change

(iv) Find the molar heat of solution of potassium nitrate

12.  When 1.6g of ammonium nitrate were dissolved in 100cm3 of water, the temperature

dropped by 6ºC. Calculate its enthalpy change. (Density of water = 1g/cm3,

specific heat capacity is 4.2kJ kg-1K-1)

13.  Sodium hydrogen carbonate was strongly heated.

a) Write an equation for the reaction

b) The grid below shows part of the periodic table. Use it to answer the questions that follow. The

letters are not the actual symbols.

i) Write the equation for the reaction that occurs between elements L and D

ii) The oxide of G reacts with both hydrochloric acid and sodium hydroxide. What is the nature of

the oxide of G?

iii) Explain why elements H has a higher boiling points than element D.

iv) State one use of element E

v) Compare and explain the atomic radius of B and C

vi) 11.5g of L was completely burnt in oxygen .Calculate the volume of gas that was used.

(L = 23, molar gas volume at room temperature is 24dm3)

14.  A student has been provided with sodium hydroxide solution of 2M and hydrobromic acid

of 4M. He was asked to investigate the equation for the reaction between these two substances and

hence determine the molar enthalpy of neutralization. He carried out the reaction and obtained the

following results:-

 Vol. of 4M Hydrobromic acid added to 20cm3 of 2M NaOH Temperature of the mixture (oC) 4.0 26.8 6.0 30.0 8.0 33.2 10.0 36.0 12.0 35.2 14.0 34.4 20.0 30.8

(a) Draw a graph of the temperature of the mixture (vertical axis against the volume of the

(b) Using the graph estimate the temperature of the mixture when 17cm3 of the acid was added

(c) Both solutions were at room temperature at the start of the experiment. Use your graph to

estimate the room temperature (½mk)

(d) What is the significance of the highest temperature of the solution mixture? *

(e) The temperature of the mixture increased during the first additions of the acid. Why did the

temperature increase? *

(f) Suggest a reason why the temperature decreased during the latter part of the experiment

(g) Use your graph to determine the volume of 4M Hydrobromic acid which just neutralize

20cm3 of 2M NaOH *

(h) How many moles of Hydrobromic acid are present in your answer in (g) above?  *

(i) How many moles of NaOH are present in 20cm3 of 2M of NaOH solution? *

(j) Use your answers in (h) and (i) above to write an equation of the reaction taking place

in the experiment. Explain clearly how you have used your answers (1½mks)

(k) Determine the molar enthalpy of neutralization of hydrobromic acid  (1½mks)

15.  (a) The following results were obtained in an experiment to determine the enthalpy of solution

of sodium hydroxide

Mass of plastic beaker = 8.0g

Mass of plastic beaker + distilled water = 108.15g

Mass of plastic beaker + distilled water + sodium hydroxide = 114.35g

The table below shows the temperature at fixed times after mixing

 Time/seconds 0 30 60 90 120 150 180 210 Temperature ( oC) 15 21 29 28 27 26 26 25

(i) Plot a graph of temperature (y-axis) against time (x-axis)

(ii) From your graph, determine the maximum temperature attained

(iii) Determine the temperature change of the reaction

(iv) Calculate the number of moles of sodium hydroxide used in the experiment

(Na = 11, H = 1, O = 16)

(v) Use your results to determine the molar enthalpy solution of sodium hydroxide. (Density of

solution is 1g cm-3 , specific heat capacity of solution = 4.18 KJ-1K-1)

(b) Below is an energy level diagram of the exothermic reaction

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890KJ

follow

(b) (i) Which H values will have negative sign?

(ii) What physical change is being represented where enthalpy change H4 is involved?  ( ½mk)

(iii) In terms of H1, H2, H3 and H4, give the overall enthalpy change for the reaction:-

H2(g) + ½ O2(g)   H2O(l)

(iv) Is the reaction in (iii) above exothermic or endothermic?

16.  (I) Study the graph below and answer the questions which follow:

(a) Distinguish between molar latent heat of fusion and molar latent heat of vaporization

(b) (i) Explain the changes occurring between points

BC ………………………………………… CD ………………………

(ii) In an experiment to determine molar enthalpy of neutralization of hydrochloric acid using

potassium hydroxide, the data below was obtained. The concentration of potassium hydroxide

used was 0.5M

 Volume of 0.5M KOH (cm3) 0 5 10 15 20 25 30 35 Total volume of acid + Base 20 25 30 35 40 45 50 55 Temperature (oC) 24 26 27 28 29 29 28 27

(i) Plot a graph of temperature (y-axis) against volume of potassium hydroxide used

(ii) From your graph:

(a) Determine the temperature change

(b) Find the volume of potassium hydroxide which completely neutralized 20cm3 of the acid

(iii) Calculate the heat change for the reaction (C = 4.2Jg-1K-1 density of solution = 1g/dm3)

(iv) Calculate the molar enthalpy of neutralization of hydrochloric acid with potassium hydroxide

17.   A typical electrolysis cell uses a current of 40,000 amperes. Calculate the mass

(in Kg of aluminium produced in one hour). (Al = 27) (Faraday = 96500Coloumbs )

18.  (a) Biogas is a mixture of mainly Carbon (IV) Oxide and methane.

(i) Give a reason why biogas can be used as a fuel

(ii) Other than fractional distillation, describe a method that can be used to determine the

percentage of methane in biogas

19.   Consider the following equilibrium reaction.

H2(g) + Cl2(g) 2HCl(g) H= -74.4KJ

a) State and explain the effect of formation of hydrogen chloride if pressure was increased

in the equation above

20.  Turning of fossil fuels has adverse environmental effects:-

a) Name two pollutants from the burning of petroleum products

b) Give one precaution taken to minimise the pollution by fossil fuels

21.  (a) Define molar heat of neutralization

(b) The rise in temperature when 50cm3 of sodium hydroxide is reacted with two acids is given

in the table below:-

 Acid 50cm3 of HCl 50cm of Oxalic acid Temp rise (oC) 7 4

(i) Explain the difference in the temperature.

22.  Calculate the latent heat of vaporization of water

H2O(l) H2O(g)

Given the following thermo chemical equations:-

H2(g) + ½O2(g)   H2O(g)
H= -242KJ/Mol

H2(g) + ½O2(g)   H2O(l)
H= -286KJ/Mol

23.  (a) Define the term fuel

(b) State four reasons why wood fuel is chosen for domestic cooking

24. The setup bellow was used to investigate the changes that take place when sodium hydroxide

pellets dissolve in water.

a) Why is a plastic beaker used instead of a metallic beaker?

b) State and explain the observations made in the above reaction

25.  (a) What is a fuel?  (1mark)

(b)Other than the cost, state two other factors to consider when choosing a fuel.

26. The equation below represents changes in the physical state of ions metal:

Fe(s)   Fe(l) DH= + 15.4kjmol-1

Fe(l) Fe(g) DH=+354kjmol-1

a) Calculate the amount of heat energy required to change 10kg of solid iron to gaseous iron

Fe = 56

b) Iodine can react with chlorine as shown below:-

I2(g) + Cl(g) 2lcl(s) DH= -68kJ

Determine the molar enthalpy change for this reaction

c) Draw an energy level diagram for the reaction in (b) above

27.  Study the diagram below and answer the questions that follow:

a) What do ∆H1 and ∆H2 represent?

∆H1 …………………………………………………………………….

∆H2 ……………………………………………………………………..

b) Write an expression to show the relationship between ∆H1, ∆H2 and ∆H3.

## Energy changes in chemical and physical processes Answers

1. (a) H = 120 x 4.2 x 4.5  ( ½mk)

1000

= + 2.268KJ (½mk)

(b) RFM of KNO3 = 39 + 14 + 48 = 101

6g 2.268KJ

101g 101 X 2.268 (½mk)

6

= +38.178KJ mol -1 (½mk

2.  (i) Heat evolved when one mole of a substance is completely burnt in oxygen

(ii) RFM of C2H5OH = 46

Molar mass = 46g

Heating value = 1370 KJ

46g

= 29.78KJ/g (with units)

3.  Ca(q) + C(q) + 3/2 O2 (g)

4.  a) C2H6O(l) + 3O(g) ___________ 2CO2(g) + 3H2O

b) DH = MCDT

200 X 4.2 X 32.5 = -27.3Kj

1000

0.92g C2H6O __________ – 27.3Kj

46g ” __________ ?

46g X 27.3Kj = -1365Kj

0.92

DHC C2 HSO4 = -1365Kj mol

5.  i) U,V,Y,Z All the 4 or nay 3 exclusively correct penalize ½ mk if wrong answer

ii) YZ is/are included any 2 correct ½ mk

6.  (a) 611-389 = +222KJ

(b) H = +222 – (611 – 100)

= -289KJ

(c) Exothermic reaction

7.  2C(s) + 3H2(g) + ½ O2 (g) ∆ Hf CH3CH2OH(l)

2CO2(g) + 3H2(g)

2CO2(g) + 3H2O(l)

∆ Hf + ∆H3 = ∆H1 + ∆H2

∴∆Hf = ∆H1 + ∆H2∆H3
√½

= -393 x 2 + -286 x 3 +1386 √1

= -786 – 858 + 1386

= -1644 + 1386 √1

∆Hf  = -258 KJmol-1
√½

8. a) i) the yield of NH3 would be lowered √ ½ any supply of heat makes NH3 to decompose to

N2 and H2

ii)the yield of NH3 would be increased

b)a catalyst accelerate the rates of both forward and reverse reactions equally√ ½ . Equilibrium

position is not affected by a catalyst√ ½

c)

9.  a) Breaking of ‘C = C’ = +610 KJ

Breaking of ‘Br – Br’ = +193

803

Formation of 2C – Br = -560

Formation of c-c   +243 Kj

-346

– 103KJ√ 2 marks

b) Addition reaction/ halogenation

10.   H H

C = C + H – H H – C – C – H

H H H H

Bond breaking  Bond formation

4 C-H – 4×410 = 1640 6C – H  6×410

C = C – 1 x610 = 610 = 2460

H – H – 1×436 = 436 C – C – 3 45

2686 2805

H = 2686 – 2805

= -119 Kj/Mol

11.  (i) Graph

labeling -*TZM*

plotting – *TZM*

scale – *TZM*

line – *TZM*

total 5mks

(ii) Shown on the graph -*TZM*

(iii) Heat change = MCT

= 50 x 4.2 x 10.2

100

= 2.142kJ

(iv) RFM of KNO3 = 39 + 14 + 48

= 101

H = 2.142 x 101 = -10.71Kjmol-1

20.2

12. MCT = 100 X 4.2 X 6 = 2.52 Kj

1000

Moles of NH4NO3 = 1.6 = 0.02 moles

80

If 0.02 mol _________ 2.52 Kj

1 mol ______________ 1 X 2.52 = +126KJ/ mol

0.02

13.  a) 2 NaHCO3 (g) ________ Na2CO3(g) + H2O(1) + CO2(g)

b)  i) 2L(g) + D2(g) ________ 2LD(g)

ii) Amphoteric oxide

iii) Element H has a giant atomic structure with strong covalent bonds throughout its

structure while D has simple molecular structure with weak Vander wall forces (2 m)

– Used in florescent tubes  (Any two correct use)

v) C has a smaller atomic radius than B because it has stronger nuclear charge// more

number of protons which attract the outer energy level electrons more firmly  (2 mks)

vi) 4L(s) + O2(g) _________ 2 L2O(g)

Moles of L = 11.5 = 0.5 moles

23

Moles of O2 = 0.5 = 0.125 moles

4

Volume of O2 = 0.125 mol X 24 = 3 dm3

4L (s) + O2(g) __________ 2L2O(s)

If 4 x23g _______________ 24dm3

11.5g of L ________ 11.5 x 24 = 3dm3

4×23

14.  (a) Drawn on the graph  A = ½ mk

S = ½ mk

P = ½ mk

C = ½ mk

b) 32.5oC + 1 Read from the student’s correctly plotted graph.

c) 20oC + O.5 Line is extrapolated downwards from the student’s correct graph.

d) It is end point/ complete neutralization.

e) The reaction is exothermic hence as reaction proceeded more heat was produced.

f) Reaction was complete hence solution lost heat through radiation to the surrounding.

g) 10.2 cm3
+ 0.1.  Read from the student’s correct graph.

h) Moles = M x V

1000

= 10.2 x 4 √ ½  = 0.0408 moles √ ½

1000

i) Moles = M x V

1000

= 2 x 20
√ ½  = 0.04 moles √ ½

1000

j) HBr   :  NaOH

0.0408 :  0.04

0.0408 :  0.04

0.04 0.04

1   :  1

HBr(aq) + NaOH NaBr(aq) + H2O(l)

k) ∆H = MC ∆t

= -30.2g x 4.2J x 16.3

g0c

= -2067.49J √ ½

Ans. in (h) = -2067.49 J.

∴ 1 Mole = 1 x 2067.49J
√ ½ e.g. 1 x 2067.49

Ans in “h” 0.0408

= -Ans. e.g 50673.82 J mol-1

Or 50.67382KJ mol-1
√ ½

15.   a)(ii) Max. temperature attained : 290c

(iii) Temperature change o the reaction = (29-115)0c

= 140c

Mass of NaOH used = (114.35 – 108 .15)g

= 6.2g

R.F.M of NaOH = 40g

Moles of NaOH used = 6.2 moles

40

= 0.155moles

(v) Heat released = Mass X Specific X Temperature

Heat capacity change

Mass of water used = (108.15 – 8)g

= 100.15g

 Heat released = 100.15 X 4.18 X 14 kj

1000

=100.15kj

0.155 moles NaOH 5.861 kj

1 mole NaOH 1x 5.861 kj mole-1

0.155

= -37.8 kjmol-1

(b) i) H3 and H4

ii) Condensation

iii) H
= H1 + H2 + H3 + H4

iv) Exothermic.

16.  I – a – Latent heat of fusion is the heat change that occurs when one mole of a solid substance

changes into liquid at constant temperature.

– Latent heat of vapourization is the heat change that occurs when one mole of liquid

substance changes into gas at constant temperature.

b – BC – The liquid loses heat as it cools hence decrease in kinetic energy of the particles

– CD – The liquid changes to solid as temperature remains constant at freezing point.

II. (i) Scale – *TZM*

Plot – *TZM*

Line

(ii) Should be shown on the graph – if not shown penalize ( ½ mk)

(iii) Heat change = m x c x T

Where m = (vol. of acid (20cm3) + volume of bas in (b) above) x 1g/cm3

T-as read form the graph

(iv) moles of acid

Moles of base = 0.5 x volume in (b) above

1000

Mole ratio acid: Base = 1:1

Moles of acid heat change in (iii)above

1mole ?

Molar heat change = 1 x heat in (iii)

Moles of acid

17.   Q = 40000 x 60 x 60 = 144000000c

Mass of Al = 144000000 x 27

3 x 96500

= 13.43kg

18.  (a) (i) Contains methane which is a fuel or contains methane which can burn

(ii) Pass a known volume of biogas through Sodium hydroxide (Potassium hydroxide) solution to absorb Carbon (IV) Oxide. Measure the volume of remaining gas

% = Volume of methane x 100

Volume of Biogas

19.  a) No effect – Reaction is not accompanied by volume changes/ similar volumes of

reactants and products

20.  a) – carbon IV Oxide;

– Sulphur IV Oxide;

(b) Availed low sulphur diesel/ availed unleaded petrol

21.  (a) Heat change that occurs when one mole of hydrogen combines with one mole of hydroxide

ions. //Heat evolved when one mole of water s formed during reaction of H+ and OH ions

(b) HCl produces a higher temperature rise than oxalic acid;

HCl is a stronger acid than oxalic acid;

22. H2O(l)   H2   H2O(g)

H2  = – H1 + H3

= H3 H1

= -242 – 286

= -242 + 286

= +44KJ/mol (No units of sign = ½mk)

23.  (a) Chemical substance that burns to produce useful amount of heat.

(b)  (i) Its cheap

(ii) Its readily available (½mk)

(iii) It burns slowly  (½mk)

(iv) Does not produce poisonous gas.  (½mk)

24.  a) Metallic beaker would make most of the heat be lost to the environment

b)  – Thermometer reading increased

– The reaction is exothermic

25.  a) A substance that produce heat energy when burnt

b) 1. Availability

2. ease of transport

26.  a) 1 mole Fe (56) required _____________ 15.4 + 354

= 396.5Kj

10,000 (10 kg) ____________ ?

10,000g X 369.5 Kj

56g

= 6596.285Kj

b)  – 68Kj = – 34 Kj   ½

2

27.  a) ∆H1 – Lattice energy 1

∆H2 – Hydrogen energy 1

b) ∆H3 = ∆H2 + ∆H1 1

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