7 energy change

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Classified Chemistry IGCSE Paper_3_Topic 7

7 – Energy Change Mostafa Barakat (0100 165 44 62)

114

Topic 7

Production of energy

Endothermic reactions

Exothermic reactions

Bond energy (calculations)

Electricity from chemical reactions





115

1) Nov 2001 Q (4_e, f)

(e) The diagram below represents a simple cell.

(i) Write an ionic equation for the reaction that occurs at the zinc electrode.…………………………………………………………………………………………

………………………………………………………………………………………[1]

(ii) How could the voltage of the cell be increased?…………………………………………………………………………………………

………………………………………………………………………………………[1]

(f) A different type of cell is drawn below.

(i) The pH of the solution increases. Give the name of the ion formed.

…………………………………………………………………………………………………………………………………………………………………………………[1]

(ii) Complete the equation that represents the formation of this ion.

2 2.......... .......... ............O H O [2]

Higher reactivity metal instead of Zn or lower instead of iron

or increase concentration of acid

Zn Zn + 2 e2+

hydroxide ion (OH )

2 4 e 4 OH--

-





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2) June 2003 Q (5_f)

(f) The reactions of these metals with oxygen are exothermic.

( ) 2( ) ( )2

s g s Ba O BaO

(i) Give an example of bond forming in this reaction.

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

(ii) Explain using the idea of bond breaking and forming why this reaction isexothermic.

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

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

3) June 2004 Q (1_a)

1 It was reported from America that a turbine engine, the size of a button,might replace batteries. The engine would be built from silicon which hassuitable properties for this purpose.

(a) (i) Why are batteries a convenient source of energy?

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

(ii) The engine will run on a small pack of jet fuel. What other chemical isneeded to burn this fuel?

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

4) Nov 2004 Q (1_c)

1 (c) Respiration and photosynthesis are two of the processes that determinethe percentage of oxygen and of carbon dioxide in the air.

(i) Name another process that changes the percentages of these two gases in air.

………………………………………………………………………………………[1]

(ii) The equation for photosynthesis is given below.

2 2 6 12 6 2

6 6 6CO H O C H O O

This is an endothermic reaction.

Complete the reaction for respiration.

6 12 6 26 ..................... ....................C H O O

This is an …………………………………. reaction.

[2]

barium----oxygen

the energy needed to break the bonds was less than that released when bonds formed.

batteries are portable

The other chemical is oxygen or air

combustion or burning

6 CO 6 H O22

exothermic





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5) Nov 2004 Q (4_b)

(b) The following diagram shows a simple cell.

(i) Predict how the voltage of the cell would change if the tin electrode was

replaced with a silver one.…………………………………………………………………………………………

………………………………………………………………………………………[1]

(ii) Which electrode would go into the solution as positive ions? Give a reasonfor your choice.

…………………………………………………………………………………………

………………………………………………………………………………………[1]

(iii) State how you can predict the direction of the electron flow in cells of thistype.

…………………………………………………………………………………………

………………………………………………………………………………………[1]

6) Nov 2005 Q (7_c)

7 (c) (i) Complete the following table that describes the bond breaking andforming in the reaction between nitrogen and hydrogen to form ammonia.

[3]

the voltage would increase because there was a bigger difference in reactivity

the zinc electrode would go into solution as it loses electrons more easily

or it is more reactive

the electrons flow from the more reactive to the less reactive metal.

endothermic

endothermic

exothermic

H - H





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(ii) Explain, using the above data, why the forward reaction is exothermic.

…………………………………………………………………………………………

……………………………………………………………………………………….[2]

7) June 2006 Q (6_a, c, d)6 (a) Exothermic reactions produce heat energy.

An important fuel is methane, natural gas. The equation for its combustion isas follows.

4 2 2 22 2CH O CO H O

(i) In chemical reactions bonds are broken and new bonds are formed.

Using this reaction give an example of

a bond that is broken, ………………………………………………………………..

a bond that is formed. ……………………………………………………………[2]

(ii) Explain, using the idea of bonds forming and breaking, why this reaction isexothermic, that is it produces heat energy.

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

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

(c) Cell reactions are both exothermic and redox. They produce electrical

energy as well as heat energy.(i) The diagram shows a simple cell.

Which substance in this cell is the reductant and which ion is the oxidant?

Reductant …………………………………………………………………………….

Oxidant …………………………………………………………………………......[2]

(ii) How could the voltage of this cell be increased?

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

because more energy is released by bond forming than is used in bond

energy change = –2328 +( 945 + 1308) = –75kJ

carbon - hydrogen bond (C - H)

carbon = oxygen bond (C = O)

more energy is released in bond forming than is used in bond breaking

zinc

hydrogen ion (H )

replace zinc with a more reactive metal or replace iron with a less reactive metal

+





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(iii) What is the important large scale use, relating to iron and steel, of thistype of cell reaction?

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

(d) Cells can be set up with inert electrodes and the electrolytes as oxidantand reductant.

The potassium manganate(VII) is the oxidant and the potassium iodide is thereductant.

(i) Describe the colour change that would be observed in the left hand beaker.

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

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

(ii) Write an ionic equation for the reaction in the right hand beaker.

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

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

sacrificial protection or galvanising

the colour change from purple to colourless

2 I + 2e I2

-





120

8) Nov 2007 Q (6_a, i, ii, iii, iv)

6) The alcohols form a homologous series. The first four members aremethanol, ethanol, propan-1-ol and butan-1-ol.

(a) One characteristic of a homologous series is that the physical properties

vary in a predictable way. The table below gives the heats of combustion of the first three alcohols.

(i) The minus sign indicates that there is less chemical energy in the productsthan in the reactants. What form of energy is given out by the reaction?

………………………………………………………………………………………[1]

(ii) Is the reaction exothermic or endothermic?

………………………………………………………………………………………[1]

(iii) Complete the equation for the complete combustion of ethanol.

2 5 2.......... ................... ..................C H OH O [2]

(iv) Determine the heat of combustion of butan-1-ol by plotting the heats of combustion of the first three alcohols against the number of carbon atoms per molecule.

- 2660

heat (energy)

exothermic

3 2 CO 3 H O2 2





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Number of carbon atoms per molecule

The heat of combustion of butan-1-ol = ………………….. kJ / mol

[3]

x

x

x

- 2660





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9) June 2009 Q (7_a)

7 Hydrogen reacts with the halogens to form hydrogen halides.

(a) Bond energy is the amount of energy, in kJ, that must be supplied(endothermic) to break one mole of a bond.

Use the above data to show that the following reaction is exothermic.

2 H H Cl Cl H Cl ............................................................................................................................

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

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

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

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

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

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

10) Nov 2009 Q (3_c)

(c) Zinc electrodes have been used in cells for many years, one of the firstwas the Daniel cell in 1831.

(i) Give an explanation for the following in terms of atoms and ions.

observation at zinc electrode – the electrode becomes smaller

Explanation ……………………………………………………………………………

……………………………………………………………………………………….[1]

total endothermic change (energy used breaking bonds) = 436 + 242 = 678 kJ

total exothermic change (energy released in bond formation) = 2 x 431 = - 862 kJ

change for reaction = 678 - 862 = -184 kJ

at the zinc electrode - zinc atoms change into zinc ions (go into solution).





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observation at copper electrode – the electrode becomes bigger

Explanation ……………………………………………………………………………

……………………………………………………………………………………….[1]

(ii) When a current flows, charged particles move around the circuit.

What type of particle moves through the electrolytes?

……………………………………………………………………………………….[1]

Which particle moves through the wires and the voltmeter?

……………………………………………………………………………………….[1]

11) June 2010 [32] Q (5_b)

5 Fuel cells are used in spacecraft to produce electrical energy.

(b) Hydrogen and oxygen react to form water 2 2 2

2 2 H O H O (i) Give an example of bond breaking in the above reaction.

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

(ii) Give an example of bond forming in the above reaction.

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

(iii) Is the change given in (i) exothermic or endothermic?

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

at the copper electrode – copper ions from the solution change into copper

atoms (deposited on the electrode).

ions

electrons

oxygen(=)oxygen ( O = O )

oxygen(–)hydrogen ( O - H )

endothermic





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12) Nov 2010 [31] Q (3_c)

3 The diagram shows a cell. This is a device which produces electricalenergy. The reaction in a cell is a redox reaction and involves electrontransfer

(i) Complete the sentence.

A cell will change ........................................... energy into electrical energy. [1]

(ii) Draw an arrow on the diagram to show the direction of the electron flow.[1]

(iii) In the left hand beaker, the colour changes from brown to colourless.

Complete the equation for the reaction.

Br 2 + ............... ...............

[2]

(iv) Is the change in (iii) oxidation or reduction? Give a reason for your choice.

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

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

(v) Complete the following description of the reaction in the right hand beaker.

Fe2+ changes into .......................... [1]

(vi) When a solution of bromine is replaced by a solution of chlorine, the

voltage increases. When a solution of bromine is replaced by a solution of iodine, the voltage decreases.

Suggest an explanation for this difference.

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

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

chemical

2 e 2Br-

The reaction is reduction because it is electron gain

Fe3+

"the more reactive the halogen, the higher the voltage" the chlorine is more reactive then

bromine whereas iodine is less active than iodine





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13) June 2011 [32] Q (5)

5 The diagram shows a simple cell.

(a) Write an equation for the overall reaction occurring in the cell.

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

(b) Explain why all cell reactions are exothermic and redox.

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

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

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

(c) Which electrode, zinc or iron, is the negative electrode? Give a reason for your choice.

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

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

(d) Suggest two ways of increasing the voltage of this cell.

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

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

14) Nov 2011[33] Q (7_c)

(iii) Bond energy is the amount of energy, in kJ, which must be supplied to

break one mole of the bond.

Use the data in the table to show that the following reaction is exothermic

Zn + H SO42 4ZnSO + H2

the cell reaction is exothermic because chemical energy was being converted into electrical

energy and it is redox because electrons are lost and gained

zinc because zinc is more reactive than iron

1) by replacing zinc with a more reactive metal such as magnesium

2) byreplacing iron with a less reactive metal such as copper





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

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

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

15) June 2012 [31] Q (8_a)

8 Iron and steel rust when exposed to water and oxygen. Rust is hydratediron(III) oxide.

(a) The following cell can be used to investigate rusting

(i) What is a cell?

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

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

(ii) Which electrode will be oxidised and become smaller? Explain your choice

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

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

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

(iii) What measurements would you need make to find the rate of rusting of the electrode

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

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

(iv) Suggest an explanation why the addition of salt to the water increases the

rate of rusting........................................................................................................................ [1]

bonds broken = (+ 436) + (+ 610) = +1046 kj/mol

bonds formed = (–415 × 2) + (–346) = –1176 kj/mol

heat change = 1046 - 1176 = –130 kJ/mol so more energy released than absorbed

cell is the device that convert chemical energy into electric energy

The electrode at which oxidation takes place is electrode B

because it is the electrode which supplies electrons to external circuit

decrease the mass of electrode B per time

the water containing dissolved sodium chloride is a better conductor.





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16) Nov 2012 [32] Q (6_d)

(d) In the apparatus used in (c), the power supply is removed andimmediately replaced by a voltmeter

A reading on the voltmeter shows that electrical energy is being produced.Suggest an explanation for how this energy is produced.

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

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

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

17) Nov 2012 [33] Q (7_d)

7 Ammonia is made by the Haber process.

2( ) 2( ) 3( )3 2 g g g N H NH

(d) (i) What is the difference between an endothermic and an exothermicreaction?

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

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

(ii) Bond breaking is an endothermic process. Bond energy is the amount of energy needed to break or form one mole of the bond. Complete the table andexplain why the forward reaction is exothermic.

it is a cell; hydrogen reacts with oxygen and this reaction produces energy where chemical

energy converted into electric energy and electrons flow from the anode to the cathode

endothermic takes in heat but exothermic given out heat

endothermic

exothermic2x3x388 = - 2328





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

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

....................................................................................................................... [3]18) June 2013 [31] Q (5_b)

(b) A reactivity series can also be established by measuring the voltage of simple cells. The diagram shows a simple cell.

Results from cells using the metals tin, cadmium, zinc and copper are given inthe table below.

Write the four metals in order of increasing reactivity and explain how youused the data in the table to determine this order.

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

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

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

heat taken in = 944 + 1308 = 2252 Kj

heat given out = -2328

heat change = 2252 - 2328 = -76 Kj

the forward reaction is exothermic because the sign of energy change is negative

Cu < Sn < Cd < Znbecause the biggest difference in the voltage in copper zinc cell and the bigger the

difference in reactivity, the bigger the voltage

7 energy change

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