31

CHEMICAL ENERGETICS WS 1

1 The table below shows the enthalpy changes needed to calculate the lattice enthalpy of
calcium chloride, CaCl 2.

process enthalpy change /kJmoi-1

first ionisation energy

+590
of calcium
second ionisation energy
+1150
of calcium
electron affinity of
-348
chlorine
enthalpy change of formation
-796
for calcium chloride .
• •. . ........... .
. �
enthalpy change of
atomisation for calcium - --- +178

enthalpy change of
atomisation for chlorine
+122
\�
(a) The Born-Haber cycle below can be used to calculate the lattice enthalpy for calcium
chloride.

Ca2+(g) + 2Cl (g) + 2e-

11H = .................. kJmo1-1

flH= . ................. kJmo1-1
Ca2+(g) + CZ2(g) + 2e-
'
L�
-
11H = .................. kJmo1-1 ca2+(g) + 2cz-(g)
ca+(g) + CZ2(g) + e-

11H = .................. kJmo1-1

Ca(g) + Cl2(9)
lattic e enthalpy of
11H = .................. kJmo1-1 calcium chloride

Ca(s) + Cl2(9)

11H = .................. kJmo1-1

' CaCZ2(s)

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(i) Use the table of enthalpy changes to complete the Born-Haber cycle by putting in
the correct numerical values on the appropriate dotted line. [3]
(ii) Use the Born-Haber cycle to calculate the lattice enthalpy of calcium chloride.

answer ........................... kJ mol-1 [2]

(iii) Describe how, and explain why, the lattice enthalpy of magnesium fluoride differs
from that of calcium chloride.

............................................................................................................................ [3]
(b) Explain why the first ionisation energy of calcium is less positive than the second
ionisation energy.

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

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2 (a) Write an equation to represent the lattice energy of sodium oxide, Na2O.

......................................................................................................................................[ 11

(b) The Born-Haber cycle shown may be used to calculate the lattice energy of sodium
oxide.

!l.H a1( oxygen )

(i) In the spaces below, identify the species A and 8 in the cycle, including the
appropriate state symbols.

species A ................................... species B ...................................

(ii) Identify the enthalpy changes labelled by the numbers 1 to 4 in the cycle.

1 ................................................................................................................................
2 ................................................................................................................................

3 ................................................................................................................................

4 ................................................................................................................................
[3]

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(c) Use your cycle, the following data, and further data from the Data Booklet to calculate a
value for the lattice energy of sodium oxide.

Data: enthalpy change of atomisation for Na(s) +107 kJ moI- 1

first electron affinity of oxygen -141 kJ moI- 1
second electron affinity of oxygen +798 kJ moI- 1
enthalpy change of formation of N a2O(s) -414 kJ moI- 1
enthalpy change of atomisation for oxygen = half the bond energy for 02.

[3]

(d) (i) How would you expect the magnitude of lattice energy of magnesium oxide to
compare with that of sodium oxide? Explain your reasoning.

(ii) State a use of magnesium oxide, and explain how the use relates to your answer in
part (d) (i).

[4]
[W'02 02]

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3 One of the reasons the melting point of magnesium chloride is quite high is because it has a
fairly high lattice energy.

(d) (i) Explain the term lattice energy.

(ii) Write a balanced equation including state symbols to represent the lattice energy
of magnesium chloride.

...............................................................................................................................(4)

(e) Suggest, with an explanation in each case, how the lattice energy of magnesium
chloride might compare with that of

(i) sodium chloride, NaCl,

(ii) calcium chloride, CaCl2.

...............................................................................................................................(4)

(f) Use the following data to calculate a value for the lattice energy of sodium chloride.

l:lH1 (NaCl} = -411 kJ moI-1

/:lH81 (Na) = 107kJ moI-1
l:lHat (Cl} = 122 kJ moI-1
first ionisation energy of Na = 494 kJ moI-1
elect ron affinity of Cl = -349 kJ moI-1

lattice energy of NaCl = ........................................ kJ moI-1 (3]

[5'04 Q1]

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4 Taken together, nitrogen and oxygen make up 99% of the air. Oxygen is by far the more
reactive of the two gases, and most of the substances that react with air combine with the
oxygen rather than with the nitrogen.

(a) State one reason why the molecule of nitrogen, N2, is so unreactive.

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

Despite the apparent lack of reactivity of N 2, nitrogen atoms have been found to form bonds
with almost all of the elements in the Periodic Table. Lithium metal reacts with nitrogen gas
at room temperature to give lithium nitride, Li3 N. Magnesium produces magnesium nitride,
Mg3N2, as well as magnesium oxide, when heated in air.

(b) Calculate the lattice energy of magnesium nitride using the following data, in addition to
relevant data from the Data Booklet.

enthalpy change value/kJ mo1-1

atomisation of Mg(s) +148
total of electron affinities for
+2148
the change N(g) ➔ N3---(g)
enthalpy of formation of
-461
Mg3Nis)

lattice energy = ...........................kJ mo1- 1 [3]

[5'11 1 Q1]

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5 (a) (i) What is meant by the term lattice energy?

(ii) Write an equation to represent the lattice energy of MgO.

[3]

(b) The apparatus shown in the diagram can be used to measure the enthalpy change of
formation of magnesium oxide, dH7(MgO).

---- ====.:::::::
to suction pump

stirrer

oxygen gas --1..

►---- -
-
small electric heater
(to ignite magnesium)

List the measurements you would need to make using this apparatus in order to calculate
dHf(MgO)

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

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(c) Use the following data, together with appropriate data from the Data Booklet, to calculate
a value of �Hi(MgO).

lattice energy of MgO(s) = -3791 kJ mol- 1

enthalpy change of atomisation of Mg = +148kJ m o I 1-
electron affinity of the oxygen atom = -141 kJ moI- 1

electron affinity of the oxygen anion, o- = + 798kJ m o I 1-

�Hi(MgO)= .......................... kJ moI- 1

[3]

(d) Write equations, including state symbols, for the reactions, if any, of the following two
oxides with water. Suggest values for the pH of the resulting solutions.

pH of resulting
oxide equation
solution

MgO

[3]
[S'12 1 Q1]

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6 (i) Write a chemical equation representing the lattice energy of AgBr.

(ii) Use the following data to calculate a value for the lattice energy of AgBr(s).
first ionisation energy of silver = +731 kJ moI-1
electron affinity of bromine = -325 kJ m o I -1
enthalpy change of atomisation of silver = +285 kJ moI-1
enthalpy change of atomisation of bromine = +112 kJ moI-1
enthalpy change of formation of AgBr(s) = -100 kJ m o I -1

(iii) How might the lattic energy of AgCl compare to that of AgBr? Explain your
answer.

[4 ]
[W'OS Q1]

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7
Use the following data and data from the Data Booklet to construct a Born-Haber
cycle and calculate the lattice energy of Bas.

standard enthalpy change of formation of BaS(s) -460 kJ m o I -1

standard enthalpy change of atomisation of Ba(s) +180 kJ moI-1

standard enthalpy change of atomisation of S(s) +279 kJ moI-1

electron affinity of the sulfur atom -200 kJ m o I -1

electron aft nity of the s- ion +640kJ moI-1

[3]

[W'09 2 Q1]

8 Use the following data, together with relevant data from the Data Booklet, to construct a
Born-Haber cycle and calculate a value for the lattice energy of zinc chloride.

standard enthalpy change of formation of ZnC '2 -415 kJ moI-1

standard enthalpy change of atomisation of Zn(s) +131 kJ moI-1
electron affinity per mole of chlorine atoms -349 kJ moI-1

lattice energy= ............................................... kJ moI-1 [3)

[5'07 Q1]

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9 Calcium chloride, CaC�, is an important industrial chemical used in refrigeration plants, for
de-icing roads and for giving greater strength to concrete.

(a) Show by means of an equation what is meant by the lattice energy of calcium chloride.

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

(b) Suggest, with an explanation, how the lattice energies of the following salts might
compare in magnitude with that of calcium chloride.

(i) calcium fluoride, CaF2

(ii) calcium sulfide, CaS

[3]

(c) Use the following data, together with additional data from the Data Booklet, to calculate
the lattice energy of CaC�.

standard enthalpy change of formation of CaC� -796 kJ moI- 1

standard enthalpy change of atomisation of Ca(s) +178 kJ moI-1

electron affinity per mole of chlorine atoms -349 kJ moI- 1

enthalpy

lattice energy= ............................................ kJ moI-1 [3]

[W'091O2]

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10 (i) Write an equation to represent the lattice energy of PbCZ2. Show state symbols.

(ii) Use the following data, together with appropriate data from the Data Booklet, to calculate
a value for the lattice energy o f PbCZ2.

electron affinity of chlorine = -349 kJ m o I 1-

enthalpy change of atomisation of lead = +195 kJ m o I 1-
enthalpy change of formation of PbCZi(s) = -359 kJ m o I 1-

lattice energy = ............................. kJ m o I 1-

(iii) How might the lattice energy of PbCZ 2 compare to that of Pb8r2? Explain your answer.

[6]
[5'14 2 02]

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11 Use the following data, together with relevant data from the Data Booklet, to calculate
a value for the lattice energy of strontium chloride. You may find it helpful to construct a
Born-Haber cycle.

electron affinity per mole of chlorine atoms -349kJ m o I -1

standard enthalpy of atomisation of Sr(s) +164k J m o I -1
standard enthalpy of formation of SrCZ2(s) -830kJ m o I -1

lattice energy= .................. kJ m o I1-

[5]
[W'14 3 Q1]

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12 Most car air bags contain a capsule of sodium azide, NaN3. In a crash, the NaN3 decomposes into
its elements.

(a) Write an equation for the decomposition of NaN3 .

. ....... .... ....... .... ........... .... .... ... .... .... ........... .... ....... .... ....... .... ....... .... .... ....... .... .... ... ...... .... .... ... .. (1]

(b) Complete the 'dot-and-cross' diagram for the azide ion, N3-.

Use the following key for the electrons.

• electrons from central nitrogen atom

x electrons from the other two nitrogen atoms
□ added electron(s) responsible for the overall negative charge

N N N

[3]

(c) Lattice energies are always negativ e showing that they represent exothermic changes.

(i) Explain what is meant by the term lattice energy.

....................................................................................................................................... [2]
(ii) Explain why lattice energy represents an exothermic change.

. .... .. .. ....... .. .. .... ... .... .... ....... .... .... .. . .... .... .. ..... .... .. .. ... .... .. .. ....... .... .... ... .... .... ....... .... .. .... ..... (1]

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(iii) Use the following data and any relevant data from the Data Booklet to calculate the
standard enthalpy change of formation, t:i.Hi, of NaN3(s).
Include a sign in your answer. Show all your working.

lattice energy, t:i.Ht11, of NaN3(s) -732 kJ m o I 1-

standard enthalpy change of atomisation, t:i.H;, of Na(g) + 107kJ moI-1

standard enthalpy change, t:i.H", for H-Ni{g) + e- ➔ N3-(g) + 142 kJ moI-1

t:i.Ht of NaN3 (s) = ............................. kJ m o l 1- [3]

(iv) The lattice energy, t:i.Ht11, of RbN3(s) is-636kJmoI-1 .

Suggest why the lattice energy of NaN3(s), -732 kJ m o l 1- , is more exothermic than that of
RbN3 (s).

... ........ .... ....... ... ........... ............ .......... ........... ... ........... ........ ... ........... ........... .................. . [1]
[W'16 2 02]

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13 (a) Complete the table using ticks (✓) to indicate whether the sign of each type of energy change,
under standard conditions, is always positive, always negative or could be either positive or
negative.

always always either positive

energy change
positive negative or negative

electron affinity

enthalpy change of atomisation

ionisation energy

lattice energy
[2]

(b) The Born-Haber cycle for magnesium chloride is shown.

Mg2+(g) + 2CZ(g) + 2e-

11Hs
l1H4
Mg2+(g) + 2 cz-(g)
Mg+(g) + 2CZ(g) + e -

l1H3
Mg(g) + 2CZ(g)

l1H2
Mg(g) + Cli{g) flH 6

11H1
Mg(s) + Cli{g)

l1H7
MgCZi{s)

(i) Explain why 11H4 is greater than 11H3.

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

(ii) What names are given to the enthalpy changes 11H6 and 11H7?

11H6 . . . . . . . . . . . . . . . . . . . . . . . ................................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........................................ .

l1H7 .. ......... ..................................................... ......... ......... ....................................................

[1l
[M'17 2 02]

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14 Calcium metal reacts with chlorine gas to form calcium chloride, CaCZ2.

(i) Write an equation, including state symbols, to represent the lattice energy of
calcium chloride, CaCZ2.

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

(ii) Complete a fully labelled Born-Haber cycle that could be used to calculate the lattice
energy, t.H�u,for calcium chloride.

[2]

(iii) Use your answer to (ii) and the following data, together with relevant data from the
Data Booklet, to calculate a value for t.H�tt for calcium chloride.

standard enthalpy change of formation of CaCZi{s), t.Hi -796kJ mo1-1

standard enthalpy change of atomisation of Ca(s), t.H:i +178kJ mo1- 1

electron affinity of chlorine atoms -349kJ mo1-1

I
IL

t.H�tt = ............................... kJ mo1-1 (3]

[M'16 2 02]

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15 (i) Use the data in the table below, and relevant data from the Data Booklet, to calculate the
lattice energy, tiHti11, of potassium oxide, Kp(s).

energy change value/ kJ m o 1 1-

enthalpy change of atomisation of potassium, t:.H:i K(s) +89

electron affinity of O(g) -141
electron affinity of o - (g) +798
enthalpy change of formation of potassium oxide, AHf Kp(s) -361

t:.H�11 = .............................. kJ mo1-1 [3]

(ii) State whether the lattice energy of Na2O would be more negative, less negative or the
same as that of K2O. Give reasons for your answer.

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

[5'17 2 Q1]

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16 (c) (i) The equation for which 6.H is the lattice energy for MgCl is shown.

Use the equation, the following data, and relevant data from the Data Booklet to calculate
a value for the lattice energy of MgCl. You might find it helpful to construct an energy cycle.

electron affinity of Cl(g) = -349 kJ m o I 1-

enthalpy change of atomisation of Mg(s) = +147 kJ mol-1
enthalpy change of formation of MgC l(s) = -106kJ m o I 1-

lattice energy MgCl = ............................. kJ m o I 1- [3]

(ii) Suggest how the lattice energies of MgCl 2 and NaCl will compare to that of MgCl.
Explain your answers.

MgCl2 and MgCl .................................................................................................................

NaCl and MgCZ. ...................................................................................................................

[3]

(d) Define the term electron affinity.

.............................................................................................................................................. [2]
[W'17 2 OS]

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CHEMICAL ENERGETICS WS 2

1 (c) The magnesium ions in seawater are mainly associated with chloride ions.

(i) Use the following flHf values to calculate a value for the flH-e- of the following
reaction.

species flH fe- /kJ mo1-1

MgCl 2 (s) -641

Mg2+(aq) -467

Cl (aq) -167

(ii) Use your answer to explain why MgCl 2 is very soluble in water.

[2]
[5'03 02]

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2 (a) (i) What is meant by the term enthalpy change of hydration, !J..H�yd ?

(ii) Write an equation that represents the /J..Hhyd of the Mg 2+ ion.

(iii) Suggest a reason why /J..Hhyd of the Mg2+ ion is greater than /J..Hhyd of the Ca2+ ion.

(iv) Suggest why it is impossible to determine the enthalpy change of hydration of the
oxide ion, 02- .

[5]

(b) The enthalpy change of solution for MgCZ 2, L'1H:01 (MgCZ i{s)), is represented by the
following equation.

Describe the simple apparatus you could use, and the measurements you would make,
in order to determine a value for l1H:01 (MgCZ2(s)) in the laboratory.

..................................................................................................................................... [4]

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(c) The table below lists data relevant to the formation of MgCZi(aq).

enthalpy change value/kJ m o 1 -1

AHf(MgCZ i(s)) -641

L\H1(MgCZi(aq)) -801
lattice energy of MgCZ i(s) -2526
L\H�viMg 2+(g)) -1890

By constructing relevant thermochemical cycles, use the above data to calculate a value for

AH!01= ....................................... kJ mol-1

AH�yd= ....................................... kJ m o 1 -1
[3]
[S'12 2 Q1]

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3 (b) Using Ag2S04 as an example, complete the following Hess' Law energy cycle relating the
• lattice energy, t..H�11,
• enthalpy change of solution, t..H:O., and
• enthalpy change of hydration, t..H�yd·

On your diagram:
• include the relevant species in the two empty boxes,
• label each enthalpy change with its appropriate symbol,
• complete the remaining two arrows showing the correct direction of enthalpy change.

[4]
[5'151 Q4]

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4 (d) (i) What is meant by the term standard enthalpy change of hydration, fl.Hhyd?

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

(ii) Use the following data to calculate the lattice energy, fl.H�,v of calcium nitrate, Ca(NO3h(s).
You may find it helpful to construct an energy cycle.

enthalpy change value

fl.Hhyd (Ca2+(g)) -1650 kJ m o I 1-

fl.Hhyd (NO3- ( g)) -314kJmoI-1

enthalpy change of solution for Ca(NO3h(s) -19 k J m o I 1-

fl.H�11 Ca(NO3h(s) = ........................... kJ moI- 1 [3]

(e) The standard enthalpy change of hydration for Ba2+, fl.Hhyd (Ba2+(g)), is -1305 kJ m o I 1- .

Suggest an explanation for why the fl.Hhyd of the Ba2+ ion is less exothermic than the fl.Hhyd of
the Ca2+ ion.

.............................................................................................................................................. [2]
[W'151 Q1]

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5 (a) The dissolving of an ionic compound in water is accompanied by an energy change, the
enthalpy change of solution, D.Hsoi ·

Describe, in terms of bond breaking and bond making, what happens to the solid ionic lattice
when an ionic compound dissolves in water.

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

(b) (i) What is meant by the term enthalpy change ofsolution, D.Hso1?

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

(ii) Use the following data to calculate the standard enthalpy change of hydration, D.Hhyd• of
chloride ions, CZ-(g).
You may find it helpful to construct an energy cycle.

enthalpy change value

D.Hhyd (Mg2•(g)) -1925kJ mol-1

lattice energy of MgCZi{s) -2524kJ moI-1

enthalpy change of solution for MgCZ2(s) -155kJ moI-1

D.Hhyd ( C Z (- g)) = ......................... kJ m o I 1- [2]

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(iii) The enthalpy change of hydration for Na·, t:,.H�yd (Na·(g)), is -410 kJ m o I 1- .

Suggest an explanation for why the t:,.Hiyd of the Na· ion is less exothermic than the t:,.Hiyd
of the Mg2• ion.

....................................................................................................................................... [2]
[W'15 3 Q1]

6 The table lists the standard enthalpy changes of formation, t:,.Hi, for some compounds and aqueous
ions.

species t:,.Hi IkJ mol-1

Ba2·(aq) -538
O H - (aq) -230
COi{g) -394
BaCOis) -1216
H2O{I) -286

(b) (i) Reaction 1 occurs when COi{g) is bubbled through an aqueous solution of Ba(OH}i.

Use the data in the table to calculate the standard enthalpy change for reaction 1, t:,.H�1 .

reaction 1

t:,.H�1 = ............................. kJ m o I1- [2]

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If COi(g) is bubbled through an aqueous s olution of Ba(OH)i for a long time, the precipitated
BaCOis) dissolves, as shown in reaction 2.

reaction 2

The standard enthalpy change for reaction 2, 11H72 , = -2 6kJ m o I 1- .

(ii) Use this information and the data in the table to calculate the standard enthalpy change of
formation of the HCO3-(aq) ion.

llHf HCO 3-(aq) = .............................. kJ mol-1 [2]

(iii) The overall process is shown by reaction 3.

Use your answer to (ii), and the data given in the table, to calculate the standard enthalpy
change for reaction 3, /lH�3.

reaction 3

11H73 = .............................. kJ moI-1 [1]

(iv) How would the value of /lH�3 compare with the value of /lH�4 for the similar reaction with
Ca(OH)i(aq) as shown in reaction 4?
Explain your answer.

reaction 4

....................................................................................................................................... [2]
[5'17 1 Q1]

ENERGETICS WS 2