F325 Enthalpy and Entropy hw ms
1.	(a)	(i)	Ca+ is smaller than Ca/ proton : electron ratio in Ca+ > Ca (1)
greater attraction from nucleus (1)	2
(ii)	“oxide” ion, O– and electron are both negative (1)
hence energy is required to overcome repulsion (1)	2
(b)	completes Born-Haber cycle showing 1st IE↑  2nd IE↑  1st EA ↓ 2nd
EA↑
and LE↓(1)(1)(1) (lose 1 mark for each error/omission)
LE = –(1)3451 kJ mol–1 (1)	5
(c)	differences in size of lattice enthalpies linked to ionic sizes/attraction
using more/less exothermic rather than bigger or smaller. (1)
	Mg2+ is smaller/Mg2+ has greater charge density(1)
hence has stronger attraction for O2– (1)	3
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2.	(a)	(i)	Ionisation energy refers to removing electrons that
are attracted to the nucleus / energy needed to
overcome the force of attraction between outer
electrons and nucleus (1)	1
(ii)	Electron affinity involves an electron (being
gained) experiencing attraction to the nucleus (1)	1
 (b)	(i)	Correct state symbols (1);
Allow 1 error or omission in state symbols.
Providing formula has correct state symbols once in cycle this is sufficient
	Correct formula (1);
	Correct cycle with labelling or energy values (1)	3
(ii)	= +178 + 249 + 798 + (–141) + 1150 + 590 + (–3459) (1)
= –635 kJ mol–1 (1)	2
Final answer must have correct units
+635 kJ mol–1 scores 0
(iii)	Ionic radius of iron(II) less (than that of calcium ion) /
charge density of Fe2+ greater (than that of Ca2+) /. ora (1)	1
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3.	(a)	Atomisation of Na = (+)218 / 2 × (+) 109 (1);
Ionisation of Na = (+)990 / 2 × (+)495 (1);
Any other two correct enthalpy changes (1);
Last two correct enthalpy change (1)	4
(b)	–791 + 141 – 247 – 990 – 218 – 416 (1);
–2521 (1)	2
Allow ecf from part (a) e.g. –2026 if only 1 mole of Na → Na+
–2412 if only 1 mole of
Na (s) → Na (g)
–1917 if only 1 mole of Na throughout
Allow full marks for –2521 with no working out
(c)	Calcium chloride (1)
If wrong salt chosen maximum of 2 marks (the comparison of the ions)
	And
	Br– has larger ionic radius than Cl– / Br– has lower charge
density than Cl– / ora (1);
Not Br has larger radius
	K+ has a lower charge than Ca2+ / K+ has lower charge
density than Ca2+ / K+ has a larger ionic radius than Ca2+ / ora (1);
Not K has lower charge
Not K+ has larger atomic radius
	Strongest attraction between ions (when smallest radius and
highest charge) / strongest attraction between ions (with the
highest charge density) / ora (1)	4
Penalise use of atoms rather than ions just once in this question
[10]

4.	(a)	(i)	Electron affinity -696 (1 mark);
Atomisation of Cl2 +244 (1 mark);
From top to bottom
2nd IE +1150,
1st IE +590,
atomisation of Ca +178
formation -796 (1 mark)	3
Allow 244, 1150, 590 and 176 i.e. without plus sign
(ii)	-796 - 178 - 590 - 1150 - 244 + 696 (1);
	But
	-2262 (with no working) (2)	2
Allow ecf from the wrong figures on the Born-Haber cycle
1 error max one mark
2 errors 0 mark
(iii)	Magnesium fluoride more exothermic than calcium chloride / ora
Answer must refer to the correct particle.
	because
	Ionic radius of Mg2+ is less than that of Ca2+ / charge density
of magnesium ion is greater than that of calcium ion / ora (1);
Ionic radius of F- is less than that of Cl– / charge density
of fluoride ion is greater than that of chloride ion / ora (1);
Not Mg or magnesium has a smaller radius or fluorine has a smaller radius
	Stronger (electrostatic) attraction between cation and anion
in MgF2 than in CaCl2 / stronger ionic bonds in MgF2 (1)	3
Allow magnesium or fluorine has a smaller ionic radius
 (b)	Any two from
For second ionisation energy the electron lost is closer to the nucleus / AW (1);
For second ionisation energy the electron is lost from a particle that
is already positive (1);
For second ionisation energy there is one more proton than electron (1)
So outer electron more firmly attracted to the nucleus (1)	2
Allow ora
[10]

5.	(i)	525 kJ mol–1 (1)	1
(ii)	193.6 J K–1 mol–1 (1)	1
(iii)	uses ∆G = ∆H – T∆S (1)
To be feasible, ∆G = 0 or ∆G < 0 (1)
minimum T = ∆H / ∆S (1)
Converts ∆S from J to kJ/÷1000 or converts ∆H from kJ to J (1)
2712 K/ 2438 °C / 2439 °C (1) (units essential)	5
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