Section 1.1 Section 1.2 Section 1.3 - The Student Room

SECTION 4
Section 4.4
1 The entropies increase for the first four alkanes as the
molecules become heavier and composed of more atoms
(the number of energy levels increases with the number
of atoms). Pentane is a liquid and so has a lower entropy
than butane.
2 a Entropy decrease, because the number of moles of gas
is reduced by half as reaction proceeds.
b Entropy increase; the number of moles of gas doubles
during the reaction and a solid has much lower
entropy than a gas.
c Entropy decrease; 2 moles of gaseous reactants are
replaced by 1 mole of solid.
d Entropy decrease; 5 moles of gaseous oxygen are
removed, the only product is a solid.
e Entropy increase; 5 moles of gaseous product are
formed.
Section 4.5
1 a When 1 mole of sodium fluoride is formed from 1 mole
of Na + (g) and 1 mole of F – (g), 915 kJ mol –1 of energy are
released. This is the lattice enthalpy of sodium fluoride.
b The lattice enthalpy becomes more negative as the
ionic radii decrease.
2 a LiF; Li + has a smaller radius than Na + and attracts F –
ions more strongly.
b Na 2
O; Na + has a smaller radius than Rb + and attracts
O 2– more strongly.
c MgO; Mg 2+ is smaller and more highly charged than
Na + , and attracts O 2– more strongly.
d KF; F – has a smaller radius than Cl – and attracts K +
more strongly.
3 a SrF 2
; Sr 2+ is smaller and more highly charged than Rb +
and will attract F – more strongly.
b By the same arguments as in a, BaO should have the
more exothermic lattice enthalpy.
c Cu 2+ is more highly charged than Cu + , so CuO should
have the more exothermic lattice enthalpy.
4 a Li + attracts water molecules more strongly than Na +
because of its smaller size.
b Mg 2+ attracts water molecules more strongly than
Ca 2+ because of its smaller size.
c Ca 2+ and Na + have similar sizes, but Ca 2+ is more highly
charged and so attracts water molecules more strongly.
5 a The ions in the lattice attract each other less strongly
as the size of the anion increases from F – to Cl – .
b DH hyd
becomes less exothermic as the anion becomes
bigger and attracts water molecules less strongly.
c
AgF(s) + aq
–DH LE
DH solution
Ag + (g) + F – (g) + aq
Ag + (aq) + F – (aq)
DH hyd
(Ag + )
+
DH hyd
(F – )
3 Students’ answers should be based on the following
deductions.
a
b
c
d
e
DS sys
DS surr
Explanation
/J K –1 mol –1 /J K –1 mol –1
+203 –44 Spontaneous: total
entropy change positive
+63 +329 Spontaneous: total
entropy change positive
+25 –604 Not spontaneous: total
entropy change negative
+209 +416 Spontaneous: total
entropy change positive
–4 –6.7 Not spontaneous: total
entropy change negative
4 Values for both DS sys
and DS surr
are negative. Therefore
DS total
must always be negative, whatever value of T is
chosen, and the process can never be spontaneous.
Enthalpy
AgCl(s) + aq
–DH LE
DH solution
Ag + (g) + Cl – (g) + aq
Ag + (aq) + Cl – (aq)
DH hyd
(Ag + )
+
DH hyd
(Cl – )
Both silver halides have endothermic enthalpy
changes of solution and the enthalpy level diagrams
will have the form shown below.
–DH LE
AgX(s) + aq
Ag + (g) + X – (g) + aq
Ag + (aq) + X – (aq)
DH solution
d DH solution
= –DH LE
+ DH hyd
(Ag + ) + DH hyd
(X – )
DH solution
(AgF) = +6 kJ mol –1 ;
DH solution
(AgCl) = +95 kJ mol –1
DH hyd (Ag + )
+
DH hyd (X – )
e AgF may be soluble in water. AgCl will be insoluble.
6 a Mg(OH) 2
+ 152 kJ mol –1
Ca(OH) 2
+ 7 kJ mol –1
b Ca(OH) 2
: enthalpy change of solution much less
endothermic.
c Entropy changes of the processes.
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