BORNHABER CYCLES

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BORN-HABER CYCLES A guide for A level students
2008 SPECIFICATIONS
KNOCKHARDY PUBLISHING
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BORN-HABER CYCLES
INTRODUCTION This Powerpoint show is one of
several produced to help students understand
selected topics at AS and A2 level Chemistry. It
is based on the requirements of the AQA and OCR
specifications but is suitable for other
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may be used for classroom teaching using an
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this, and the full range of AS and A2 topics, are
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BORN-HABER CYCLES

• CONTENTS
• Lattice Enthalpy
• Definition of enthalpy changes
• Born-Haber cycle for sodium chloride
• Calculation of Lattice Enthalpy
• Born-Haber cycle for magnesium chloride

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Lattice Enthalpy Definition(s)
THERE ARE TWO DEFINITIONS OF LATTICE ENTHALPY 1.
Lattice Formation Enthalpy The
enthalpy change when ONE MOLE of an ionic
lattice is formed from its isolated
gaseous ions. Example Na(g) Cl(g)
Na Cl(s)
2. Lattice Dissociation Enthalpy The
enthalpy change when ONE MOLE of an ionic
lattice dissociates into isolated gaseous
ions. Example Na Cl(s)
Na(g) Cl(g) MAKE SURE YOU CHECK WHICH
IS BEING USED
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Lattice Enthalpy Definition(s)
1. Lattice Formation Enthalpy The
enthalpy change when ONE MOLE of an ionic
lattice is formed from its isolated
gaseous ions. Values highly EXOTHERMIC
strong electrostatic attraction between
oppositely charged ions a lot of energy is
released as the bond is formed relative
values are governed by the charge density of the
ions. Example Na(g) Cl(g)
Na Cl(s)
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Lattice Enthalpy Definition(s)
2. Lattice Dissociation Enthalpy The
enthalpy change when ONE MOLE of an ionic
lattice dissociates into isolated gaseous
ions. Values highly ENDOTHERMIC strong
electrostatic attraction between oppositely
charged ions a lot of energy must be put in
to overcome the attraction relative values
are governed by the charge density of the
ions. Example Na Cl(s)
Na(g) Cl(g)
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Calculating Lattice Enthalpy
SPECIAL POINTS you CANNOT MEASURE LATTICE
ENTHALPY DIRECTLY it is CALCULATED USING A
BORN-HABER CYCLE
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Calculating Lattice Enthalpy
SPECIAL POINTS you CANNOT MEASURE LATTICE
ENTHALPY DIRECTLY it is CALCULATED USING A
BORN-HABER CYCLE greater charge densities of
ions greater attraction larger
lattice enthalpy
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Calculating Lattice Enthalpy
SPECIAL POINTS you CANNOT MEASURE LATTICE
ENTHALPY DIRECTLY it is CALCULATED USING A
BORN-HABER CYCLE greater charge densities of
ions greater attraction larger
lattice enthalpy Effects Melting point the
higher the lattice enthalpy, the higher the
melting point of an ionic compound Solubility so
lubility of ionic compounds is affected by the
relative values of Lattice and Hydration
Enthalpies
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Lattice Enthalpy Values
Cl Br F O2- Na -780 -742 -918 -2478
K -711 -679 -817 -2232 Rb -685 -656 -783 Mg
2 -2256 -3791 Ca2 -2259
Units kJ mol-1
Smaller ions will have a greater attraction for
each other because of their higher charge
density. They will have larger Lattice Enthalpies
and larger melting points because of the extra
energy which must be put in to separate the
oppositely charged ions.
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Lattice Enthalpy Values
Cl Br F O2- Na -780 -742 -918 -2478
K -711 -679 -817 -2232 Rb -685 -656 -783 Mg
2 -2256 -3791 Ca2 -2259
Smaller ions will have a greater attraction for
each other because of their higher charge
density. They will have larger Lattice Enthalpies
and larger melting points because of the extra
energy which must be put in to separate the
oppositely charged ions.
The sodium ion has the same charge as a potassium
ion but is smaller. It has a higher charge
density so will have a more effective attraction
for the chloride ion. More energy will be
released when they come together.
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Born-Haber Cycle For Sodium Chloride
kJ mol-1 Enthalpy of formation of NaCl
Na(s) ½Cl2(g) gt NaCl(s)
411 Enthalpy of sublimation of sodium Na(s)
gt Na(g) 108 Enthalpy of atomisation
of chlorine ½Cl2(g) gt Cl(g)
121 Ist Ionisation Energy of sodium Na(g)
gt Na(g) e 500 Electron Affinity
of chlorine Cl(g) e gt Cl(g)
364 Lattice Enthalpy of NaCl Na(g)
Cl(g) gt NaCl(s) ?
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Born-Haber Cycle - NaCl
Enthalpy of formation of NaCl Na(s) ½Cl2(g)
gt NaCl(s)
1
Na(s) ½Cl2(g)
This is an exothermic process so energy is
released. Sodium chloride has a lower enthalpy
than the elements which made it. VALUE - 411 kJ
mol-1
1
NaCl(s)
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Born-Haber Cycle - NaCl
Enthalpy of formation of NaCl Na(s) ½Cl2(g)
gt NaCl(s) Enthalpy of sublimation of
sodium Na(s) gt Na(g)
1
2
Na(g) ½Cl2(g)
2
Na(s) ½Cl2(g)
This is an endothermic process. Energy is needed
to separate the atoms. Sublimation involves
going directly from solid to gas. VALUE 108
kJ mol-1
1
NaCl(s)
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Born-Haber Cycle - NaCl
Enthalpy of formation of NaCl Na(s) ½Cl2(g)
gt NaCl(s) Enthalpy of sublimation of
sodium Na(s) gt Na(g) Enthalpy of
atomisation of chlorine ½Cl2(g) gt Cl(g)
1
2
3
Na(g) Cl(g)
3
Na(g) ½Cl2(g)
2
Na(s) ½Cl2(g)
Breaking covalent bonds is an endothermic
process. Energy is needed to overcome the
attraction the atomic nuclei have for the shared
pair of electrons. VALUE 121 kJ mol-1
1
NaCl(s)
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Born-Haber Cycle - NaCl
Enthalpy of formation of NaCl Na(s) ½Cl2(g)
gt NaCl(s) Enthalpy of sublimation of
sodium Na(s) gt Na(g) Enthalpy of
atomisation of chlorine ½Cl2(g) gt
Cl(g) Ist Ionisation Energy of sodium Na(g)
gt Na(g) e
1
Na(g) Cl(g)
2
4
3
Na(g) Cl(g)
4
3
Na(g) ½Cl2(g)
2
Na(s) ½Cl2(g)
All Ionisation Energies are endothermic. Energy
is needed to overcome the attraction the protons
in the nucleus have for the electron being
removed. VALUE 500 kJ mol-1
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NaCl(s)
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Born-Haber Cycle - NaCl
Enthalpy of formation of NaCl Na(s) ½Cl2(g)
gt NaCl(s) Enthalpy of sublimation of
sodium Na(s) gt Na(g) Enthalpy of
atomisation of chlorine ½Cl2(g) gt
Cl(g) Ist Ionisation Energy of sodium Na(g)
gt Na(g) e Electron Affinity of
chlorine Cl(g) e gt Cl(g)
1
Na(g) Cl(g)
2
5
4
3
Na(g) Cl(g)
Na(g) Cl(g)
4
3
Na(g) ½Cl2(g)
5
2
Na(s) ½Cl2(g)
Electron affinity is exothermic. Energy is
released as the nucleus attracts an electron to
the outer shell of a chlorine atom. VALUE - 364
kJ mol-1
1
NaCl(s)
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Born-Haber Cycle - NaCl
Enthalpy of formation of NaCl Na(s) ½Cl2(g)
gt NaCl(s) Enthalpy of sublimation of
sodium Na(s) gt Na(g) Enthalpy of
atomisation of chlorine ½Cl2(g) gt
Cl(g) Ist Ionisation Energy of sodium Na(g)
gt Na(g) e Electron Affinity of
chlorine Cl(g) e gt
Cl(g) Lattice Enthalpy of NaCl Na(g)
Cl(g) gt NaCl(s)
1
Na(g) Cl(g)
2
5
4
3
Na(g) Cl(g)
Na(g) Cl(g)
4
3
Na(g) ½Cl2(g)
5
6
2
Na(s) ½Cl2(g)
6
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Lattice Enthalpy is exothermic. Oppositely
charged ions are attracted to each other.
NaCl(s)
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Born-Haber Cycle - NaCl
CALCULATING THE LATTICE ENTHALPY Apply Hesss Law
Na(g) Cl(g)
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- - - - The
minus shows you are going in the opposite
direction to the definition - (-364) -
(500) - (121) - (108) (-411) - 776 kJ
mol-1
1
5
6
4
3
2
4
Na(g) Cl(g)
Na(g) Cl(g)
3
Na(g) ½Cl2(g)
6
2
Na(s) ½Cl2(g)
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NaCl(s)
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Born-Haber Cycle - NaCl
CALCULATING THE LATTICE ENTHALPY Apply Hesss Law
Na(g) Cl(g)
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- - - - The
minus shows you are going in the opposite
direction to the definition - (-364) -
(500) - (121) - (108) (-411) - 776 kJ
mol-1 OR Ignore the signs and just use the
values If you go up you add, if you come down
you subtract the value - -
- - (364) - (500) - (121) - (108)
- (411) - 776 kJ mol-1
1
5
6
4
3
2
4
Na(g) Cl(g)
Na(g) Cl(g)
3
Na(g) ½Cl2(g)
6
2
Na(s) ½Cl2(g)
1
1
6
5
4
3
2
NaCl(s)
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Born-Haber Cycle - MgCl2
Enthalpy of formation of MgCl2 Mg(s) Cl2(g)
gt MgCl2(s) Enthalpy of sublimation of
magnesium Mg(s) gt Mg(g) Enthalpy of
atomisation of chlorine ½Cl2(g) gt
Cl(g) x2 Ist Ionisation Energy of
magnesium Mg(g) gt Mg(g) e 2nd
Ionisation Energy of magnesium Mg(g) gt
Mg2(g) e Electron Affinity of
chlorine Cl(g) e gt Cl(g)
x2 Lattice Enthalpy of MgCl2 Mg2(g) 2Cl(g)
gt MgCl2(s)
1
Mg2(g) 2Cl(g)
2
5
6
Mg(g) 2Cl(g)
3
4
4
Mg2(g) 2Cl(g)
Mg(g) 2Cl(g)
3
5
Mg(g) Cl2(g)
2
7
6
Mg(s) Cl2(g)
1
7
MgCl2(s)
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BORN-HABER CYCLES THE END
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