Pages in this set

Page 1

Preview of page 1

Enthalpy change, H
The enthalpy change is the amount of heat taken in or given out at
constant pressure during any physical or chemical change. The size
of any enthalpy change depends on the pressure and the
temperature as well as on the amount of substance used. Chemists

Page 2

Preview of page 2
H kJ mol
at /

C (graphite) +715
Na (s) +109
K (s) +90
Mg (s) +150

In the case of bond fission, a diatomic molecule will produce two moles of atoms, so the enthalpy
of atomisation is half the bond dissociation enthalpy. Thus, for chlorine:

½Cl2 Cl H…

Page 3

Preview of page 3
The enthalpy of lattice formation is the converse of this, i.e. the standard enthalpy change that
accompanies the formation of one mole of a solid ionic lattice from its gaseous ions.

For example:

NaCl(s) Na+ (g) + Cl-(g) HL
= +771 kJ mol-1 (=dissociation)

If lattice dissociation is used as…

Page 4

Preview of page 4
H -H
L = ½H
sub - H
diss - H
i- H
ea + f

HL = -109 ­ 121 ­ 494 ­(-364) +(-411)
= -771 kJ mol-1

The ionic model
The ionic model can be used to calculate a theoretically-based value for lattice dissociation
enthalpy. It makes the…

Page 5

Preview of page 5
Reactions in which there is a release of energy (exothermic processes) and those where there is an
increase in entropy may sometimes conflict. The two are brought together in the relationship:

G = H ­ TS

G must be negative (or zero) for spontaneous change.


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »