2. ΔHθ – The amount of heat absorbed/evolved
when the molar quantities of reactants as
stated in the equation react together under
std conditions.
Std conditions : P = 1 atm
T = 298K
Substances in their normal
physical states
3. ΔH f
θ - The heat change required to produce
1 mol of the substance from its elements
under standard conditions.
Ag(s) + ½ Cl2 (g) AgCl (s) ; ΔH f
θ = -127
kJmol-1
ΔH c
θ - The heat energy evolved when 1 mol
of the substance is completely burnt in
excess oxygen under standard conditions.
C2 H4(g) + 3O2 (g) 2CO2 (g) +2 H2O (l) ;
ΔH c
θ = -1411 kJmol-1
4. Hess’s law states that overall heat change in a
chemical reaction is constant and not
dependent on the route taken.
X
A + B C + D
(initial) (final)
Y Z
ΔHθ Route 1 = ΔHθ Route 2
X = Y + Z
7. Def : The enthalpy change when 1 mol of
an ionic compound is broken apart into
its constituent gaseous ions under
standard conditions.
NaCl(s) Na+(g) +Cl-(g) ;
ΔH latt
θ =+771 kJmol-1
8. E.g.1
Draw a Born Haber cycle for RbI. Hence use the
data below to calculate the ΔH
f
of RbI.
ΔH a Rb = +86 kJmol-1
ΔH a I = +107 kJmol-1
ΔH latt Rbl = +609 kJmol-1
IE Rb = +402 kJmol-1
I(g) + e I- (g) ; ΔH = -314 kJmol-1