STANDARD MOLAR ENTHALPY OF FORMATION

• Enthalpy change when 1 mol of species is formed in its Standard State at a Specified Temperature from the most stable forms of its constituent elements in their standard forms (at the same temperature).

MOST STABLE FORM OF ELEMENT

• = Form favored in Equilibrium at 1 Atmosphere and specified temp. (usually 298.15 K)

• e. g. for C at 298.15 K and 1 atmos., most stable form is GRAPHITE (not diamond!)

STANDARD ENTHALPY OF FORMATION FOR CO2

• Hf0 CO2(gas) = Standard Enthalpy of

Reaction for:

• C (s, graphite) + O2 (g) CO2(g)

• Nomenclature:– Hf

0 Std. State at 25 0C understood

Formation

ENTHALPY OF FORMATION

• N.B. Hf0 for an ELEMENT in its

Standard State = 0– If not in its Standard State = 0

– e.g. For C (s, graphite)C (s, diamond)

H0 = 1.895 kJ mol –1

CALCULATION OF H FROM TABLE OF H

VALUES• 1. Break down steps of reaction

into:– (a) Decomposition of Reactants into

Elements in Standard Forms– (b) Formation of Products from

Elements in Standard States– 2. Apply HESS’S LAW

HESS’S LAW

Add 2 (or more) Reactions to give New Reaction, then Add Enthalpies in same manner to give Enthalpy of New Reaction

HESS’S LAW

CALCULATE ENTHALPY OF FORMATION FOR CO• Need Reaction:

C(s, graphite) + ½ O2 CO2 (g)

H= ??

CALCULATE ENTHALPY OF FORMATION FOR CO

(cont.)I. C(s, graphite) + O2 (g) CO2 (g) ,

H0 = -393.5 kJ mol –1

II. CO2 (g) – ½ O2 (g) CO (g) ,

H0 = +283.0 kJ mol –1

APPLICATION OF HESS’S LAW

• ADD I + II:

C(s, graphite) + O2 (g) CO2 (g) ,

H0 = -393.5 kJ mol –1

II. CO2 (g) – ½ O2 (g) CO (g) ,

H = + 283.0 kJ mol –1

C(s, graphite) + ½ O2 (g) CO (g)

H0 = -393.5 kJ + 283.0 kJ = -110.5 kJ mol-1

HESS’S LAW

DIFFERENT ALLOTROPIC & PHYSICAL FORMS OF

ELEMENTS• e.g. red, whitewhite and black P

– different forms of S– C (graphite and diamond)– Spacing and arrangement of

atoms is different in Graphite and DiamondDiamond,, and requires energy input to effect the transition from one form to the other.

BOND ENTHALPIES

• Energy used to BREAK Specific Bond in Gas Phase Reaction

• N.B. Bond Enthalpies are ALWAYS +.