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STANDARD MOLAR ENTHALPY OF FORMATION

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.

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STANDARD MOLAR ENTHALPY OF FORMATION

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  1. 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 elementsin their standard forms (at the same temperature).

  2. 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!)

  3. STANDARD ENTHALPY OF FORMATION FOR CO2 • Hf0 CO2(gas) = Standard Enthalpy of Reaction for: • C (s, graphite) + O2 (g) CO2(g) • Nomenclature: • Hf0Std. State at 25 0C understood Formation

  4. 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

  5. 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

  6. HESS’S LAW Add 2 (or more) Reactions to give New Reaction, then Add Enthalpies in same manner to give Enthalpy of New Reaction

  7. HESS’S LAW

  8. CALCULATE ENTHALPY OF FORMATION FOR CO • Need Reaction: C(s, graphite) + ½ O2 CO2 (g) H= ??

  9. 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

  10. 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

  11. HESS’S LAW

  12. DIFFERENT ALLOTROPIC & PHYSICAL FORMS OF ELEMENTS • e.g.red, white and black P • different forms of S • C (graphite and diamond) • Spacing and arrangement of atoms is different in Graphite and Diamond, and requires energy input to effect the transition from one form to the other.

  13. BOND ENTHALPIES • Energy used to BREAK Specific Bond in Gas Phase Reaction • N.B.Bond Enthalpies are ALWAYS +.

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