Energetics
Enthalpy Change (delta H)
- heat change at constant pressure
- Exothermic reaction: energy in forming bond > breaking bond. Diagram illustration.
- Endothermic reaction: energy in forming bond < breaking bond. Diagram illustration.
Standard Enthalpy Change
- Standard Enthalpy Change refer to reaction which occurs at a pressure of 1 atm, a temperature of 298K
(25*C) and the solutions have concentration of 1 mol per dm3. (98/8(a)(i))
- Standard Enthalpy (heat) of formation refer to the formation of 1 mole of substance from its elements
under standard conditions. e.g. formation of common salt. (98/8(a)(ii))
- Standard Enthalpy (heat) of combustion is the enthalpy change when 1 mole of substance burns
completely under standard conditions. e.g. complete combustion of propane.
- Standard Enthalpy (heat) of neutralization is the enthalpy change when 1 mole of water is formed from
neutralization between and acid and an alkali under standard conditions.
e.g. reaction between H2SO4(aq) and NaOH(aq)
- Standard enthalpy (heat) of solution is the enthalpy change when 1 mole of substance is dissolved in a
specific amount of solvent. e.g. dissolution of NaCl in water
Hess' Law
- Heat change for a chemical change is independent of the path by which chemical change takes place.
- Born-Haber Cycle: Delta H (reaction) = - Delta H (f, reactants) + Delta H (f, products)
+----------------+ Delta H (reaction) +---------------+
| Reactant | -------------------------------> p; | Products |
¡@ +----------------+ +----------------+
^ ^
| +--------------+ |
--------------------- | Elemments | ----------------
Delta H (f, reactants) +--------------+ Delta H (f, products)
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