Energetics Flashcards
Define standard enthalpy change of formation.
Heat change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions of 298K and 1 bar.
Can be exothermic or endothermic
Define standard enthalpy change of combustion.
Heat change when one mole of a substance is completely burned in excess oxygen under standard conditions of 298K and 1 bar.
Always exothermic
Define standard enthalpy change of neutralisation.
Heat change when one mole of water is formed when an acid neutralises a base and is carried out in an infinitely dilute aqueous solution under standard conditions of 298K and 1 bar.
Always exothermic
Define bond energy.
Heat change when one mole of covalent bond between 2 atoms in a gaseous molecule are broken.
Always endothermic
Define lattice energy.
Heat change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions of 298K and 1 bar.
Always exothermic
Define Hess’ Law.
Hess’ Law states that the enthalpy changes in a chemical reaction is independent of the pathway which the reaction takes place provided that the intial and final states of the system remain the same.
Given bond energies, how to calculate △H of reaction?
△H = sum of bond energy of reactants - sum of bond energy of products
Given △Hc, how to calculate △H of reaction?
△H = sum of △Hc of reactants - sum of △Hc of products
Given △Hf, how to calculate △H of reaction?
△H = sum of △Hf of products - sum of △Hf of reactants
Is bond breaking exothermic or endothermic?
Endothermic.
Is bond forming exothermic or endothermic?
Exothermic.
What are the two limitations of using bond energies in calculating △H?
- Bond energies are average values and not specific to particular bonds in any molecule.
- Bond energies are only applicable to molecules in gaseous state.
