Energetics Flashcards
Enthalpy change
Amount of heat energy taken in or given out during any change in system where the pressure is constant
Exothermic reaction
One where energy is transferred from the system to the surroundings. The products have less energy than the reactants
Endothermic reaction
One where energy is transferred from the surroundings to the system. The products have more energy than the surroundings
Standard enthalpy change of formation
Energy required to form 1 mole of a compound from its elements under standard conditions, all reactants and products being in their standard states
Standard conditions
298 K, 100 kPa, solutions of 1 mol dm^-3
Enthalpy of formation of an element
0 kJ mol^-1
Standard enthalpy change of combustion
enthalpy change when 1 mole of a substance is combusted completely in excess oxygen with all reactants and products being in their standard states
Hess’s law
The total enthalpy change for a reaction is independent of the route taken
Calculating an enthalpy change of reaction from formation information
= enthalpy of formation of products - enthalpy of formation of reactants
Calculating an enthalpy change of reaction from combustion information
= enthalpy of combustion of reactants - enthalpy of combustion of products
Mean bond energies
The mean energy needed to break a covalent bond averaged for that bond over several different molecules
Bond breaking is an…
endothermic process (+ve)
Bond making is an…
exothermic process (-ve)
In terms of bonds, change of enthalpy =
bonds broken - bonds made
Enthalpy changes from mean energies are more/less accurate than using formation or combustion data?
Less accurate because the mean bond energies are not exact
