Energetics Flashcards
Definition of enthalpy change
Heat energy change at a constant pressure
Exothermic reaction
ΔH is negative
Heat is lost to the surroundings
products have less energy than the reactants
Endothermic reaction
ΔH is positive
Heat is absorbed
products have more energy than the reactants
Define standard enthalpy of formation
The enthalpy change when 1 mol of compound is formed from its elements under standard conditions.
Define standard enthalpy of combustion
The enthalpy change when 1 mol of a substance is completely combusted in oxygen under standard conditions.
What are the standard conditions for enthalpy change?
100kPa pressure
298K (room temp/25)
solution at 1 mol dm^3
All substances should have their normal state at 298K
Equation for heat change
q=mcΔT
q =mass X specific heat capacity X temp change
ΔH formula
ΔH = (q/1000) / moles
change cm to dm^3 before working out moles
Hess’s law
States that the total enthalpy change for a reaction is independent of the route followed (provided the reactants and products are the same).
ΔH when enthalpy of combustion data is given
ΔcH reactants - ΔcH products
ΔH when enthalpy of formation data is given
ΔfH products - ΔfH reactants
Formula for mean bond enthalpy
ΔH bonds broken - ΔH bonds made
Define mean bond enthalpy
Enthalpy change when 1 mol of covalent bonds
is broken, in the gaseous phase, averaged over a number of compounds
Why is mean bond enthalpy different from Hess’s law?
Mean bond enthalpy uses averages
