Chem2 - Energetics Flashcards
Standard enthalpy change of a reaction definition
Is the enthalpy change when the reaction occurs in the molar quatities shown in the chemical equation in standard states under standard conditions
Enthalpy change definition
Is the heat energy transferred in a reaction at constant pressure, it is measured in kJ mol^-1
Standard enthalpy change of formation definition
Is the enthalpy change when one mole of a compound is formed from it’s elements in their standard states under standard conditions
Define standard enthalpy change of combustion
Is the enthalpy change when 1 mole of a substance is completely burned in a plentiful supply of oxygen under standard conditions
State Hess’ Law
The overall enthalpy change is independant of the reaction route taken
State equationsused when calculating /_\ H (enthalpy change)
q=mc/\T
kJ
mass /\H = —————
n= ——— N# moles
Mr
Why can data book values differ from values found from calirometry?
Heat loss for exothermic
Heat gain for endothermic
Average bond enthalpies…
Enthalpy change of a reaction equals?
Enthalpy change of a reaction=
Total energy absorbed - Total energy released
(Energy needed to - (Energy needed to
break bonds) make bonds)
What is a mean bond enthalpy?
Is the average enthalpy required to break or make one mole of a bond, this is taken as an average from many molecules containing the bond.
Therefore mean bond enthalpies are not exact because it is an average and they are each different in different environments e.g. H-OH
And the O-H bond enthalpies are different
A reaction which releases heat is?
Exothermic
Negative enthalpy change
Releases energy
A reaction which absorbs heat?
Endothermic
Positive enthalpy change
Gains energy
