2.2 Thermochemistry Flashcards
energy change of a reaction in joules
E=mc∆T
Where m is the mass of water or solution and c is the specific heat capacity of water
4.18 Jg-1K-1
Energy = J
eg enthalpy change calculation
Hess’s law states
the enthalpy change of a reaction is independent of the route of the reaction.
Enthalpy changes have the symbol
ΔH and the units kJmol
−1
.
enthalpy change of formation
enthalpy change of combustion
All enthalpy changes are measured in standard conditions:
25˚C or 298 K and 1 atmosphere (atm) or 101kPa pressure.
Exothermic enthalpy changes
have a negative sign and endothermic enthalpy changes have a positive sign.
0
EG of enthalpy change of formation
bond enthalpy value
The amount of energy that is needed to break a bond
this is the same amount of energy that is given out when that bond is made
Breaking bonds requires energy. Energy is given out when bonds are made.
Average bond enthalpy
is the energy required to break one mole of a bond in a gaseous species under standard conditions.
enthalpy change is negative.
exothermic reaction
exo and endo sign change
Exothermic enthalpy changes have a negative sign and endothermic enthalpy changes have a positive sign.
🔤 Anagram: “PINE”
Each letter stands for:
P – Positive
I – In (heat goes in → endothermic)
N – Negative
E – Exit (heat exits → exothermic)
🔁 So:
PINE = Positive In, Negative Exit
→ Endothermic is Positive (In)
→ Exothermic is Negative (Exit)
The enthalpy change for the reaction can then be calculated:
