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Enthalpy - Coggle Diagram
Enthalpy
Hess' law and enthalpy changes
allows for enthalpy changes to be determined indirectly
states that, if a reaction can take place by two routes, and the starting ad finishing conditions are the same, the total enthalpy change is the same for each route.
▲r
H
= sum ▲f
H
products - sum ▲f
H
reactants
▲r
H
= sum ▲c
H
reactants - sum ▲c
H
products
▲r
H
enthalpy change of reaction
H
A measure of the heat energy in a chemical system
▲
H
=
H
(products) -
H
(reactants
Standard conditions
100 kPa
298K (25°C)
1 Moldm-3
Enthalpy Change
Formation
when one mole of a compound is formed from its elements
All elements have an enthalpy change of formation of 0 kJMol-1
Combustion
Enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
Neutralisation
energy change when an acid and base form one mole of H2O under stand. condit.
E
a
(Activation Energy)
Minimum energy required for a reaction to take place
From the system to the surroundings
An exothermic Change
▲
H
is -ve
From the surroundings to the system
An endothermic change
▲
H
is +ve
Calculating enthalpy changes
q = mc▲T
heat energy
c - specific heat capacity
4.18 Jg-1K-1 for water
energy required to raise the temp of 1g of a substance by 1 K
mass
