thermochemistry
study of heat changes during chemical reactions
endothermic reactions:
- energy profile diagrams
- energy of reactants vs products
energy gets absorbed by the products so the energy of products is higher
temperature of surroundings decreases
exothermic reactions:
- energy profile diagrams
- energy of reactants vs products
- name of the type of energy released
energy is released by products so the energy of reactants is higher
temperature of surroundings increases
energy released is known as surplus energy
enthalpy change:
- definition
- symbol
- units
change in heat energy at constant pressure
delta H
kJ/mol
what is the symbol for a substance that is in a standard state under standard conditions?
delta H ^ (circle with horizontal line in it)
standard conditions examples
pressure: 100kPa
temperature: 25 degrees/298K
concentration: 1 mol/dm3
standard molar enthalpy of formation
enthalpy change when 1 mole of a compound is formed from its elements; all reactants and products are in standard states under standard conditions
symbol: delta (superscript F) H ^ (circle with horizontal line in it)
what is the value of enthalpy formation of elements?
0 (cannot form an element)
standard molar enthalpy of combustion
enthalpy change when 1 mole of a substance is burnt in excess oxygen; all reactants and products are in standard states under standard conditions
symbol: delta (superscript C) H ^ (circle with horizontal line in it)
what is the value of enthalpy combustion of the products formed?
0 (cannot burn water or CO2)
standard molar enthalpy of formation for ammonia:
(1/2)N2(g) + (3/2)H2(g) — NH3(g)
must include state symbols and only 1 mol of compound must be formed
standard molar enthalpy of combustion for methane:
CH4(g) + 2O2(g) — CO2(g) + (2)H2O(l)
must include state symbols and only 1 mol of reactant must be burnt
Hess’s law
in a reaction, the enthalpy change is independent of the route taken
enthalpy change of A — B = enthalpy change of A —- C —- B
usefulness of Hess’s law:
useful for reactions with a high activation energy
helps determine enthalpy changes for smaller reactions as it may not be possible to find enthalpy change from direct reaction
useful for slow reactions
examples of smaller reactions that Hess’s law uses to find enthalpy change of direct route:
standard enthalpy change of formation
standard enthalpy change of combustion
Hess’s law cycle using standard enthalpies of formation:
reactants —- products
underneath, write elements in standard states under standard conditions (include state symbols)
equation for Hess’s law cycle for the standard enthalpy of formation
total standard enthalpy change of products - total standard enthalpy change of reactants
equation for Hess’s law cycle for the standard enthalpy of combustion
reactants - products
bond enthalpy
enthalpy required to break 1 mole of covalent bonds with all species being in the gaseous state
mean bond enthalpy:
mean enthalpy change when 1 mole of covalent bonds break (is averaged out over a range of compounds)
bond enthalpy equation
bonds broken - bonds formed
reactants - products
if a compound isn’t a gas, add enthalpy of vaporisation to it
why is the bond enthalpy value for O=O not considered to be a mean bond enthalpy value?
Oxygen is the only substance with O=O
how to calculate heat change using specific heat capacity:
q = mc x temp change
q = heat energy (J)
m = mass of the solution (g)
c = SHC (J/gK)
temp change: K (if in degrees celsius, no conversion needed as change would be same)
how to calculate enthalpy change from heat change:
delta H: q/n
delta H: enthalpy change (kJ/mol)
q: heat change (kJ)
n: moles (mol)
