Enthalpy
L.O.:
Understand that energy is conserved.
Explain that all chemical reactions are
accompanied by enthalpy changes.
Explain that enthalpy change can be
exothermic or endothermic.
In pairs:
1) Discuss the meaning of the terms
endothermic and exothermic reactions.
2) Find examples of endothermic and
exothermic reactions.
Exothermic: gives out heat as they proceed.
Endothermic: takes heat form the
surroundings.
Enthalpy change, ∆H: heat change at a
constant pressure.
It varies with conditions (temperature,
pressure, concentration of solution,
etc)
ΔH=
Change in enthalpy
• Greek capital letter delta = Δ means “a change”
in science
• Lower case delta is δ and means a “small
amount” in science
• The chemical “system” relates to all of the
chemicals in the considered equation
• In reality, energy cannot be gained or lost
overall, it simply moves between systems
Enthalpy change, ∆H: heat change at a constant
pressure.
It varies with conditions (temperature, pressure,
concentration of solution, etc)
Standard conditions:
Pressure 100 KPa (atmospheric pressure)
Temperature of 298 K.
Standard Enthalpy change:
Standard state of an element is the state at which it
exists at 298K and 100 KPa.
Standard enthalpy change, ∆Hѳ298: enthalpy change
measured under standards conditions.
L.O.:
Construct a simple enthalpy profile diagram
for a reaction.
Explain the term activation energy using
enthalpy profile diagrams.
Enthalpy level diagram
Positive or negative?
• T increases?
• If so, then delta H is negative!
• T decreases?
• If so then delta H is positive!
Standard molar enthalpy of formation, ∆Hѳf , is the
enthalpy change when one mole of compound is formed
from its constituents under standard conditions, all the
reactants and products in their standard states.
• Example(s):
C(s,graphite) + O2(g) ———> CO2(g)
H2(g) + ½O2(g) ———> H2O(l)
2C(s,graphite) + ½O2(g) +
3H2(g) ——>C2H5OH(l)
Standard molar enthalpy of combustion, ∆Hѳc , is the
enthalpy change when one mole of compound is
burned in oxygen under standard conditions, all the
reactants and products in their standard states.
Example(s):
C(s, graphite) + O2(g) ———> CO2(g)
H2(g) +
½O2(g)
———>
H2O(l)
C2H5OH(l) + 3O2(g) ———> 2CO2(g) + 3H2O(l)