Enthalpy

1
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.

2

• In pairs
• 1) Discuss the meaning of the terms endothermic
  and exothermic reactions.
• 2) Find examples of endothermic and exothermic
  reactions.

3

• Exothermic gives out heat as they proceed.
• Endothermic takes heat form the surroundings.

4

• Enthalpy change, ?H heat change at a constant
  pressure.
• It varies with conditions (temperature, pressure,
  concentration of solution, etc)

5
Change in enthalpy
?H

• Greek capital letter delta ? means a change
  in science
• Lower case delta is d 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

6

• 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, ?H?298 enthalpy change
  measured under standards conditions.

7

• L.O.
• Construct a simple enthalpy profile diagram for a
  reaction.
• Explain the term activation energy using enthalpy
  profile diagrams.

8
Enthalpy level diagram
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10
Positive or negative?

• T increases?
• If so, then delta H is negative!
• T decreases?
• If so then delta H is positive!

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• Standard molar enthalpy of formation, ?H?f , 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) gt CO2(g)
• H2(g) ½O2(g) gt H2O(l)
• 2C(s,graphite) ½O2(g) 3H2(g)
  gtC2H5OH(l)

14

• Standard molar enthalpy of combustion, ?H?c , 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) gt CO2(g)
• H2(g) ½O2(g) gt H2O(l)
• C2H5OH(l) 3O2(g) gt 2CO2(g) 3H2O(l)