Thermochemistry

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Chapter 5.1-5.3

• Thermochemistry

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Thermochemistry

• Thermo means heat or energy
• Energy Capacity of doing work
• Work Force x displacement

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Types of Energy

• Kinetic Energy (KE)
• Energy due to motion
• Ek ½ mv2
• Where m mass v velocity

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Types of Energy

• Potential energy (PE)
• Energy due to position
• Gravity

higher PE lower PE
Units Calorie (cal), Joule (J) 1 Cal 4.184
J
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• System
• Where we focus our attention
• Surroundings
• Everything else

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First Law of Thermodynamics

• Energy is conserved
• Piston pushed up
  work (W)
• done by
  the gas (say 6 J)

10 J added (q)
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First Law of Thermodynamics

• What happens to the difference (10 6)?
• Retained inside. (causes Internal energy change,
  ?E) 10 4 6
• q (Heat applied) ? q ?E W (first law
  Thermodynamics)
• or ?E q-W (here -W, Work done by the gas)
• ?E q W (here W, Work done on the gas)
• Note Two different sign conventions exist

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• Enthalpy (H)

?E q-W (work done by a gas)
q ?E W
q ?E P ?V
?H ?E P ?V (constant pressure)
H E PV
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Enthalpy or heat Content (H)

• Begin End
• 2H2 O2 ? 2H2O
• Say 484 kJ 1 kJ
• Note 484 kJ is the Heat content of the reactants
  (HR) and 1 kJ is the heat content of the products
  (HP).

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Enthalpy or heat Content (H)

• ?H Hp HR
• 1 484
• -483 kJ (exothermic)
• Here, initial heat contentgt Final heat cont.
• What happens to the difference?
• It is given off.

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Enthalpy or heat Content (H)

• 2H2O ? 2H2 O2
• 1 kJ (HR) 484 kJ (HP)
• Is heat absorbed or given off?
• Heat is absorbed
• ?H Hp HR
• 484 1
• 483 kJ (endothermic)
• Here, initial heat cont lt Final heat cont