Enthalpy and Entropy

1
Enthalpy and Entropy
2

• Enthalpy change, ?H
• a measure of the energy released or
  absorbed by the substance when bonds are broken
  and formed during a reaction.
• The Basic Rules of Enthalpy
• When bonds are formed, energy is released
• When bonds are broken, energy is absorbed

3

• Enthalpy change can be calculated
• ?H Hproducts Hreactants
• If products have stronger bonds, products will be
  more stable, energy is released,Exo
• If products have weaker bonds, products will be
  less stable, energy is absorbed, Endo
• All substances like the lowest energy state

4
Heat of Formation, ?Hf

• The change in energy when one mole of a compound
  is formed from elements under standard
  conditions.
• ?Hf zero for a pure element
• - ?Hf exothermic, more stable
• ?Hf endothermic, less stable
• ?H S ?Hf products - S ?Hf reactants

5

• Calculate the ?H for the following
• 2 CH3OH(g) 3 O2(g) ? 2 CO2(g) 4 H2O(g)
• CH3OH -201 O2 0
• CO2 -394 H2O -242

6

• The answer is 1354 kJ

7
Bond Energy

• The energy required to break a bond, an
  endothermic process, SO the number is always
  positive. When the bond is formed. energy equal
  to the bond energy is released.
• ?H S energy of broken - S energy of formed
• Calculate for the following
• 2 H2(g) O2(g) ? 2 H2O(g)

8

• H-H 436 kJ/mol
• OO 499 kJ/mol
• O-H 463 kJ/mol

9

• Answer is -481 kJ

10
Hesss Law

• If a reaction can be described as a series of
  steps, then ?H for the overall reaction is the
  sum of all the steps.
• So, C2H2(g) H2O(l) ? C2H5OH(l) ?H ?
• C2H2(g) 3 O2(g) ? 2 CO2(g) 2 H2O(l) ?H
  -1411
• 2CO2(g) 3H2O(l) ? C2H5OH(l) 3O2(g) ?H 1368

11

• The answer is
• -1411 1368 kJ -43 kJ

12
Entropy, ?S

• measure of randomness or disorder the greater
  the disorder, the greater the entropy. Zero
  entropy is defined as a solid crystal at
• 0 K, so all substances have a positive value

13
Entropy Change

• ?S S ?S products - S ?Sreactants