THERMOCHEMISTRY

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THERMOCHEMISTRY
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Heating Curve of Water
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THINGS TO REMEMBER

• ?H for temperature changes uses the formula
• ?H c m ?T and the table of specific heats.
• ?H for phase changes uses the formula
• ?H m Hp and the table of phase changes
• ?H for CHEMICAL REACTIONS uses the formula
• ?H Hproducts Hreactants and the standard
  enthalpies of formation table
• Heat of formation for a free element is 0

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ENVIRONMENT
H is positive
H is negative
SYSTEM (chemical reaction)
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?H is negative
?H
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?H is positive
?H
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Standard Heat of Formation (?Hof)

• The amount of heat energy gained or lost when one
  mole of the substance is formed from its elements
  under standard conditions ( 25oC and 101.3 kPa)

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IMPORTANT POINTS

1. Balance the equation so that ONE MOLE OF THE
   REACTANT is used.
2. Remember the diatomic molecules (H2, N2, O2, F2,
   Cl2, Br2 and I2) and write them correctly.
3. The reactants must be elements, not polyatomic
   ions.

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The formation reaction for liquid water is
described by the following equationH2 (g)
½O2 (g) ? H2O (l) 285.8 kJ

• or
• H2 (g) ½O2 (g) ? H2O(l) ?Hof -285.5 kJ

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HESSS LAW ?H ??Hproducts ??Hreactants

• The heat of reaction can be determine by
  subtracting the sum of heats of formation of the
  reactants from the sum of the heats of formation
  of the products

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Example

• Calculate ?H for the combustion of benzene, C6H6,
  as shown by the following reaction
• C6H6 (l) 15/2 O2 (g) ? 6 CO2 (g) 3 H2O(l)

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Using the formula ?H ??Hproducts
??Hreactants

• C6H6 (l) 15/2 O2 (g) ? 6 CO2 (g)
  3 H2O(l)

(49.0) 15/2 x (0) 6 x (-393.5) 3 x (-285.5)
49.0 49.0 -3218.4 -3218.4
??Hreactants ??Hreactants ??Hproducts ??Hproducts
?H -3218.4 kJ (49.0kJ) -3267.4 kJ ?H -3218.4 kJ (49.0kJ) -3267.4 kJ ?H -3218.4 kJ (49.0kJ) -3267.4 kJ ?H -3218.4 kJ (49.0kJ) -3267.4 kJ ?H -3218.4 kJ (49.0kJ) -3267.4 kJ