Enthalpy (H)

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Enthalpy (H)

• H - amount of heat change that occurs during a
  chemical reaction
• Change in enthalpy
• ?H H products H reactants
• Negative -?H exothermic
• Positive ?H endothermic

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Practice Problem

• Calculate the amount of heat generated when 5.2
  grams of C2H6O is burned according to the
  following equation.
• C2H6O O2 ? CO2 H2O ?H -276 KJ

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Practice Problem

• How many grams of sugar should a person eat to
  obtain their needed 2000 calories of intake for a
  day?
• C6H12O6 6 O2 ? 6 CO2 H2O ?H
  -2801 KJ

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1 food Calorie C 1000 calories 4184 J
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Calorimetry the measurement of heat change

• Specific heat (s) the amount of heat needed to
  raise the temperature of one gram of the
  substance by 1 ºC.
• Heat capacity (C) - the amount of heat needed to
  raise the temperature of a given quantity of a
  substance by 1 ºC.

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Calorimetry

• heat (mass) (specific heat) (?T)
• q m s ?t

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Standard enthalpy of formation

• ?Hfº the heat change that results when one
  mole of a compound is formed from its elements
  at 1 atm.

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Standard enthalpy of formation

• Heat of formation is calculated based on an
  arbitrary reference point like that of sea level
• With the most stable form of the element is set
  at 0

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Direct method of determination

• Cgraphite O2 (g) ? CO2 (g) ?Hfº
  -393.5

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Indirect - by calculation

• Hesss Law when reactants are converted to
  products, the change in enthalpy is the same
  whether the reaction takes place in one step or
  in a series of steps.

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Practice exercise page 221

• Calculate the standard enthalpy of formation of
  cabon disulfide (CS2) from its elements, given
  that
• Cgraphite O2 (g) ? CO2 (g) ?Hfº
  -393.5
• Srhombic O2 (g) ? SO2 (g) ?Hfº
  -296.1
• CS2 (l) 3O2 (g) ? CO2 (g) 2SO2
  ?Hfº -1072

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Calculate the ?Hrxnº

• ?H º rxn ?Hfº products - ?Hfº reactants

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Practice problem page 222

• Benzene (c6H6) burns in air to produce carbon
  dioxide and liquid water. Calculate the heat
  released in KJ/g of the compound reacted with
  oxygen. The standard enthalpy of formation of
  benzene is 49.04 KJ/mol

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Homework

• Chapter 6
• 12, 13, 18, 20, 30, 36, 38, 42,
• 44, 46, 48, 56, 58, 60, 66, 72,
• 78, 80