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154 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
The heat content of a system at constant pressure is called the
enthalpy (H) of the system. The heat absorbed or released during a
change in a system at constant pressure is the change in enthalpy
(⌬H). The enthalpy change for a chemical reaction is called the
enthalpy of reaction or heat of reaction and is defined by this
equation.
⌬H rxn ϭ H products Ϫ H reactants
In an endothermic reaction, heat is absorbed and ⌬H rxn is positive.
In an exothermic reaction, heat is released and ⌬H rxn is negative.
15.3 Thermochemical Equations
A thermochemical equation is a balanced chemical equation that
includes the physical states of the reactants and products and the
change in enthalpy. For example, the thermochemical equation for
the combustion of ethanol is as follows.
C 2 H 5 OH(l) ϩ 3O 2 (g) 0 2CO 2 (g) ϩ 3H 2 O(l) ⌬H comb ϭ Ϫ1367 kJ
The enthalpy change for the complete burning of one mole of a
substance is the enthalpy (heat) of combustion (⌬H comb ) for that
substance. Heat is also absorbed or released during changes of state.
The heat required to vaporize one mole of a liquid is called its
molar enthalpy (heat) of vaporization (⌬H vap ). The heat required
to melt one mole of a solid is its molar enthalpy (heat) of fusion
(⌬H fus ).
Example Problem 15-3
Calculating Enthalpy of Reaction
The enthalpy of combustion for methanol (CH 3 OH) is Ϫ726 kJ/mol.
How much heat is released when 82.1 g of methanol is burned?
The enthalpy of combustion is negative, so the reaction is
exothermic and heat is released. The molar mass of methanol is
32.05 g/mol. First, calculate the number of moles of methanol that
is burned.
82.1 g CH 3 OH ϫ
ϭ 2.56 mol CH 3 OH
1 mol CH 3 OH
ᎏᎏ
32.05 g CH 3 OH
154 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
The heat content of a system at constant pressure is called the
enthalpy (H) of the system. The heat absorbed or released during a
change in a system at constant pressure is the change in enthalpy
(⌬H). The enthalpy change for a chemical reaction is called the
enthalpy of reaction or heat of reaction and is defined by this
equation.
⌬H rxn ϭ H products Ϫ H reactants
In an endothermic reaction, heat is absorbed and ⌬H rxn is positive.
In an exothermic reaction, heat is released and ⌬H rxn is negative.
15.3 Thermochemical Equations
A thermochemical equation is a balanced chemical equation that
includes the physical states of the reactants and products and the
change in enthalpy. For example, the thermochemical equation for
the combustion of ethanol is as follows.
C 2 H 5 OH(l) ϩ 3O 2 (g) 0 2CO 2 (g) ϩ 3H 2 O(l) ⌬H comb ϭ Ϫ1367 kJ
The enthalpy change for the complete burning of one mole of a
substance is the enthalpy (heat) of combustion (⌬H comb ) for that
substance. Heat is also absorbed or released during changes of state.
The heat required to vaporize one mole of a liquid is called its
molar enthalpy (heat) of vaporization (⌬H vap ). The heat required
to melt one mole of a solid is its molar enthalpy (heat) of fusion
(⌬H fus ).
Example Problem 15-3
Calculating Enthalpy of Reaction
The enthalpy of combustion for methanol (CH 3 OH) is Ϫ726 kJ/mol.
How much heat is released when 82.1 g of methanol is burned?
The enthalpy of combustion is negative, so the reaction is
exothermic and heat is released. The molar mass of methanol is
32.05 g/mol. First, calculate the number of moles of methanol that
is burned.
82.1 g CH 3 OH ϫ
ϭ 2.56 mol CH 3 OH
1 mol CH 3 OH
ᎏᎏ
32.05 g CH 3 OH