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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
157
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
Standard enthalpy (heat) of formation The standard state of a
substance is the normal state of the substance at 298 K (25°C) and
one atmosphere pressure. For example, the standard state of water is
a liquid, whereas the standard state of helium is a monatomic gas.
The change in enthalpy that accompanies the formation of one mole
of a compound in its standard state from its constituent elements in
their standard states is called the standard enthalpy (heat) of
formation (⌬H° f ) of the compound. For example, the standard heat
of formation for magnesium oxide is given by this equation.
Mg(s) ϩ O 2 (g) 0 MgO(s) ⌬H° f ϭ Ϫ602 kJ
Standard enthalpies of formation may be used with Hess’s law to
calculate enthalpies of reaction under standard conditions (⌬H° rxn ).
The following formula summarizes the procedure.
⌬H° rxn ϭ ⌺⌬H° f (products) Ϫ ⌺⌬H° f (reactants)
The equation states that the standard heat of reaction equals the sum
of the standard heats of formation of the products minus the sum of
the standard heats of formation of the reactants. In this equation, the
standard heat of formation of an element in its standard state is zero.
Example Problem 15-5
Enthalpy Change from Standard Enthalpies of Formation
Use standard enthalpies of formation from Table A-8 in the back of this
book to calculate ⌬H° rxn for the reaction of calcium metal with water
to form calcium hydroxide and hydrogen gas.
Ca(s) ϩ 2H 2 O(l) 0 Ca(OH) 2 (s) ϩ H 2 (g)
Use the following formula.
⌬H° rxn ϭ ⌺⌬H° f (products) Ϫ ⌺⌬H° f (reactants)
Because gaseous hydrogen and solid calcium are both elements in
their standard states, their heats of formation are zero. The heat of
reaction then becomes as follows.
⌬H° rxn ϭ ⌬H° f (Ca(OH) 2 ) Ϫ (2)⌬H° f (H 2 O(l))
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