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N-1
EXPERIMENT N: ENTHALPY OF REACTION
OBJECTIVES
In this experiment you will use a simple calorimeter to measure the heat given off or absorbed in the
course of some chemical reactions. You will also apply Hess's Law in order to obtain the heat of a
reaction that cannot be (easily) measured directly. Specifically, in Part I of this experiment you will
measure the heats of reaction of magnesium and of magnesium oxide with hydronium ion and then
determine the enthalpy (heat) of formation water using Hess's Law. In Part II, you will determine the
heat of reaction for the conversion of sodium sulfate to sodium sulfate decahydrate (Glauber's salt). In
Part III, you will determine the heats of neutralization of a strong acid, hydrochloric acid, by a strong
base, sodium hydroxide and a weak acid, acetic acid, by sodium hydroxide. The heats of neutralization
will be compared and from this information, the heat of dissociation for the weak acid will be
determined.
THEORETICAL CONSIDERATIONS
The standard enthalpy (heat) of formation, ?H
f
? of a compound is defined as the change in enthalpy
when one mole of the compound, in its standard state, is formed from its elements, in their standard
states. The standard state of an element is the most stable physical form (solid, liquid or gas) and the
most stable allotrope of the element at one atmosphere pressure and 25
C. By definition, the
standard enthalpy of formation of an element in its standard state is zero. For example, the
standard state of carbon is solid graphite, C
(gr)
, and the standard state of oxygen is a diatomic gas, O
2(g)
.
The standard enthalpy of formation of carbon monoxide, CO
(g)
, is the heat evolved when one mole of
C
(gr)
reacts with one-half mole of O
2(g)
when the initial and final temperature of all reactants and products
is 25
C and they are at 1 atmosphere pressure. The standard enthalpy of formation of carbon
monoxide is difficult to measure using the reaction of carbon graphite and oxygen gas because some
carbon dioxide, CO
2(g)
, also forms. This complication can be avoided by using Hess' s Law.
Hess's Law states that, if a chemical reaction can be written as the sum of two or more
reactions, then ?H° for this reaction equals the sum of the ?H° values of the two or more
reactions. For example, to find the ?H
f
for carbon monoxide (below):
