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N-3
Mg
(s)
+
1
2
O
2(g)
MgO
(s)
?H
8
(8)
Reversing equation (6) and adding this to equations (7) and (8), gives the desired reaction:
1
2
O2
(g)
+ H
2(g)
H2O
(l)
(9)
Therefore, the enthalpy change for this reaction given by Hess's Law is:
?H
f
H2O
(l)
= ?H
7
+ ?H
8
+ (- ?H
6
).
In this experiment, you will obtain ?H
6
and ?H
7
from experimental measurements.
?H
6
is determined from the heat produced when magnesium powder is dissolved in an acidic solution,
and ?H
7
from the heat produced when magnesium oxide powder is dissolved in an acidic solution. ?H
8
is ?H
f
of MgO
(s)
, which is well known, having a value of –601.7 kJ/mol.
In Part II of this experiment, the heat of reaction, ?H
rxn
, to form sodium sulfate decahydrate
(Na2SO
4
10 H2O) from anhydrous sodium sulfate (Na2SO
4
) and water will be determined:
Na2SO
4(s)
+ 10 H2O
(l)
?Na2SO
4
10 H2O
(s)
?H
10
(10)
This can be determined indirectly from the heat of solution of Na2SO
4(s)
, and from the heat of solution of
Na2SO
4
10 H2O
(s)
:
Na2SO
4(s)
H2O
2 Na
+
(aq)
+ SO
4
2-
(aq)
?H
11
(11)
Na2SO
4
10 H2O
(s)
H2O
2 Na
+
(aq)
+ SO
4
2-
(aq)
+ 10 H2O
(l)
?H
12
(12)
The heat of solution of a compound is the heat absorbed or evolved when one mole of the compound is
dissolved in water to form a dilute aqueous solution. Since equation (10) can be obtained by adding the
reverse of equation (12) to equation (11), the heat of reaction is:
?H
10
= ?H
11
+ (- ?H
12
).
You will measure ?H
12
, but ?H
11
is too small to measure with the simple calorimeter system used here,
so the value -4.43 kJ/mol is provided from thermochemical tables.
