Parr Instrument 6755 - Manual (Page 26)

5
Calculations
P a r r I n s t r u m e n t C o m p a n y 24
Figure 3
Exothermic Reaction
Example - Exothermic Reaction
Problem: Determine the change in enthalpy for
solid sodium sulfate, Na
2
SO
4
, when dissolved in
a 5 gram/liter aqueous solution of barium chlo-
ride, BaCl
2
• 2H
2
O.
Na
2
SO
4
m = 0.1458 grams
Ba ++ solution = 100.00 grams
Corrected temperature rise
∆T
C
= 0.042 °C (from Figure 3)
T(0.63R) = 24.885 °C
Energy equivalent
e = 121.46 cal/°C
Energy evolved
Q = (∆T
C
) (e)
= (0.042) (121.46)
= 5.1013 calories
Enthalpy change
=
=
= -34.99 cal/g @ 24.885 °C
Or, multiplying by 142.04 (the molecular weight
of Na
2
SO
4
)
= (-34.99) (142.04)
= -4.970 Kcal/mole @ 24.885 °C
0.1458 g solid Na
2
SO
4
dissolved in
100.00 g of 5 g/l aqueous
BaCl
2
• 2H
2
O
Temperature at
time of mixing
24.852 °C
R = ∆T
C
0.042 °C
0.63R 0.026 °C
T
i
24.859 °C
T(0.63R) = T
i
+ 0.63R 24.885 °C
T
f
= T
i
+ R 24.901 °C