Parr Instrument 6755 - Manual (Page 25)

Calculations
6755 5
w w w . p a r r i n s t . c o m 23
Calculating the Energy Change
The energy change, Q, measured in this calorimeter
is calculated by multiplying the net corrected tem-
perature change, ∆T
c
, by the energy equivalent, e, of
the calorimeter and its contents.
Q = (∆T
c
)(e)
If ∆T
c
is measured in °C and e is expressed in calo-
ries per °C, Q will be reported in calories. (The
energy equivalent, e, is determined by a standard-
ization procedure).
The change in enthalpy, ∆H, at the mean reaction
temperature is equal to -Q divided by the amount of
sample used in the experiment, expressed either in
moles or grams.
where:
T is the temperature at the 0.63R point on the
thermogram.
Enthalpy values are usually expressed in kilocalories
per mole.
Procedures for converting enthalpy changes, ∆H, to
thermodynamic standard conditions and for using
∆H in other computations can be obtained from
thermodynamics or thermochemistry textbooks, or
from literature references.
Reading the Thermogram
In order to determine the net temperature change
produced by the reaction, it is necessary to interpo-
late a point on the thermogram at which the tem-
perature reached 63 percent of its total rise. This can
be done easily by following Figure 2, although other
variations of this method can be used as well.
1. Place a straight edge over the preperiod drift line
and extend this line well past the point at which
the reaction was initiated.
2. Move the straight edge to the postperiod drift
line and extrapolate this line backward to the
mixing time. If there are fl uctuations in the
drift lines due to noise or other variations in the
signal, use the best average when drawing these
extrapolations.
3. Using a centimeter scale, measure the vertical
distance, R, between the two extrapolated lines
at a point near the middle of the reaction period.
4. Multiply the distance, R, by 0.63.
5. Set the zero end of the centimeter scale on the
extrapolated preperiod drift line and move the
scale along this line to locate a vertical intercept
with the thermogram which is exactly 0.63R
above the preperiod drift line. Draw a vertical
line through this point to intercept both drift
lines.
6. Read the initial temperature, and the fi nal tem-
perature, at the points of intersection with the
drift lines and subtract to determine the correct-
ed temperature rise, ∆T (see Figure 2)
∆T
c
= T
f
- T
i
Figure 2
Thermogram