CHM 101/102
Hess’s Law
General Chemistry 101/102
Laboratory Manual
University of North Carolina at Wilmington
Laboratory Manual
CHM 101/102
Hess’s Law
•
Purpose
 To use Hess’s Law to determine the enthalpy
change (DH) for a chemical reaction that cannot
be easily measured by normal experimental
methods.
•
Safety and Waste Management
 The concentration of HCl used in this experiment
will eat holes in your clothing and will cause burns
to your skin. Handle with care.
 The products of the reactions in this experiment
can be poured down the sink and flushed with
water.
Laboratory Manual
CHM 101/102
Hess’s Law
•
Background
 When a chemical reaction is carried out in
aqueous solution, the heat lost or gained by the
reaction is equal to the heat gained or lost by the
solution.
(1)
qsolution = - qreaction
The minus sign is necessary since heat loss
is an exothermic process (q is negative)
while heat gain is an endothermic process (q
is positive).
Laboratory Manual
CHM 101/102
Hess’s Law
•
Background
 The enthalpy change (DH) for a chemical reaction
taking place in aqueous solution can be calculated
from the following relationship :
specific heat capacity of
solution (4.18 J/gºC)
DHrxn =
mass of solution
(in grams)
temperature change
(in ºC)
-qsoln
= s x g x DT
moles reactant
moles reactant
Laboratory Manual
CHM 101/102
Hess’s Law
•
Background
 In today’s experiment, you will determine the value
of DH for this reaction
(1)
CaCO3(s) + H2O(l)
Ca(OH)2(s) + CO2(g)
DHrxn(1)
 By experimentally determining the value of DH for
each of these two reactions
(2)
CaCO3(s) + 2 HCl(aq)
CaCl2(s) + H2O(l) + CO2(g)
DHrxn(2)
(3)
Ca(OH)2(s) + 2 HCl(aq)
CaCl2(s) + 2 H2O(l)
DHrxn(3)
 According to Hess’s Law
DHrxn(1) = DHrxn(2) + (-DHrxn(3))
Laboratory Manual
CHM 101/102
Hess’s Law
•
Procedure
 Weigh 0.90 g of CaCO3 into a coffee
cup calorimeter. Add 5.0 mL of water
to the calorimeter and stir the mixture.
 Place a temperature probe in the calorimeter
and start measuring the temperature.
 After 1 minute, pour 20.0 mL of
HCl(aq) into the calorimeter. Stir
the solution and continue measuring
the temperature until it finally begins
to decrease.
 Clean and dry the calorimeter and repeat this
procedure using 0.60 g of Ca(OH)2.
Laboratory Manual
CHM 101/102
Hess’s Law
•
Procedure
Laboratory Manual
CHM 101/102
power connector and
serial/USB connector
USB connector on computer
temperature probe connected to
the Microlab unit
Laboratory Manual
2
CHM 101/102
1
graph area
3
data table
Laboratory Manual
live data