Experiment A8- Calorimetry - 1 EXPERIMENT A8 CALORIMETRY...

1
E
XPERIMENT
A8: C
ALORIMETRY
Learning Outcomes
Upon completion of this lab, the student will be able to:
1) Measure the heat of a reaction under constant pressure conditions.
2) Calculate the enthalpy change for a reaction using the enthalpy change of
two other reactions and Hess’s law of heat summation.
Introduction
Enthalpy and Calorimetry
Enthalpy is defined as the heat of the reaction measured under constant pressure
conditions. Since most reactions conducted in a laboratory are under constant
pressure (i.e. atmospheric pressure), a measurement of the heat of the reaction is
also a measure of the enthalpy change for that reaction. Heat of a reaction can be
measured using a calorimeter.
A calorimeter requires two components: 1) an insulated container that minimizes
loss of heat to the surroundings and 2) a thermometer to measure the temperature
before and after a chemical reaction. A commonly used insulated container is a
Styrofoam cup.
Hess’s Law
According to the additive properties of enthalpy change ( Δ H), Hess’s law of heat
summation states that states that the total enthalpy change during the complete
course of a chemical reaction is the same whether the reaction is made in one step
or in several steps. For instance, assume the enthalpy change for reactions 1 and 2
below are respectively, Δ H
1
and Δ H
2
.
Reaction 1: A + B ! C + D Δ H
1
Reaction 2: C + E ! B + F Δ H
2
Then according to Hess’s law the enthalpy change for reaction 3, which is the sum of
reactions 1 and 2, shown below:
Reaction 3: A + E ! D + F Δ H
3
Δ H
3
is the sum of Δ H
1
and Δ H
2
Therefore: Δ H
3
= Δ H
1
+ Δ H
2

2
Experimental Design
In this experiment two sets of reactions will be performed. The first set of reactions
involves sodium hydroxide and hydrochloric acid. The enthalpy change for the
following three reactions: 1) solubilization of solid sodium hydroxide in water 2)
reaction between solid sodium hydroxide and aqueous hydrochloric acid and 3)
reaction between aqueous solutions of sodium hydroxide and hydrochloric acid will
be measured using the calorimetry method. The molecular equations for these three
reactions are shown below:
1. NaOH
(s)
! NaOH
(aq)
Δ H
1
2. NaOH
(s)
+ HCl
(aq)
! NaCl
(aq)
+ H
2
O
(l)
Δ H
2
3. NaOH
(aq)
+ HCl
(aq)
! NaCl
(aq)
+ H
2
O
(l)
Δ H
3
Using the measured values of Δ H
1
and Δ H
2
, a calculated value of Δ H
3
can be obtained
using Hess’s law. The calculated and measured values of Δ H
3
will then be compared
with each other to confirm Hess’s law.