# CHM110H5 Study Guide - Midterm Guide: Heat Capacity, Ammonium Chloride, Sodium Acetate

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9 Feb 2016
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Experiment 5
Measurement of Enthalpy of Reaction by Calorimetry
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Introduction
The study of reaction enthalpy is measuring the energy change of a reaction. If the energy
is needed, the change in energy will be positive or an endothermic reaction. However, if energy
is released, the change in energy will be negative or an exothermic reaction. The purpose of this
experiment was to measure the change in energy of several reactions and also the enthalpies of
neutralization of a strong and a weak acid. To perform the experiment, the system was assumed
to be a closed system, where heat remains within the system. A Styrofoam cup was used as a
calorimeter to insulate the heat released or absorbed from the reaction. The temperature change
from a reaction, which occurs inside the calorimeter, can be measured if the total heat capacity of
the system (calorimeter and contents) is known.
For instance, the heat q in Joules can be expressed as (1)
q=C p∆ T
where V is the volume in mL, ρ is its density in g/mL,
Cp
is the specific heat capacity in
J/gK, and
∆ T
(= Tfinal – Tinitial) is the change in temperature in K.
In the first part of the experiment, the specific heat capacity of the calorimeter is measured
and calculated using the formula (2):
|Heat lost by hot water| = |Heat gained by cold water| + |Heat gained by calorimeter|
where heat lost by hot water is qh, heat lost by cold water is qc and heat gained by calorimeter is
Cp
cal T
After obtaining
Cp
cal
, the value will be used in obtaining the heats of neutralization of
a strong acid, HCl and a strong base, NaOH:
NaOH(aq)+HCl(aq)
Na+(aq)+Cl-(aq)+H2O
To obtain the heat released by the reaction of either the strong or weak acid with the strong base,
the formula below will be used (3):
|Total heat lost| = |Heat absorbed by acid| + |Heat absorbed by base|+ |Heat absorbed by calorimeter|
where heat lost by acid is qa, heat lost by base is qb and heat gained by calorimeter is
Cp
cal T .
In the next part of the lab, the change in energy is observed when a salt, NH4Cl and NaA is
reacted with water.
NH4Cl(s)+H2O(l)
NH4+(aq)+Cl-(aq)
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