CHEM 103 MODULE 3 NOTES.
3.1: THERMOCHEMISTRY
Thermodynamics is the study of the relationship between heat and other forms of energy, particularly mechanical work. Thermochemistry is the part of thermodynamics that deals with the quantity of heat given off or absorbed during a chemical reaction. ...
chem 103 module 3 notes 31 thermochemistry thermodynamics is the study of the relationship between heat and other forms of energy
particularly mechanical work thermochemistry is the part of therm
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CHEM 103 MODULE 3 NOTES.
3.1: THERMOCHEMISTRY
Thermodynamics is the study of the relationship between heat and other forms
of energy, particularly mechanical work. Thermochemistry is the part of
thermodynamics that deals with the quantity of heat given off or absorbed
during a chemical reaction. The quantity of heat given off or absorbed during
a physical change or temperature change can also be studied, and we will
refer to this process as calorimetry.
In order to adequately discuss thermochemistry, we need to define some
common terms.
System - the object (or substance) being studied
Open system - a system that permits the transfer of mass and energy with the
surroundings
Closed system - a system that permits the transfer of energy but not mass
with the surroundings
Isolated system - a system that does not permit the transfer of energy or
mass with the surroundings
Surroundings - the rest of the universe interacting with the system
Energy - the potential or capacity to move matter: the ability to do work
(unit is J = joule)
Work - the amount of energy transferred by a force acting through a distance
Kinetic energy - the energy possessed by an object by virtue of its motion (unit
is J = joule)
Potential energy - the energy possessed by an object by virtue of its position
(unit is J
= joule)
Heat (q) - the thermal energy transferred between system and surroundings
due to a difference in temperature between them (unit is J = joule)
Enthalpy - the total energy of a system
Heat of reaction - (ΔH) the amount of heat (q) gained or lost during a
chemical reaction
Exothermic - a reaction with a - ΔH
Endothermic - a reaction with a + ΔH
You must be able to use the terms above to describe a thermochemical
system.
Calorimetry
The energy change that accompanies a physical, temperature, or chemical
change is determined by carrying out the process in a device known as a
calorimeter. The calorimeter is able to measure the amount of heat absorbed
or evolved as a process takes place. A Styrofoam coffee cup calorimeter can
be used to measure an energy change that takes place at constant
pressure. An enclosed bomb calorimeter is used to measure an energy
change that takes place at constant volume with a change in pressure.
We will use the term calorimetry to refer particularly to measuring energy
changes that accompany temperature and physical change (state change or
phase change) processes.
,Temperature change calorimetry measures the thermal energy change
occurring as a system at higher temperature transfers kinetic energy to a
system at lower temperature, which is reflected by a change in temperature
for the overall system. This is demonstrated below by adding a 15.6-gram
piece of aluminum (heated to 100oC) to a
45.6 gram sample of water at 26.7oC in a coffee cup calorimeter. The final
temperature
, of this system can be predicted using the equations below and several facts
about the materials (Al and H2O).
Heat temp change = qtemp change = mass x specific heat (heat capacity) x temp
change = m x c x ∆t
The specific heat data for most substances is known and can be found in
data tables. The specific heat value for liquid water (not steam or ice) is
4.184 J/g oC and for aluminum is 0.899 J/g oC.
In the example given, the hot Al transfers heat to the colder water until the
two substances reach the same temperature. This can be predicted by
setting the heat lost by the Al equal to the heat gained by the water as in
the equation below:
(mAl x cAl x ∆tAl) = (mH2O x cH2O x ∆tH2O)
However, since the Al is losing heat, we'll use a negative sign in front of the
heat loss equation.
- (mAl x cAl x ∆tAl) = (mH2O x cH2O x ∆tH2O)
We know ∆t = Tempmixture - Tempinitial, so we can substitute the data to get:
- [15.6 g x 0.899 J/g oC x (Tmix - 100oC)] = [(45.6 g x 4.184 J/g oC x (Tmix -
26.70oC)]
This final temperature appears to make sense since the value should be
between 100oC and 26.7oC, but closer to 26.7oC since a greater amount of
water than Al was used.
Incidentally, the actual experimental final temperature of the mixture is only
31.4oC since the coffee cup, cover, and thermometer have absorbed some
thermal energy. This absorption of thermal energy by the calorimeter can be
corrected for by determining the heat capacity of the calorimeter.
Phase change calorimetry measures the energy change occurring as a
substance changes from one phase (state) to another, such as water
melting or boiling, or ice freezing or steam condensing. In this case, no
temperature change occurs, but the energy change causes the particles of
the substance to form or break intermolecular bonds and change from one
state to another. The equations used to do phase change calorimetry
calculations are shown below:
Phase changes of solid to liquid or liquid to
solid:
qs↔i = mass x Heat of Fusion = m x ∆Hfusion
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