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
Phase Changes of liquid to gas or gas to liquid:
ql↔g = mass x Heat of Vaporization = m x ∆Hvapor
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