5. ThermoChemistry



The science of transformation of energy.


The capacity to do work or to transfer heat.


Work is one means of transferring energy from our system to the surroundings.  Work is given the symbol w in equations. Work has several aspects to it.  There is mechanical work, electrical work, light-energy work and work of expansion or contraction of a gas (PV work).  In chemistry, we generally are only interested in PV work and will use the symbol w to refer only to that type of work.  All other types of work will be referred to using a prime on the symbol, w'.

.One of the most common definitions for work is when a force f displaces an object by a distance \Delta x does work w = f \times \Delta x.  (also known as w = f \times d).  This is a mechanical work and serves as the basis for our definition of PV work.

.Work can also be done by pushing back the atmosphere (piston in a cylinder). In this case, one pushes back against the atmospheric pressure P_{atm} and moves the piston a distance \Delta x. The force needed to push back the atmosphere is actually P_{atm}\times A where A is the area of the piston. Hence, we can calculate the magnitude of the work done as

\[|w| = P_{atm}\times A \times \Delta x = P_{atm}  \times \Delta V\]

We call this atmospheric work.  The only thing left to decide is the sign convention. (Described later)  Since the system increased in volume and hence, did the work of pushing back the atmosphere (positive value for ΔV), it has used up energy so we can now write down a more complete definition for PV work as:

w = -Patm × ΔV

In chemical situations, we are normally interested only in the atmospheric work done when gases are evolved or used up in the reaction.

Kinetic and Potential Energy

both of these can be forms of work and of heat energyKE = 1/2 mv2
PE = mgh    lifting mass m  by height h  against
acceleration due to gravity, g.  Since mg is the ‘force” of gravity and  is really a distance, we can write.
PE = f × d     force times distance.

Work is organized energy, lifting a book, etc.

Heat (defined later) is random (disorganized) energy that is released or absorbed, often as a result of some work being done, e.g., sound (energy) dissipates through air, when a book drops and hits the floor and the floor gets warmer (molecules vibrate more rapidly) spreading out from the spot where the hit occurred.  Thus, the work done to raise the book (increasing its potential energy) is converted to kinetic energy upon release and then to thermal energy (energy is released as heat) as the book strikes the floor.

One can also transfer energy into or out of a sample via light-energy and electrical work
(We include this type of work in our catchall term w’ ).SI units of energy (work or heat):Let’s start with work: w has dimension of force times distance.
The SI units are N m = kg m2 s-2 = Joules (J). These are the same units used for heat transfer.


Heat is the Energy transferred between the system and the surroundings as a result of a temperature differential.  This is the only kind of energy transfer that does not involve doing work. Heat is given the symbol q in equations.

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