Energy = Heat + Work



Any time we consider energy changes, we must think about the direction of energy transfer between the chemical system and the surroundings. When energy in any form goes into the chemical system (increasing its total energy) the energy change is a positive number. When energy is lost from the system (total energy decreases, becomes more stable), the energy change is a negative number.


Work

What is work? (No, studying chemistry is fun!)

work = force x distance


When we discuss work in chemical systems, we relate the work done on the system (w) to changes in pressure (P) and volume (V). At constant pressure:

w = -PV


If pressure is in units of atmospheres and volume is in liters, the work term will have units of liter-atmospheres (L atm). This can be converted to joules as follows:

1 J = = 9.869 x 10-3 L atm


Imagine that a quantity of air in a balloon is our initial state. If we supply an external force by squeezing the balloon, we decrease its volume. We've added energy to the system in this way.



In most chemical reactions the energy change associated with heat loss or gain is much greater than the work energy. The work done by or to a chemical system is important in:
  1. gas phase reactions where the number of product molecules is different than the number of reactant molecules, or
  2. when there is a change in phase between gas and liquid.



Temperature

Temperature and heat are different quantities. The temperature of a material is a physical property that we can measure with a thermometer or thermocouple. A thermometer relies on the regular change of volume of a liquid, either alcohol or mercury, with temperature to determine the temperature of some other material.

Temperature is related to the AVERAGE kinetic energy of the molecules or atoms in the material. The kinetic energy of molecules includes:
  1. translation; movement through a fluid (gas or liquid)
  2. vibration; stretches and bends of the molecule's bonds
  3. rotations of the molecule
The kinetic energy of all atoms or molecules in a material can be represented by a distribution function.

In the graph at right you can see 4 curves. The area under each of the curves represents the total population of molecules in a sample of material.

At the lowest temperature, T1, the range of kinetic energy values for the molecules in the sample is narrow so most of the molecules have energies close to the average kinetic energy of the sample.

As the temperature increases, the range broadens. At temperature T3, the average kinetic energy is higher than the average kinetic energy at temperatures T1 and T2 and lower than the average kinetic energy of the sample at temperature T4. However, some of the molecules in the sample have energies below the averages of T1 and T2. Others have energies even greater than the average of the sample at T4.

There are 3 common temperature scales. For all calculations involving thermodynamics, use the Kelvin scale for temperature. Of course, either Celsius or Kelvin is fine for T.
  1. Farenheit
      Water freezes at 32 deg F and boils at 212 deg F (at 1 atm).

  2. Celsius or centigrade
      Water freezes at 0 deg C and boils at 100 deg C (at 1 atm).
      (Tdeg C x 9/5) + 32 = Tdeg F

  3. Kelvin
      This is the absolute scale. At 0 K, molecular motion is zero. TK = Tdeg C + 273.15



Heat

Heat is thermal energy, that is energy not associated with work.

For any pure substance, there is a specific quantity of heat energy required to increase the temperature of 1 gram of the substance by 1 degree. This is called the specific heat or heat capacity.

q = m x C x T

Specific Heat at 25 deg C
substance C (J g-1 deg-1)
air 1.01
copper 0.385
ethanol 2.44
hydrogen 14.3
silica 0.703
water 4.18




Enthalpy

For constant pressure processes, the heat change is the enthalpy change or H. On the surface of the Earth, the pressure is constant at any given location at close to 1 atmosphere.




Professor Patricia Shapley, University of Illinois, 2011