Heat and Wrok:

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Heat and work are both forms of energy. They are also related forms, in that one can be transformed into the other. Heat energy (such as steam engines) can be used to do work (such as pushing a train down the track). Work can be transformed into heat, such as might be experienced by rubbing your hands together to warm them up.

Work and heat can both be described using the same unit of measure. Sometimes the calorie is the unit of measure, and refers to the amount of heat required to raise one (1) gram of water one (1) degree Celsius. Heat energy is measured in kilocalories, or 1000 calories. Typically, we use the SI units of Joules (J) and kilojoules (kJ). One calorie of heat is equivalent to 4.187 J. You will also encounter the term specific heat, the heat required to raise one (1) gram of a material one (1) degree Celsius. Specific heat, given by the symbol "C", is generally defined as:

Where:

C = specific heat in cal/g-°C
q = heat added in calories,
m = mass in grams
ΔT = rise in temperature of the material in °C.

The value of C for water is 1.00 cal/g-°C.

The values for specific heat that are reported in the literature are usually
listed at a specific pressure and/or volume, and you need to pay attention to
these settings when using values from textbooks in problems or computer models.

Example Problem:

If a 2.34 g substance at 22°C with a specific heat of 3.88 cal/g-°C is heated with 124 cal of energy, what is the new temperature of the substance?

Answer:

new T = 22 + 13.7 = 35.7°C

Two other common heat variables are the heat of fusion and the heat of vaporization.
Heat of fusion is the heat required to melt a substance at its melting temperature, while
the heat of vaporization is the heat required to evaporate the substance at its boiling point.

Chemical work is primarily related to that of expansion. In physics, work is defined as:

Where:

w = work, in joules (N×m) (or calories, but we are using primarily SI units)
d = distance in meters
f = opposing force in Newtons (kg*m/s2)

In chemical reactions, work is generally defined as :

w = distance × (area × pressure)

The value of distance times area is actually the volume. If we imagine a reaction taking place in a container of some volume, we measure work by pressure times the change in volume.

w = ΔV × P

Where:

ΔV is the change in volume, in liters

If ΔV=0, then no work is done.

Example Problem:

Calculate the work that must be done at standard temperature and pressure
(STP is 0°C and 1 atm) to make room for the products of the octane combustion:



Answer:

  Knowing that 25 moles of gas are replaced by 34 moles of gas in this reaction, we can
  calculate a net increase of 9 moles of gas. Knowing the molar volume of an ideal gas at
  STP (22.4 L/mol), the change in volume and the work of expansion can be calculated

  dV = 9 moles ∗ 22.4 L/mol = 202 L

  The external pressure is 1.0 atm (standard pressure), so the work required is:

  w = dV ∗ P = 202 L ∗ 1.00 atm = 202 l-atm

  Using the conversion factor of 1 L-atm = 101 J, the amount of work in joules is:

  w = 202 L-atm ∗ 101 j/L-atm = 2000 J, or 2kJ of energy.

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