Thermodynamics and Thermochemistry

Laws of Thermodynamics

Internal energy

• Internal energy (U) represents the total energy of a system (i.e., the sum of chemical, electrical, mechanical or any other type of energy).

• Internal energy of a system may change when:

  • Heat passes into or out of the system
  • Work is done on or by the system
  • Matter enters or leaves the system

Work

• For an adiabatic system which does not permit the transfer of heat through its boundary (shown in the figure), a change in its internal energy can be brought by doing some work on it.

• Initial state of the system, (1)

Temperature = T₁

Internal energy = U₁

• When some mechanical work is done, the new state (2) is obtained.

Temperature at state 2 = T₂

Internal energy at state 2 = U₂

• It is found that T₂ >T₁

Change in temperature, ΔT = T₂ − T₁

Change in internal energy, ΔU = U₂ − U₁

• The value of internal energy (U) is the characteristic of the state of a system.

• The adiabatic work (W_ad) required to bring about a change of state is equal to the change in internal energy.

ΔU = U₂ − U₁ = W_ad

• Thus, internal energy (U) of the system is a state function.

• When work is done on the system, W_ad = + ve

• When work is done by the system, W_ad = − ve

Heat

• Internal energy of the system can also be changed by transfer of heat from the surroundings to the system or vice versa, without doing any work.

• This exchange of energy, which is a result of temperature difference, is called heat (q).

• A system which allows heat transfer through its boundary is shown in the figure.

• At constant volume, when no work is done, the change in internal energy is, ΔU = q

• When heat is transferred from the surroundings to the system, q is positive.

• When heat is transferred from the system to the surroundings, q is negative.

General Case

• When change in state is brought about both by doing work (W) and by transfer of heat (q):

Change in internal energy, ΔU = q + W

• If W = 0 and q = 0 (i.e., no transfer of energy as heat or as work), then

ΔU = 0

This means that for an isolated system, ΔU =…