Fun With Physics

The Four Laws Of Thermodynamics

The following are summaries of the four laws of thermodynamics.  Notice that the last one is called the Third Law so the numbering starts with zero.

It is assumed that you know the definitions of the words used here

Zeroth Law of Thermodynamics:                                                                     

There is a state function, called temperature which has the symbol T, which has the following relationship to heat, q :

addition of heat to a system will increase the temperature of the system.

if two closed system (together isolated), with different temperatures are brought into thermal contact, then the temperatures of the two systems will change to approach the same temperature.  That is, the temperature of the system which is at a higher temperature will decrease and the temperature of the system with the lower temperature will increase.  They will eventually have the same temperature.

The zeroth law leads to the general idea of heat capacity.  The symbols Cp and C v are used for this (constant pressure and constant volume) but for solid there is usually little difference between these two.  Using the relationship at constant volume (and therefore  Cv ) between a change in temperature, Δ T , of a substance and the amount of heat transferred, q,  to this substance is given by:

 q = Cv ΔT 

First Law of Thermodynamics                                                                           

There is a state function, the internal energy E (in some texts U), which has the following properties:

in an isolated system E remains constant

addition of work, symbol w, to a closed system will increase the internal energy by the amount of work expended.

This can be express by the following relation ship for a change in internal energy and work, w, done on a closed system:

         ΔE   =  q   +  w         

                                       

Definition of enthalpy, H and  ΔH

Use of internal energy or change in internal energy,  Δ E , is not very convenient in chemistry.  The reason for this is that when chemical reactions occur or samples are heated, the volume does not stay constant.  If one is therefore interested in only q, the  ΔE is complicated by an additional w.  To avoid this a new quantity called enthalpy is defined, given the symbol H.

    H = E + PV     or

    ΔH = ΔE + PΔV

Since at constant pressure PΔV = -- w if no other external form of work is present, then:

   ΔH = w + q + PΔ V 

and

   ΔH = q

Therefore at constant pressure ΔH will yield the heat transferred.  All thermodynamic tables use this as the tabulated "heat of reaction," etc.