Thermochemistry

Question. What is first law of thermodynamics? How does it explain that (i) qv=∆E (ii) qp=∆H 

Answer. 

First law of thermodynamics: This law states that energy is neither created nor destroyed but can be changed from one form to another form. First law of thermodynamics is also called law of conservation of energy. OR Total energy of a system and its surroundings remains constant. 

Explanation. Consider a gas enclosed in a cylinder fitted with a frictionless piston. Let us assume that E1 be initial energy of the system. Now, there are two ways of transferring energy to system: by heating the system or by doing work on the system. We transfer the energy by both ways. If q is quantity of heat supplied to system and w is work done on the system by compressing piston from V1 to V2 and internal energy of system changes to E2, then

                                          ∆E= q + w ………. (i) 

      Equation (i) is mathematical form of first law of thermodynamics

Here ∆E is change in internal energy, q is quantity of heat supplied to the system and w is work done on the system. In laboratory, most work is pressure volume work, at constant pressure, we have 

                                         W = P∆V  

Substituting into equation i,

we get                               ∆E= q + P∆V ………….. (ii)

If work is done by the system due to expansion, then above equation can be written as 

                                         ∆E= q − P∆V …………… (iii) 

When volume of the gas is not allowed to change, then ∆V = 0 then equation (iii) becomes 

                                        ∆E=qv 

 This shows that change in internal energy at constant volume is equal to heat absorbed by system. When volume of the gas is allowed to change under constant pressure, the equation(iii) becomes

                      

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