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First Law Or Law Of Conservation Of Energy - (Concept)

First Law or Law of Conservation of Energy 

It was introduced by Helmholtz and according to it "Energy can neither be created nor destroyed but can be converted from one form to another or the total energy of the universe is constant",

It can also be written as: 

  • Energy of an isolated system must remain constant, although it may be transformed from one form to another.

  • Energy in one form, if it disappears will make its appearance in an exactly equivalent in another form.

  • When work is transformed into heat or heat into work, the quantity of work is mechanically equivalent to the quantity of heat. 

  • It is never possible to construct a perpetual motion machine that could produce work without consuming any energy. 

Thus if heat is supplied to a system it is never lost but it is partly converted into internal energy and partly in doing work in the system that is,

Heat supplied = Work done by the system + Increase in internal energy

So increase in internal energy = Heat supplied - work done by the system 

\textrm{ie. } \Delta E =q+w \ \ \ \ \ \ \ [ \because \textup{ work done by the system is -w}]

Mathematical Formulation of the First Law

If a system absorbs 'q' amount of heat and its state changes from X to Y and this heat is used up.

(i) On increasing the internal energy of the system

 \begin{array}{l}{\mathrm{\Delta E=E_Y-E_X}}\end{array}

(ii) In order to do some external work (W) on the surrounding by the system. 

From the first law, we get the relation

\\\Delta \mathrm{E}=\mathrm{Q}-\mathrm{W}\textup{ (that is, work done by the system =W)} \\ d \mathrm{E}=d \mathrm{Q}-d \mathrm{W}\textrm{ or }d \mathrm{E}=d \mathrm{Q}-\mathrm{P} d \mathrm{V}\) \\\textrm{Work done by the system or in expansion}


OR

 

\\\Delta \mathrm{E}=\mathrm{Q}+\mathrm{W}\textup{ (that is, work done by the system =W)} \\ d \mathrm{E}=d \mathrm{Q}+d \mathrm{W}\textrm{ or }d \mathrm{E}=d \mathrm{Q}+\mathrm{P} d \mathrm{V}\) \\\textrm{Work done by the system or in compression}

For the take of simplicity, remember the formula \\\Delta \mathrm{E}=\mathrm{Q}+\mathrm{W} and when the work is done by the system, work is negative, and when the work is done on the system.

Exam Chapter
JEE MAIN Chemical Thermodynamics
Chemistry Part I Textbook for Class XI
Page No. : 164
Line : 1

Let us consider the general case in which a change of state is brought about both by doing work and by the transfer of heat. 


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