Question

a) Define following
(i) First law of thermodynamics.
(ii) Standard enthalpy of formation.
b) In a process 701 J of heat is absorbed by a system and 395 J of work is done by the system. What is the change in internal energy for the process?

Answer 1

• (a)
  

(i) First law of thermodynamics $\to$

The first law, also known as the law of conservation of energy, states that energy cannot be created or destroyed in an isolated system.

(ii) Standard enthalpy of formation $\to$

The standard enthalpy of formation of a compound is the change of enthalpy during the formation of $1$ mole of the substance from its constituent elements, with all substances in their standard states.

• (b) Given that the heat is absorbed by the system and work is done by the system.
  

Therefore,

$q=701J$

$w=-395J$

Now from first law of thermodynamics,

$\Delta U=q+w$

$\Rightarrow \Delta U=701+(-395)$

$\Rightarrow \Delta U=306J$

Hence the change in internal energy for the given process is $306J$.