Question

a) Define the following
A.First law of thermodynamics
B.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 of the process?

Answer 1

The heat supplied to the system is related to internal energy (formula given). We shall substitute the given values in the equation to find out the change in internal energy in the second part of the question. The standard enthalpy of formation is defined as the change in enthalpy of a compound during its formation from its constituent atoms.
Formula used:
$\Delta {\text{U = q - W}}$ where q is the heat, $\Delta {\text{U}}$ is the change in internal energy, P is the pressure of the gas and W is the work done.

Complete step by step answer:
A)Thermodynamics refers to the study of relation of heat and other forms of energy. It describes how thermal energy is converted to and from other forms of energy.
The first law of thermodynamics states that, in a process without the transfer of matter, the change in internal energy is equal to the difference of the heat supplied to the system and the work done. It can also we written as:
$\Delta {\text{U = q - W}}$ (Eq. 1)
B)The law is also an expression of the principle of conservation of energy, that energy can only be transformed from one state to another, it cannot be created or destroyed.
The standard enthalpy of formation is the change in enthalpy of the compound during the formation of 1 mole of the substance from its constituent elements., with all the substances in their standard state. In their standard states, like oxygen as a gas, water as a liquid etc. all have a standard enthalpy of formation of zero.

b)In the second part of the question, to calculate the internal energy of the process, we shall substitute the appropriate values in Eq. 1:
q = 701 J, W = 395 J
$\Rightarrow \Delta {\text{U = }}701{\text{ }} - {\text{ }}395{\text{ }} = {\text{ }}306{\text{ J}}$
So, the internal energy of the process will be 306 J.

Note:
The work done in a gaseous system is defined as the change in the temperature and pressure. It is written as:
${\text{W = }}\Delta \left( {{\text{PV}}} \right){\text{ = P}}\Delta {\text{V + V}}\Delta {\text{P}}$
In the equation to calculate change in enthalpy of a reaction, $\Delta {\text{H = }}\Delta {\text{U - P}}\Delta {\text{V}}$ , the enthalpy change can also regarded as the heat absorbed by the system. Thus, $\Delta {\text{H}}$ will be equal to heat.