Question

Is energy a state function?

Answer 1

State function is an inherent property of any system whose overall values depend only on the state of the system. State function is independent of the path and the manner by which the particular state of the system is reached.

Complete answer:
State function is also known as point function or function of state because it only depends on the thermodynamic state of the system; it may be in solid state, liquid state, gaseous state, suspension or emulsion.
The changes in the state function only depends on the initial state of the system and the final state of the system.
Every substance in nature has some fixed amount of energy which is also known as internal energy. Energy of the system depends on the nature of substance, temperature, and pressure.
Energy in substance can exist in many forms like internal energy, kinetic energy, potential energy.
Change in internal energy is always considered as the difference between the internal energies of the two states of the system.
For example- two substances $A$ and $B$ have internal energy of magnitude ${U_A}$ and ${U_B}$. Then the difference between the internal energies between these two states will be expressed as:
$\Delta U =$ ${U_B}$ $-$ ${U_A}$
The value we get as the difference between the internal energies of the system are fixed in nature and will be independent of the path chosen between two different states $A$ and $B$.
Thus, we can say that energy of any system is a state function. This means that $\Delta U$ depends only on the initial state and the final state of the system and remains independent of the path. This is also explained as, $\Delta U$ will be the same even if the change is brought about differently.
As we see, internal energy of the system is state function which is a form of energy hence, energy is also considered as state function.
Temperature, volume, pressure, moles and mass are also examples of state function.
$\Rightarrow$ From the above discussion we see that energy depends only on the state of the system hence, known as state function.

Note:
Other properties which also come under state functions are enthalpy, entropy and internal energy. According to the first law of thermodynamics energy is the sum of heat and work which is expressed as $\left( {E = q + w} \right)$. Energy here is a state function but heat and work are path functions.