What is Internal Energy: Overview, Questions, Preparation

Every substance or molecule has a certain amount of energy present in them. The definition, formula, and the factors affecting this energy are discussed below

Internal energy is the energy that is inherently present in a system. It develops due to the motion of the particles in a substance or molecule.

The letter U denotes the internal energy of a system. There are two energies responsible for internal energy - kinetic energy and potential energy. The kinetic energy is generated due to the motion of the particles in the molecule. The vibration of these atoms causes potential energy to generate. 

Therefore, for a closed system, the formula for internal energy is as follows:

ΔU = q + W

Where,

ΔU is the changing internal energy. 

q is the total heat generated in the system.

W is the total work done.

The SI units of measurement of internal energy are J or joules. The SI base unit is m2⋅kg/s2.

The various components of the internal energy of a system are as follows:

• Thermal energy

Molecular rotation and vibration generate energy. This energy generated releases heat in the system. This heat is known as sensible heat that forms a part of the internal energy. 

Also, when a molecule changes from liquid to gaseous state, latent heat will be generated. This heat also forms a component of internal energy.

• Chemical energy

The chemical bonds formed in a molecule between different atoms or ions release energy. This energy is a component of the internal energy in a system.

• Nuclear energy

The bonds present in the nucleus of an atom generate energy in the atom. This energy forms an essential component of internal energy.

• When a molecule goes from a higher temperature to a lower temperature, there is a heat transfer. This may affect the internal energy of the system.
• When two components are mixed, heat is released. This heat may lower the internal energy of the system.
• Changes in the physical properties of a system because of expansion or compression may affect its internal energy.
• A chemical reaction between molecules may change the chemical energy of a system. This change may also affect the internal energy.

In class 11, the chapter thermodynamics explains and defines the three laws of thermodynamics. It distinguishes between the different types of states and explains different processes. It carries a weightage of 20 marks in the exam. 

1. Calculate the difference in the internal energy of the working fluid. The heat passed during the compression stroke is 70 KJ/kg, and the work is 250 KJ/kg.

Answer:

Work input W = – 250 KJ/kg

Q = – 70 KJ/kg

Using the formula,

Q =  ΔU – W

ΔU = – 70 + 250

ΔU = 180 KJ/kg

2. On burning 1g of benzoic acid (molecular mass = 122), the heat evolved is 3150cal at 25oC. Calculate U.

Answer:

The heat evolved while burning 1 mol (122g) of benzoic acid is

=−3150×122/1

= -384,300 cal

Hence ΔU = -384.3 Kcal mol-1

Q: How is the internal energy of a system measured?

Q: Does internal energy depend on mass?

Q: What is the relationship between the internal energy and temperature of a system?

Q: Is internal energy a state function?

Q: What is the significance of internal energy?