Some Basic Thermodynamic Process that You Should Know

What is the thermodynamic process?

A thermodynamic process is an operation by which a thermodynamic system changes from one state to another state. This process is always accompanied by a change in energy. In the case of open system change in the matter may also take place.

Consider a cup of tea kept on the table. If you observe it you can see the vapors which are the gaseous form of water. Now as time goes, the temperature of tea decreases. Or we can say there is a change in the state of tea.

This indicates a thermodynamic process which is carried out in an open atmosphere known as an isobaric process – one of the types of thermodynamic process.

Let get into some important and basic types of thermodynamic processes.

Types of Thermodynamic Processes

# Isothermal process

You may have performed an experiment by carrying in the thermostat. The thermostat helps to maintain the temperature of the system. The process by which the temperature of the system remains constant.

Even heat enters or leaves the system but the temperature remains constant throughout the process. If the reaction carrying in the system is exothermic, heat is given out to the surroundings instantaneously.

That’s why the temperature of the system doesn’t rise at all. On the other hand, if the reaction is endothermic (heat is absorbed for the reaction) heat is absorbed by the system. Hence again temperature doesn’t fall at any stage.

Change in temperature dT=0

As we know internal energy is directly proportional to the temperature. But here the change in temperature is zero or you can say it is constant. So there is no change in internal energy of the system.

If you consider the first law of thermodynamics which dQ= dU+dW. It gets modified for this process because the change in temperature is zero (dT)=0. Obviously, the change in internal energy is also zero (dU=0).

Modified first law of thermodynamics for the isothermal process is dQ= dW. This suggests that heat supplied to the system get converted into the external work done.

Opposite to isothermal process, no heat enters or leave the system during any step of the process. Let try to understand first case i.e heat doesn’t enter the system. When the reaction is endothermic and heat doesn’t enter the system, the temperature of the system will obviously decrease. Because the heat inside the system will be absorbed the reacting substance inside the system.

If the reaction is exothermic, the temperature will increase. Now consider the second case – heat doesn’t leave the system. If the reaction is exothermic, the temperature of the system will increase and if the reaction is endothermic then the temperature will decrease. Such processes are generally carried out in insulated containers

Change in heat dQ=0

Now for the adiabatic process, the first law of thermodynamics gets modified because the change in heat is zero (dQ)=0.

Modified first law of thermodynamics for the adiabatic process is dU= – dW. This suggests that the internal energy of the system increases when external workdone on the system. On the other hand, if external work done by the system, the internal energy of the system decreases.

# Isobaric process

As the word suggests, isobaric means pressure remains constant. In this process pressure of the system during the change of the state remains constant.  Consider the example of the boiling point of water and its vaporization. These processes occur at the same atmospheric pressure. Isobaric processes are carried out in the open atmosphere.

change in pressure dP=0

# Isochoric process

In this process, the volume of the system remains constant throughout the process. Imagine a non-expandable container or closed container filled with a substance. Now if we heat the system, the substance will expand but the volume of the system remains the same. That’s why such processes are carried out in closed containers.

change in volume dV=0

Now for the isochoric process, the first law of thermodynamics gets modified because the change in heat is zero (dV)=0.

Modified first law of thermodynamics for the adiabatic process is dU= dQ. This suggests that the heat supplied to the system get converted into internal energy of the system.

# Reversible process

The process in which change is carried out so slowly that the system and surroundings are always in equilibrium. This process can be reversed by making infinitesimal changes in the property of the system. The reversible process takes the infinite number of steps to occur and takes infinite time to complete. So the perfect reversible process is impossible.

# Irreversible process

This concept is exactly opposite to the reversible process. In the reversible process, system and the surrounding is in thermodynamic equilibrium throughout the process. But in the irreversible process system and surrounding is in equilibrium but in initial and final states. This process occurs in a single step and cannot be reversed. Natural processes are a good example of an irreversible process.

Conditions for thermodynamic equilibrium

A system is said to be in thermodynamic equilibrium with surrounding if three equilibriums – mechanical, thermal and chemical equilibrium is achieved.
Mechanical equilibrium – When work done by the system or on the system is zero.
Thermal equilibrium – When the temperature of the system remains constant throughout along with surrounding.
Chemical equilibrium – When the composition of the system remains constant and definite.