What is Thermodynamics? 

Thermodynamics is the branch of physics which consists of analyzing and understanding the concept of the transfer of energy, temperature, pressure and work done by or to the system accompanying a chemical reaction.

If a system that is cold by nature comes in contact with a body whose temperature is higher, then there is a heat transfer from the hotter body to the colder body. For example, when ice is placed on a heated stove, the ice melts rapidly. The word 'thermodynamic' comes from 'thermo,' meaning temperature, and 'dynamics,' meaning flow of the particles in a state of matter. Thus, thermodynamics is the flow of energy in the form of temperature, heat, pressure and energy. 

In physics, the components of thermodynamics are the system, surroundings, and boundary. The systems are divided into three types: open systems, closed systems, and isolated systems. The physical properties of the system depend on the intensive properties  (which are things like pressure, temperature, specific heat, and density) and extensive properties (like heat capacity, mass, volume, and Gibb's free energy).

Heat and Temperature 

"Direction of heat flow."

Heat energy is the transfer of energy between substances or systems due to a temperature difference between them. Heat is conserved and can also be transferred. Heat can be converted from potential energy to kinetic energy, or electrical energy to electromagnetic energy, like a motor engine and light bulb, respectively.

Temperature is the amount of heat transferred by a system depending on the number of molecules and the velocity of the molecules. The higher the temperature, the velocity of molecules transfers heat at a more rapid rate. This factor rises to the boiling points and melting points of the substance or system. A heat engine is a system that converts thermal energy to mechanical energy. 

Heat Transfer and Specific Heat 

In a physical system, the amount of heat required to increase the temperature of a certain mass of a system by a certain specific amount is known as the specific heat capacity of the system. It depends on the number of atoms of the system. For example, one gram of aluminum can absorb and transfer heat more than one gram of copper due to a difference in the number of atoms.

There are three modes of heat transfer: conduction, convection and radiation.

  • Conduction is a molecular transfer of heat energy when there is a direct contact between the substances. It occurs only in solid-state matter.
  • Convection is the transfer of heat energy between fluid state of matter (liquid or gas) where the molecules of the fluids make contact with a solid substance that transfers the heat energy. The rate of heat transfer depends on the surface area of the solid state system.
  • Radiation is the emission of electromagnetic energy with the help of photons which carry the heat energy. The surface's radiation level depends on how much electromagnetic radiation is absorbed by the surface.  

Internal Energy

In physics, all the substances or systems possess a definite amount of energy, which depends on factors like temperature, pressure and volume, where the resultant energy of the system, which is called the net energy, is additive in nature. The net energy is the sum of all the internal energy of the system. 

The Carnot Cycle 

The Carnot cycle is a heat engine that encapsulates the depended of the fundamental factors like pressure, volume, temperature of gases and the input energy required to change the form of the system and do work outside the system. The compression of gas increases its temperature and thus the air around it becomes hot. This heat can be removed by heat exchange, which expands the air and cools it. An air conditioner is an example of this. The heating of a gas increases its pressure by expansion and this pressure is installed into a piston, converting the thermal energy to kinetic energy.  

The Laws of Thermodynamics 

There are four laws of Thermodynamics: 

The zeroth law of thermodynamics states that when the first system is in thermal equilibrium with the third system, and when the second system is in thermal equilibrium with the third system, then the first and the second system are also in equilibrium with each other. This law mainly depends on the temperature and the property or state of matter. 

The first law of thermodynamics states that the net increase in the energy of  a system is equivalent to the increase in the thermal energy and the work done by the system. This is nothing but the additive nature of the internal energy of the system, external heat transferred to the system, and the work done by the system. This law is also called the law of conservation of energy. 

"First law"

The second law of thermodynamics states that heat energy cannot be transferred from a lower temperature to a higher temperature without any external addition of energy, and at a particular temperature, the heat cannot be converted into work. 

The entropy of a closed system is a quantitative measure of the amount of thermal energy that is not present to do work. When the thermodynamic system leaves out unused energy, the increase of unused energy of that system is the increase of the entropy of the system. It is the measure of randomness in a closed system. For example, if we mix two different liquids of two different temperatures and chemical components, after it has become one, it is impossible to separate them back to the same original temperature and composition of substances. It is heat energy per unit time. 

The equation of entropy is a measure of randomness of a system. Hence ΔS=ΔSfinal -Sinitial. The comparison of entropy of three states of matter is: Sgas > Sliquid >Ssolid 

So, as per the second law, the entropy of a closed system reaches its peak value as the time increases. 

The third law of thermodynamics states that the entropy of a pure crystal at absolute zero is zero. If the crystal's system is perfectly ordered, if the temperature has a positive value, then there is a movement of particles inside the crystal that eventually causes a disorder. Hence, the statistical measure of a physical system is positive, hence the entropy is positive.  


Enthalpy is defined as the statistical mechanics of the heat stored in the system under a certain condition. The formula for enthalpy is H=E+PV. H is the enthalpy, E is the energy and PV is a constant. Now due to the heat transfer and change in heat and energy, the equation becomes  

Types of Thermodynamic Process

First, thermodynamics can be categorized into spontaneous and non-spontaneous processes, then further fundamentally classified into six types of processes, which are: 

  • Adiabatic process
  • Reversible process
  • Irreversible process
  • Isothermal process
  • Isobaric process
  • Isochoric process

Real-Life Applications of Thermodynamics

  • This law is seen in air conditioners, refrigerators, heat pumps, and freezers. 
  • Thermodynamics are seen in plenty of chemical reactions in chemical and manufacturing industries. 
  • All automobile vehicles works on the Carnot's cycle and thermodynamic principles. 
  • Cooking our day to day food is only possible due to the thermodynamic process. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for    

  • Bachelor in Physics and Chemistry 
  • Master in Physics and Chemistry 

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