What is Physical Chemistry?

Chemistry is an interdisciplinary science concerned with studying the properties of matter and the way substances interact with each other. Physical chemistry combines chemistry findings with physics. It aims to explain the principles of matter interactions and energy exchange at the atomic and molecular level. A degree in physical chemistry enables people to find employment in many sectors that use physical and chemical principles and experimental methods such as science, technical, or medical institutes or development control labs in the pharmaceutical, chemical, food, or other industries. Physical chemists can also work as teachers and professors at many schools or universities that teach physical chemistry or related subjects. Physical chemistry has several branches, but the most well-known branch is called thermodynamics.

What is Thermodynamics?

Thermodynamics is a science concerned with studying principles behind energy and changes in energy. Thermodynamics was developed to understand heat as one of the two principle processes (apart from work), especially the work of heat engines and the conversion of heat. Thermodynamics not only studies heat as a physical phenomenon, but also the chemical reactions that result in heat production or removal. This science can give answers to questions related to the outcome of the chemical interaction, the concentrations of products and reactants following the reaction, and the amount of heat that will be produced during these reactions. Thermodynamics gives scientists information on the ways energy is stored, transferred,  released or used. However, thermodynamics does not provide answers related to the time needed for chemical reactions to take place, nor the mechanisms of such reactions.  

What is the First Law of Thermodynamics?

The First Law of Thermodynamics explains the fundamental concepts of energy, work, and heat as well as the relation between these three elements. There are two versions of this law. The simpler version states: “Energy is conserved.” The more complex version explains “Energy can neither be created nor destroyed. However, it can be transformed from one form to another. ” There is one additional definition of the first law of thermodynamics that is based on Einstein’s theory of relativity. In his equation E= mc2, Einstein explained that mass and energy correlate with each other. Therefore, the first law of thermodynamics can be formulated as follows: “The total mass and energy of an isolated system remain unchanged ” or “The total energy of a system and its surroundings must remain constant; however, it can change from one form to another. ”

Can the First Law of Thermodynamics be expressed through a Mathematical Equation?

The First Law of Thermodynamics can be explained through the statement of the following mathematical scenario: A system in its initial state and with internal energy E1 receives heat q that can either increase its internal energy, be used for work, or both. In the event that heat q increases the internal energy, the whole system’s energy value changes to E2. The mathematical formula that describes this relationship is as follows: E2 – E1 = delta E (a total increase in internal energy). 

If the amount of work done by the system is labelled with W, then the law of conservation of energy would suggest the amount of heat absorbed by the system equals the sum of the increase in internal energy of the system and work done by the system:

delta E + (–W) = delta E – W             or                     delta E = q + W .

The minus sign of work (–W) is used to mark the work done by the system in the surroundings. It suggests that the internal energy of the system decreases with work. It also indicates that when a system receives a certain amount of heat “q”,and performs a certain amount of work “W”, its internal energy increases by delta E. For small infinitesimal changes, the previous equation can be written as follows: delta E = delta q + delta W... where delta q is the amount of heat given to the system, delta E represents infinitesimal changes in the internal energy and delta W represents the work done by the system. If only considering expansion work (pressure-volume work) as the work resulting from a change in volume, then the equation would go as follows: delta W = –P delta V.


Kirshenbaum, G. S., Pearce, E. M., & Zaikov, G. E. (2012). New Steps in Physical Chemistry, Chemical Physics, and Biochemical Physics. Hauppauge, N.Y.: Nova Science Publishers, Inc.

Mortimer, R. G. (2005). Mathematics for Physical Chemistry. Burlington, Mass: Academic Press.

Singh, N. B. (2009). Physical Chemistry. New Delhi: New Age International.

Linder, B. (2011). Elementary Physical Chemistry. Singapore: World Scientific.


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