Chemical thermodynamics and its importance in engineering

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Thermodynamics is the branch of science that studies the relationship between heat, temperature, work, energy and their relation to energy, radiation and the physical properties of matter. It is often defined as a branch of physics that focuses on the transfer of energy, with heat and work at the heart of the theory. But when these interactions occur for chemical reactions, the research area is often referred to as chemical thermodynamics.

What is thermodynamics in terms of chemistry?

We can think of basic chemical thermodynamics as a branch of thermodynamics that studies thermal effects in chemical reactions – the conversion of chemical energy into thermal energy under the laws of thermodynamics.

Let’s review these laws:

  • The zero law of thermodynamics states that if two independent thermodynamic systems are in thermal equilibrium with a third system (meaning that there is no net flow of thermal energy between them), then they are also in equilibrium. thermal with each other.
  • The first law of thermodynamics says that energy cannot be created or destroyed, but only transformed or transferred.
  • The second law of thermodynamics states that entropy always tends to increase over time.
  • The third law of thermodynamics establishes that the entropy of a system at absolute zero of temperature remains at a constant value. These systems are in a basic state and the entropy increases only with the degeneration of this basic state.

The foundations of chemical thermodynamics are also based on two earlier laws, respectively from 1789 and 1840. They are both considered to be the clearest antecedents of the first law of thermodynamics, which, not coincidentally, is also known as name of law of conservation of energy.

  • The law of Lavoisier and Laplace expresses that the changes of energy which come from any transformation are equivalent and opposite to the changes of energy which come from the inverse processes.
  • Hess’ law is also called constant heat summation law because it postulates that the enthalpy changes occurring in chemical reactions are additive – they do not depend on the number of steps taken to obtain the reaction.

Basic state functions of chemical thermodynamics

Enthalpy is a property of a thermodynamic system which is defined as the flow of thermal energy that the system releases or absorbs from the environment at constant pressure. It measures these energy variations in Joules (J).

You can calculate it with the formula H = E + PV, where H is enthalpy, E is internal energy, P is pressure, and V is volume.

Enthalpy is one of the main state functions (a property whose value does not depend on the path taken to reach that specific value) which are fundamental in chemical thermodynamics. The others are:

  • Internal energy (U) represents the energy contained in a thermodynamic system. Thermodynamics is largely concerned with changes in internal energy. In a closed system, internal energy changes (ΔU) are due to heat transfer and thermodynamic work made by the system on its environment. This relationship can be described by the equation
    ΔU = Q – W, where Q is the net heat transfer and W is the net work done. Note that this also describes the first law of thermodynamics.
  • Entropy (S) is a thermodynamic quantity which measures, in Joules per Kelvin, the amount of thermal energy that is not available for conversion to mechanical work. EIntropy is often defined as the level of molecular disorder or the randomness of a system.
  • Gibbs free energy (G) is the maximum amount of work without expansion that can be obtained from a thermodynamically closed system (it is a system that can exchange heat and work with its environment but no matter ). It is derived using the formula ΔG = ΔH − TΔS, where G is the Gibbs free energy change, ΔH is the enthalpy change, T is the temperature (in Kelvin) and ΔS is the change d entropy.
  • Helmholtz free energy is also often considered a primary state function in thermodynamics. It measures the “useful” work that can be obtained from a closed thermodynamic system at constant temperature, volume and number of particles. It uses the equation F = U – TS where F is the free energy of Helmholtz, U is the internal energy of the system, T is the absolute temperature and S is the entropy.

So… Is thermodynamics physics or chemistry?

Although thermodynamics is most often classified as a branch of physics, it also applies to chemistry. It can also be used to describe and explain both steam engines and chemical reactions. After all, thermodynamics deals with thermal and mechanical (work) energies, which have a place in both physical and chemical phenomena.

Physics and chemistry are in fact strongly linked beyond thermodynamics. The laws of physics often explain the behavior of chemical compounds. For example, we can use electrical forces to elucidate chemical reactions in which there is an exchange of ions and electrons.

Physics and chemistry study matter and the energies that interact with it, but with different scopes and approaches – and a few exceptions that apply to physics but not chemistry, like dark matter or quarks. Yet these disciplines are not radically separated and that is why there are interdisciplinary fields that encompass them, such as physical chemistry, chemical physics, electrochemistry, nanotechnology and thermodynamics.

Thermodynamics itself also has several branches. Apart from chemical thermodynamics, there are other branches, including:

    • Classic thermodynamics. Since it was founded before the discovery of atomic structures in the 19th century, classical thermodynamics deals only with the relationships between the macroscopic and measurable properties of matter.
    • Statistical thermodynamics. Also known as Statistical Equilibrium Mechanics, it is often seen as a link between the mechanics and thermodynamics of macroscopic systems. Using statistical methods and probability theory, statistical thermodynamics uses molecular properties to predict the behavior of macroscopic amounts of compounds.
    • Equilibrium thermodynamics. It focuses on the transformations of matter and energy within systems in thermodynamic equilibrium (meaning that there is no heat or energy flow between them). This concept is the basis of the zero law of thermodynamics.

How do chemical engineers use thermodynamics?

Chemical engineering is at the interface of physics and chemistry. To understand how chemical engineers can benefit from thermodynamics in their work, we need to clarify what exactly chemical engineering is in the first place.

Chemical engineering was created as a profession by the English engineer George E. Davis, who wrote the Chemical Engineering Manual (1904), based on 12 lectures presented at the Manchester School of Technology. At the time, he described chemical engineers as people who applied chemical and mechanical knowledge “to the use of chemical action” at the manufacturing scale.

Chemical engineers are engaged in chemical production as well as in the design and manufacture of products through chemical processes, including the research of equipment and methods to do so. According to United States Labor Office, they can be involved in the manufacture of fuels, synthetic rubber, batteries, paints, explosives, fertilizers, plastics, detergents, textiles, cement, paper and many more. They often work in industrial factories, refineries or laboratories.

Chemical engineers apply the principles of chemistry and physics to convert raw materials into end products. Specifically, they can use thermodynamics to define phase and chemical equilibrium states that could enable them to design chemical reactors, more efficient mixing and separation processes, and controlled equilibrium operations.

The thermodynamics of chemical engineering is actually a course in the field of chemical engineering at prestigious academic institutions like MIT. Thermodynamics applied to chemical engineering can help these professionals calculate the amount of work that certain fuels can produce, the best temperature and pressure for certain chemical processes, etc.

In other words, we can say that you don’t need chemistry for thermodynamics, but chemistry and in particular chemical engineering rely on thermodynamics to guide the understanding, analysis and design of processes. chemical.


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