Chemical thermodynamics and its importance for technology

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Thermodynamics is the branch of science that explores the relationship between heat, temperature, work, and energy their relationship to energy, radiation and the physical properties of matter. It is often defined as a branch of physics that focuses on energy transfer, with heat and work at the heart of the theory. But when these interactions are during chemical reaction, the research field is often called chemical thermodynamics.

What is thermodynamics in chemical terms?

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

Let’s look at these laws:

  • The zeroth law of thermodynamics states that when two independent thermodynamic systems are in thermal equilibrium with a third system (i.e. there is no net flow of thermal energy between them), they are also in thermal equilibrium with each other.
  • The first law of thermodynamics says that energy cannot be created or destroyed, but only converted or transferred.
  • The second law of thermodynamics says that Entropy always has a tendency to increase over time.
  • The third law of thermodynamics says that the entropy of a system remains constant at absolute zero temperature. These systems are in a basic state, and entropy only increases with the degeneracy of that basic state.

The fundamentals of chemical thermodynamics are also based on two earlier laws from the years 1789 and 1840. They are both considered the clearest precursors of the first law of thermodynamics, which is not by chance also known as the law of conservation of energy.

  • Lavoisier and Laplace’s law states that the energy changes that result from a transformation are equivalent and opposite to the energy changes that result from the reverse processes.
  • Hess’s law is also called the law of constant heat sum because it postulates that the enthalpy changes that occur in chemical reactions are additive – they do not depend on the number of steps required to achieve the reaction.

Fundamental state functions of chemical thermodynamics

Enthalpy is a property of a thermodynamic system that is defined as the flow of thermal energy that the system emits or receives from the environment at constant pressure. It measures these fluctuations in energy in joules (J).

You can calculate it using 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 most important state functions (a property the value of which does not depend on the path taken to reach that particular value) that is of fundamental importance in chemical thermodynamics. The others are:

  • Internal energy (U) represents the energy that a thermodynamic system contains. Thermodynamics is mainly concerned with changes in internal energy. In a closed system, Changes in internal energy (ΔU) are due to heat transfer and thermodynamic work that the system does to its environment. This relationship can be expressed 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 that measures in joules per kelvin the amount of thermal energy that is not available for conversion into mechanical work. E.ntropy is often defined as the degree of molecular disorder or randomness in a system.
  • Gibbs free energy (G) is the maximum amount of non-expansion work that can be obtained from a thermodynamically closed system (this is a system that can exchange heat and work with its surroundings, but not matter). It is derived from the formula ΔG = ΔH − TΔS, where ΔG is the change in Gibbs energy, ΔH is the change in enthalpy, T is the temperature (in Kelvin) and ΔS is the change in entropy.
  • The Helmholtz free energy is also often seen as the main 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 Helmholtz free energy, 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 is also applicable to chemistry. It can be used equally to describe and explain steam engines and chemical reactions. Finally, thermodynamics deals with thermal and mechanical energies (work) that have their place in both physical and chemical phenomena.

Physics and chemistry are actually closely related beyond thermodynamics. Physical laws often explain the behavior of chemical compounds. For example, we can use electrical forces to elucidate chemical reactions in which ions and electrons are exchanged.

Both physics and chemistry study matter and the energies that interact with it, but with different applications and approaches – and some exceptions that apply to physics but not to chemistry, such as dark matter or quarks. However, these disciplines are not radically separated, and therefore there are interdisciplinary areas that encompass both, such as physical chemistry, chemical physics, electrochemistry, nanotechnology, and thermodynamics.

Thermodynamics itself also has several branches. In addition to chemical thermodynamics, there are other branches including:

    • Classical thermodynamics. Since it was founded before the discovery of atomic structures in the 19th century, classical thermodynamics only deals with the relationships between macroscopic and measurable properties of matter.
    • Statistical Thermodynamics. This is also known as statistical equilibrium mechanics and is often used as a link between the mechanics and thermodynamics of macroscopic systems. Statistical thermodynamics uses statistical methods and probability theory and uses molecular properties to predict the behavior of macroscopic sets of compounds.
    • Equilibrium thermodynamics. It focuses on the conversion of matter and energy within systems in thermodynamic equilibrium (i.e. there is no heat or any type of energy flow between them). This concept is the basis of the zeroth 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 first need to clarify what chemical engineering actually is.

Chemical engineering was established as a profession by the English engineer George E. Davis who the Chemical engineering manual (1904), based on 12 lectures at the Manchester School of Technology. At that time he described chemical engineers as people who applied chemical and mechanical knowledge “to the use of chemical effects” on a production scale.

Chemical engineers are dedicated to chemical production and the design and manufacture of products through chemical processes, including research into equipment and methods for doing so. According to US Labor Office, they can be involved in the production of fuels, synthetic rubber, batteries, paints, explosives, fertilizers, plastics, detergents, textiles, cement, paper and many others. They often work in industrial plants, refineries or laboratories.

Chemical engineers apply chemical and physical principles to convert raw materials into end products. In particular, they can use thermodynamics to define phase and chemical equilibrium states that enable them to develop more efficient chemical reactors, mixing and separating processes, and equilibrium-controlled operations.

Chemical Engineering Thermodynamics is actually a chemical engineering course at prestigious academic institutions such as MIT. Thermodynamics applied to chemical engineering can help these professionals calculate the workload of specific fuels, what temperature and pressure are best for specific chemical processes, etc.

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

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