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’s often defined as a branch of physics that focuses on energy transfer, with heat and work at the core of the theory. But when these interactions occur during chemical reactions, the research field is often termed chemical thermodynamics.
What is Thermodynamics in Chemistry 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 into thermal energy within the framework of the laws of thermodynamics.
Let’s review these laws:
- The zeroth law of thermodynamics states that if two independent thermodynamic systems are in thermal equilibrium with a third system (meaning there is no net flow of thermal energy between them), then they’re also in thermal equilibrium with each other.
- The first law of thermodynamics says that energy can’t be created or destroyed, but only transformed or transferred.
- The second law of thermodynamics affirms that entropy always has a tendency to increase over time.
- The third law of thermodynamics establishes that the entropy of a system at absolute zero temperature remains at a constant value. These systems are at a basic state and entropy only increases with the degeneracy of that basic state.
The bases of chemical thermodynamics are also founded on two previous laws, from 1789 and 1840, respectively. They’re both considered the clearest antecedents of the first law of thermodynamics, which, not by chance, is also known as the law of conservation of energy.
- The law of Lavoisier and Laplace expresses that the energy changes that come from any transformation are equivalent and opposite to the energy changes that come from the reverse processes.
- The law of Hess is also called the law of constant heat summation because it postulates that enthalpy variations occurring during chemical reactions are additive —they don’t 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 that 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 doesn’t depend on the path taken to reach that specific value) that are fundamental in chemical thermodynamics. The other ones are:
- Internal energy (U) represents the energy that a thermodynamic system contains. Thermodynamics is largely concerned with changes in internal energy.
In a closed system, changes in internal energy (ΔU) are due to heat transfer and thermodynamic work done by the system on its surroundings. 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 that measures, in Joules per Kelvin, the amount of thermal energy that’s unavailable for conversion into mechanical work. Entropy is often defined as the level 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 using the formula ΔG=ΔH−TΔS, where ΔG is the change in Gibbs free energy, ΔH is the change in enthalpy, T is the temperature (in Kelvin), and ΔS is the change in entropy.
- Helmholtz free energy is also often seen as a main state function in thermodynamics. It measures the "useful" work that can be obtained from a closed thermodynamic system at a constant temperature, volume, and particle number. 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 entropy.
So… Is Thermodynamics Physics or Chemistry?
Even if thermodynamics is most frequently classified as a branch of physics, it is also applicable to chemistry. It can be equally used to describe and explain both steam engines and chemical reactions. After all, thermodynamics deals with thermal and mechanical energies (work), which have a place in both physical and chemical phenomena.
Physics and chemistry are actually heavily linked beyond thermodynamics. Laws of physics often explain the behavior of chemical compounds. For example, we can use electric forces to elucidate chemical reactions in which there is an exchange of ions and electrons.
Both physics and chemistry study matter and the energies that interact with it, but with different scopes and approaches —and some exceptions that apply to physics but not chemistry, such as dark matter or quarks. Still, these disciplines are not radically separated and this is why there are interdisciplinary fields that encompass the two of them, such as physical chemistry, chemical physics, electrochemistry, nanotechnology, and thermodynamics.
Thermodynamics themselves have several branches as well. Apart from chemical thermodynamics, there are other branches, including:
- Classical thermodynamics. Given that it was founded before the discovery of atomic structures in the 19th Century, classical thermodynamics deals with the relationships between macroscopic and measurable properties of matter only.
- Statistical thermodynamics. Also known as equilibrium statistical mechanics, this is often taken as a nexus between the mechanics and thermodynamics of macroscopic systems. Making use of statistical methods and probability theory, statistical thermodynamics uses molecular properties to predict the behavior of macroscopic quantities of compounds.
- Equilibrium thermodynamics. It focuses on transformations of matter and energy within systems in thermodynamic equilibrium (meaning there is no heat or any kind of energy flow between them). This concept is the base 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 must clarify what exactly is chemical engineering in the first place.
Chemical engineering was established as a profession by English engineer George E. Davis, who wrote the Handbook of Chemical Engineering (1904), based on 12 lectures presented at Manchester School of Technology. At the time, he described chemical engineers as people who applied chemical and mechanical knowledge “to the utilization of chemical action” on a manufacturing scale.
Chemical engineers devote themselves to chemical production and the design and fabrication of products through chemical processes, including research on equipment and methods to do so. According to the U.S. Bureau of Labor, they can be involved in the manufacturing of fuels, synthetic rubber, batteries, paint, explosives, fertilizers, plastics, detergents, textiles, cement, paper, and many others. They often work at industrial plants, refineries, or laboratories.
Chemical engineers apply principles of chemistry and physics to convert raw materials into final products. Specifically, they can use thermodynamics to define states of phase and chemical equilibrium that could allow them to design more efficient chemical reactors, mixing and separation processes, and equilibrium controlled operations.
Chemical engineering thermodynamics is actually a course within the chemical engineering area in prestigious academic institutions like MIT. Thermodynamics applied to chemical engineering can help these professionals calculate the amount of work that certain fuels can produce, what temperature and pressure are best for certain chemical processes, etc.
In other words, we can say that you don’t need chemistry for thermodynamics, but chemistry and especially chemical engineering rely on thermodynamics to guide the comprehension, analysis, and design of chemical processes.