A Level Chemistry Revision: Physical Chemistry – Thermodynamics

Thermodynamics is the study of how energy, particularly heat, dissipates in a system, and how the mass of substances flows as they’re converted from one form to another. In chemistry, thermodynamics is focused on how energy is transferred and transformed during chemical reactions. Studying thermodynamics allows A level chemistry students to understand how energy flows in a system. 

Your revision in thermodynamics will encompass the three laws of thermodynamics, properties of systems, enthalpy, the Gibbs free energy, thermochemical equations, and other related topics. You may, therefore, find it very helpful to practise solving some of the chemical reaction problems that require calculations when you’re revising for A level chemistry.

What Are the Laws of Thermodynamics?

The Laws of Thermodynamics are:

  • 1st Law: Otherwise known as the law of conservation of energy, the first law of thermodynamics states that in an isolated system, energy can neither be created nor destroyed.
  • 2nd Law: The second law of thermodynamics is also known as the law of entropy or disorder. It states that the entropy or degree of energy disorder of any isolated system is always on an upward trajectory.
  • 3rd Law: This law states that the entropy of an isolated system approaches a constant value as the temperature of the system approaches absolute zero (0 Kelvin or −273.15 °C).
  • Zeroth Law: This is more fundamental than the first three laws, but was developed after the other laws were established. Therefore, it had to be given a lower number, which is zero.It states that if two systems are both in thermal equilibrium with a third system, then the two systems are logically in thermal equilibrium with each other.

The ideas behind the currently established Laws of Thermodynamics can be traced back to the ideas of heat in ancient times. The Laws as they’re known today are interwoven with the history and developments in physics and chemistry, particularly associated with the breakthroughs in combustion engines during the nineteenth and twentieth centuries. The ordinal ranking of the Laws, however, was not established until quite recently.

Although not its current ordinal ranking, the first established thermodynamic principle was formulated by Sadi Carnot in 1824. His book ‘Reflections on the Motive Power of Fire’ laid down the first thermodynamic principle, which eventually became known as the second law of thermodynamics.

It took almost four decades after the publication of Carnot’s books before Rudolf Clausius and William Thomson formalised the first and second laws of thermodynamics. More than half a century later in 1912, the third law of thermodynamics was developed by Walther Nernst, though he first developed it as a theorem over a period of six years from 1906 to 1912.

Entropy vector illustration
The second law of thermodynamics is also known as the law of entropy or disorder

What Are Systems and Surroundings?

In physical chemistry, systems and surroundings are important thermodynamic concepts. A system is defined as the part of the universe being investigated. It could be anything from the reactants in a test tube or the atmosphere of a distant planet being analysed through spectroscopy. A system can be open, closed, or isolated, as follows:

  • Open systems: Open systems allow the natural exchange of matter and energy with the surroundings. Our bodies are a good example of an open system. We exchange both matter and energy with our surroundings, taking in food, water, and oxygen, while giving off carbon dioxide and body wastes like urine.
  • Closed systems: Closed systems allow the exchange of energy with the surroundings but prevent the natural exchange of matter. A good example of a closed system is a sealed terrarium with plants and microorganisms that recycle the organic materials and air inside.
  • Isolated systems: These systems do not allow the exchange of matter or energy with the surroundings. While it’s almost impossible to create an isolated system under normal conditions, one good approximation is a sealed Thermos jug, which is an insulated water container that can hold the heat of water for an extended period.

Surroundings are part of the universe other than the system being investigated. The surroundings can interact with a system in several ways. For example, the air outside the combustion engine of a car is part of the surroundings that interact with the fuel of the engine. The air is the source of oxygen needed to burn the fuel.

Given specific parameters, like room temperature and atmospheric pressure, that are maintained constant at standard levels, chemical reactions and behaviours can be accurately predicted. You can also calculate the precise amount of energy necessary to reverse a reaction.

Different types of thermodynamic systems

What is Gibbs Free Energy?

In chemical thermodynamics, Gibbs free energy, or simply Gibbs energy, is a thermodynamic potential. It can be used to calculate the maximum amount of reversible work that can be done by a system under certain conditions, such as constant temperature and pressure. It’s the maximum possible amount of non-expansion work that can be derived from a closed system.

Josiah Willard Gibbs developed this concept in the 1870s. It was originally named as the ‘available energy’ in a system. In 1873, Gibbs published a research paper entitled ‘Graphical Methods in the Thermodynamics of Fluids.’ In it, he expounded on the available energy concept and how his equation could be used to predict the behaviour of a closed system.

Gibbs energy is the energy of a chemical reaction that can be applied to do work. It’s the sum of a system’s enthalpy and the product of the temperature and the entropy of a system. The complete formula can be written as follows:

G=U+PV−TS

Here:

  • U  is internal energy (SI unit: joule)
  • P  is pressure (SI unit: pascal)
  • V  is volume (SI unit: m3 )
  • T is temperature (SI unit: kelvin)
  • S  is entropy (SI unit: joule/kelvin)
  • H  is the enthalpy (SI unit: joule)

In your A level chemistry exam, you may be asked to solve some problems involving enthalpy and Gibbs free energy. Most problems will require only basic knowledge of algebra and how to manipulate equations. However, you will need to understand the underlying concepts of thermodynamics to be able to correctly answer the problems.

For more A level chemistry resources to help you revise and for information on studying chemistry A level, check out our A Level Chemistry Resources hub.

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