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Chemistry LibreTexts

15: Thermodynamics of Chemical Equilibria

Trying to introduce chemical thermodynamics to beginning students is always problematic; to do it "properly" requires a degree of rigor that rarely succeeds for more than a small fraction of the class. Although a full formal development is rarely appropriate at this level, I believe that the value of developing students' understanding of the fundamental concepts is generally underappreciated. This requires some understanding of the ways thermal energy is dispersed in matter— something that is not a part of classical thermodynamics and is not supported by most textbooks, but which is in keeping with the molecular focus of modern chemical science.

The equilibrium value for a reversible reaction is an important quantity that characterizes a chemical reaction, but what factors govern its value? In particular, is there any way that we can predict the value of the equilibrium constant of a reaction solely from information about the products and reactants themselves, without any knowledge at all about the mechanism or other details of the reaction? The answer is yes, and this turns out to be the central purpose of chemical thermodynamics:

The purpose of thermodynamics is to predict the equilibrium composition of a system from the properties of its components.

Don’t let the significance of this pass you by; it means that we can say with complete certainty whether or not a given change is possible, and if it is possible, to what extent it will occur— without the need to study the particular reaction in question. To a large extent, this is what makes chemistry a science, rather than a mere cataloging of facts.

  • 23.1: Energy Spreading Drives Spontaneous Change
    Processes that proceed in a definite direction when left to themselves and in the absence of any attempt to drive them in reverse — are known as natural processor spontaneous changes. In many cases our everyday life experiences teach us the direction in which spontaneous change can occur, and anything that runs counter to these expectations is immediately sensed as weird.
  • 23.2: Entropy Rules
    Previously, we explained how the tendency of thermal energy to disperse as widely as possible is what drives all spontaneous processes, including, of course chemical reactions. We now need to understand how the direction and extent of the spreading and sharing of energy can be related to measurable thermodynamic properties of substances— that is, of reactants and products.
  • 23.3: The Second Law of Thermodynamics
    The First Law of thermodynamics, expressed as ΔU = q + w, is essentially a statement of the law of conservation of energy. The significance of this law is that it tells us that any proposed process that would violate this condition can be dismissed as impossible, without even inquiring further into the details of the process.
  • 23.4: Free Energy and the Gibbs Function
    In this unit we introduce a new thermodynamic function, the free energy, which turns out to be the single most useful criterion for predicting the direction of a chemical reaction and the composition of the system at equilibrium. As we will explain near the bottom of this page, the term "free energy", although still widely used, is rather misleading, so we will often refer to it as the "Gibbs function" or "Gibbs energy."
  • 23.5: Thermodynamics of Mixing and Dilution
    This lesson goes somewhat beyond what is covered in most first-year courses, and can usually be skipped by students in non-honors beginners' courses. The concepts presented here are not especially complicated, but they don't really become essential until one gets into more advanced courses in chemistry, physiology, and similar subjects.
  • 23.6: Free energy and Equilibrium
    Under conditions of constant temperature and pressure, chemical change will tend to occur in whatever direction leads to a decrease in the value of the Gibbs free energy. In this lesson we will see how G varies with the composition of the system as reactants change into products. When G falls as far as it can, all net change comes to a stop. The equilibrium composition of the mixture is determined by ΔG° which also defines the equilibrium constant K.
  • 23.7: Some Applications of Entropy and Free Energy
    Thermodynamics may appear at first to be a rather esoteric subject, but when you think about it, almost every chemical (and biological) process is governed by changes in entropy and free energy. Examples such as those given below should help you connect these concepts with the real world.
  • 23.8: Quantum states, Microstates, and Energy spreading in Reactions
    Entropy (S) is a state function whose value increases with an increase in the number of available microstates.For a given system, the greater the number of microstates, the higher the entropy. During a spontaneous process, the entropy of the universe increases.