Lecture 16. Introduction to thermochemistry
Thursday 21 March 2024
Energy types and units (review). Thermodynamics and the terminology of thermodynamics: System and surroundings. State of a system and state variables. State functions. Heat, work, internal energy and the first law of thermodynamics. Heat capacity.
Reading: Tro NJ. Chemistry: Structure and Properties (3rd ed.) - Ch.9, §9.1 - §9.4, §9.6 - §9.8 (pp.383-395, pp.397-406).
Summary
The topics treated here form the some of the foundations of what can be termed chemical thermodynamics. Often we'll refer to classical thermodynamics in this context. A starting point for our discussion is to consider the nature of energy, its characterization as in kinetic or potential forms, its measurement and units. This leads us to the laws of classical thermodynamics, which set the stage for carefully defining and validating the state of thermal equilibrium and the physical quantity of temperature along with its measurement. These are often taken for granted perhaps due mainly to familiarity and personal experience lending an aura of common sense.
From that point, we are able to progress to coherent and logical discussions of internal energy, Heat and heat capacity, work, and the measurement of these thermodynamically-defined quantities, and the First Law of Thermodynamics. All this relies as well on a schematic view of world or universe as divided into a system which occupies the center of our experimental and/or theoretical attention, and everything else which we term the surroundings. These are separated by a boundary, which is usually a definite or physical boundary, although it may in some instances be imaginary or purely conceptual. Normally, we are only concerned justifiably as a practical matter only our immediate surroundings, not the entire rest of the universe.
The concept of an isolated system is quite important. Since it does not exchange any matter or energy with its surroundings, an isolated system is analogous to a universe unto itself, and yields the simplest applications of the laws of thermodynamics.
A state function is a function whose value is dependent only on the state of a system, and the changes in a state function when a system changes from one state to another are path-independent. This means that no matter what path (sequence of changes in chemical composition and other state variables) a system takes in going from state 1 to state 2, the change in value of the state function is the same. Another way of expressing this is to say that the value of a state function is determined solely by its current state, and not by its "history" (exactly how it got to that state).
The measurement of heat energy is called calorimetry. To make use of calorimetry, we define several versions of heat capacity, in particular Ctotal, an extensive quantity, units J/°C), and an intensive specific heat capacity (cs, units J/g·°C. In this course one of the accompanying labs is devoted to a method of constant-pressure calorimetry, in which the enthalpy of reaction for the dissolution of an ionic compound and an acid-base neutralization.
Ultimately, a principal aim of chemical thermodynamics is to understand and enable a quantitative definition of what we'll often refer to in this course as chemical potential energy. This in turn provides the means to predict whether or why a chemical reaction occurs spontaneously (i.e. without any input of energy whatsoever) or alternatively would have to be driven by energy input (e.g. by heating) in order to proceed to any significant extent.
A full treament of spontaneity and chemical potential energy must include the introduction of the concept of entropy and the second law of thermodynamics.