CHEM 101
General Chemistry

J. D. Cronk    Syllabus    Previous lecture | Next lecture

Lecture 16. Introduction to thermochemistry

Thursday 17 March 2022

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 (2nd ed.) - Ch.9, pp.367-389.


Summary

The nature of energy: Forms of energy, kinetic and potential, energy units.

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).

Heat and heat capacity

We define several versions of heat capacity: total heat capacity (Ctotal, an extensive quantity, units J/°C), specific heat capacity (cs, units J/g·°C), and molar heat capacity (cM, units J/mol·°C). The latter two definitions are intensive quantities.

For details, see the Heat and heat capacity topics webpage.

The topics treated in chapter 9 of our text form the foundations of what can be termed chemical thermodynamics. Ultimately, a principal aim of such a treatment 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.