Lecture 20. Intermolecular forces
Thursday 4 April 2023
Survey of the phases of matter. Gases. Condensed phases and solutions. Phase equilibria. Chemical equilibria.
Reading: Tro NJ. Chemistry: Structure and Properties (3rd ed.) - TBA
Summary
We spend the last part of the course considering the properties of the common phases of matter. We'll place special emphasis on the state of dynamic equilibrium that can be characterized for phase transitions, saturation of solutions, and chemical reactions.
The study of the properties of gases leads to an example of an equation of state, a mathematical relationship expressing the interdependence of state variables. A remarkable feature is that the macroscopic equation of state can be predicted from a simple model of the atoms or molecules that compose a gas.
Intermolecular forces account for the existence of condensed phases.
The study of the rates of chemical reactions is known as chemical kinetics. The connection between kinetics and equilibrium is of great interest in the pursuit of understanding the dynamic nature of the latter. The dynamic features of equilibria in physical and chemical systems arise from a balance of the rates of opposing processes.
The labs this week and next in CHEM101L investigate chemical equilibrium first qualitatively, then quantitatively.
A qualitative principle, known as Le Châtelier's Principle, allows us to predict the direction of change in equilibrium when conditions of concentration, temperature, and other system variables change. The principle can be summarized by saying that when equilibrium is perturbed (changed, disturbed), the system will respond by a spontaneous re-establishment of a new equilibrium in the direction that counteracts the perturbation. Analysis of a physicochemical system according to Le Châtelier's Principle corresponds quantitatively to comparison of the reaction quotient Q to Keq. When the perturbation induces a change in temperature, Keq itself changes, in accordance with the enthalpy change. If a chemical reaction is endothermic (ΔHrxn > 0), Keq increases with temperature; for an exothermic (ΔHrxn < 0) reaction, Keq decreases with increasing temperature.