Thermodynamics: the Study of Energy Transformations

SCH 4U Energy and Equilibrium: The Laws of Thermodynamics (Read Chapter 7.7 p.494 – 511)

Thermodynamics: The study of energy transformations.

Three empirically derived, fundamental laws of nature:

First Law of Thermodynamics: The total amount of energy in the universe is constant. Energy can neither be created nor destroyed, but can be transferred from one object or place to another, or transformed from one form to another.

Second Law of Thermodynamics: All changes, either directly or indirectly, increase the entropy of the universe.

Δ Suniverse > 0

symbol for entropy: S (measure of disorder or randomness)

Third Law of Thermodynamics: The entropy of a perfectly ordered pure crystalline substance is zero at absolute zero. S = 0 when T = 0 K

Why do some physical & chemical changes occur spontaneously while others do not?

Spontaneous rxn - one that, given the necessary activation energy, proceeds without continuous outside input e.g. rxn of sodium with water, burning of paper, rusting of iron, dissolving of NaCl. e.g. of non-spontaneous rxns: photosynthesis, frying an egg, building a house.

Spontaneity – determined by enthalpy (ΔH) and entropy (ΔS) We know that exothermic rxns (ΔH is – ve, i.e. products have less energy than reactants) tend to be spontaneous over endothermic rxns (ΔH is + ve).

However, some endothermic rxns water e.g. NH4Cl(s) + heat NH4Cl(aq) are spontaneous even at low temperatures.

What’s happening? Another driving force of rxns:

The Tendency to Greater Disorder: Entropy

Δ S = Sproducts – Sreactants

Δ S > 0 when Sproducts > Sreactants (i.e. disorder has ↑)

Δ S < 0 when Sproducts < Sreactants (i.e. disorder has ↓)

Note: the universe favours greater disorder!

Read together:  Last 2 paragraphs p.496  Predicting the signs of Δ S p.497  Sample Problem p.498  Practice p.498

nd Recall: 2 Law of Thermodynamics (Δ Suniverse >0) Sometimes we may think we “beat” this Law of the Universe because our “system” in question has achieved greater order. But to do so the “surroundings” must have ↑ in disorder.

i.e. Δ Suniverse = Δ Ssystem + Δ Ssurroundings

 See worksheet “Spontaneity of Chemical Reactions”  Read together p.500 – 501 from “The second law has ...”  Read p.500 – 502  Read “Equilibrium and Δ G” p.508 – 509

 Note: you do not need to know how to do the specific numerical calculations from p.506 – 511

Gibbs free energy (or simply “free energy”)  In the late 1800’s American physicist Josiah Willard Gibbs helped formulate an equation that linked together the concepts of energy change (Δ H), entropy change (Δ S), and temperature of a reaction & that predicts if it will occur spontaneously

Gibbs-Hemholtz equation:

hwk. p.512 #1-6, 10