Thermodynamics

Chapter 19 Brown-LeMay

I. Review of Concepts

• Thermodynamics – area dealing with energy and relationships • First Law of Thermo – law of conservation of energy U = q + w the change in internal energy of a system equals q + w • Internal energy is a state function – one that is independent of the pathway (altitude- state function, miles traveled is not a state function

Concept review

• Heat (q) is the energy that moves into or out of a system because of temperature differences between the system and the surroundings • Work (w) – energy exchange that results when a force (f) moves an object through a distance (d) w = f x d • Work done to the system is positive work done by the system is negative

Concept review

• Work in a chemical system can de described by the pressure and change in volume of a system • W = -p v u = q p + w cons. pressure u = q p – p v • H is the reaction q at constant pressure H = q p

Delta H

• • More precisely H is the quantity u + pv H = u + pv • H = u + pv H = q p – pv + pv • H = q p • All U,P,V are state functions so H is also a state function H = n H 0 f(products) n H 0 f (reactants)

II. Entropy and the Second Law

• Spontaneous reactions – are a physical or chemical change that occurs by themselves • When delta H is neg – exothermic the reaction tend to be spontaneous • Spontaneous reactions are non-reversible • When delta H is positive – endothermic the reaction may or may not be spontaneous

II. Entropy and the Second Law

• Endothermic reactions are/not spontaneous because of the naturally tendency to ether move towards or away from a random state • Entropy S or S- is a measure of randomness or disorder of a system (J/K) state function (influenced by temp and pressure)

The Second Law

• The entropy of a system and its surrounds always increases in a spontaneous process • s = s (final) – s (initial) • s+ = increase in disorder • s- = decrease in disorder • different from energy because it can be created

Second Law

• Delta S = entropy created + q/T • Delta S > q/T spontaneous • Delta S = q/T equilibrium • For a process to be spontaneous at a given temperature, the change in entropy of the system is greater than the heat of they system divided by the absolute temperature

Second Law (spontaneous rxn)

• Delta s > q p /T = Delta H/T = Delta H/T – Delta S < 0 H – T S <0 spontaneous H – T S >0 non-spontaneous H – T S =0 Equilibrium You can often look up Delta H and Delta S values form tables and determine if a reaction is spontaneous as written under certain conditions

The Third Law & Predicting Entropy Change

• Third Law – a substance that is perfectly crystalline at 0 Kelvin has an entropy of zero – as the temp of a substance is raised disorder increases as it absorbs heat

Changes in Entropy

• In general Entropy increases as * Liquids or solutions are formed from solids * Gases are formed from either solids or liquids * The number of molecules of gas increases during a chemical reaction * the temperature of a substance in increased.

• Ludwig Boltzmann – developed a method to calculate entropies of substances based on the number of possible arrangements of the particles of a system • Standard Entropy – S o – the entropy value for the standard state of the species (indicated by the subscript sign) • Gas – pure sub at 1 atm tempt 25 0 C or 298 k – Solutions are one molar.

• Calc S o = Sum n S o reactants products - Sum n S o

Free Energy and Spontaneity

• The Gibbs free energy of a substance combines its enthalpy and entropy into a single quantity that describes the total amount of energy available for use.

• G = H – TS • At const temp G = H – T S • If G is neg(-) the rxn is spontaneous • If G is pos(+) the rxn non-spontaneous • If G is zero the rxn is at equilibrium