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 = qp + w cons. pressure

u = qp – p v

• H is the reaction q at constant pressure

H = qp

Delta H

• More precisely H is the quantity u + pv• H = u + pv

H = u + pv

H = qp – pv + pv

• H = qp

• All U,P,V are state functions so H is also a state function

• H = n H0f(products) - n H0f (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 > qp/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 – So – the entropy value for the standard state of the species (indicated by the subscript sign)

• Gas – pure sub at 1 atm tempt 250C or 298 k – Solutions are one molar.

• Calc So = Sum n So products - Sum n So

reactants

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