Important terms to remember• System• Surrounding• Work done• Heat• Spontaneous process• Non-spontaneous process
System and SurroundingA system is defined as the part of universe which isunder study and the rest of the universe is know as thesurroundings. System is separated from surroundingsthrough boundaries.
Spontaneity and Randomness Diffusion of gases Melting of iceThe degree of randomness is known as ENTROPYSpreading of ink in water Evaporation of water
Second Law of ThermodynamicsIt is impossible to construct a device which operating in a cycle, has thesole effect of extracting heat from a reservoir and performing anequivalent amount of work.It is impossible for a self acting machine, working in a cyclic process andunaided by any external agency to transfer heat from a body at lowertemperature to a body at higher temperature.
Second Law of ThermodynamicsWhenever a spontaneous process takes place, it isaccompanied by an increase in the total entropy of theuniverse( system and surrounding) Diffusion of gases
Entropy and spontaneity• Entropy is positive : the process if spontaneous• Entropy change is zero: Equilibrium• Entropy change is negative: non-spontaneous.
Entropy in various types of systems• Isolated system: ΔS is positive• Open or closed system: ΔS = ΔS system + ΔS surrounding
Adiabatic change and entropy• For irreversible change: ΔS > q irrev/ T• For reversible change: q= 0 ΔS = 0• During an adiabatic change the entropy of a system increases if the change is irreversible while it remains constant if the change is reversible.
Entropy change of the universe in a isothermal reversible process• ΔS system= q/T• ΔS surrounding= -q/T• Total change is entropy = 0• This means that in a reversible isothermal process there is no change in the entropy and the entropy of the universe remains constant.
Entropy change of the universe in an irreversible process• Entropy of universe increases in an irreversible process.
Helmholtz free energy• Or work function is represented by A and is defined as: A = E-TS Where E is internal energy T is temperature S is entropy• Change is Helmholtz free energy ΔA = ΔE – T ΔS
Gibb’s free energy• The maximum amount of energy available to a system during a process for doing useful work under constant temperature and pressure conditions is called Gibb’s free energy. G = H – TS• Change in Gibb’s free energy: ΔG = ΔH – T ΔS
Relationship between Gibb’s freeenergy and Helmholtz free energy ΔG = ΔA + P ΔV
Gibb’s free energy and work done Decrease in free energy may be regarded as• ΔA = -w a measure of the net work done by a system• of constantPtemperature and pressure. ΔG = -w + ΔV• - ΔG = w – P ΔV
Gibb’s free energy and spontaneity• When ΔG is negative : spontaneous• When ΔG is zero: equilibrium• When ΔG is positive: non-spontaneous
Standard free energy change and equilibrium constant• The standard free energy change is defined as the free energy change for a process for a specified temperature in which the reactants in their standard state are converted to products in their standard state. ΔG 0 = Σ Δ f G 0 products – ΣΔ f G 0 reactants• In terms of equilibrium constant ΔG 0 = - RT log e K
Home work• For a reaction K = 1.8 x 10-7 at 300 K. What of the value of ΔG 0 at this temperature? R = 8314 J K-1 mol -1• Define the term entropy. How does T delta S determine the spontaneity of a reaction?