Chapter 3: The Second Law of Thermodynamics

- 2nd law - deduction of S from 2nd law - how to calculate entropy changes in processes - relation btwn entropy and equilibrium

the Kelvin-Planck statement of the 2nd law of thermodynamics asserts what?

the impossibility of the complete conversion of heat to work in a cyclic process

from the 2nd law, dq_rev/T is the differential of …?

a state function, entropy S

the entropy change in a process where the integral must be evaluated using a reversible path from state 1 to state 2 is ΔS =?

ΔS = ∫(1 to 2)*dq_rev/T

2nd law proves that the entropy of an isolated system must _________ in an irreversible process

• increase, iso. sys., irr. proc.

thermodynamic equilibrium in an isolated system is reached when —

the system’s entropy is maximized

calculation of ΔS for a reversible process is done using what equation?

dS = dq_rev/T

calculation of ΔS for an irreversible process is done by finding…?

a reversible path btwn the initial and final states

calculation of ΔS for a reversible phase change is ..?

ΔS = ΔH/T

calculation of ΔS for constant-pressure heating using differential…

dS = dq_rev/T = (C_P/T)dT

state function entropy, S possesses the property that for an isolated system, the equilibrium position corresponds to ..

..maximum entropy

what type of property is entropy?

• macroscopic, not molecular

• is it intensive or extensive or neither??

for a heat engine to produce continuous mechanical work, what must it do?

it must exchange heat with 2 bodies at different temperatures, absorbing heat from hot body and discarding heat to cold body

• engine can’t function continuously w/o cold body for discard of heat

Kelvin-Planck statement of 2nd law of thermodynamics

it’s impossible for system to undergo a cyclic process whose sole effects are the flow of heat into the system from a heat reservoir and the performance of an equivalent amount of work by system on surroundings

(from cold reservoir and flow of an equal amount of heat out of system into hot reservoir)

what is meant by heat bath or heat reservoir?

• body in internal equilibrium at constant temperature and large enough for flow of heat btwn it and system to cause no significant change in temp of reservoir

2nd law says it’s impossible to build a cyclic machine that…

converts heat into work with 100% efficiency

evidence for 2nd law

• failure to construct such a machine

  • if machine exited, could use atmosphere as heat reservoir and continuously withdraw energy from it and convert into work

• 2nd law leads to conclusions abt equilibrium in chem sys. and conclusions are verified

2nd law shows that vapour pressure of pure substance varies with temperature according to eq. :

dP/dT = ΔH(T*ΔV)

where ΔH = heat of vaporization

P(not) = P(0) is a pure system

True

more disorder, higher entropy is associated with some phases

gases have highest entropy because they have most disorder and movement, then liquids, then solids