Ch 18 - Chemical Thermodynamics

spontaneous process (ΔG or ΔS_total)

• happen under indicated conditions

• may occur quick or slow

Are all process spontaneous in both directions?

• no some are only spontaneous in one

• ex: melting ice is spontaneous, but freezing water is not

entropy (ΔS)

• the measure of disorderness in a system

• how spread out a systems energy is

• cannot be measured with an absolute valu e

the second law of thermodynamics

• spontaneous processes always result in an overall increase in entropy of the universe

As solid → liquid → gas, the entropy (ΔS)

is increasingly greater than 0

As solid ← liquid ← gas, the entropy (ΔS)

is decreasingly less than 0

When is the entropy higher? What equation represents this?

• when there are more energetically equivalent ways to arrange components of a system

• S = k lnW

  • k = Boltzman’s constant

  • W = number of energetically equilibrium arrangements (microstates) possible for the system

What is Boltzman’s constant derived from?

• the gas constant R and Avogadro’s number

• k = R/N_A

• k = 1.38 × 10^-23

third law of thermodynamics

• used to calculate ΔS

• entropy of pure, perfectly ordered crystalline substance at absolute of zero is 0

• can be used to predict the sign of ΔS

• at absolute 0, there is no thermal energy and particle (in a perefectly order crystalline structure) that exists in a single microstate

What unites do standard molar entropies (S°) have?

J/mol K

What are the standard thermodynamic conditions

298K, 1 atm gas P, and 1 M solute concentration

How can changes in entropy (ΔS°) be calculated

• from S° values

• ΔS°_rxn = m (S° products) - n (S° reactants)

What is the overall change in entropy equal to?

to the change in entropy of the system + the change in entropy of the surroundings

When will ΔS° be positive?

• when ΔS°_sys and ΔS°_surr are both positive

• when ΔS°_sys is negative but less than ΔS°_surr

• when ΔS°_surr is negative but less than ΔS°_sys

isothermal process

a system that causes no temperature change on the surrounding

For heat transfer, what does q_{sys =}

-q_surr

What is the equation for ΔS_surr

ΔS_surr = -q_sys/T OR = -ΔH_sys/T

is the equation for ΔS_univ

ΔS_univ = ΔS_sys - ΔH_sys/T