Unit 6

Question 1

Mean molar bond enthalpy (ΔH°b)

Answer 1

-the average molar enthalpy change accompanying the dissociation (breaking) of a given kind of bond in the gas phase
-all elements must first be in gas form (via molar atomization)

Question 2

molar atomization enthalpy (ΔH°a)

Answer 2

-the enthalpy change associated with atomizing an element from its most stable form to a mole of gaseous atoms

Question 3

enthalpy change of reaction ΔH°rx

Answer 3

ΔH°rx = Σ np ΔH°f (products) - Σ nr ΔH°f (reactants)

Question 4

enthalpy change of gas phase reaction

Answer 4

ΔH°rx = Σ ΔH°b (reactants) - Σ ΔH°b (products)

Question 5

enthalpy change of gas-phase compound

Answer 5

ΔH°f = Σ ΔH°a (elements) - Σ ΔH°a (compound)

Question 6

2nd law of thermodynamics

Answer 6

-in any spontaneous process, there is always an increase in entropy of the universe

Question 7

entropy (S)

Answer 7

-a thermodynamic state function that describes the total number of “microstates” that are available to a system
-a measure of how spread out the energy is in a system, or how many different ways the system’s particles can be arranged
-a measure of how spread out the energy is in a system, or how many different ways the system’s particles can be arranged

Question 8

microstates

Answer 8

-microstates are assigned to “states”
-the more microstates there are for a given state, the greater probability for that state existing
-nature spontaneously proceeds towards the state with the highest probability (state with most microstates)
-the state in which gas atoms are evenly distributed has the greatest number of microstates and the greatest probability

Question 9

calculating number of microstates

Answer 9

-volume ratio ^ # of particles

Question 10

Boltzmann’s equation

Answer 10

S = Kb ln Ω
(Kb is Boltzmann’s constant, R/Na)

Question 11

ΔS for an expanded gas

Answer 11

-ΔS = Rln(V2/V1) for 1 mole
-ΔS = nRln(V2/V1) for n moles
ΔS = qrev / T

Question 12

qrev

Answer 12

qrev = nRTln(V2/V1)

Question 13

ΔS for a heated gas

Answer 13

-gas expansion is reversible if Pint = Pext at every point in the gas expansion
-heating process is reversible if T(heat source) = T(heated body) at every point
-in isothermal reversible gas expansion, both gas expansion and the accompanying heat flow from surroundings to system are considered to be reversible

Question 14

ΔS under constant V

Answer 14

-q = ΔU = nCvΔT
-ΔS = qrev/T = nCvln(T2/T1)

Question 15

ΔS under constant P

Answer 15

-q = ΔH = nCpΔT
-ΔS = nCpln(T2/T1)
-ΔS = nCVln(T2/T1) + nRln(T2/T1)
- (Cp = Cv + R)