Chapter 16 Notes

spontaneous process

process that takes place w/o continuous E input from external source

rxns are spontaneous when the change results in

a decrease in system energy

energy

the ability to do work or transform heat

system

part of the universe isolated for study

chemical system

atoms involved in a chemical rxn

chemical rxns

rearrage system atoms

factors that influence spontanteity

enthalpy, entropy

enthalpy

ability to transfer heat (H)

heat E of a system

-dH

negative enthalpy change

exothermic

decrease in system E

favors spontaneity

+dH

positive enthalpy change

endothermic

increase in system E

disfavors spontaneity

entropy (S)

state function that is a measure of matter and/or dispersal w/i a system, determined by # syst molecules; often described as a measure of system disorder

+dS

increasing disorder

favors spontaneity

how can an endothermic rxn be spontaneous

MELTING OF ICE:

is an endothermic process (-dH)

HOWEVER

as ice melts entropy increases (more disordered as liquid than solid) which means there is a positive dS

+dS change is large enough to overcome the unfavorable entropy change (+dH)

first law thermodynamics

conservation of energy (not created or destroyed)

second law thermodynamics

all spontaneous processes involve an increase in entropy of the universe

(as time goes on the universe moves into a more disordered state)

molecular motion influence on entropy

rotational, transformational, vibrational

rotational molecular motion

spinning (not mving in XYZ planes)

transformational molecular motion

movement within a X, Y, or Z plane

vibrational molecular motion

mvmt w/i a molecule; mvmt of atoms in a molecule relative to each other

SUBTYPES: bond bending, bond stretching

bond bending

subtype of vibrational molecular motion

changes bond angles within a molecule

bond stretching

subtype of vibrational molecular motion

changes bond length

TYPES: asymmetric stretch, symmetric stretch

asymmetric stretch

type of bond stretching (vibrational motion)

lengths of bonds don’t change the same for all bonds

symmetric stretching

type of bond stretching (vibrational motion)

lengths of bonds all change equally

microstate

possible configuration / arrangement of matter and E w/i a system

ie: methane (CH4) vs ethane (CH6)

ethane has more bonds which means it has more possible microstates, so it has greater enthalpy (S)

influence of temperature on entropy (S)

higher temp = more motion = more randomness = more entropy (S)

increasing temperature increases entropy

temperature

measure of average KE a molecule has

RECAP entropy @ molecular level influenced by phase, comp, temp

PHASE - g → l → s = decreasing S

COMPOSITION - increasing molecule size = increasing S

TEMP - increasing temp = increasing S

standard entropy

S associated for one mol of substance at 1 barr pressure

larger atoms have different energies associated with microstates

larger molar mass = more entropy

large atoms

atoms can be large in number or bonds/atoms AND in molar mass

third law thermodynamics

entropy of a perfect crystal at absolute zero is zero (cannot get slower than stopping)

0K

theoretical point (standard entropy S⁰ will never happen unless @ 0K)

dS⁰

sum of the standard entropies of the products - reactants

gibbs free energy

thermodynamic property defines in terms of system entropy; all spontaneous processes involves a DECREASE in G

cannot talk about G in absolute terms

since cannot talk about H in absolute terms and G = H - TS, we cannot talk ab G in absolute terms so we talk about the change

dG = dH - TdS

+dG

nonspontaneous in FORWARD direction, spontaneous in REVERSE direction

H S G favoring spontaneity

-dH

+dS

-dG

H S G disfavoring spontaneity

+dH

-dS

+dG

+dG

nonspontaneous in FORWARD, spontaneous in REVERSE

dG = 0

nonspontaneous in either direction; system is at equilibrium

standard gibbs free energy of formation

dG⁰_f change in free energy accompanying the formation of one mol of substance from its elements in their standard states

dG⁰_f =

sum of G formation of products times their coefficients - reactants

gibbs free energy and temperature

G = H - TS

temp adds weight to determine if sign S overpowers sign H

G (-) when H (_) and S (_)

-H and +S bc both favor spot

G (+) when H (_) and S (_)

+H and -S bc both disfavor spot

G (-) at high T and G (+) at low T when

H and S are both positive, bc the strength of T term will determine if S overpowers H or not

if large T, S > H, which means a larger +S than +H, so rxn is spontaneous at large temps

if small T, S < H, which means a larger +H than +S, so rxn is nonspontaneous at small temps

G (+) at low T and G (-) at high T when

H and S are both (+), then strength of T term determines if S overpowers H

if small T then S will not overpower H, and +H means nonspontaneous low T

if large T then S will overpower H, and large +S means spontaneous at high T

when is dG spontaneity temperature dependent

when the signs of entropy (S) and enthalpy (H) match

the relative free E of R and P (dG⁰) determines

the relative abundance of R and P at EQ (K)

large -dG = sppt 4ward, EQ when [P] > [R], means large K

large +dG = spot reverse, EQ [P] < [R], means small K

intermediate dG = closer to EQ (Emin), small E diff btwn R and P, appreciable amnts R and P, so K btwn 10^-3 and 10³