CHEM12 - unit 2 - equilibrium

adding PICS! Le chat principle; graphs; ICE tables

double arrow meaning ⇌

chemical reaction is reversible

• many chem rxn are

• eg. A + B → C and C → A + B

in a system AT equilibrium…

1. forward rate + reverse rate

2. microscopic processes continue but since rate (f) = rate (r), no macroscopic changes

3. system close so temperature constant

4. same equilibrium whether reactants or products

eg. N2O4 → 2 NO2

-if we start w only N2O4 in flash

-rate (f) is high bc [N2O4] is high

-rate (r) increases as [N3O2}

dynamic equilibrium def

• rxn has not stopped.

• -rare (f) (and rate) =

• as if nothing chnaging

how to predict spontaneous or not

-if nothing else considered

consider:

1. enthalpy changes

2. entropy changes

-if non considered:

• min. enthalpy favored

  • reactants in endothermic

  • products in endothermic

  • think about if endo/exo

• max. entropy favoured

  • look at states + determine which side max entropy

  • add up in total as a “side”

units / abbreviations for heat/enthalpy and entropy

• kJ enthalpy aka change in H

• kJ entropy aka change in S

order the states to determine which side max entropy

solid < liq < aq < gas < more mols of gas

eg. NaCl (s) → NaCl (aq)

• s is increasing → products favoured

eg. N2 (g) + 3H2 (g) → 2NH3 (g)

• s is decreasing → reactants favoured (4 mols of gas vs 2 mols of gas)

which types are most stable

-lowest energy state most favorable

entropy changes def

• why

: entropy is the amount of disorder or randomness in substances

• has to do w the arrangement of particles + which state

order the states w/ entropy

• solid, liquid, gas

• how measure entropy

solid → very order, rows → less entropy

liquid: less order, not super in rows → lil high entropy

gas: no order → max entropy + preferred

• measure entropy by adding up the total number of mols of gas on each side IF NO GAS then do aq then…

  • evaluate/add up the diff states individually

1. if P favour min. enthalpy + max. entropy…

2. if R favour min enthalpy + maximum entropy

3. if both tendencies oppose each other

• think about if endo/exo?

• “favour” just means what side it has these wanted qualities

1. spontaneous + go to completion

   • min H: products + max S: products

2. non-spontaneous + not occur

   • min H: reactants + max S: reactants

3. spontaneous + reach equilibrium

   • min H: products + max S: reactants

   • min H: reactants + max S: products

spontaneous meaning
+ when happen

chem rxn does happen

eg. min H + max S are on products side

eg. min H is on R; max S is on P

eg. min H is on P; max S is on R

this rxn will ___ and will ___

P4 (s) + 6H2 (g) + 37kJ → 4 PH3 (g)

min H: reactants bc endo

max S: reactants bc 6 mols of gas vs 4 mols of gas

→ when measuring max S, only care about mols of gas UNLESS NOT THERE, then move down list

not occur & non-spontaneous

his rxn will ___ and will ___

Na2CO3 (s) + 2HCl (aq) → 2NaCl (aq) + CO2 (g) + H2O (l) + 27.7 kJ

min H: products bc exo

max S: products bc 0 mols of gas vs 1 mol of gas

go to completion & spontaneous

Le Chatelier’s Principle

“if a closed system at equilibrium is subjected to a stress, processes will occur that tend to counteract the stress imposed”

ie. the equilibrium will shift to compensate for the stress imposed & a NEW equilibrium will be established

how many stressors there are
(for Le Chatelier’s Principal)

+summary of rules for each

4 stressors:

1. effect of concentration change (partial pressure of gas)

   • if [ ] increases, equilibrium will shift AWAY from substance

     • to get rid of

   • if [ ] decrease, equilibrium will shift TOWARDS from substance

     • to replenish

2. effect of volume/pressure change (total pressure)

   • if vol decrease/pressure increase, eqb will shift TOWARDS with LESS mols of gas

     • to spread out/disperse or make less mols of gas

   • if vol increases/pressure decrease, eqb will shift TOWARDS with MORE mols of gas

     • to relax

3. effect of temperature change

   • if temp increases, eqb shift AWAY from heat term

     • want to use it up

   • if temp decreases, eqb shift TOWARDS heat term

     • want to make more heat

4. effect of catalyst

   • no shift since new path for both rates faster

LCP: effect of concentration change (partial pressure of gas)

-increase [ ]

-decrease [ ]

• shift where

• HOW

• end result

-increase [ ] of reactants

• shift right

• forward increases bc reactants changed

  + forward rate slows back down as reactants are consumed
  + reverse rate increases as now more products to reform reactants

• new equilibrium

  • reactants decreased

  • products increased

-decrease [ ] of reactants

• shift left

• forward decreases bc reactants changed

  + forward faster as more reactants
  + reverse rate decreases as has reactants to make products

• new equilibrium

  • reactants increased

  • products decreased

partial pressure vs total pressure

→ look at onenote?

• partial pressure is individual concentration of one gas

• total pressure is total pressure

LCP: effect of volume/pressure change (total pressure change)

• effect what

-decrease vol/increase pressure

-increase volume/decrease pressure

• shift where

• HOW → don’t need to know

• end result

-effects gas only (can have other states just not effect)

-decrease vol/increase pressure

• shift to LESS mols of gas

• yay

• new equilibrium

  • direction where shift to will increase

  • shifted from will decrease

-increase vol/decrease pressure

• shift to MORE mols of gas

• yay

• new equilibrium

  • direction where shift to will increase

  • shifted from will decrease

LCP: effect of temperature change

• does it effect endo/exo differently? → depends on what

-increase temp

-decrease temp

• shift where

• HOW

• end result

-effects endo/exo differently → depends on Activation Energy + endo/exo

-endo:

-increase temp

• shift right

• forward rate changes bc Ea(f) > Ea(r) (want bigger one bc increase)

• new equilibrium

  • reactants decreased

  • products increased

-decrease temp

• shift left

• reverse rate changes bc Ea(r) < Ea(f) (want smaller one bc decrease)

• new equilirbium

  • reactants increased

  • products decreased

-exo:

-increase temp

• shift left

• reverse rate changes bc Ea(r) > Ea(f) (want bigger one bc increase)

• new equilibrium

  • reactants increased

  • products decreased

-decrease temp

• shift right

• forward rate changes bc Ea(f) < Ea(r) (want smaller one bc decrease)

• new equilibrium

  • reactants decreased

  • products increased

explain how change in temperature affects which rate does the change

if increase, the rate thats bigger does the change

if decrease, the rate that is smaller does the change

the size of rate depends on if exo/endo

LCP: effect of catalyst

since catalyst provides an alternate path with lower Ea, BOTH rates will speed up

• equilibrium will NOT shift

• no difference in [ ] of reactants, products

LCP: graphs of stressors

• general things

• then specifics

• won’t go back to same line as beginning since new equilibrium

  • unless catalyst!

• mol ratio matters (use x to show)

1. effect of concentration change (2step)

   • if directly/stress (increase/decrease), need to do vertical line

   • for [ ] if there’s another reactant thats not increased, will be decrease curve (NOT VERTICAL bc not directly) → shift

2. effect of volume/pressure change (2step)

   • if vol decrease, all initial increase

   • if vol increase, all initial decrease

   • think about shift (+ end result) then determine which is increase or decrease

   • if equal mols of gas on each side = no shift = increase + new plateau

3. effect of temperature change

   • look at heat + shift → second arrows to determine (increase/decrease)

   • NO VERTICAL LINES bc temperature is a gradual thing

4. effect of adding catalyst

   • for concentration graph, same line, no change

   • for rate, increases vertical line then new plateau

eg of each type of graph?

Keq def

equilibrium constant:

-ratio of [products]:[reactants] in a equilibrium at a constant temperature

Keq = [P] / [R]

-if coefficient/mol ratio = do exponents

-if any reactants/products, multiply them tgt

-ONLY FOR GAS, ignore solids + liquids

why can we ignore solids + liquids when calc Keq

-in solids, # of moles in a given volume is constant (fixed density) therefore constant [ ].

-same w liquids, constant [ ].

what does it mean when Keq:

• smaller than 1

• bigger than 1

• equal to 1

1. less than 1 = [R] > [P] (favour reactants)

2. more than 1 = [P] > [R] (favour products)

3. equal 1 = [R] = [P]

what factor(s) change value of Keq?

temperature → think of exo/endo

ICE TABLE TIPS

• label everything ([ ]eq ; [ ]i)

• define what x is

• fill out table properly

• C row: be careful of mol ratios (some can be 2x, x)

• some intials are given in mols → need to convert to M

• some are 0 at start (not givens)

  • shift towards this to make

• remember shifts will determine if add or subtract change

• be careful which can be used for Keq or Ktrial

• adding to get final [ ]eq → remember decimal place sigfigs

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using Q in eq:

• when unknown is how many mols added at start

Q or Trial Keq or Ktrial

• equal to Keq

• smaller than Keq

• larger than Keq

ratio of [product] : [reactants] at initial conditions (NOT at eqb)

• use this to find out where shift when compared to Keq

• Q = Keq → rxn at eqb

• Q < Keq → will shift right (to make more products)

  • think that Q needs to increase → make more numerator/products

• Q > Keq → will shift left (to make more reactants)

  • think Q needs to decrease → make more demon/reactants

relationships of Keq

1. reverse

A + B → C + D (Keq = 5)

C + D → A + B (Keq = 1/5)

2. doubled

A + B → C + D (Keq = 5)

2A + 2B → 2C + 2D (Keq = 5²)

3. halved

A + B → C + D (Keq = 5)

½ A + ½ B → ½ C + ½ D (Keq = 5^1/2)