Chemical Equilibrium
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Reversible vs Irreversible Rxn
• Irreversible rxn → one direction only
– React → prod
• Examples: combustion rxn, decomposition of food
• Reversible rxn → both directions
– React → prod and Prod → react
– When rates of the two opposing reactions are equal, you get chemical equilibrium!
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Irreversible reaction Reversible reaction
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Static vs Dynamic Equilibrium
• Static
– Completely immobile, system remains at a given point
• Dynamic
– Two opposing processes occurring at same rate so that no visible change takes place
– Involves reversible reactions
– Three types
• Phase
• Solubility
• **Chemical** (what we’re really concentrating on)
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Dynamic Equilibrium
• Phase: single substance found in several phases as a result of physical changes
– Sealed bottled water
• Solubility: solute is dissolved in a solvent and an excess of solute is in contact with the saturated solution
– Sugar in coffee
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Chemical Equilibrium • Def: Dynamic Equilibrium which is a result from
two opposing chemical reactions that occur at the same rate, leaving composition of the reaction system unchanged – Rxn must be reversible
• Equilibrium must be the result of chemical change between a reactant and a product
You are in a state of dynamic equilibrium when the forward rxn rate is equal to the reverse rxn rate
revfor rr
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Chemical Equilibrium
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Conditions for Attaining Equilibrium
• Must have a reversible reaction
• Change must occur in a closed system
• **Macroscopic properties are constant**
– Macroscopic = visible to naked eye
– Colour, volume, pH, temperature, pressure
• It by itself it is not proof
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Le Chatelier’s Principle
If the conditions of a system in a state of equilibrium change, the system will react to partially oppose this change until it attains a new state of equilibrium
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Le Chatelier’s Principle
• Factors that affect state of chemical equilibrium
– Concentration
– Temperature
– Pressure
• The story of Mr. Catalyst
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Key about Le Chatelier
Whatever side you place the stress on, the reaction will shift in the opposite direction
to relieve that stress and get back to equilibrium
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Systems in disequilibrium…eutrophication
• Eutrophication
– Fertilization of surface water by previous scare nutrients
• ↑ nutrients ↑ phytoplankton, benthic and epiplagic algae and bacterial biomass
• ↑ anoxia in deep water
• ↑ mortality of fish and shellfish
• ↓ in water transparency
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Equilibrium Constant
• Relationship between [prod.] and [react], (at a given temp) each raised to the power of their stochiometric coefficients
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Now, equilibrium is defined as
revfor rr
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Equilibrium Constant
• Given the equation
dDcCbBaA
ba
c
cBA
DCK
][][
][][
Kc : Equilibrium Constant [C], [D] Conc. of prods (mol/L) [A], [B] Conc. of react (mol/L) a,b,c,d Stoich. Coeff of balanced equation
Just like in rate law, only consider gases or substances in solution
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Equilibrium Constant
• Just like you can get Kc for the forward rxn, can get it for the reverse
for
rev
c
cK
K1
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Effect of Temperature on K
• Only temperature can change K; conc. P and V will not alter K – Why temp must specified when providing K
• What’s happening? – Le Chatelier!
– When shifting equili, react or prods are being made as a result of temperature
– New equili won’t have conc. proportions that are the same from initial equili
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How to calculate K→ R.I.C.E/I.C.E
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How to calculate K→ R.I.C.E/I.C.E
• Stands for
– Reaction (if you Google it, some have the R, some don’t)
– Initial
– Change
– Equilibrium
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Acids and Bases:2 key theories
Arrhenius theory of acids and bases
• Acid: sub. when dissociates in water produces H+
• Base: sub. when dissociates in water produce OH-
Limitations to this theory!
• H+ wouldn’t exist in water, would form H3O+
• Can’t explain why solutions with NH3 are basic
• Cant’s explain why some salt solutions without H or O present basic properties
Brønsted-Lowry theory of acids and bases
• Acid: sub. which a proton can be removed – Proton donor
• Base: sub. that can remove proton from acid – Proton acceptor
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Conjugate acid
Conjugate base
Conjugate acid-base pair
Conjugate acid-base pair
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pH and pOH
• pH: Measure of the acidity of solution
– Concentration of H3O+ ions per mol/L in a solution
• pOH: Measure of the bascisty of solution
– Concentration of OH- ions per mol/L in a solution
pHOH
OHpH
10][
]log[
3
3
pOHOH
OHpOH
10][
]log[
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Ionization constant of water
• pH of water is 7….why?
]][[
2
/100.1][][
3
)()(3)(2
7
3
OHOHK
OHOHOH
LmolxOHOH
w
aqaql
Kw : Ionization constant of water [H3O+] Conc. of hydronioum ion (mol/L) [OH-] Conc. of hydroxide ion (mol/L
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Kw always equals 1.0 x 10^-14 @ 25°C
14
77
7
3
100.1
]100.1][100.1[
/100.1][][
xK
xxK
LmolxOHOH
w
w
Using this logic, at any given moment, knowing either the pH or pOH, you can calculate the number of hydronium or hydroxide ions
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Acidity and Basicity Constants
• Indicates the strength of the acid or base at equilibrium
– Weaker acid/base → smaller value of acidity/basicity constants
][
]][[ 3
HA
AOHKa
][
]][[
B
OHHBKb
)()()(2)( aqaqlaq OHHBOHB )()(3)(2)( aqaqlaq AOHOHHA
Ka /Kb : Acidity/Basicity constant [H3O+]/ [OH-] : Conc. of hydronioum/ hydroxide ion (mol/L) [HA]/[B]: Conc. of un-dissociated acid/base (mol/L) [A-]/[HB+]: Conc. of conjugate base/acid (mol/L) A. Di Lallo
Solubility Product Constant
• Similar stories to Ionization Constant
– But talking about salts dissolving now!
mn
sp
aqaqsmn
YXK
mYnXYX
][][
)()()(
Ksp: Solubility product constant [X+], [Y-] : Conc. of ions (mol/L) n, m: Coeff of each ions in balanced equ
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