32
Chemical Equilibrium A. Di Lallo
Chemical Equilibrium
A. Di Lallo
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!
A. Di Lallo
Irreversible reaction Reversible reaction
A. Di Lallo
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)
A. Di Lallo
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
A. Di Lallo
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
A. Di Lallo
Chemical Equilibrium
A. Di Lallo
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
A. Di Lallo
A. Di Lallo
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
A. Di Lallo
A. Di Lallo
Le Chatelier’s Principle
• Factors that affect state of chemical equilibrium
– Concentration
– Temperature
– Pressure
• The story of Mr. Catalyst
A. Di Lallo
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
A. Di Lallo
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
A. Di Lallo
A. Di Lallo
A. Di Lallo
A. Di Lallo
Equilibrium Constant
• Relationship between [prod.] and [react], (at a given temp) each raised to the power of their stochiometric coefficients
A. Di Lallo
Now, equilibrium is defined as
revfor rr
A. Di Lallo
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
A. Di Lallo
Equilibrium Constant
• Just like you can get Kc for the forward rxn, can get it for the reverse
for
rev
c
cK
K1
A. Di Lallo
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
A. Di Lallo
How to calculate K→ R.I.C.E/I.C.E
A. Di Lallo
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
A. Di Lallo
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
A. Di Lallo
Conjugate acid
Conjugate base
Conjugate acid-base pair
Conjugate acid-base pair
A. Di Lallo
A. Di Lallo
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[
A. Di Lallo
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
A. Di Lallo
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
A. Di Lallo
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
A. Di Lallo
