Chemical Equilibrium
1. What is chemical Equilibria
2. demo
3. What is it
4. LeChatelier’s Principle
5. Stresses – Concentration, Pressure, Temperature, Volume
6. Percent Reaction
7. Equilibrium Expression – Ke, Law of Mass Action
1. Demo – textbook pg 423 Shakin’ the Blues
250mL of water, add 6 drops methylene blue and 5.0g of potassium hydroxide, and 3.0 g glucose
or dextrose to 250 mL flask.
Stir mixture until solids have dissolved
Stopper the flask and set it on the bench. Observe the colour of the solution
Shake solution, note changes.
Set on table and leave it standing until another change is noticed
Repeat 2 previous steps.
Compare to a reaction like HCl and Zn which once complete is done.
Chemical Equilibria
aA + bB
cC + dD
*when the rate of the forward = the rate of the reverse
in Chemical Equilibrium OR dynamic equilibrium
we say the system is
This means …
1. A, B, C and D are all present in the container
2. Concentration of all species remains constant.
3. At the macroscopic level (visual) nothing seems to be going on (pressure,
temp, colour)
4. At the microscopic level (atomic) both reactions are taking place at the same
rate (forward and reverse)
Lets just take a look at …
A system that goes to equilibrium
Driving either the forward Rxn or reverse Rxn to
proceed when a system is at equilibrium
Le Chatelier`s Principle
When a stress is applied to a
System at equilibrium, the
System WILL react so as to
relieve that stress
one of the reactions will
take place  either
the FORWARD or REVERSE
Quick Terminology
Rxn that takes place We say equilibrium
shifts to the
Favored Direction
Start to produce
Forward Reaction
Right
Forward
Products
Reverse Reaction
Left
Reverse
Reactants
The Stressors:
1. Concentration Stress
If you increase the [ ] of the reactants/products,
the system will react to decrease the [ ] of the
reactants/products which will cause an increase
in the [ ] of the products/reactants
Eg. : N2 (g) + 3 H2 (g)  2 NH3 (g) ΔHR= -300kJ
Stress:
consider the equilibrium : Fe3+ ( aq ) + SCN- 
FeSCN2+
( yellow )
adding more SCN- ---->
stress too much SCN- to relieve stress equilibrium shifts to the _____________ producing
more
causing the colour to __________________
( colourless )
( deep red )
Consider the equ. SO2(g) + 1/2 O2 (g) SO3(g)
In what direction would the equation shift if
More O2 is pumped in :
A: to the right
B: to the left
Some SO2 is removed from the system
A: to the right
B: To the left
Consider the equ. SO2(g) + 1/2 O2 (g) SO3(g), If SO3 is removed from the
system, the concentration of O2 will increase
A: True
B: False
2. Temperature Stress
If you add heat ( T) ‘ENDO’ rxn is favoured
If you remove heat ( T) “EXO” rxn is favoured
Ex. N2 (g) + 3 H2 (g)  2 NH3 (g) ΔHR= -300kJ
a) Label forward / reverse as endo or exo
Consider the reaction 2N2O <--> 2N2 + O2 energy, is this an endo or exo reaction?
A: ENDO
B: EXO
Plickers:
Consider the reaction N204 + energy <--> 2NO2.
This is an exothermic reaction.
A: True
B: False
Consider the reaction 2N2O <--> 2N2 + O2 energy, if you were to add heat to this reaction,
in which direction would the equation shift?
A: Forward
B: Reverse
Consider the reaction N204 + energy <--> 2NO2.
This is an exothermic reaction
A: True
B: False
Consider the reaction N2O4 + energy <--> 2NO2.,
in which direction would the reaction shift and
what would happen to the concentration of the
product?
A: Right, Increase
B: Right, Decrease
C: Left, Increase
D: Left, Decrease
consider the equ. N2O4 (g) + 59 kJ  2 NO2 (g)
(colourless)
(brown/reddish)
-
increase in temp. ; stress is too much energy to relieve the stress equ. shifts to the
using up some of the energy ; reddish brown colour
________________
since there increase in
conc. and a decrease
in
conc.
3. Pressure Stress
If you increase pressure the system will react to
decrease the pressure (goes toward the side with
the lower # of gas phase particles)
Eg. : N2 (g) + 3 H2 (g)  2 NH3 (g) ΔHR= -300kJ
Stress:
Increase in P = shift to the side with least
# of gas moles
Decrease in P = shift to the side with
more moles of gas
Discuss with a partner, then click for the right answer.
Consider the equation N2O4 (g) + energy  2 NO2
(g)
How will an increase in pressure affect the system?
A: shift the equilibrium to the right
B: shift the equilibrium to the left
Because the left side has the least # of moles of gas
(thus reducing the stress of increased pressure)
4. Volume Stress:
Translate to a pressure stress (gr. 11 V/P
relationships)
increase V = decrease P
decrease V= increase P
5. Addition of a Catalyst: NO EFFECT, just speeds
up reaction
6. Addition of an INERT gas:  not altogether
sure yet
Discuss with a partner, then vote as a class – explain your reasons. Click for the answer,
Then click again for the explanation.
Consider the reaction Eg. N2O4 (g) + energy  2 NO2 (g), how would the system react
To being moved from a 1L container to a 2L container?
A: equilibrium will move in the forward direction
B: equilibrium will move in the reverse direction
An increase in volume = a decrease in pressure so the system will shift towards the side
With more moles of gas (this will increase the pressure again, reducing the stress of
Decreased pressure due to increased volume)
Four types of equilibria
1. Gaseous
2. Solubility
3. Phase
4. Acid/Base
Smartboard for percent reaction.
Law of Mass Action
At equilibrium ALL species are present … the
concentrations are constant
Consider: aA + bB < -- > cC + dD
we have something known as the equilibrium
constant Ke for a reaction at a given temp
** only if (g) or (aq)
** if (l) or (s) sub in the #1
High Ke values indicate that the products are favoured (or
the forward reaction).
ex. 2CO(g) + O2 < -- > 2CO2 (g)
Ke = 3.3 x 1091 at 25 ̊C
Small Ke values indicate that the reactants are favoured
(or the reverse reaction).
NO2(g) + NO (g) < -- > N2O (g) + O2 (g)
Ke = 0.914 @ 25 ̊C
Go to smart board for examples
Gaseous/ Chemical Equilibria
Go to Smart Board
Lets look at …
H2 (g)
Initial (i)
Change (c)
Equilibrium (e)
____
____
____
+
I2 (g)  2 HI (g)
____
____
____
____
____
____
Solubility Equilibria
smartboard
ACID/BASE EQUILIBRIA
Preview/Review
1. Determining pH
1. pH = -log[H+1 ]
2. pOH = log [OH-1]
3. pH + pOH = 14
2. Defn’s
monoprotic/ monobasic – one
diprotic/dibasic = 2
polyprotic/polybasic =
.3.Neutralization
# moles H+1 = # moles OH-1
MH+1VH+1 = MOH-VOH# moles H+ = MOH-VOH4. Acid / Base Equation
Something a little different… determining pH of
a mixture of an acid and base
1. Calculate the # _____________ and # of
____________ (MxV = n) (BECAREFUL –
write out equation)
2. Identify the one in _______________.
3. Calculate [ __________________]
=
=
Recall: Arrhenius Definition
ACID: Proton (H+1) donor
BASE: Hydroxide (OH-1) donor
Bronsted-Lowrey Definition
ACID: is a proton (H+1) donor (AD)
BASE: is a proton (H+1) acceptor (BA)
Acids donate H+1 ions, bases accept H+1 generating OH-1. This
is donated to or accepted from H20.
Eg. H – Acid + H2O
Base + H2O
H3O + Acid-1
Hydronium ion = H+1 = proton
H-Base + OH-1
Strong Acid (SA): acid that ionizes 100%
no equilibria
eg. HCl
H+ + Cl-1
HCl, HNO3, H2SO4, HBr, HI, HClO4
Strong Base (SB): base that ionizes 100%
eg. NaOH
Na+ + OH-1
•
LiOH, NaOH, KOH, RbOH, CsOH
Weak Acid:
• An acid that does not ionize 100%
• An equilibria sets-up
Acetic Acid CH3COO-H
H2O (l) + CH3COO-H (aq) < --- > H3O+1 (aq) + CH3COO-1 (aq)
Or
CH3COO-H (aq) < --- > CH3COO-1 (aq) + H+1 (aq)
Weak Base:
• A base that does not ionize 100%
• An equilibria is set up
Ex Ammonia NH3
NH3 (aq) + H2O (l) < --- > NH4+1 (aq) + OH-1 (aq)
Or
NH3 (aq) < --- > NH4+1 (aq) ) + OH-1 (aq)
Weak Acid
HA
HA < -- > H+1 + A-1
Weak Base
NH3
NH3 (aq) < --- > NH4+1 (aq) ) + OH-1 (aq)
-1
Acidic eq. because a H+1 Basic eq. because we have OH
ions
ion.
Ka = [A-1] [ H+1]
[HA]
Kb = [NH4+1][OH-1]
NH3
Handout – do acidic/basic
equilibriums
Then to smart board for
determining pH of weak
acid or base.
Do example then in
partners try the 2
examples on white page
then take up.
Titrations
• formerly known as neutralization reactions
ACID + BASE  SALT + H2O
• If the salt produces a strong conjugate as one
of its ions, hydrolysis takes place and it will
create an acidic/basic solution instead of a
neutral one
Ex. HCl + NH3  NH4Cl + H2O
Buffers in the Blood
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The pH of blood is 7.35 – 7.45
Changes in pH below 6.8 and above 8.0 may result in
death
The major buffer system in the body fluid is H2CO3/HCO3Some CO2, the end product of cellular metabolism, is
carried to the lungs for elimination, and the rest dissolves
in body fluids, forming carbonic acid that dissociates to
produce bicarbonate (HCO3-) and hydronium (H3O+) ions.
More of the HCO3- is supplied by the kidneys.
CO2 + H2O ↔ H2CO3
H2CO3 + H2O ↔ H3O+ + HCO3-
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Carbonate buffer
H2CO3 + H2O ↔ H3O+ + HCO3
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Excess acid (H3O+) in the body is
neutralized by HCO3H2CO3 + H2O ← H3O+ + HCO3Equilibrium shifts left
Excess base (OH-) reacts with the carbonic
acid (H2CO3)
H2CO3 + OH- → H2O + HCO3Equilibrium shifts right
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The effectiveness of the blood buffer
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If the pH of 100 mL of distilled water is 7.35 and
one drop of 0.05 M HCl is added, the pH will
change to 7.00.
To change 100 mL of “normal” blood from pH of
7.35 to 7.00, approximately 25 mL of 0.05 M HCl
is needed.
With 5.5 L of blood in the average body, more
than 1300 mL of HCl would be required to make
the same change in pH.
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Importance of the bicarbonatecarbonic acid buffering system
1.
2.
3.
H2CO3 dissociates into CO2 and H2O,
allowing H3O+ to be eliminated as CO2 by
the lungs
Changes in PCO2 modify the ventilation
rate
HCO3- concentration can be altered by
kidneys
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Interpretation of Clinical Blood Gas Data
H2O
CO2 + H2O ↔ H2CO3 ↔ H3O+ + HCO3-
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The concentration of carbonic acid in the body is
associated with the partial pressure of CO2.
When CO2 level rises, producing more H2CO3, the
equilibrium produces more H3O+, which lowers
the pH – acidosis.
Decreasing of CO2 level due to a hyperventilation,
which expels large amounts of CO2, leads to a
lowering in the partial pressure of CO2 below
normal and the shift of the equilibrium from
H2CO3 to CO2 and H2O. This shift decreases
H3O+ and raises blood pH – alkalosis.
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Respiratory Acidosis: CO2 ↑ pH ↓
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Symptoms: Failue to ventilate, suppression of
breathing, disorientation, weakness, coma
Causes: Lung disease blocking gas diffusion (e.g.,
emphysema, pneumonia, bronchitis, and asthma);
depression of respiratory center by drugs,
cardiopulmonary arrest, stroke, poliomyelitis, or
nervous system disorders
Treatment: Correction of disorder, infusion of
bicarbonate
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Respiratory Acidosis: CO2 ↑ pH ↓
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Symptoms: Failue to ventilate, suppression of
breathing, disorientation, weakness, coma
Causes: Lung disease blocking gas diffusion (e.g.,
emphysema, pneumonia, bronchitis, and asthma);
depression of respiratory center by drugs,
cardiopulmonary arrest, stroke, poliomyelitis, or
nervous system disorders
Treatment: Correction of disorder, infusion of
bicarbonate
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Metabolic (Nonrespiratory)
Alkalosis: H+ ↓ pH ↑
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Symptoms: Depressed breathing, apathy,
confusion
Causes: Vomiting, diseases of the adrenal
glands, ingestions of access alkali
Treatment: Infusion of saline solution,
treatment of underlying diseases
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