Equilibrium
Chapter 14 p. 496
Standard
HS-PS1-6.
Refine the design of a
chemical system by specifying a change
in conditions that would produce
increased amounts of products at
equilibrium
PET sim: reaction rates
Mission to Mars
Equilibrium
 Many chemical reactions ‘go to
completion’
 But some chemical reactions are
‘reversible’:
 forward reaction () where reactants
become products
 reverse reaction () where products
become reactants
Teacher notes: equilibrium arrows
The chemical reaction that feeds the world - Daniel D. Dulek
Equilibrium Model:
Toothpick Lab
Graph (Part A):
Make a graph of Number of Toothpicks (final) in
containers R and P against transfer round. Use
different colors for the R and P lines.
Remember:
 Title
 Independent (x) vs dependent variable (y)
 Design scale appropriately (and use ruler)
 axis labels
 Include legend
Equilibrium Model:
Toothpick Lab
Think about it….
1. What changes can be observed in the
number of toothpicks in R and P during
the ten transfer rounds?
Now write about it…..
Formal writing, using academic vocab
Suggested ~ 2 sentences
Equilibrium Model:
Toothpick Lab
1. What changes can be observed in the
number of toothpicks in R and P during the
ten transfer rounds?
2.What would happen if you continued
doing the transfer for the entire class period
(100+ transfer rounds!!). How many
toothpicks would there be in R and P at the
end of the period? Why? Explain.
Equilibrium Model:
Toothpick Lab
2.What would happen if you continued doing the
transfer for the entire class period (100+ transfer
rounds!!). How many toothpicks would there be in R
and P at the end of the period? Why? Explain.
3a. What do the toothpicks represent? What do the
cups represent?
3b. What would happen if we changed the parameters
of our experiment to 1/4 forward and 1/3 back?
3c. A model always has limitations (it cannot be taken
literally). What are some of the limitations of the
toothpick model?
Equilibrium model:
Toothpick lab
3a. What do the toothpicks represent? What do
the cups represent?
3b. What would happen if we changed the
parameters of our experiment to 1/4 forward and
1/3 back?
3c. A model always has limitations (it cannot be
taken literally). What are some of the limitations
of the toothpick model?
4. Compare your new graph to the original graph
we made (where there was a forward reaction
only). How are they similar/different? Why?
Graph
4. Compare your new graph A to the
original graph we made (where there was a
forward reaction only). How are they
similar/different? Why?
5. Draw a tangent on your new graph P line
at:
 transfer round 2
 transfer round 6
What is the slope at transfer round 6?
 What does this indicate?
Equilibrium Model:
Think about it…..
6. In this activity, the reactions between R
and P appear to stop when no further
changes were observed. Do chemical
reactions actually stop when they reach
equilibrium? Explain.
PhET: Salts and Solubility
Equilibrium model
Think about it….
Part B “Does the position of equilibrium
depend on the initial number of reactants?”
1. Calculate the P/R ratio from Number of
Toothpicks (final) transfer round 6 (2 sig
figs) for both Parts A and B.
2. Does the P/R ratio depend on the initial
number of reactants? Yes or No.
ANS = NO
Frayer Summarizer:
Equilibrium
definition/facts/diagram/examples
On the basis of this experiment, write your
own definition of the word ‘equilibrium’
(closed book)
Frayer
Equilibrium definition =
“A state of balance…
in which the rate…..
of the forward reaction equals…..
the rate of the reverse reaction...and…..
the concentrations……
of products and reactants……
remain unchanged.”
Frayer: Equilibrium Facts
 a dynamic condition
 Forward and reverse reactions continuously
occur, but there is no overall change.
 Does equilibrium mean there are equal
amounts in R and P?
No, equilibrium does not have to be a 50:50 mix
Diagram: draw a representation of equilibrium
(be creative)
Frayer – Facts : Closed
System
What is the difference between a CLOSED
SYSTEM and an open system?
Draw it!
Equilibrium Model:
Think about it…..
Part C: ‘Does the position of equilibrium
depend upon the starting point?’
Hypothesis = ??
One of the toothpicks in your experiment is
marked with black ink. Keep an eye on
that particular toothpick during your
experiment.
Where was the labelled toothpick at
transfer round 6?
“Radioisotope labeling”
Equilibrium model
Think about it…..
Part C
‘Does the position of equilibrium depend
upon the starting point?’
Calculate the P/R ratio.
How does the data for Part A and C differ?
How is the data similar? What does this
data tell us about equilibrium?
Equilibrium Model:
Toothpick Lab
HONS: Using algebra, calculate the
number of molecules (toothpicks) that
would be present in R and P at equilibrium
if we started with 100 in R and zero in P.*
14.2 Equilibrium Constant
Frayer
Look up the definition if equilibrium constant (Keq)
in your textbook and copy it onto your Frayer
(definition section).
See Skills Toolkit p. 503
Read/notes p. 505 Keq shows if the reaction is
favorable
see Fig 7 and Table 2
Calculating Equilibrium
Constant**
p. 504 # 1,2 Calculating Keq from [R][P]
P. 506 # 1,2 Calculating [P] from Keq & [R]
Practice:
p. 522-523 # 26-34 evens only (5)
Review: Solubility
Consider the dissolving of calcium fluoride
‘Sparingly Soluble Salt’
CaF2 (s) ↔ Ca2+(aq) +
2F- (aq)
‘Dissociation’
Solubility = 3.4 x 10-4 mol/L
Demo: sparingly soluble salt
PET sim: salts&solubility
Solubility Product
Constant, Ksp See p. 507-508
Solubility can also be quantified by the
‘Solubility Product Constant’, Ksp
Sample Problem C
Calculate the Ksp of CaF2
Procedure: read Skills Toolkit p. 508
ANS: Ksp = 1.6 x 10-10
Solubility Product
Constant, Ksp See p. 507-508
What do the numbers in Table 3 p.508 tell
us about solubility?
Which salt listed has the lowest solubility?
Ag2S
Ksp = 1.1 x 10-49
Practice
Write the equilibrium expression for the Ksp
of Ag2S
Ag2S(s) ↔ 2Ag+(aq) +
S2- (aq)
Solubility Product
Constant, Ksp
Handout: Ion Sheet
p. 509 #1,2,3 Calculating Ksp from
Solubility
Practice
p. 523 # 35-38
Per 2 h’wk p. 523 # 35,36 (practice)
Solubility Product
Constant, Ksp
p. 510 #1,2,3,4 Calcing Ion Conc using Ksp
Practice
p. 523-524 # 39-43 odds only (3)
Equilibrium Model:
Think about it……
Part D “What happens when more reactants are
added to a system at equilibrium”
Hypothesis = ?
Compare the results obtained in Parts A,B&C
with the results from part D.
“Does the P/R ratio depend upon the initial
number of reactants? Yes/No. Explain.”
NO b/c……
Equilibrium Water Games
Part 1
Prediction
I predict that at equilibrium, the amount of water
will be:
A) All of the water in P
B) Most of the water in P, a small amount in R
C) Half the water in P and the other half in R
D) Most of the water in R, a small amount in P
E) All of the water in R
F) There will not be an equilibrium with this
system
Equilibrium Water Games
Part 1
Think about it……
“Why was most of the water in P and a
smaller amount in R? How can this
model be applied to a chemical system at
equilibrium?”
Closed System
What is the difference between a CLOSED
SYSTEM and an open system?
Draw it!
Equilibrium Water Games
Part 2
Prediction:
Think about it…..
Predict what you think will happen to the
equilibrium water levels if we add extra
water to the Reactant side and then
continue the transfer.
‘Stress’
Equilibrium Water Game
Part 2
3. Explain:
Le Chatelier’s Principle:
“If we have a system at equilibrium...and…
we apply a stress to it…..
it will shift by undoing the stress.”
read p. 512 paragraph 3
Chemistry Standard
9a “Students know how to use Le
Chatelier’s Principle to predict the effect
of changes in:
 concentration
 temperature
 pressure.”
Homework
read Sec 14.2 p. 512-516
Le Chatelier:
concentration
Try process of elimination (POE)
NGSS
Refine the design of a chemical system by
specifying a change in conditions that would
produce increased amounts of products at
equilibrium.
* [Clarification Statement: Emphasis is on the
application of Le Chatelier’s Principle and on
refining
designs of chemical reaction systems, including
descriptions of the connection between changes
made at the macroscopic level and what happens
at the molecular level.
Examples of designs could include different ways
to increase product formation including adding
reactants or removing products.]
Equilibrium
Le Chatelier’s Principle
“If we have a system at equilibrium...and…
we apply a stress to it…..
it will shift by undoing the change.”
Le Chatelier’s Principle
Notes:
Worksheet - Equilibrium and Le Chat’s
Principle
p. 4,5,6
p.6 Problems
What are the five things that a chemical
engineer could do to make more ammonia
(shift equilibrium to the right)?
Typical Le Chatelier’s
Principle Problem
1. Start with a system at equilibrium
2. Stress applied:



Change in concentration
Change in pressure
Change in temperature
3. System shifts to relieve stress. Predict:


Does reaction rate increases in forward direction?
OR
Does reaction rate increases in reverse direction?
4. New equilibrium established
Practice question
For choice B, count the total number of moles of gas on both sides of the equation.
Pressure change?
 How many moles of gas are on the left side of
the eqn?
 Left = 2 moles of gas
 How many moles of gas are on the right side of
the equation?
 Right = 2 moles of gas
 Does changing pressure have any effect on the
equilibrium?
 No, so the answer CANNOT be B
Practice question
ANS = C
Practice
Reminder:
Exothermic reaction
 Like cycling from the
Valley to the beach
 energy released by
system to surroundings
 Temp of surroundings
increases
 Reactants have higher
energy than products
 ‘downhill’
 E.g. combustion
Temperature?
 Is this reaction endothermic or exothermic?
 Exothermic, because the system produces
energy.
 Choice A: If we add heat to the rxn (STRESS) it
will try to remove the added energy (per Le
Chat).
 How? By moving to the left  (SHIFT)
 So choice A cannot be correct.
Pressure?
 How many moles of gas are on the left side of
the eqn?
 Left = 5 moles of gas
 How many moles of gas are on the right side of
the equation?
 Right = 2 moles of gas (careful)
 Does changing pressure (STRESS) have any
effect on the equilibrium? Yes
 Choice D: system SHIFTS to less gas side
(right)
Practice question
For choice A or C, count the total number of moles of gas on both sides of the equation.
ANS = A, greater number of moles of gas side
Practice question:
pressure
ANS = A, smaller number of moles of gas side