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Agenda
• Please hand in your entry-level
assessment in MY basket!
• Introduction to Reaction Rates
• Short Ted Ed Video
• Assign Group Project!
**HW: Complete intro to rates ws, Vocab p.
662, list of 8 words posted on my website!
LAB tomorrow: Shoes, Hair etc.
Rates
A rate measures changes
that occur within an
interval of time.
Reaction Rates
Rxn speed:
Examples
Very fast
Mg and hydrochloric acid, sodium
and water, burning a match,
combustion of fuel, etc.
Moderate
Food spoiling, rusting of iron,
baking a cake, food digestion,
cooking, etc.
Very slow
Forming diamonds, weathering of
rocks, setting concrete, silver
tarnishing, etc.
Reaction Rate
• Reaction Rate can be defined as:
– The rate of formation of a product, or the
rate of disappearance of a reactant.
Collision Theory
• In order for reactions to occur:
– Molecules must collide
– orientation of molecules must be correct
– There must be enough energy (activation
energy…we will come back to this)
Factors That Affect Reaction Rates
The rate of a chemical reaction
depends on 4 things:
1.Temperature
2.Concentration
3.Particle size
4.Adding a catalyst ( we will
discuss this later)
Temperature
Increasing the temperature raises
the frequency of collisions and the
number of particles that have
enough kinetic energy to get over
the energy barrier.
Concentration
Increasing the concentration
causes an increase in the
frequency of collisions.
Particle Size
The smaller the particle size,
the larger the surface area for
a given mass of particles.
vs
Activation Energy (Ea)
The minimum energy colliding particles
must have in order to react.
•During a reaction there is an inbetween state called the activated
complex or transition state which is the
unstable arrangement of atoms at the
peak of the activation energy barrier.
(Exists for a very short amount of time,
~10-13 seconds.)
Energy Changes in a Reaction
Energy
Ea of forward rxn
Ea
Energy of
reactants
Energy of
products
Enthalpy (ΔH)
of reaction
Time
Catalyst: Increases the rate of
a reaction by lowering the Ea.
Pt
2H2(g) + O2(g)  2H2O(l)
Inhibitor: A substance that
decreases the rate of a
reaction by increasing Ea.
Adding a Catalyst
Video
•Here’s a video about
how to get a date…and
speed up the rate of a
chemical reaction 
Agenda
• Lab: Alka Seltzer and Reaction Rates
• Review HW
• If time, continue brainstorming ideas for
group project
**HW: Complete Lab, continue working on
group project, Vocab p.622 (8 terms are
listed on my website under Class Homework
Assignments)
Agenda
• Please turn in your Lab to MY basket!
• CR Final on Thermo. Tuesday 3/24!!
• Equilibrium Lesson
• Computer Research Group Project
**HW: Complete Le Chatelier practice ws,
continue working on group project, Vocab
p.622 (8 terms are listed on my website
under Class Homework Assignments)
Equilibrium and Le
Chatelier’s principle
Equilibrium (Eq)
• Equilibrium can involve physical
changes (i.e. phase changes) or
chemical changes (i.e. chemical
reactions)
• We will begin by discussing equilibria
involving physical changes.
Equilibrium (Eq)
• Physical equilibria require a closed
system at a constant temperature.
– Ex. Evaporation, dissolving
Equilibrium (Eq)
Chemical equilibrium occurs whenever
two opposite reactions occur at the
same rate.
Example: 2SO2 + O2 ⇌ 2SO3
The double sided
arrow indicates that
a reaction is
reversible!
Equilibrium (Eq)
• Although the rate of the forward
reaction equals the rate of the
reverse reaction, this does not mean
that the amounts of products equal
the reactants.
• It simply means that the amounts of
products and reactants are constant!
Equilibrium (Eq)
Le Châtelier’s
Principle: If a
stress is applied to
a system in
equilibrium, the
reaction will shift
in the direction to
relieve that stress.
(1) Change in Concentration
An increase in concentration
causes the system to shift in the
direction that will use that
substance up.
A decrease in concentration
causes the system to shift in the
direction that will make more of
that substance.
Example:
2SO2(g) + O2(g) ⇌ 2SO3(g)
shift right
[SO2],_________
shift left
[SO3],_________
[O2], _________
shift left
Note: Changing the concentration of a pure solid
or pure liquid has no effect on the equilibrium.
Example:
2Na(s) + Cl2(g) ⇌ 2NaCl(s)
[NaCl],_________
no change
no change
 [Na], _________
(2) Change in Temperature
Depends on whether energy is
absorbed or energy is released.
Exothermic = energy released
Endothermic = energy absorbed
(2) Change in Temperature
If energy is absorbed;
A+B⇌C+D
A + B + 100KJ ⇌ C + D
If energy is released;
A+B⇌C+D
A + B ⇌ C + D + 100KJ
**It may help to think of heat as a
reactant or product**
Example:
2SO2(g) + O2(g) ⇌ 2SO3(g) + 135kJ
 temp, shift left
______________
 temp, shift right
______________
Example:
286kJ + 2H2(g) + O2(g) ⇌ 2H2O(l)
 temp, shift right
______________
 temp, shift left
______________
(3) Change in Pressure
Affects gases only.
Increase pressure:
Will shift toward
the side with less
moles of gas.
Example:
2Na(s) + Cl2(g) ⇌ 2NaCl(s)
1 mole of gas
0 moles of gas
 pressure, shift right
______________
(3) Change in Pressure
Decrease pressure: Will shift
toward the side with more
moles of gas.
Example:
2Na(s) + Cl2(g) ⇌ 2NaCl(s)
1 mole of gas
0 moles of gas
______________
 pressure, shift left
N2(g) + O2(g) ⇌ 2NO(g)
 pressure,no change
______________
2 moles of gas
2 moles of gas
Agenda
• Warm Up
• Short Keq lesson
• Computer Activity demonstrating Le
Chatelier’s principle
**HW: Finish Keq practice, Finish Computer
Activity, Study for Thermo. CR on Tuesday,
Study for QUEST on Wednesday, continue
working on group project due Thursday!!
Warm Up
• For the following reaction, predict the
direction the equilibrium will shift for
each applied stress.
PCl5(g) + 92.5 kJ ⇌ PCl3(g) + Cl2(g)
1.
2.
3.
4.
Temperature increases
Increase [Cl2]
Decrease [PCl3]
Increase pressure
Recall, equilibrium is reached
when the rates of the forward
and reverse reactions are
equal. There is no change in
the amount of reactants and
products (i.e. it is constant)
Equilibrium Constant (Keq)
For the general reaction,
aA + bB ⇌ cC + dD
once the reaction has reached
equilibrium, the concentrations of
reactants and products are related in the
following manner:
[C]c[D]d
[A]a[B]b
= Keq
Keq>1, products are
favored at equilibrium
Keq<1, reactants are
favored at equilibrium
**Important!**
•As previously discussed with
LeChâtelier’s principle, pure solids and
pure liquids have no effect on
equilibrium. Do not put them into the
equilibrium expression!
•Only gas and aqueous concentrations
can be used in the Keq expression!
•Concentrations are always in the unit
M for Molarity (we will discuss this
more next unit.)
Example
For the following reaction,
2CO(g) + O2(g) ⇌ 2CO2(g)
the equilibrium concentrations for CO is 1.5M and
for CO2 is 0.81M. What is the concentration of
the O2? (Keq=15.3)
[CO2]2
Keq=
[CO]2[O2]
[0.81]2
= 15.3
2
[1.5] [x]
[O2] = 0.019M