Reaction Rates and Chemical Equilibrium

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Reaction Rates and Chemical Equilibrium

• What you will Learn
• The factors that control the rate of a reaction
• How energy influences a reaction
• How reversible reactions behave
• The factors that control reversible reactions

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Reaction Rates

• The rate at which reactants are used up in a
  chemical reaction
• For example, how fast the wood is used up in a
  campfire

3
Reaction Rates

• The rate at which products are formed in a
  chemical reaction
• For example, how fast the ashes form in a campfire

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Reaction Rates

• The reaction rate is the decrease in
  concentration of reactants or the increase in
  concentration of products with time.

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Reaction Rates

• Using H2 I2 ? 2HI,
• Rate kH2I2
• This means that the rate of the reaction depends
  on both of the concentrations in Molarity.

• k rate constant
• For fast reactions this would be a large number
• Its unique for every reaction

H2 hydrogen gas concentration in
molarity I2 iodine gas concentration in
molarity
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Reaction Rates

• k is a constant
• This means that if you have a specific amount of
  reactants, the reaction will always take the same
  amount of time.

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Reaction Rates

• However, some conditions can change the value of
  k
• The surface area of the solid reactants
• The pressure on gaseous reactants
• The temperature of the reactants
• The presence of catalysts

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Reaction Rates

• Factors that affect the rate of a reaction
• Concentration of reactants
• Surface area of solid reactants
• Pressure on gas reactants
• Temperature of reactants
• Catalysts
• KNOW THESE THIS IS SOOOOOOOOOPER IMPORTANT!

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Factor 1 Concentration

• When the reactants are concentrated
• The particles have a greater chance of colliding
  and therefore making a reaction

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Factor 1 Concentration
When there are few molecules of each reactant
left, the collisions happen less often and the
reaction is slow to finish completely
When there are many molecules of each reactant,
the collisions happen often and the reaction is
quick
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Factor 1 Concentration

• Remember,
• Higher concentration can be made by
• using more solute
• by using a smaller space.
• (smaller volume)

• Examples of reactions that depend on
  concentration
• A piece of marble in dilute acid will dissolve
  slowly.
• Inside a car engine, the gas and the oxygen in
  the air are crammed into a tiny cylinder, thus
  increasing the concentration and making the
  reaction fast.

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Factor 2 Surface Area

• If the reactants are solid, then the amount of
  surface area affects the reaction rate.
• Large pieces react slowly because most of the
  reactant particles are trapped inside the piece.

The smaller pieces have more exposed surface for
reactions to take place.
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Factor 2 Surface Area

• Examples of reactions where surface area affected
  the reaction rate
• Exploding grain silo
• The grain dust builds up and even a tiny spark
  can cause all the grain to burn at once. BOOM!
• Starting a campfire
• Big pieces of wood are hard to burn alone because
  there is not enough surface area

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Factor 3 Gas Pressure

• Increasing the pressure on gas reactants causes
  the molecules to collide more often increasing
  the reaction rate.

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Factor 3 Gas Pressure

• Most of the time, pressure is increased by
  shrinking the volume of the reacting gases.

Notice how the particles strike each other more
often now that they have less space
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Factor 3 Gas Pressure

• Examples of reactions where gas pressure affected
  the reaction rate
• The dissolving of CO2 into water.
• CO2(g) H2O ? H2CO3
• Soda pop has pressure applied to it before its
  bottled to help the CO2 dissolve.
• The Haber Process for making NH3
• N2(g) H2(g) ? NH3(g)
• The process includes a step where the reaction is
  placed under high pressure to help the reaction
  speed up

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Factor 4 Temperature

• Temperature increases cause the reaction rate to
  go up
• Because the increased motion of the particles,
  increases the chance of a collision
• Because every reaction requires activation energy
  to start

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Factor 4 Temperature

• Increased motion of the particles, increases the
  chance of a collision
• Faster particles cover more distance and the
  chance of a collision is higher
• Though not all collisions end up causing the
  particles to react, the added temperature
  increases the odds of getting any collision

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Factor 4 Temperature
The further a particle goes in one second,
increases its chance of a collision
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Factor 4 Temperature

• Because every reaction requires activation energy
  to start

Activation energy is the energy required to have
a reaction take place.
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Factor 4 Temperature
Lets speed it up a little
Too Slow, so they bounced off
When two particles collide they require
Activation Energy to react.
Still not enough combined energy
Finally these particles had enough combined
energy to react!
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Factor 4 Temperature

• This diagram shows how the reactants start at an
  energy lower than the activation energy.

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Factor 4 Temperature

• When the particles collide, if they have enough
  kinetic energy, they will exceed the activation
  energy.

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Factor 4 Temperature

• Next an activated complex forms from the energy
  of the collision which then breaks apart to form
  the products of the reaction.

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Factor 4 Temperature

• The difference between the activation energy and
  the energy released by the activated complex is
  called the Heat of the Reaction

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Factor 4 Temperature

• This diagram shows how kinetic energy varies
  among the particles of a material

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Factor 4 Temperature

• The orange section is the amount of the particles
  at T1 that have the required activation energy to
  react.

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Factor 4 Temperature

• Notice how the T2 curve has more of the particles
  over the activation energy line

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Factor 4 Temperature

• This curve shows that if the temperature is
  higher, more particles have the required kinetic
  energy to react.

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Factor 4 Temperature

• Higher Temp Faster Reaction Rate

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Factor 5 Catalysts

• Catalysts help increase the rate of a reaction by
  lowering the activation energy needed.
• Catalysts are not used up in the reaction
• They allow the molecules to line up more neatly,
  improving the quality of the collisions

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Factor 5 Catalysts
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Factor 5 Catalysts
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Factor 5 Catalysts

• Higher Temp Faster Reaction Rate

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Factor 5 Catalysts

• Lowered activation energy more particles are
  allowed to react faster reaction rate.

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Review

• Concentration higher faster
• More or Closer
• Surface Area small pieces faster
• No protection
• Pressure higher faster
• Closer
• Temperature higher faster
• More and Better collisions
• Catalysts faster
• Matchmaker

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Homework

• Read pgs. 532-535
• Do Pg.535 4,6,7
• Read pgs 536-541
• Do pg 541 11-14

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Reversible Reactions

• Some reactions are reversible
• They can form products and then reverse and
  reform the reactants.
• A double arrow ?? is used to symbolize a
  reversible reaction in the balanced equation

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Reversible Reactions

• Examples include
• Salt dissolving and undissolving
• NaCl(s) H2O ?? Na1(aq) Cl-1(aq)
• Formation of ammonia
• N2(g) 3H2(g) ?? 2NH3(g)

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Energy Considerations

• In every reversible reaction there is an
  endothermic reaction and an exothermic reaction

Reactants
Reactants
Products
Exothermic ?
?Endothermic
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Energy Considerations

• The exothermic reaction sets up a chain reaction
  where the energy released by one reaction gives
  energy for many more.

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Energy Considerations

• The Endothermic reaction gets its energy from the
  residual energy of the exothermic reaction.

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Chemical Equilibrium

• At first when a reaction begins, the reactants
  begin forming products

• As the reactants are used up, the rate of the
  forward reaction slows down

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Chemical Equilibrium

• When there are more products, the reverse
  reaction rate begins to increase

• The forward and reverse reactions happen at the
  same time

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Chemical Equilibrium

• When the rate forward the rate back you have
  chemical equilibrium

• The rate forward the rate back as long as the
  factors that control rates stay the same

• If a system is in Chem EQ then the concentrations
  must be constant

7 reactant particles
3 product particles
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Chemical Equilibrium

• Its like a school with 100 single boys and 100
  single girls
• At first the reaction is one way BGC
• Soon the C is high enough for break ups to start
  happening

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Chemical Equilibrium

• Simultaneously, the single boys and girls are
  getting used up making couples
• So the rate of the hook-ups is dropping and the
  rate of the break ups is rising

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Chemical Equilibrium

• This continues until the number of break-ups per
  day the hook-ups per day
• This would be the same as chemical equilibrium

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Chemical Equilibrium

• Once the rates are equal, the amount of boys,
  girls, and couples will no longer change.
• So, once a chemical system is in equilibrium, the
  concentration of the reactants and products will
  stop changing.

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Chemical Equilibrium

• The reaction will have seemed to stopped but it
  hasnt.
• Just like the couples keep forming and breaking
  up forever
• So, the products keep forming and keep breaking
  up in chemical equilibrium forever

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The Equilibrium Constant, Keq

• At equilibrium the total ratio of products to
  reactants stays constant
• The ratio of the concentrations of the products
  divided by the concentrations of the reactants is
  called the Keq
• Keq products / reactants

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The Equilibrium Constant, Keq

• Big Keq means
• Keq gt 1
• the reaction makes products more easily
• The reaction is exothermic

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The Equilibrium Constant, Keq

• Small Keq means
• Keq lt 1
• the reaction makes reactants more easily
• The reaction is endothermic

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The Equilibrium Constant, Keq

• For the reaction aA bB ? cC dD

A
B
C
D
a
b
c
d




Keq




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The Equilibrium Constant, Keq

• Sample question1
• Write the expression for the Keq for the
  following reaction
• N2 3H2 ? 2NH3
• Keq products/reactants
• Keq NH32/ N2H23

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The Equilibrium Constant, Keq

• Sample question2 Calculate the value of the Keq
  for the following reaction
• N2 3H2 ? 2NH3
• where at equilibrium
• N2 .30M, H2 .20M, and NH3 .005M
• Keq NH32/ N2H23
• Keq .0052/ .30.203
• Keq .0104 (no units)

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Practice Problems

• 1. 2H2 O2 ? 2H2O
• If H2 .020M, O2 .010M
• H2O .35M, What is the Keq?

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Practice Problems

• 2. 3H2 N2 ? 2NH3
• If H2 .40M, N2 .20M
• Keq .035, What is the NH3?

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Homework

• Read pgs. 559-568
• Do pg 565 1, Pg 568 3-4, 6, 8, 9

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LeChateliers Principle

• Purpose
• 1. To explain how changes in the concentration of
  the materials in equilibrium can cause the
  equilibrium to shift
• 2. To explain how changes in a systems
  environment can cause equilibrium to shift

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LeChateliers Principle

• What are the five conditions that affect a
  reaction rate?
• 1. Concentration
• 2. Temperature
• 3. Pressure
• 4. Catalysts
• 5. Surface Area

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LeChateliers Principle

• For reversible reactions, only three of these
  apply
• 1. Concentration
• 2. Temperature
• 3. Pressure

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LeChateliers Principle

• LeChateliers Principle says,
• A system in equilibrium will respond to a stress
  by
• trying to reverse the stress.

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LeChateliers Principle

• Examples of LeChateliers Principle
• 1. A system with added materials will try to get
  rid of them

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LeChateliers Principle

• Examples of LeChateliers Principle
• 2. A system that is heated will try to cool down
  by reacting endothermicly

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LeChateliers Principle

• Examples of LeChateliers Principle
• 3. A system that is squished will try to make
  room by reacting into fewer molecules

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LeChateliers Principle Concentration Change

• When the concentration of a reactant is
  increased
• The forward rate increases, and equilibrium is
  unbalanced
• N2 ?

3H2 ?
2NH3
N2
?
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LeChateliers Principle Concentration Change

• When the concentration of a
• reactant is increased
• The added reactants get used up
• N2 ?

3H2 ?
2NH3
N2
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LeChateliers Principle Concentration Change

• When the concentration of a
• reactant is increased
• The forward rate starts to slow down
• ?

3H2 ?
2NH3
N2
?
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LeChateliers Principle Concentration Change

• When the concentration of a reactant increase
• The products increase

3H2 ?
2NH3
N2
?
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LeChateliers Principle Concentration Change

• When the concentration of a reactant increase
• The reverse rate speeds up

3H2
2NH3
N2
?
?
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LeChateliers Principle Concentration Change

• When the concentration of a reactant increase
• The new equilibrium is when
• forward rate the reverse rate

3H2
2NH3
N2
?
?
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LeChateliers Principle Concentration Change

• Overall, the reaction responded to the stress by
  reversing it.
• More reactant was added to create the stress
• The reactant was used up to reverse the stress

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LeChateliers Principle Concentration Change

• This would work the same way if a product was
  added to a system in equilibrium

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LeChateliers Principle Concentration Change

• Corrections made when a stress is applied to a
  system in equilibrium are called shifts

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LeChateliers Principle Concentration Change

• For example, in the following reaction,
• N2 3H2 ?? 2NH3
• If more N2 is added, then the reaction will use
  it up by reacting more rapidly in the right
  direction.
• Therefore, the reaction will shift right

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LeChateliers Principle Concentration Change

• What would happen if the amount of NH3 increased?
• The reaction going back to the left would
  increase to use up the NH3,
• Therefore the reaction would shift left.

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LeChateliers Principle Concentration Change

• Dropping a concentration also adds a stress to
  the system
• If in the N2 3H2 ?? 2NH3 reaction, some N2 was
  removed which way would the reaction shift?

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LeChateliers Principle Concentration Change

• The lower amount of N2 would lower the forward
  reaction rate.
• The NH3 would react backwards at the same rate as
  before.
• So the reverse rate would be faster and products
  would form reactants at a faster rate.
• So the reaction would shift to the left.

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LeChateliers Principle Concentration Change

• So the reverse rate would be faster and products
  would form reactants at a faster rate.
• So the reaction would shift to the left.

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LeChateliers Principle Concentration Change

• Simply, if a concentration on one side is
  increased, then the reaction shifts away from the
  increase.
• If the concentration is lowered on one side of
  the reaction, then the system will shift towards
  the decrease.

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LeChateliers Principle Temperature Change

• Consider the same reaction, but include energy
  this time
• N2 3H2 ?? 2NH3 92kJ
• Think of the kJ as another reactant

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LeChateliers Principle Temperature Change

• So if this system is cooled which way does the
  reaction shift?
• Right towards the decrease in kJ

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LeChateliers Principle Temperature Change

• So if this system is heated which way does the
  reaction shift?
• Left away from the increase in kJ

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LeChateliers Principle Pressure Change

• Pressure changes only affect gases in reversible
  reactions
• Gases can compress by forming compounds that take
  up less space

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LeChateliers Principle Pressure Change

• In the following reaction,
• N2(g) 3H2(g) ?? 2NH3(g)
• There are 4 molecules of gas on the left side
• And 2 molecules of gas on the right side

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LeChateliers Principle Pressure Change

• N2(g) 3H2(g) ?? 2NH3(g)
• If the system is put under higher pressure,
• They will shift to the side with fewer molecules
  the right side.

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LeChateliers Principle Pressure Change

• N2(g) 3H2(g) ?? 2NH3(g)
• Likewise, if the system is put under lower
  pressure
• They will shift to the side with more molecules
  the left side.

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Homework

• Read pgs 569-574
• Do pg574 10-15