Reaction%20Rate%20and%20Equilibrium

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Reaction Rate and Equilibrium
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Video World of Chemistry Chemical Kinetics

• Use this video as an introduction to this concept.

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Reaction Rate

• Not all reactions occur at the same speed.
• Some reactions are very slow while others are
  fast.

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Reaction Rate

• The reaction rate of a chemical reaction is a
  measurement of the increase in the concentration
  of a product or the decrease in the concentration
  of a reactant as the reaction proceeds over time.

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N2 3H2 ? 2NH3

• The units generally used to express reaction rate
  are mol/L . sec.
• What does it mean if the rate of the reaction
  above was given as 4.5 x 10-2 mol N2/L . sec?

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C(s) O2(g) ? CO2(g)

• Occurs slowly at room T and normal O2.
• Can speed up the reaction by increasing the T or
  increasing O2.
• We can explain these changes in reaction rate
  using the collision theory.

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Collision Theory

• All substances are comprised of millions of tiny
  particles in constant motion. These particles are
  colliding with each other constantly in any
  substance.
• All collisions between particles do not result in
  a reaction.
• There are two factors that determine whether or
  not a reaction will occur between two particles
  that are colliding.

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Collision Theory

• Substances most come into contact, (collide) with
  enough energy.

Activation Energy has been supplied by the force
of the collision.
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Activation Energy

• The activation energy is the amount of energy
  that must be available in order for a reaction to
  occur.

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• The sparks generated by striking steel against a
  flint provide the activation energy to initiate
  combustion in this Bunsen burner.
• The blue flame will sustain itself after the
  sparks are extinguished because the continued
  combustion of the flame is now providing the
  necessary energy through an exothermic reaction.

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Activation Energy
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Collision Theory

• Substances most come into contact, (collide) in
  the correct orientation (facing the correct way).

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Collision Theory

• The collision theory states that reacting
  substances most come into contact, (collide) with
  enough activation energy, and in the correct
  orientation (facing the correct way), so that
  their electron shells can rearrange to form the
  products of the reaction.
• Therefore any factor which changes the frequency
  (how often), or energy of the collisions will
  change the rate of the reaction.

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Five Factors AffectingReaction Rate

• nature of the reacting substances
• concentration
• surface area
• temperature
• catalysts

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Nature of the reacting substances

• The type, strength, and number of chemical bonds
  or attractions between atoms differ from one
  substance to another.
• These differences determine the energy and
  orientation of the reacting particles that is
  necessary to create an effective collision
  resulting in a reaction.

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TNT vs Gunpowder

• Explosive materials which react very violently
  are known as high explosives. In contrast, there
  are some materials that react more slowly. These
  are known as low explosives. They release a large
  amount of energy, but due to the relatively slow
  rate of reaction.

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TNT vs Gunpowder
Gunpowder is a mixture of potassium nitrate
(KNO3), sulphur (S8) and charcoal (C). It is a
high explosive.

• TNT C6H2(NO2)3CH3 is a high explosive.

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As the concentration of the reactants increases,
the reaction rate increases.

• Why?

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Concentration

• Concentration of the reactant refers to the
  number of reactant particles within a given
  volume.
• If the concentration of the reactants increases
  there will be a greater number of collisions.
• The greater the number of total collisions, the
  greater the number of effective collisions
  (collisions that will form product) and the
  greater the rate.

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Concentration
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Concentration and Reaction Rate
Steel wool burning in air
Steel wool burning in pure oxygen
A (21 oxygen) (100
oxygen)
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As the surface area of the reactants increases,
the reaction rate increases.

• Why?

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Surface Area

• Increasing the surface area of the reactants
  results in a higher number of reaction sites.
• Reaction sites - specific sites on molecules at
  which reactions occur.
• Increasing the number of reaction sites increases
  the number of total collisions.
• The greater the number of total collisions, the
  greater the number of effective collisions
  (collisions that will form product) and the
  greater the rate.

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Surface Area
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Reaction Rate and Surface Area Lycopodium
Powder Exit slideshow and play using Windows
Media Player. There is a delay if we try to show
this file within the slide show.
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As the temperature of the reactants increases,
the reaction rate increases.

• Why?

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Temperature

• Increasing the temperature increases the kinetic
  energy of the particles.
• This results in more frequent collisions and more
  energetic collisions.
• Therefore not only are there more collisions but
  also a greater percentage of the collisions have
  the needed activation energy.

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Both light sticks have been activated however the
one on the left was placed in ice water and the
one on the right in boiling water.
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Temperature and Reaction Rate
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Catalysts increase the rate of reactions.

• Why?

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Catalysts

• Catalysts lower the activation energy by
  providing an alternate pathway by which the
  reaction can occur at a lower energy.
• This results in a greater percentage of the
  collisions having the necessary energy to be
  effective resulting in an increase in reaction
  rate.
• Catalysts are remain unchanged at the end of a
  reaction.

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Catalysts lower the activation energy
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Catalysts lower the activation energy
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Left Partially caramelized cube sugar, Right
burning cube sugar with ash as catalyst
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Enzymes

• Enzymes act as catalysts that lower the
  activation energy of a chemical reaction within a
  living organism.
• Enzymes carry out their function of lowering
  activation energy by temporarily combining with
  the chemicals involved in the reaction. These
  chemicals that the enzyme combines with are
  called the substrate.
• When the enzyme and substrate combine, the
  substrate is changed to a different chemical
  called the product. The enzyme is not consumed or
  altered by the reaction.

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Enzymes
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Enzymes

• Enzymes are specific for their substrate A
  particular substrate molecule will combine
  temporarily with one enzyme type, and the active
  site of a particular enzyme will fit only one
  kind of substrate. For example, the enzyme
  sucrase will attach only to the substrate
  sucrose.

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Enzymes
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Catalysts Beakmans World 625
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Homework

• Worksheet Reaction Rate

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Reversible Reactions

• N2 3H2 ? 2NH3
• Reactions can normally be reversed.
• 2NH3 ? N2 3H2
• Reversible reactions are often indicated by a
  double arrow (?).
• N2 3H2 ? 2NH3
• This shows the forward and reverse reaction.

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For a reversible reactionsuch as

• A B ? C D
• there are actually two reactions
• Forward reaction A B ? C D
• Backward reaction C D ? A B

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• A B ? C D
• Forward reaction
• A B ? C D
  
• A and B are used up
• C and D are formed
• Backward reaction
• C D ? A B
• C and D are used up
• A and B are formed

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At beginning of the reaction

• A B ? C D
• There is only A and B in the reaction container
  (no C and D formed yet)
• Forward reaction is very fast
• No backward reaction occurs yet

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A little later

• A B ? C D
• Forward reaction is still fast (since the
  container still has mainly A and B)
• Some C and D have been formed
• The backward reaction starts
• Backward reaction is very slow (since there is
  only a small amount of C and D)

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Still some time later

• A B ? C D
• More A and B have been used up
• The forward reaction slows down
• More C and D have been formed
• The backward reaction speed up

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Eventually

• A B ? C D
• the point is reached where the speed (rate) of
  the two reactions become equal.
• The system is then said to be in
• EQUILIBRIUM

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Chemical Equilibrium

• At equilibrium
• The rate of the forward reaction becomes equal to
  the rate of the reverse reaction.
• How could this graph be adjusted and still show
  equilibrium?

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Chemical Equilibrium

• At equilibrium
• The forward and reverse reactions continue at
  equal rates in both directions.
• For this reason we often refer to a dynamic
  equilibrium

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Dynamic Equilibrium
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Dynamic Equilibrium

• It often appears that a reaction at equilibrium
  has stopped. This however is only somewhat
  true.
• What would happen if the person stopped running?

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Chemical Equilibrium

• When equilibrium is
• Reached
• There is no further change in the amounts
  (concentrations) of reactant and product.
• Concentrations at equilibrium are constant (not
  equal).

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Chemical Equilibrium

• N2 3H2 ? 2NH3
• A chemical reaction is at equilibrium when the
  forward and reverse reactions are occurring at
  the same rate.
• A reaction that has reached equilibrium is
  assigned an equilibrium constant (Keq or just K).

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The equilibrium constant expression

• N2 3H2 ? 2NH3
• All we need to write an equilibrium expression is
  a balanced equation.

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Write the equilibrium expression for4HCl
O2?2Cl2 2H2O

• If we know the concentrations (molarities) we can
  calculate a numerical value for K.

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Given CO 0.200, H2O 0.500, H2 0.32
and CO2 0.42 Find K forCO H2O ? H2 CO2
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Given H2S 0.706, H2 0.222 and S2
0.111 Find K for2H2S ? 2H2 S2
K 0.0110
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Given K 0.0875 and N2O4 0.0172M. Find
NO2 forN2O4 ? 2NO2
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Given K 0.0140 and H2 and I2 are each 2.00
x 10-4M. Find HI for 2HI ? H2 I2
HI 0.00169 M
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LeChâteliers Principle

• states that when a stress is applied to a system
  at equilibrium, the system will respond (shift)
  in a manner that attempts to undo the stress.

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Stresses are

• Change in concentration ( )
• (adding or removing substances)
• Change in temperature
• (heating or cooling the system)
• Change in pressure
• (increasing or decreasing pressure)

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• CONCENTRATION CHANGE
• Increase concentration of a reactant
• (add more nitrogen)
• N2 3H2 ? 2NH3 heat
• Equilibrium shifts to the right
• (FORWARD reaction is favored because it will use
  up the nitrogen)

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• CONCENTRATION CHANGE
• Increase concentration of a product
• (add more ammonia)
• N2 3H2 ? 2NH3 heat
• Equilibrium shifts to the left
• (REVERSE reaction is favored because it will use
  up the ammonia)

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• CONCENTRATION CHANGE
• Decrease concentration of a reactant
• (remove some nitrogen)
• N2 3H2 ? 2NH3 heat
• Equilibrium shifts to the left
• (REVERSE reaction is favored because it will
  replace the nitrogen)

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• CONCENTRATION CHANGE
• Decrease concentration of a product
• (remove the ammonia)
• N2 3H2 ? 2NH3 heat
• Equilibrium shifts to the right
• (FORWARD reaction is favored because it will
  replace the ammonia)

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• TEMPERATURE CHANGE
• Increase the temperature.
• (heat is added)
• N2 3H2 ? 2NH3 heat
• Equilibrium shifts to the left
• (REVERSE reaction is favored because it will use
  up the added heat)

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• TEMPERATURE CHANGE
• Decrease the temperature.
• (heat is removed)
• N2 3H2 ? 2NH3 heat
• Equilibrium shifts to the right
• (FORWARD reaction is favored because it will
  replace the heat that was removed)

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Pressure Change

• Pressure can change by adjusting the volume.

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Pressure Changes

• The side of the reaction with the greater number
  of moles of gas will create higher pressure.
• The side of the reaction with the lesser number
  of moles of gas will create lower pressure.

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• PRESSURE CHANGE
• Increase the pressure.
• (volume of the container is decreased)
• N2(g) 3H2(g) ? 2NH3(g) heat
• Equilibrium shifts to the right
• (FORWARD reaction is favored because it will
  change 4 moles of gas into 2 moles of gas
  therefore returning to a lower pressure)

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• PRESSURE CHANGE
• Decrease the pressure.
• (volume of the container is increased)
• N2(g) 3H2(g) ? 2NH3(g) heat
• Equilibrium shifts to the left
• (REVERSE reaction is favored because it will
  change 2 moles of gas into 4 moles of gas
  therefore returning to a higher pressure)

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Le Chateliers Principle
Disturbance Equilibrium Shift
Add more NO2
Add more N2O4.
Remove NO2
Add a catalyst..
no shift
Decrease pressure
Decrease temperature.
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Homework

• Worksheet Equilibrium