Chemical Reactions and Collision Theory

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Chemical Reactions and Collision Theory
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What are Chemical Reactions?

• A chemical reaction is a process in which at
  least one new substance is produced as a result
  of a chemical change.

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Chemical Reaction Terminology

• In chemical reactions there are reactants and
  products.
• Reactants are the chemicals that react together
  to form a product.
• Reactants are on the left side of a chemical
  equation.
• A B ? AB

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Chemical Reaction Terminology

• Products are the chemicals formed in a chemical
  reaction.
• Products are on the right side of the chemical
  reaction equation.
• A B ? AB

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Chemical Reaction Terminology

• So, whats the arrow in the middle of the
  equation?
• A B ? AB
• The arrow represents what happens when A runs
  into B. It is the chemical reaction.

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Chemical Reaction Terminology

• Making cookies is a lot like a chemical reaction.
• When you make cookies what are the reactants?
• What are the products?
• And what is the reaction, how do you make them?
  (i.e. what does the arrow represent?)

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Chemical Reaction Terminology

• Reactants flour, sugar, butter, chocolate
  chips, etc.
• Products cookies.
• The reaction (the arrow) mixing, and stirring,
  and baking.
• Flour Sugar Butter Chocolate Chips
  Cookies

Mixing Stirring Baking
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Collision Theory and Chemical Reactions

• Picture a container with two different reactants,
  A and B, where the chemical equation is
  ?

B
B
A
A
B
A
A
B
B
A
B
A
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Collision Theory and Chemical Reactions

• Reactants must interact (that is, collide) with
  one another before any reaction can occur. In
  other words, must hit before
  can form.

B
A
B
A
B
A
B
B
A
A
B
A
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Collision Theory and Chemical Reactions

• If reactants must collide to react, then the
  frequency of collisions (the number of collisons
  per amount of time) would affect how quickly the
  reactants form the product.
• How could you increase the frequency (how often)
  the reactants (molecules and
  )collide?

B
A
A
B
B
A
B
A
B
A
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Collision Theory and Chemical Reactions

• You could increase the temperature, increase the
  size of the reactants, or increase the
  concentration of the reactants.
• How would each of these increase the frequency of
  collisions and thereby increase the rate of the
  reaction?
• Predict and record your answers in your notebook
  before proceeding.

B
A
A
B
B
A
B
A
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Collision Theory and Chemical Reactions -
Temperature

• Increasing the temperature would increase how
  quickly molecules A and B are moving in the
  container and would thereby increase the chances
  of the two reactants colliding.
• If the reactants collide more frequently then
  they form product more frequently, and the
  reaction rate (how quickly product is formed) is
  increased.

B
A
A
B
B
A
B
A
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Collision Theory and Chemical Reactions Size

• Increasing the size of the reactants would
  increase the likelihood of collision, and the
  therefore the frequency of collision would
  increase and so would the reaction rate.

A
B
A
B
B
B
A
A
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Collision Theory and Chemical Reactions -
Concentration

• Increasing the concentration of the reactants
  would increase the number of particles that could
  collide and form product.
• With more reactants colliding and forming
  product, the reaction rate increases.

B
B
B
B
A
A
B
A
A
B
B
A
B
A
A
A
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Collision Theory and Chemical Reactions
Activation Energy

• Colliding particles must possess a certain
  minimum amount of energy, called the activation
  energy, if a collision is to be effective (that
  is, result in a reaction).
• When two reactant particles collide they do not
  always result in the formation of product.
  Sometimes, they rebound unchanged.
• The collision theory states that in order for a
  reaction to occur, particles must collide with a
  certain minimum energy. That is the kinetic
  energies of the colliding particles must add to a
  certain minimum value.
• The activation energy is the minimum combined
  kinetic energy that reactant particles must
  possess in order for their collision to result in
  a reaction.

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Collision Theory and Chemical Reactions
Activation Energy

• Increasing the temperature of the reactants has
  two effects 1) It increases the rate of movement
  (kinetic energy) of the reactants making
  collision more likely, and 2) It increases the
  kinetic energy of the reactants making collisions
  more likely to be effective (more likely to form
  product) by overcoming the activation energy.
• In the case of the above pictured reaction,
  increasing the temperature increases the speed of
  movement and the kinetic energy of A and B.
  Think of it as having an increased kinetic energy
  allows A and B to overcome the energy barrier
  (the activation energy) that is required for A
  and B to combine and form product.

B
A
A
B
B
A
B
A
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Collision Theory and Chemical Reactions
Activation Energy

• Heres a drawing to help.
• This drawing shows that when reactants dont have
  enough energy they cant collide with enough
  energy to produce product.
• The kinetic energy increases with temperature and
  allows the reactants to overcome the activation
  energy and collide with enough energy to form
  product.

B
reaction
A
Increased kinetic Energy from Increased temperatur
e
Activation Energy
B
A
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The Effects of Catalysts on Reaction Rates

• A catalyst is a substance that increases reaction
  rate without being consumed in the reaction.
• Catalysts increase reaction rates by providing
  alternate reaction pathways that have lower
  activation energies than the original,
  uncatalyzed pathway.
• In biological systems, enzymes are catalysts.
• Enzymes catalyze (speed up) reactions in animals
  that would not otherwise occur at appreciable
  rates at normal animal body temperature because
  the temperature is not enough to overcome the
  activation energy.

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The Effects of Catalysts on Reaction Rates

• Heres a drawing to show how catalysts increase
  reaction rates by lowering the activation energy.
• Compare this drawing to the previous drawing
  where temperature was increased to increase the
  kinetic energy. Here, its the activation energy
  that decreases. The kinetic energy of the
  reactants doesnt increase. With a lower
  activation energy, collisions are more likely to
  contain sufficient energy to lead to product
  formation.

Catalysts decrease The activation energy Allowing
the reaction To proceed at a lower Energy level.
B
A
reaction
B
A
Activation Energy
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Experiment on Reaction Rates

• For background information
• H2O2 (hydrogen peroxide) catalase (yeast) ?
  O2(gas) catalase (yeast)
• The above reaction involves the production of
  oxygen from hydrogen peroxide when the enzyme
  catalase (found in yeast) is added.
• Remember, catalase is found on both the reactant
  and product side because it is a catalyst and is
  not consumed in the reaction.

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Experiment on Reaction Rates

• Materials needed
• Hydrogen peroxide from the local store
• Regular baking yeast from the store
• You decide what else you need

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Experiment on Reaction Rates

• Design an experiment using hydrogen peroxide and
  yeast and any other equipment you need that will
  answer the following questions?
• Is oxygen produced from hydrogen peroxide without
  adding yeast? Why?
• What component of yeast catalyzes the formation
  of oxygen from hydrogen peroxide? How does a
  catalyst affect the energy of activation?
• How does the temperature of the hydrogen peroxide
  affect the rate of oxygen production?
• How could you test to know that the gas produced
  is oxygen?
• Record your experimental design and the answers
  to these questions in your notebook.

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• Now proceed back to the assignment page to
  complete part 2 on dynamic equilibrium and Le
  Chateliers principle.