KINETICS

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KINETICS

• How Fast Does A Reaction Occur?

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Energy Diagrams

• Reactants always start a reaction so they are on
  the left side of the diagram.

• Products are on the right.

Reactants
Products
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Exothermic Reactions

• The exothermic reaction gives off heat because
  the products are at a lower energy level than the
  reactants.

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Graph of anExothermic Reaction
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Graph of anExothermic Reaction

• In an exothermic graph, the reactants have
  greater energy than the products.

• The change in energy is a negative value.

Products
Reactants
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Exothermic And Endothermic Reactions

• The endothermic reaction absorbs heat because the
  products are at a higher energy level than the
  reactants.

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Graph of anEndothermic Reaction
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Graph of anEndothermic Reaction

• In an endothermic graph, the products have
  greater energy than the reactants.

• The change in energy is a positive value.

Products
Reactants
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Exothermic And Endothermic Reactions

• Scientists have observed that the energy released
  in the formation of a compound from its elements
  is always identical to the energy required to
  decompose that compound into its elements.

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Energy Diagrams
Activation energy
activated complex

• Activation energy is the minimum amount of energy
  that reacting particles must have to form the
  activated complex.

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Energy Diagrams
activated complex

• The activated complex is a short-lived, unstable
  arrangement of atoms that may break apart and
  re-form the reactants or may form products.

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Energy Diagrams
Activation energy

• To calculate the activation energy, subtract the
  energy of the reactants from the energy at the
  top of the peak.

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Energy Diagrams
Enthalpy/heat of reaction (?H)

• The enthalpy or heat of reaction (?H) is the
  amount of heat released or absorbed in the
  reaction.

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Energy Diagrams
Enthalpy/heat of reaction (?H)

• To determine ?H, take the energy of the products
  and subtract the energy of the reactants.

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Problem

• The heat content of the reactants of the forward
  reaction is about ______ kilojoules.

(80 kJ)
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Problem

• The heat content of the products of the forward
  reaction is about ______ kilojoules.

(160 kJ)
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Problem

• The heat content of the activated complex of the
  forward reaction is about _____ kilojoules.

(240 kJ)
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Problem

• The activation energy of the forward reaction is
  about _____ kilojoules.

(160 kJ)
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Problem

• The heat of reaction (?H) of the forward reaction
  is about _____ kilojoules.

(80 kJ)
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Problem

• The forward reaction is (endothermic or
  exothermic).

(endothermic - products are higher in energy)
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Problem

• The heat content of the reactants of the reverse
  reaction is about ______ kilojoules.

(160 kJ)
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Problem

• The heat content of the products of the reverse
  reaction is about ______ kilojoules.

(80 kJ)
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Problem

• The heat content of the activated complex of the
  reverse reaction is about _____ kilojoules.

(240 kJ)
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Problem

• The activation energy of the reverse reaction is
  about _____ kilojoules.

(80 kJ)
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Problem

• The heat of reaction (?H) of the reverse reaction
  is about _____ kilojoules.

(- 80 kJ)
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Problem

• The reverse reaction is (endothermic or
  exothermic).

(exothermic - products are lower in energy)
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Energy Diagrams

• The activation energy can be lowered by adding a
  catalyst.

effect of the catalyst
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Energy Diagrams

• The catalyst lowers the activation energy by
  providing an alternate pathway for the reaction
  to occur.

effect of the catalyst
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Expressing Reaction Rates

• As you know, some chemical reactions are fast and
  others are slow however, fast and slow are
  inexact, relative terms.
• Chemists often need to be more specific.

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Expressing Reaction Rates

• We generally define the average rate of an action
  or process to be the change in a given quantity
  during a specific period of time.

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Expressing Reaction Rates

• Reaction rates cannot be calculated from balanced
  equations as stoichiometric amounts can.
• Reaction rates are determined experimentally by
  measuring the concentrations of reactants and/or
  products in an actual chemical reaction.

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Collision Theory

• According to the collision theory, atoms, ions,
  and molecules must collide with each other in
  order to react.

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Collision Theory

• The following three statements summarize the
  collision theory.
• 1. Particles must collide in order to react.
• 2. The particles must collide with the correct
  orientation.

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Collision Theory

• 3. The particles must collide with enough energy
  to form an unstable activated complex, also
  called a transition state, which is an
  intermediate particle made up of the joined
  reactants.

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Collision Theory
activated complex
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Collision Theory

• The minimum amount of energy that colliding
  particles must have in order to form an activated
  complex is called the activation energy of the
  reaction.
• Particles that collide with less energy than the
  activation energy cannot form an activated
  complex.

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Collision Theory

• In an exothermic reaction, molecules collide with
  enough energy to overcome the activation
  energy
  barrier,
  form an activated
  complex, then
  release energy
  and
  form products
  at a lower energy
  level.

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Reactants
Energy
Products
Reaction coordinate
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Activated Complex or Transition State
Reactants
Energy
Products
Reaction coordinate
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Activation Energy
Reactants
Energy
Products
Reaction coordinate
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Reactants
Energy
Overall energy change
Products
Reaction coordinate
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Collision Theory

• In the reverse endothermic reaction, the reactant
  molecules lying at a low energy level must absorb
  energy
  to
  overcome the
  activation energy
  barrier and form
  high-energy
  products.

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Factors Affecting Reaction Rates

• The reaction rate for almost any chemical
  reaction can be modified by varying the
  conditions of the reaction.

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Factors Affecting Reaction Rates

• An important factor that affects the rate of a
  chemical reaction is the reactive nature of the
  reactants. As you know, some substances react
  more readily than others. The more reactive a
  substance is, the faster the reaction rate.

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Factors Affecting Reaction Rates

1. Another important factor that affects the rate of
   a chemical reaction is the concentration of the
   reactants. Reactions speed up when the
   concentrations of reacting particles are
   increased. Increasing the number of reactants
   increases probability of collisions.

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Factors Affecting Reaction Rates

1. continued. The rate of gaseous reactions can be
   increased by pumping more gas into the reaction
   container.

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Factors Affecting Reaction Rates

1. Surface area of the reactants affects the rate of
   a chemical reaction. Increasing the surface area
   of reactants provides more opportunity for
   collisions with other reactants, thereby
   increasing the reaction rate.

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Factors Affecting Reaction Rates

1. Temperature affects the rate of a chemical
   reaction. Generally, increasing the temperature
   at which a reaction occurs increases the
   reaction rate. Raising
   the temperature
   raises both
   the collision
   frequency and the
   collision energy.

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Factors Affecting Reaction Rates

1. Adding a catalyst affects the rate of a chemical
   reaction. A catalyst is a substance that
   increases the rate of a chemical reaction without
   itself being consumed in the reaction. In fact,
   catalysts are not included in the chemical
   equation.

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Factors Affecting Reaction Rates

• A catalyst lowers the activation energy of a
  reaction by
  providing an
  alternate
  pathway for the
  reaction to
  occur (the
  blue
  line).

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Reactants
Energy
Products
Reaction coordinate
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Factors Affecting Reaction Rates

1. Compressing gases affects the rate of a chemical
   reaction. When two gases react, compressing the
   gases generally increases the rate of the
   reaction.