Aim: What factors can affect the rate of a chemical reaction?

1
Aim What factors can affect the rate of a
chemical reaction?

• Do Now
• Take out a calculator and reference tables.
• What does the word kinetic mean?

2
Collision Theory

• For a reaction to occur, particles must collide
  in a specific manner.
• Factors affecting rates of collision include
• Nature of the reactants
• Concentration
• Surface Area
• Pressure
• The presence of a catalyst
• Temperature

3
Factors affecting rates of reaction

1. Nature of reactants generally covalently bonded
   reactants are slower to react than ionic
   substances because they have more bonds to break
   which means they must have more energy.
2. Concentration - Most chemical reactions will
   take place faster if the concentration of one or
   more of the reactants is increased. Why?
3. Surface Area When more of the surface area of a
   substance is exposed, there are more chances for
   the reactant particles to collide thus increasing
   the reaction rate.

4
Factors affecting rates of reaction

• 4. Pressure Affects mainly gases. An increase
  in pressure will increase the rate of reaction.
• 5. Presence of a catalyst Catalysts are
  substances that increase the rate of reaction by
  providing an easier pathway for a reaction.
  Catalysts take part in a reaction but remain
  unchanged. Therefore, they can be used and
  reused.
• 6. Temperature Reactants with a higher
  temperature will have more kinetic energy.
  Therefore, they will have more collisions which
  will cause the reaction to occur faster.

5
Heat of Reaction

• The reactants and products in a given reaction
  have their own heat of reaction.
• The heat of reaction is the amount of heat given
  off or absorbed in a chemical reaction. Heat of
  reaction is the difference in heat content of the
  products and reactants. The symbol for heat of
  reaction is shown as ?H. It can be calculated
  using the following formula
• ?H Hproducts - Hreactants
• Why is the heat contained by the reactants and
  products different?

6
Exothermic Reactions

• Is ?H positive or negative when the reaction is
  exothermic?
• If the products have less potential energy or
  heat content than the reactants, heat is given
  off, and it is an exothermic reaction. ?H
  negative.

7
Endothermic Reactions

• Is ?H positive or negative when the reaction is
  endothermic?
• If the products have more potential energy or
  heat content than the reactants, heat is
  absorbed, and it is an endothermic reaction. ?H
  positive.

8
Using Table I

• Table I lists heat of reaction for different
  reactions. We can use this table to tell whether
  a reaction is exothermic or endothermic as well.
• Example What is the heat of reaction for the
  following reactions?
• 2C(s) 3H2(g) ? C2H6(g)
• C2H5OH(l) 3O2(g) ? 2CO2(g) 3H2O(l)

9
Activation energy

• Activation energy is the smallest amount of
  energy needed to form an activated complex.
• Whats an activated complex?
• An activated complex is the result of a collision
  between the reactants in a chemical reaction. It
  is temporary and will either break apart and
  reform the reactants or rearrange and form new
  products.

10
Parts of a Potential Energy Diagram
11
Activity/Homework

• Review Book pages 137-141. Copy and answer
  question 1-17.