Chemical Reactions

Factors Affecting Rates of Reaction

January 9, 2015

WARM-UP: Identify the type of reaction below:

a. 2 Na + Cl2 2 NaCl

b.Zn + 2 HCl ZnCl2 + H2

c. PbO2 Pb + O2

d.2 H3PO4 + 3 Ca(OH)2 Ca3(PO4)2 + H2O

e. CaCl2 + 2AgNO3 Ca(NO3)2 + 2AgCl

f. CH4 + 2 O2 CO2 + 2 H20

SYNTHESIS

S.R.

DECOMP

D.R.

D.R.COMB.

Collision Theory• Molecules must…

• Collide with each other hard enough to react

• Be in a favorable orientation

• In a successful collision, existing bonds are broken and new bonds formed while reactants become products

Collision Theory Con’t• REACTION RATE:

• Informally, reaction rate is the speed at which a chemical reaction takes place

• Formally, the reaction rate is the change in concentration of reactants or products over time

Factors that Affect Reaction Rates• 1. TEMPERATURE:

• Increasing temperature means the particles move faster

• Particles collide more frequently and with more energy

• Reactions occur faster

Factors that Affect Reaction Rates• 2. CONCENTRATION:

• Increasing concentration means the molecules are closer together

• Particles collide more frequently

• Reactions occur faster

Factors that Affect Reaction Rates• 3. PARTICLE SIZE:

• Molecules can only collide on the surface

• Smaller particles have a larger surface area

• Reactions occur faster• Dissolving speeds up a reaction

• Smallest particles are molecules or ions

Factors that Affect Reaction Rates• 4. CATALYSTS:

• A catalyst is a substance that speeds up a reaction without being used up • Example: enzymes

• Increases the number of collisions

• Reduces the activation energy• The minimum energy

needed for a reaction to occur

• Activation Energy: the minimum amount of energy required to make a reaction proceed forward• Catalysts speed up reactions by lowering

activation energy

Activation Energy

Check for Understanding

1. According to “Collision Theory”, what two conditions must be fulfilled for a reaction to occur?

2. How does increasing temp cause the reaction to occur faster?

3. Which has a larger surface area, a single stick of butter or butter cubed into small pieces? Which would cause a reaction proceed faster?

4. What is the activation energy?

Check for Understanding ANSWERS

1. The molecules must collide with enough strength to react and be in a favorable orientation

2. It speeds up the molecules and they collide more frequently

3. Cubed butter because it is smaller in size; cubed butter will react faster

4. Minimum amount of energy required for a reaction to occur

Reaction rate can be stated as:

1. The decrease in the concentration of reactants with time

or

2. The increase in the concentration of products with time

Reaction Rate

• In the reaction: A --> B

1. A is the reactant and B is the product

2. Notice that the concentration of A decreases over time and B increases

REACTANTS:Minimum # moles at 60 minMaximum # moles at 0 min

PRODUCTS:Minimum # moles at 0 minMaximum # moles at 60 min

EQUILIBRIUM:Approximately 24 min (both reactants and products have 0.5 moles)

REACTION RATE

En

erg

y

Reaction coordinate

Reactants

Products

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy: Minimum energy to make the reaction happen

Ene

rgy

Reaction coordinate

Reactants

Products

Activated Complex or Transition State

Ene

rgy

Reaction coordinate

Reactants

Products

Overall energy change

An Endothermic Reaction (Energy Absorbed)

Ene

rgy

Reactants Products®

CaCO3 (s)

CaO (s) + CO2 (g)

+176 kJ

An Exothermic Reaction (Energy Released)

Ene

rgy

Reactants Products®

C + O2

CO2

-395 kJ

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen

Activated Complex or Transition State

PE of the Reactants

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen

Activated Complex or Transition State

PE of the Reactants

PE of the Activated Complex

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen

Activated Complex or Transition State

PE of the Reactants

PE of the Activated Complex

PE of Products

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen

Activated Complex or Transition State

PE of the Reactants

PE of the Activated Complex

PE of Products

Heat of Reaction

(∆H)

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy

Activated Complex or Transition State

PE of the Reactants

PE of the Activated Complex

PE of Products

Heat of Reaction

(∆H)

PE of the Reverse Reaction