Energy Changes &
Reactions Rates
Unit 7 - Chapter 7.3 & 7.4
Chemical Bonds and Energy
Chemical reactions:
Breaking chemical bonds in the
reactants
Forming new chemical bonds in the
products.
Chemical energy: Energy stored in the
chemical bonds of a substance.
True for both reactants and products
Chemical Bonds and Energy
Energy changes in reactions  determined
by changes in chemical bonding.
Breaking chemical bonds requires energy
Forming of chemical bonds releases
energy.
For combustion reactions  Heat and light are
given off because new chemical bonds formed
Chemical Bonds and Energy
Chemical Bonds and Energy
Exothermic and Endothermic Reactions
During a chemical reaction, energy is
either released or absorbed.
Exothermic
Releases energy to its surroundings
“Exo” means Energy “exits” from the
reaction
Energy required to break reactant bonds 
less than energy released as products
formed
Chemical Bonds and Energy
 Exothermic reaction: chemical energy of reactants
is greater than chemical energy of products.
Chemical Bonds and Energy
 Chemical energy peaks before reactants change
into products.
 Peak Height = energy required to break the chemical
bonds of the reactants.
 Particles must collide with enough energy to break these
bonds, or the reaction will not occur.
Chemical Bonds and Energy
Endothermic - Absorbs energy from
surroundings
“Endo” means Energy goes “into” the
reaction
Example: Emergency Cold Packs
Energy required to break reactant
bonds  greater than energy released
by formation of products.
Chemical Bonds and Energy
Endothermic reaction - energy of products is
greater than energy of reactants.
Chemical Bonds and Energy
Conservation of Energy:
 Total Energy is same before and after a
reaction
Exothermic reaction:
Chemical energy of reactants = heat +
chemical energy of products.
Reactants  Heat + Products
Endothermic reaction:
Chemical energy of reactants + heat =
chemical energy of products.
Reactants + Heat  Products
Reaction Rates
Reaction Rates tell you:
 the rate that reactants change
into products
 Fast or slow, controlled or
uncontrolled - total amount of
energy released is the same in all
cases
Reaction Rates
 What is a Rate?
 Changes over time are expressed as
rates.
 Reaction rates are often expressed as:
How fast reactants are consumed <or>
How fast products are formed
<or>
How fast energy is absorbed or released
Some Factors Affecting
Chemical Reaction Rates:
Temperature
Surface area
Concentration
Stirring
Catalysts
Factors Affecting Reaction Rates:
General
 Reaction rate depends on how often
reactant particles collide.
•If collisions are more frequent reaction
rate increases.
•If collisions are less frequent reaction
rate decreases.
Reaction rates can be changed
• Vary conditions so collisions increase or
decrease
Factors Affecting Reaction Rates:
Temperature
 Increasing temperature increases reaction rate
 Decreasing temperature decreases reaction rate
 Example: Milk stored in a refrigerator slows down the
reactions that cause the milk to spoil.
WHY?
 Increased temperature  particles moving faster
Collide more often
 Collide with more energy
 faster rate of reaction
Factors Affecting Reaction
Rates: Surface Area
 For a given mass of material:
smaller particle sizes  greater surface area
Increased surface  increased collisions 
More particles reacting  Faster reaction rate
Examples:
Crushed ice melts much more quickly than large
block of ice
Small twigs and branches burn much more quickly
than big logs
Factors Affecting Reaction
Rates: Stirring
 Stirring reactants:
Increases particles exposure to each other
(moving faster – colliding more often)
More exposure  faster reaction rate
Example: Clothes in washing machine
Factors Affecting Reaction
Rates: Concentration
 Concentration = # of particles in a
given volume.
More particles in a given volume 
more opportunities for collisions 
reaction rate is faster
Greater the concentration  faster the
reaction rate
Factors Affecting Reaction
Rates: Concentration
 Dye solution in
left beaker is
more
concentrated than
solution in right.
 Increasing
concentration of
dye increases rate
of color change in
the material.
Factors Affecting Reaction
Rates: Gas Pressure
 For gases, concentration changes with
pressure.
 Greater the pressure of a gaseous reactant 
Greater is its concentration
Faster reaction rate.
Factors Affecting Reaction
Rates: Catalysts
 Catalyst: substance that affects the
reaction rate without being used up in
the reaction.
 Catalysts can:
 Speed up a reaction <or>
 Enable a reaction to occur at lower
temperature.
Factors Affecting Reaction
Rates: Catalysts
Ex: One of the ways to make sulfuric acid is to react
sulfur dioxide with oxygen to form sulfur trioxide.
This reaction happens very slowly without a catalyst
 [vanadium(V) oxide is a good catalyst for this reaction]
 The catalyst is neither a reactant nor a product,
so it is written over the arrow.
Factors Affecting Reaction
Rates: Catalysts
 Catalyst lowers energy required for effective
collisions
Exit Slip Questions
1. Label the following graphs as Endo or
Exothermic and mark the change in energy:
Exit Slip Questions
2. A reaction rate is the rate at which reactants
change into products over time.
True
False
Exit Slip Questions
3. What are two things that might happen to a
reaction if a catalyst is added to the reaction?
Exit Slip Questions
4. What impact will each of the following
changes have on the rate of most chemical
reactions (increase or decrease)?
3) decreasing the temperature
4) grinding a reactant into a fine powder
5) stirring the reaction mixture
6) decreasing the concentration of one of the
reactants