Unit 2-5: Energy Change During
Chemical Reactions
Conservation of Mass Review:
• When a reaction takes place, the same
number of atoms is present in the reactants
and the products.
• Atoms are never gained or lost in a chemical
reaction. They are only rearranged to form
new substances
• The Law of Conservation of Mass states that
mass is conserved in a chemical reaction.
• This means that the total mass of the products
is always equal to the total mass of the
reactants in ANY reaction (in a closed system).
Energy Changes in Chemical Reactions
1. It takes energy to break bonds
2. Energy is released when bonds are formed
• By comparing the energy used when bonds in
the reactants are broken with the energy
released when bonds in the products are
formed, you can determine whether a
chemical reaction releases energy or absorbs
energy overall.
Reactions that release energy are
called EXOTHERMIC
• In exothermic reactions, more energy is
released when the bonds are formed in the
products than is used to break the bonds in
the reactants.
Reactions that absorb energy are
called ENDOTHERMIC
• In endothermic reactions, more energy is
absorbed when the bonds in the reactants are
broken than is released when new bonds are
formed in the products.
Identifying Exothermic and
Endothermic Reactions
There are two methods for distinguishing
between exothermic and endothermic reactions
1. Monitor temperature change
– When energy is released in an exothermic
reaction, the temperature of the reaction mixture
increases.
– When energy is absorbed in an endothermic
reaction, the temperature decreases.
– You can monitor changes in temperature by
placing a thermometer in the reaction mixture.
Identifying Exothermic and
Endothermic Reactions
There are two methods for distinguishing
between exothermic and endothermic reactions
2. Calculate the enthalpy of reaction (ΔH)
– To classify the net energy output or input of
chemical reactions, you can calculate something
called the enthalpy change (ΔH) or heat of
reaction.
• this compares the energy of the reactants with the
energy of the products.
– Enthalpy is a measure of internal heat energy.
Identifying Exothermic and
Endothermic Reactions
• When you calculate the difference between the enthalpy of
the products and the enthalpy of the reactants, you find the
enthalpy change (ΔH), which can be represented
mathematically as:
ΔH = (PEproducts) - (PEreactants)
• If ΔH is negative (−) then the chemical reaction
is exothermic, because more energy is
released when the products are formed than
energy is used to break up the reactants.
• If ΔH is positive (+) then the chemical reaction
is endothermic, because less energy is
released when the products are formed than
the energy is used to break up the reactants.
• You can also use energy level diagrams to
visualize the energy change during a chemical
reaction as a result of the energies used and
released according to the equation for ΔH.
• To understand these diagrams, compare the
energy level of the reactants on the left hand
side with that of the products on the righthand side.
• Because more energy is released when the
products are formed than is used to break up
the reactants, this reaction is exothermic, and
ΔH for the reaction is negative.
100
90
POTENTIAL
ENERGY
80
Ea – Energy needed for
bonds in the Reactants to
be broken.
70
60
50
REACTANTS
40
H- Energy Difference between Reactants
and Products. (In this case H is negative so
Rx. is Exothermic
30
20
PRODUCTS
10
Question #1
2 00
18 0
16 0
14 0
12 0
10 0
REACTANTS
8 0
60
4 0
PRODUCTS
2 0
Progress of Reaction
Question #2
A2 + B2

2AB
50
45
POTENTIAL
40
ENERGY
35
30
A2 + B2
25
20
15
10
2 AB
5
Progress of Reaction
Question #3
Assignment:
• Complete the Endothermic vs. Exothermic Lab
– You will work in groups, but each person must
hand in their own lab report!
Activation Energy
• Chemical reactions take place when particles
physically collide.
• To successfully trigger a reaction, a collision
must have a minimum amount of kinetic
energy (activation energy) and the proper
alignment.
– This allows the bonds in the reactant molecules to
be broken!
• Many chemical reactions speed up when you
increase the temperature because a greater
number of particles will have the minimum
kinetic energy necessary to react.
• Many chemical reactions will also speed up
when you increase the concentration of one
reactant, as more reactant molecules are
present within a given space, so the odds of
them having the necessary energy to react is
also increased.
• During that split second in time when the
bonds in the reactants have broken but the
bonds that will exist in the products haven’t
formed yet, an unstable compound called the
activated complex forms.
– This complex has the greatest potential energy
If you want to speed up a reaction
without either increasing the
temperature or increasing the
concentration of a reactant, what
can you do?
Use a Catalyst!!
• This is a chemical that basically aligns all the reactant
molecules so that they collide more efficiently.
• This LOWERS the needed activation energy for the
reaction, speeding it up!
How does lowering the Activation
Energy increase the rate of the
reaction?
• A catalyst lowers the required activation energy but
it does NOT change the ΔH value of the reaction.
Research Project
• Find and carefully explain the chemistry
behind a catalyst used in everyday life.
– It might be a catalyst in your body
– It might be a catalyst used in industry
– Be creative!
• Once you choose your reaction, explain how
the catalyst speeds it up and why it is useful to
humans.
Research Project
• You will have one class to research and one
class to put your project together
– You may present your findings in any form you
wish
– Please have your presentation style approved by
Mrs. Boxall before you begin.