The document discusses energy changes in chemical reactions, particularly in relation to endothermic and exothermic processes, and the concept of activation energy. It outlines factors affecting reaction rates, such as surface area, temperature, concentration, pressure, catalysts, and inhibitors. Additionally, it highlights the role of enzymes as biological catalysts that facilitate essential reactions in living organisms.

1. Energy Changes and
Chemical Reactions
Chapter 9.3
p 317-323

2. Energy Changes
• To propel a space shuttle, scientists use
rocket fuel.
• The shuttle’s main engines burn almost 2
million L of liquid hydrogen and liquid oxygen.
• The reaction produces water vapor and a lot
of energy.
• The heat energy causes the water vapor to
heat to high temperatures.
• The rapidly expanding water vapor pushes
the shuttle.

3. Chemical Energy in Bonds
• Chemical bond contain a form of energy
called chemical energy
– Breaking a bond absorbs energy from the
surroundings.
– The formation of a bond releases energy to the
surroundings.
– Some chemical reactions release more energy
then they absorb.
– Others absorb more than they release
– You can detect these energy changes by
recording the temperature of the surroundings.

4. Endothermic Reactions –
Energy Absorbed
• Chemical reactions that absorb thermal
energy are endothermic reactions
• For an endothermic reaction to continue,
energy must be constantly added.
• In these reactions more energy is required
to break bonds of the reactant than is
released when the products form.
• Therefore, the overall reaction absorbs
energy.

6. Exothermic Reactions – Energy
Released
• An exothermic reaction is a chemical
reaction that releases thermal energy
• In an exothermic reaction, more energy is
released when the products form than is
required to break the bonds in the
reactants.
• Therefore, the overall reaction releases
energy

9. Activation Energy

10. Activation Energy
• Many reactions do not start by themselves.
– Paper does not burn when it touches oxygen. Fire
is required.
• All reactions require energy to start the
breaking of bonds.
• This is called activation energy.
• Activation energy is the minimum amount of
energy needed to start a chemical reaction.
– Different reactions require different activation
energies

11. Activation Energy
• The rusting of iron is a reaction that has a low
activation energy.
– The energy in the surroundings is enough to start
the process.
• Other reactions require more energy and
have high activation energy
– The burning of wood requires the energy of a
flame.
– Once the reaction starts, the reaction itself
releases enough energy to keep the reaction
going.

13. Reaction Rates
• Some reactions happen quickly, while others
happen very slowly
– Fireworks explode in seconds
– The rate of reaction is the speed at which it
occurs.
• For a chemical reaction to occur, particles
must collide in the right orientation with
enough energy to break the bonds
– Chemical reactions occur faster of particles
collide more often or move faster when they
collide.
http://youtu.be/2pXyJ7P0B0k

14. Several Factors Affect Reaction
Rates
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•
•
•
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Surface Area
Temperature
Concentration and Pressure
Catalysts
Inhibitors

15. Surface Area
• Surface area is the amount of exposed,
outer area of a solid.
• Increased surface area increases reaction
rate because more particles on the
surface if a solid come into contact with
the particles of another substance.
– Consider a piece of chalk versus the same
amount of chalk powder.
http://youtu.be/FJtwkum_QAY?t=1m3s

17. Temperature
• At higher temperatures, the average
speed of particles is greater
• This speeds reactions in two ways
– First, particles collide more often
– Second, collisions with more energy are more
likely to break chemical bonds

18. Concentration and Pressure
• Increasing the concentration of one or
more reactants increases collisions
between particles.
• More collisions results in a faster reaction
rate.
• In gases an increase in pressure pushes
gas particles closer together
• When they are close, more collisions
occur

20. Catalysts
• A catalyst is a substance that increases
reaction rate by lowering the activation
energy of a reaction.
– One way catalysts speed reactions is by helping
reactant particles contact each other more often
– A catalyst isn’t changed in a reaction, and it
doesn’t change the reactants or products
– A catalyst doesn’t increase the amount of
reactant used or the amount of product that is
made.

22. Catalyst
• Catalyst are not changed, so they are not
considered reactants
• Your body is filled with catalysts called
enzymes.
• An enzyme is a catalyst that speeds up
chemical reactions in living cells.
– The enzyme protease breaks the protein
molecules that can be absorbed by your intestine.
– Without protease the reaction would occur too
slowly and we could not survive.

23. Inhibitors
• An inhibitor is a substance that slows, or
even stops, a chemical reaction caused by
an enzyme.
• Inhibitor are often used on our foods.
Preservatives are inhibitors that slow the
processes that cause food to spoil.

24. http://www.youtube.com/watch?v=OttRV5ykP7A