Energy Changes and
• To propel a space shuttle, scientists use
• The shuttle’s main engines burn almost 2
million L of liquid hydrogen and liquid oxygen.
• The reaction produces water vapor and a lot
• The heat energy causes the water vapor to
heat to high temperatures.
• The rapidly expanding water vapor pushes
Chemical Energy in Bonds
• Chemical bond contain a form of energy
called chemical energy
– Breaking a bond absorbs energy from the
– The formation of a bond releases energy to the
– 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.
Endothermic Reactions –
• 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
Exothermic Reactions – Energy
• 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
• Therefore, the overall reaction releases
• Many reactions do not start by themselves.
– Paper does not burn when it touches oxygen. Fire
• 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
• The rusting of iron is a reaction that has a low
– The energy in the surroundings is enough to start
• Other reactions require more energy and
have high activation energy
– The burning of wood requires the energy of a
– Once the reaction starts, the reaction itself
releases enough energy to keep the reaction
• Some reactions happen quickly, while others
happen very slowly
– Fireworks explode in seconds
– The rate of reaction is the speed at which it
• 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
Several Factors Affect Reaction
Concentration and Pressure
• 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.
• 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
Concentration and Pressure
• Increasing the concentration of one or
more reactants increases collisions
• More collisions results in a faster reaction
• In gases an increase in pressure pushes
gas particles closer together
• When they are close, more collisions
• 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
• Catalyst are not changed, so they are not
• Your body is filled with catalysts called
• 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.
• An inhibitor is a substance that slows, or
even stops, a chemical reaction caused by
• Inhibitor are often used on our foods.
Preservatives are inhibitors that slow the
processes that cause food to spoil.