Energy Changes and Chemical Reactions

Chapter 9.3p 317-323

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.

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.

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.

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

Activation Energy

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

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.

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

Several Factors Affect Reaction Rates

• Surface Area

• Temperature

• Concentration and Pressure

• Catalysts

• Inhibitors

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

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

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

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.

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.

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.

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