1. Energy and Chemical Reactions
Section 7-4
Energy and Chemical Reactions
March 29, 2011
Pages 197 and 199

2. Energy and chemical reactions
All chemical reactions involve energy.
Burning is a chemical reaction that gives off energy in the form of heat and light.
In plants, photosynthesis is a reaction that uses energy from sunlight.

3. 8.1 Why chemical bonds form
It takes energy to separate atoms that are bonded together.
The same energy is released when chemical bonds form.
Atoms form bonds to reach a lower energy state.

4. Types of energy in reactions
If forming new bonds releases more energy than it takes to break the old bonds, the reaction is exothermic.
Exothermic reactions tend to keep going because each reaction releases enough energy to start the reaction in neighboring molecules.
How can the energy from an exothermic reaction be useful?

6. Types of energy in reactions
Endothermic reactions absorb energy.
These reactions need energy to keep going.
Where does the energy needed for this reaction come from?

7. Activation energy
Activation energy is the energy needed to start a reaction and break chemical bonds in the reactants.
This is why a flammable material, like gasoline, does not burn without a spark or flame.
The spark supplies the activation energy to start the reaction.

8. Activation energy
So even if you end up at a lower energy level, you have to start by adding energy
The activation energy must be supplied to break the molecules of hydrogen and oxygen apart.
Combining 4 free hydrogen and oxygen atoms into 2 water molecules releases energy.
The reaction is exothermic because the energy released by forming water is greater than the activation energy.
Once the reaction starts, it supplies its own activation energy and quickly grows

9. 10.2 Nuclear reactions
Nuclear reactions change the nucleus of an atom.
Because they affect the nucleus itself, nuclear reactions can change one element into a different element.
This means that nuclear reactions don’t balance!
Also, in nuclear reactions, some matter is converted into energy! (This is what E=mc2 means!)

11. BOOM! Fission There are two kinds of nuclear reaction
In a fission reaction, a neutron is shot at a big atom to break it into smaller pieces.
Nuclear reactors and simple nuclear bombs use fission.
BOOM!

12. Fusion
Fusion reactions happen when small atoms are combined to make bigger atoms
Big nuclear bombs use fusion.
The sun produces energy through fusion
4 Hydrogens
1 Helium