1. Chemical Reactions
Chapter 9 Section 1

2. Chemical Reactions
A chemical reaction is a process in which one or more substances change to make one or more new substances with different properties.
Signs - color or odor change, formation of a gas or a solid, and/or the release or absorption of energy.
For a chemical reaction, the chemical bonds in the starting substances must break, the atoms then rearrange, and new bonds form.

3. Reactions and Energy
Energy is needed to break chemical bonds in the starting substances. As new bonds form, energy is released.
There are 2 main types of reactions, exothermic and endothermic.
Many chemical reactions can proceed in either direction, depending on their surroundings.

4. Exothermic Reactions
A chemical reaction in which energy is released is called an exothermic reaction.
Energy is given off in several forms.
Heat
Light
Electrical energy
Example: Cellular respiration
Glucose is oxidized & energy is released (why you are “warm-blooded”).
The bonds of the products
have a lower potential
energy (more chemically stable)

5. Endothermic Reactions
A chemical reaction in which energy is absorbed from the surroundings (taken in) is called an endothermic reaction.
The bonds of the products are at a higher state of potential energy than the reactants
Photosynthesis is an example of an endothermic process.
Sunlight is the energy needed to transform H2 O and CO2 into O2 & glucose.

6. Reactions and Energy, continued
The law of conservation of energy states that energy cannot be created or destroyed but can change from one form to another.
E = mc2 shows that matter can be converted to energy & vice versa.