Section 2-4 Chemical Reactions
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Key Concept: Life depends on thousands of chemical reactions occurring in the body
Main Ideas:
1. Chemical reactions involve the breaking and forming of bonds between atoms.
2. Chemical reactions either release or absorb energy.
What is a Chemical Reaction?
Chemical Reaction – the process of breaking the bonds between atoms and then
reforming them so that the atoms are arranged into new types of molecules.
http://www.cartage.org.lb/en/themes/sciences/chemistry/inorganicchemistry/chemicslformula/Equation/chemicalequation/FG03_003.GIF
Chemical reactions always involve the breaking of bonds in reactant molecules and the
formation of new bonds in the product molecules.
Parts of a Chemical Reaction
Chemical Equation – Shorthand way to describe the overall chemical reaction.
Reactants – Elements or compounds that are the “starter materials” for a reaction
Products – Elements or compounds produced by a chemical reaction
Ex. Chemical equation for photosynthesis
6CO2 + 12H2O + sunlight
6O 2 + C6 H12O 6 + 6H2 O
Reactants
Products
Bond Energy in Chemical Reactions
What happens when bonds break and bonds form?
Energy must be added to break bonds that hold the reactant molecules together. This is
called activation energy (Ae). This amount of energy is what “activates” or gets the
reaction started.
Once the bonds are broken, the atoms are freed up and can make new molecules.
When bonds form between the atoms to make new molecules, energy is released.
The specific amount of energy that is needed to break a bond, or is releases when that
same bond forms, is called bond energy.
Two Categories of Chemical Reactions based on Energy Changes
1. Exothermic Reaction - a reaction that releases more E than what it absorbed.
Exo – Out of
Thermic - Heat
Example: Combustion/Cell Respiration
2. Endothermic Reaction - a reaction in which more E is absorbed that released.
Endo - Inside
Thermic - Heat
Example: Photosynthesis