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Chemical Reactions
Types of Chemical Bonds
 Ionic- Two elements bond by transferring electrons to create ions that
attract together (+ is attracted to - after an electron is transferred)
 Covalent- Two elements bond by sharing electrons (strongest bond
type)
 Metallic- Two metals bond and form a “common electron cloud”. This
is a cluster of shared electrons (weakest bond type)
Chemical Reactions

Chemical Reaction: a process in which the
physical and chemical properties of the
original substance change as new
substances with different physical and
chemical properties are formed
Evidence of Chemical Change
 EPOCH is an acronym that stands for evidence that a
chemical reaction has occurred.
E–
 P–
O–
C –
 H–

Effervescence (bubbles and/or gives off gas)
Precipitate (solid crystals form)
Odor (change of smell is detected)
Color change
Heat (reaction either heats up or cools down)
 Does sighting evidence of a chemical reaction mean that a
chemical reaction has undoubtedly taken place?
How do you know when a
chemical reaction takes
place?
Color Change
Precipitate Formation
How do you know when a
chemical reaction takes place?
Gas Formation
Odor
How do you know when a
chemical reaction takes place?
Temperature Change
Change in Acidity
Chemical Reaction Basics
2H2 + O2 --> 2H2O
Reactants
Products
Reactants- substance that enters into a reaction
Products- substance that is produced by a chemical reaction
Exothermic
Exothermic Reaction- heat energy EXITS
the system
- ex. Combustion, evaporation of water
- surroundings usually feel warmer
Endothermic
Endothermic Reaction- heat energy ENTERS the
system
- ex. Cold packs, melting ice
- surroundings usually feel cooler
Conservation of Mass
Atoms cannot be created or destroyed in a
chemical reaction.
 What goes in must come out.
 So we must balance equations to conserve
mass.
 There must be the exact same amount of
atoms before and after a chemical
reaction has taken place.
