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 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. – Effervescence (bubbles and/or gives off gas) – Effervescence (bubbles and/or gives off gas) – Precipitate (solid crystals form) – Precipitate (solid crystals form) – Odor (change of smell is detected) – Odor (change of smell is detected) – Color change – Color change – Heat (reaction either heats up or cools down) – Heat (reaction either heats up or cools down)  Does sighting evidence of a chemical reaction mean that a chemical reaction has undoubtedly taken place? E P O C H

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 2H 2 + O 2 --> 2H 2 O Reactants- substance that enters into a reaction Products- substance that is produced by a chemical reaction ReactantsProducts

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. Atoms cannot be created or destroyed in a chemical reaction. What goes in must come out. What goes in must come out. So we must balance equations to conserve mass. 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. There must be the exact same amount of atoms before and after a chemical reaction has taken place.