Unit #2- Writing and Balancing Chemical Equations

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Substances which enter into the chemical reaction/change


Substances which are produced by the chemical reaction

Chemical Equation

Condensed statement which indicates what has happened during a chemical reaction

What are the two types of double replacements?

1. Precipitation Double Replacement
2. Acid-Based Double Replacement

Precipitation Double Replacement

-The reactants are both ionic compounds
-Occurs in aqueous solution
-Insoluble product = precipitate (solid)
-Can write molecular, ionic, and net ionic equations

Acid-Base Double Replacement

-Reactants must be an acid or base
-Acid: substance which releases hydrogen ions into solution
-Base: substance which releases hydroxide ions into solution
-Can write molecular, ionic, and net ionic equations

Hydrochloric Acid


Sulfuric Acid


Nitric Acid


Writing Molecular Equations

-All substances are represented by element symbols/ compound formulas and phase symbols
-Must be balanced

Writing Ionic Equations

-Dissociate the aqueous compounds into their ions
-Ions are represented by element symbol, phase symbol, charge, and number of atoms

Writing Net Ionic Equations

-Remove spectator ions (ions that appear in identical form on both sides of the equation) from the ionic equation

Solubility Rules (1-3)

Compound is SOLUBLE if it contains:
1. Group 1 ions
2. Ammonium ions
3. Nitrate ions

What are the two types of single replacements?

1. Metal/Hydrogen SIngle Replacement
2. Halogen Single Replacement

Metal/Hydrogen Single Replacement

-Metallic element (or hydrogen gas) reacting with an ionic compound (or an acid)
-Will only occur if the metal is higher in the activity series than the cation in the compound
-Can write molecular, ionic, and net ionic equations

Halogen Single Replacement

-Halogen element is reacting with an ionic compound
-Halogen MUST be higher than the anion in order for a reaction to occur (period table = activity series)
-Can write molecular, ionic, and net ionic equations

What are the three types of non-aqueous reactions?

1. Synthesis
2. Decomposition
3. Hydrocarbon combustion


-Reactants are individual elements
-Product is always a single compound
-Only molecular equations are written
-Most are exothermic-energy is written on the right side


-Reactant is always a single compound
-Assume products are individual elements
-Only molecular equations are written
-All are endothermic-energy is written on the right side

Hydrocarbon Combustion

-Hydrocarbon: compound that contains hydrogen, carbon, and sometimes oxygen
-Requires oxygen as a reactant
-Products are carbon dioxide gas and gaseous water
-Only molecular equations are written
-All are exothermic-energy is written on the right side

What are the two types of energy changes in chemical reactions?

1. Endothermic Reactions
2. Exothermic Reactions

Endothermic Reaction

-Energy (usually heat) is absorbed from surroundings
-Occurs when reactant(s) are continually heated OR when the reaction vessel becomes cooler as the reaction proceeds
-Breaking chemical bonds requires energy --> decomposition = endothermic
-"Energy" is written on left side of the equation

Exothermic Reaction

-Energy (usually heat) is released into the surroundings
-Reaction vessel becomes warmer as the reaction proceeds
-Formation of chemical bonds between atoms releases energy --> most synthesis + all combustions = exothermic
-"Energy" is written on right side on the equation

Oxidation and Reduction Mnemonic



-Loss of electrons (electrons are added to the right side)
-Use element name or symbol


-Gain of electrons (electrons are added to the left side)
-Use element name or symbol

Oxidizing Agent

-Substance that is reduced
-Use exact name or formula

Reducing Agent

-Substance that is oxidized
-Use exact name or formula

Redox Reactions

-Involve the exchange of electrons between two reactions

Balancing Non-Trivial Redox Reactions

1. Write two half-reactions
2. Balance atoms except oxygen and hydrogen
3. (if needed) Balance oxygen by adding H2O
4. (if needed) Balance hydrogen by adding H+
5. Balance net electrical charge
6. Equalize electrons
7. Combine and simplify

Assigning Oxidation Numbers

1. Free Elements = 0
2. Monatomic Ions = Charge on the ion
3. Oxygen in most compounds = -2
4. Hydrogen in most compounds = +1
5. Neutral molecules sum of numbers = 0
6. Polyatomic ion sum of numbers = charge on that ion

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