Indicators of a chemical reaction-• Color change• Producing a gas• Heat/light energy released• Formation of a precipitate

Chemical equation - Describes a chemical change.

Parts of a chemical equation:

Zn + 2HCl ZnCl2 + H2

Reactant Product

Reaction symbol

Coefficient

Reactant - The substance(s) that exist before a reaction beginsWritten on left side of equation.

Product - The new substance(s) formed by the reaction.Right side of equation.

Subscript – indicates the number of atoms of an element in a compound.EX: H2O

2 atoms of hydrogen (H)1 atom of oxygen (O)

Coefficient – indicates how many units of each compound is in a chemical reaction.EX: 3 H2O

3 molecules of H2O

2H2 + O2 2H2O

Lavoisier: Matter is neither created nor destroyed in a chemical reaction.In other words, the number and type of atoms

going INTO a reaction must be the same as the number and type of atoms coming OUT.

A Balanced Chemical Equation must obey the Law of Conservation of Mass.

CH4 + O2 CO2 + H2O

Reactant Side Product Side

1 carbon atom4 hydrogen atoms2 oxygen atoms

1 carbon atom2 hydrogen atoms3 oxygen atoms

A Balanced Equation

CH4 + 2O2 CO2 + 2H2O

Reactant Side Product Side

1 carbon atom4 hydrogen atoms4 oxygen atoms

1 carbon atom4 hydrogen atoms4 oxygen atoms

1. Matter cannot be created or destroyed.2. Subscripts cannot be added, removed, or changed.3. You can only change coefficients.•4. Coefficients can only go in front of chemical compound

• NEVER in the middle of a formula.A few extra tips:

Try balancing big formulas first; save single elements for last.

If the same polyatomic ion appears on both sides of the equation, it’s usually okay to treat it as one unit.

There is no one particular way to balance equations. Some equations are harder to balance than others and might require some creativity to solve.

Balance the following equation by adjusting coefficients.

reactants products

N

H

NN22 + H + H22 NH NH33

2

2

1

3

22

2

6

33

6

Balance the following equation by adjusting coefficients.

KClOKClO33 KCl + O KCl + O22reactants products

K

Cl

O

1

1

1

1

3 2

3322

2

2

6 6

2

2

22

Balance the following equation: C2H6 + O2 CO2 + H2O44 667722

Balance the following equation: Fe + O2 Fe2O3223344

Chemical Reaction Types• synthesis – reaction where 2 or more substances combine to form

one• A + B → AB• Ex: 2H2 + O2 → 2H2O

• decomposition – reaction where a compound decomposes(breaks apart) into multiple substances

• AB → A + B• Ex: 2KClO3 → 2KCl + 3O2

• single replacement - single element replaces another element in a compound

• A + BC → AC + B• Ex: 2Ag + FeCl2 → Fe + 2AgCl

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Chemical Reaction Types Continued…• Double replacement - two elements in different

compounds switch places• AB + CD → AD + BC• Ex: NaOH + HCl → NaCl + HOH(H2O)

• Combustion Reaction - Hydrocarbon combines with oxygen to form water and carbon dioxide• hydrocarbon – any compound made from carbon and

hydrogen• hydrocarbon + O2 → CO2 + H2O• Ex: C2H4 + 3O2 → 2CO2 + 2H2O

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