Chemical Reactions

Balancing and Classifying Chemical Equations

Chemical Reactions

Chemical reactions occur when a chemical change occurs

Evidence of a Chemical Reaction

Color change Formation of a gas Odor change Formation of a

precipitate Temperature change pH change

Why do chemical reactions occur?

Chemical reactions occur because the products are more stable than the reactants.

All substances react in an effort to become more stable.

Common Symbols in Chemical Equations

Reactants and Products

Reactants (ingredients) -> Products

How many oxygen atoms are in the reactants?

How many oxygen atoms are in the products?

Balancing Equations

Balancing Equations Law of

Conservation of Matter (or mass): In a chemical

reaction, matter can be neither created nor destroyed.

In a chemical reaction, the amount of reactants equal the amount of products.

Balancing Equations Paraphrase: Law of Conservation

of Atoms: The number of atoms

of each type of element must be the same on each side of the equation.

Balancing Equations

Hydrogen and oxygen are diatomic elements.

Their subscripts cannot be changed. The subscripts on water cannot be

changed.

Hydrogen + oxygen water

H2 + O2 H2O

Balancing Equation

Count the atoms on each side. Reactant side: 2 atoms H and 2

atoms O Product side: 2 atoms H and 1

atom O

H2 + O2 H2O

Balancing EquationsH2 + O2 H2O

If the subscripts cannot be altered, how can the atoms be made equal?

Adjust the number of molecules by changing the coefficients.

Balancing Equations

Reactants: 2 atoms of H and 2 atoms of O

Products: 4 atoms of H and 2 atoms of O

H is no longer balanced!

H2 + O2 2H2O

Balancing Equations

Reactant side: 4 atoms of H and 2 atoms of O

Product side: 4 atoms of H and 2 atoms of O

It’s Balanced!

2H2 + O2 2H2O

Balancing Equations

2H2 + O2 2H2O

Balancing Equations Balancing hints:

Count all atoms and polyatomic ions occurring on each side of ->

Balance the metals first.Balance the polyatomic ions next.

Balance the other atoms.Save the oxygen and hydrogen until the end.

Diatomic elements – always exist in pairs

Remember the diatomic elements: H2, N2, O2, F2, Cl2, Br2, and I2.

Types of Reactions 1. Synthesis (or Addition or

Combination) Reactions A + B AB

2 or more substances combine to form a new compound. (A and B are elements or compounds)

small molecules join to form chains - polymerization

Fe + O2 Fe2O3

Types of Reactions

2. Decomposition Reactions AB A + B

compounds are broken apart into smaller substances

Example: Electrolysis of water 2 H2O 2 H2 +

O2

Types of Reactions

Single – Replacement ReactionAX + B BX + A

One element replaces a similar element in a compound

Fe + CuCl2 FeCl2 + Cu Look at the Metals activity series

Types of Reactions Double – Replacement Reaction

AX + BY AY + BX Ions from 2 compounds in solution

exchange places to produce 2 new compounds.

One of the compounds is usually … A precipitate A gas that bubbles off A molecular compound like water

Types of Reactions Combustion Reactions

These use O2 as a reactant One product contains O , usually

H2O____________________________________________________________________________________________________________________________

Importance of Good Air Supply2CH4 + 4O2 2CO2 + 4H2O

2CH4 + 3O2 2CO + 4H2O

2CH4 + 2O2 2C + 4H2O

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Some Metals in the Activity Series