Chemical Reactions:The dance of the atoms

Chemistry basics • Chemical reactions are the

rearrangement of atoms • The reactants are the

molecules & atoms that start the reaction.

• The products are the molecules & atoms that are formed by the reaction.

• Energy is either released or absorbed during a reaction

Energy & Chemical reactions

• Energy must be added to break chemical bonds

• Forming bonds releases energy

• Exothermic reactions release energy (often as heat)

• Endothermic Reaction require energy (often colder after reaction)

Reaction Types• Synthesis

• Decomposition

• Single displacement

• Double Displacement

• Combustion

Synthesis

At least two reactants make a product

A + B = AB

Photosynthesis is an example of a Synthesis Reaction.

6CO2 + 6 H2O C6H12O6 + O2

DecompositionLarger molecules are broken into smaller ones.

A B A + B

Respiration is an example of a Decomposition Reaction.

C6H12O6 + O2 6CO2 + 6 H2O

Single Displacement

elements trade places

AX + B A + BX

Example of a Single Displacement Reaction:

3CuCl2 + 2Al 2AlCl3 + 3Cu

The more reactive element replaces the less reactive one in Single Displacement Reactions.

Double DisplacementTwo Compounds Exchange Ions

AX + BY AY + BX

An example of Double Displacement Reaction:

Pb(NO3)2 + K2CrO4 PbCrO4 + 2KNO3

Combustion Oxygen gas is a reactant

Examples of Combustion Reactions:

2CH4 + 4O2 2CO2 + 4H2O

2CH4 + 3O2 2CO + 4H2O

2CH4 + 2O2 2C + 4H2O

When there is not enough oxygen for this Combustion Reactions carbon monoxide or soot will be produced instead of carbon

dioxide.

Reaction ReviewTwo substances are combined to make one

Oxygen gas is a reactant

Two compounds trade places

One element trades places with another

Substances are broken apart

Review

1. What kind of reaction releases heat?

2. What kind of reaction requires oxygen gas?

3. What kind of reactions is the break down of large molecules into smaller ones?