Sec. 2

Describing Chemical Reactions

N2

+

H2     NH3

chemical formula• shows ratio of elements in a compound• subscript= shows # of atoms of an

element in a molecule

H2SO4

2 hydrogen1 sulfur

4 oxygen

- Describing Chemical Reactions

chemical equation• shorter, easier way to

show chemical reactions

• uses symbols instead of words

• summarizes a reaction

Structure of an equation

• reactants= materials at beginning

• products= materials at end

• 2SO2 + O2 + 2H20 2H2SO4

H2 + O2 H2O

reactants products

hydrogen oxygen

H2 + O2 H2O2 2

2

2

2

1

Aluminum + Oxygen Aluminum Oxide

Al + O2 Al2O3

Left side: Right side:

- Observing Chemical Change

Principle of conservation of mass

• During a chemical reaction, matter is not created or destroyed

• Total mass of reactants = total mass of products

- Describing Chemical Reactions

Principle of conservation of mass

open system matter can enter from or escape to the surroundings

closed system matter is not allowed to enter or leave

Classifying Chemical Classifying Chemical ReactionsReactions

synthesis

• when 2 or more substances combine to make a more complex substance

• hand + kerchief= handkerchief

• 2SO2 + O2 + 2H20 2H2SO4

decomposition

• compounds are broken into simpler

products

• schoolbook = school + book

• 2 H2O 2 H2 + O2

replacement• 1 element replaces another in a compound

or 2 elements in compounds trade places

• television screen + door knob=

television knob + screen door

Mg + 2 H2O Mg(OH)2 + H2

replacement

synthesis a + b ab decomposition ab a+b

replacement a + b b + a