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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