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Sec. 2
Describing Chemical Reactions
N2
+
H2 NH3
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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
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- Describing Chemical Reactions
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chemical equation• shorter, easier way to
show chemical reactions
• uses symbols instead of words
• summarizes a reaction
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Structure of an equation
• reactants= materials at beginning
• products= materials at end
• 2SO2 + O2 + 2H20 2H2SO4
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H2 + O2 H2O
reactants products
hydrogen oxygen
H2 + O2 H2O2 2
2
2
2
1
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Aluminum + Oxygen Aluminum Oxide
Al + O2 Al2O3
Left side: Right side:
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- Observing Chemical Change
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Principle of conservation of mass
• During a chemical reaction, matter is not created or destroyed
• Total mass of reactants = total mass of products
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- Describing Chemical Reactions
Principle of conservation of mass
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open system matter can enter from or escape to the surroundings
closed system matter is not allowed to enter or leave
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Classifying Chemical Classifying Chemical ReactionsReactions
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synthesis
• when 2 or more substances combine to make a more complex substance
• hand + kerchief= handkerchief
• 2SO2 + O2 + 2H20 2H2SO4
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decomposition
• compounds are broken into simpler
products
• schoolbook = school + book
• 2 H2O 2 H2 + O2
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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
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replacement
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synthesis a + b ab decomposition ab a+b
replacement a + b b + a