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Ch 14 Section 2
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Objectives:y Interpret and write simple chemical formulas
yWrite and balance simple chemical equations
y Explain how a balanced equation shows the law ofconservation of mass
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Chemical Formulas:y Ashorthand way to use chemical symbols & numbers
to represent a substance
y It shows how many atoms of each kind are present in amolecule
y Formula for water: H2O one water molecule is madeup of 2 hydrogen atoms and one oxygen atom
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Chemical Formulas:y Subscripts: a # written below and to the right of a
chemical symbol in a formula
y
Example: H2O the 2 is the subscript; only 1 atom ofoxygen is present
y Example: Glucose C6H12O6 - 6 atoms of carbon, 12atoms of hydrogen, 6 atoms of oxygen
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Chemical Formulas:yWriting formulas for Covalent Compounds:
y Composed of 2 nonmetals
y Names use prefixes, representing a #y Tells how many atoms of each element are in a formula
y Examples: CO2 carbon dioxide one carbon atom, 2
oxygen atomsy Example: N2O dinitrogen monoxide 2 nitrogen
atoms, 1 oxygen atom
http://www.windows2universe.org/physical_science/chemistry/co2_molecule_big.gif
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Chemical Formulas:yWriting formulas for Ionic Compounds:
y Composed of one nonmetal & one metal
y Charge is zeroy The formula has subscripts that cause the charges of the
ions to cancel out
y Example: NaCl sodium chloride 1 sodium atom, 1chlorine atom
y Example: MgCl magnesium chloride 1 magnesiumatom, 2 chlorine atoms
http://www.teachers.ash.org.au/jfuller/chemistry/magnesiumchloride.gif
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Chemical Equations:y Away to describe reactions
y Chemical symbols and formulas are used
y Starting material: reactants
y Substances formed from a reaction: products
y Example: C + O2 CO2
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Chemical Equations:y The importance of accuracy
y Formulas and symbols can be mistaken
y Similar symbols & formulas
y CO2 carbon dioxide
y CO carbon monoxide
y Co the element cobalt
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Chemical Equations:y Must be balanced
y Atoms cannot be lost or gained in a chemical reaction;they are only rearranged
y Antoine Lavoisier Law of Conservation of Mass
mass is neither created nor destroyed in chemical orphysical changes
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http://www.chemteam.info/Gallery/Lavoisier.gif
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Chemical Equations:y How to balance an equation:
y Use coefficients a # placed in the front of a chemicalsymbol or formula
y Example: 2CO - 2 carbon monoxide molecules
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Chemical Equations:y Atoms must be counted to balance an equation
y Multiply the subscript of each element with the
coefficient
y Example: 2H2Oy 4 hydrogen atoms, 2 oxygen atoms
y
Only coefficients are changed
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Quick Quiz:
yWhat is the difference between a reactant and a
product?
yWhat is the difference between the formula CO andthe symbol Co?
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Reference:
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