iiiiiiivv i.intro to reactions ch.7– chemical reactions
TRANSCRIPT
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I II III IV V
I. Intro to Reactions
Ch.7– Chemical Reactions
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A.Signs of a Chemical Reaction
heat and light produced Formation of a gas Formation of a precipitate Color change
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B.Law of Conservation of Mass mass is neither created nor destroyed
in a chemical reaction
4 H
2 O
4 H
2 O4 g 32 g
36 g
total mass stays the same atoms can only rearrange
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C. Chemical Equations
A+B C+DREACTANTS PRODUCTS
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p. 246
C. Chemical Equations
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D. Writing Equations
Identify the substances involved. Use symbols to show:
2H2(g) + O2(g) 2H2O(g)
How many? - coefficient
Of what? - chemical formula
In what state? - physical state Remember the diatomic elements.
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Example:
Two atoms of aluminum react with three units of aqueous copper(II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride.
• How many?• Of what?• In what state?
Al 2 (s) + 3CuCl2(aq) 3 Cu(s) + 2AlCl3(aq)
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E. Describing Equations
Describing Coefficients:
individual atom = “atom”
covalent substance = “molecule”
ionic substance = “unit”
3 molecules of carbon dioxide
2 atoms of magnesium
4 units of magnesium oxide
3CO2
2Mg
4MgO
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Example
to produce
• How many?• Of what?• In what state?
Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
One atom of solid zinc reacts withtwo molecules of aqueous hydrochloric acid one unitof aqueous zinc chloride and onemolecule of hydrogen gas.
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I II III IV V
II. Balancing Equations
Chemical Reactions
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A. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient subscript = # of atoms
4. Reduce coefficients to lowest possible ratio, if necessary.
5. Check yourself before you wreck yourself
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B. Helpful Tips
one element at a time. Update ALL atom counts after adding
a coefficient. If an element appears more than
once per side, balance it last.
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Al + CuCl2 Cu + AlCl3
Al
Cu
Cl
1 1
1 1
2 3
2
3
6
3
33 2
C. Balancing Example
Aluminum and copper(II) chloride react to form copper and aluminum chloride.
2
2
6
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I II III IV V
Chemical Reactions
III. Types of Chemical Reactions
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A. Combustion
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
the burning of any substance in O2 to produce heat
A + O2 B
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Na(s)+ O2(g)
C3H8(g)+ O2(g) 5 3 4
Products: contain oxygen hydrocarbons form CO2 + H2O
CO2(g)+ H2O(g)
Na2O(s) 4 2
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B. Synthesis
the combination of 2 or more substances to form a compound
only one product
A + B AB
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H2(g) + Cl2(g) 2 HCl(g)
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Al(s)+ Cl2(g) AlCl3(s)2 3 2
Products: ionic – cross charges covalent - hard to tell
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C. Decomposition
a compound breaks down into 2 or more simpler substances
only one reactant
AB A + B
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2 H2O(l) 2 H2(g) + O2(g)
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KBr(l) K(s) + Br2(l) 2 2
Products: binary - break into elements others - hard to tell
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D. Single Replacement
one element replaces another in a compound metal replaces metal (+) nonmetal replaces nonmetal (-)
A + BC B + AC
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Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)
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Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)
Products: metal metal (+) nonmetal nonmetal (-) free element must be more active (check activity series)
Br2(l)+ NaCl(aq) N.R.
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AB + CD AD + CB
E. Double Replacement
ions in two compounds “change partners” cation of one compound combines with
anion of the other
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Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)
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Pb(NO3)2(aq)+ KI(aq) PbI2(s)+ KNO3(aq)
Products: switch negative ions one product must be insoluble (check
solubility table)
NaNO3(aq)+ KI(aq) N.R.
2 2