I II III IV V I. Intro to Reactions Unit 7 – Chemical Reactions
I II III IV V
I. Intro to Reactions
Unit 7 – Chemical Reactions
A.Signs of a Chemical Reaction Evolution of heat and light Formation of a gas Formation of a precipitate Color change
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
C. Chemical Equations
A+B C+DREACTANTS PRODUCTS
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C. Chemical Equations
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.
D. Writing Equations
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)
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
E. Describing Equations
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.
I II III IV V
Unit 7 – Chemical Reactions
II. Types of Chemical Reactions
A. Combustion
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
the burning of any substance in O2 to produce heat
A + O2 B
Na(s)+ O2(g)
C3H8(g)+ O2(g) 5 3 4
A. Combustion
Products: contain oxygen hydrocarbons form CO2 + H2O
CO2(g)+ H2O(g)
Na2O(s) 4 2
B. Synthesis
the combination of 2 or more substances to form a compound
only one product
A + B AB
B. Synthesis
H2(g) + Cl2(g) 2 HCl(g)
Al(s)+ Cl2(g) AlCl3(s)2 3 2
B. Synthesis
Products: ionic - cancel charges covalent - hard to tell
C. Decomposition
a compound breaks down into 2 or more simpler substances
only one reactant
AB A + B
C. Decomposition
2 H2O(l) 2 H2(g) + O2(g)
KBr(l) K(s) + Br2(l) 2 2
C. Decomposition
Products: binary - break into elements others - hard to tell
D. Single Replacement one element replaces another in a
compound metal replaces metal (+) nonmetal replaces nonmetal (-)
A + BC B + AC
D. Single Replacement
Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)
Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)
D. Single Replacement Products:
metal metal (+) nonmetal nonmetal (-) free element must be more active (check activity series)
Br2(l)+ NaCl(aq) N.R.
AB + CD AD + CB
E. Double Replacement
ions in two compounds “change partners” cation of one compound combines with
anion of the other
E. Double Replacement
Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)
Pb(NO3)2(aq)+ KI(aq) PbI2(s)+ KNO3(aq)
E. Double Replacement Products:
switch negative ions one product must be insoluble (check
solubility table)
NaNO3(aq)+ KI(aq) N.R.
2 2
I II III IV V
III. Balancing Equations
Unit 7 – Chemical Reactions
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. Double check atom balance!!!
B. Helpful Tips
Balance 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. Balance polyatomic ions as single
units. “1 SO4” instead of “1 S” and “4 O”
Al + CuCl2 Cu + AlCl3
Al
Cu
Cl
1 1
1 1
2 3
2
3
6
3
33 2
C. Balancing ExampleAluminum and copper(II) chloride react to form copper and aluminum chloride.
2
2
6