Chapter 8 Notes
Chemical Reactions
I. Describing Chemical Change
• Chemical Equation: Arrow separates the reactants (on left) from
the products (on right);
plus signs separate multiple reactants and products
Ex.
Fe + O2 → Fe2O3
• Symbols Used in Chemical Equations:
Be sure you have these filled in from page 206 in your textbook
• Catalyst: A substance that speeds up a reaction without being
used up in the process; often shown above the reaction arrow
Ex.
H2 + O 2
Pt
H2 O
• Coefficient: Whole numbers placed to the LEFT of reactants and
products; used to balance equations;
COEFFICIENTS AND SUBSCRIPTS MULTIPLY
• Balanced Chemical Equation: Each side of the equation (left &
right sides of the arrow) has the same number of atoms of each
element
Coefficients represent _____
the ________
ratios of products and reactants,
NOT ____________!
masses
(A Few) Rules for Balancing Chemical Equations:
1. Never change the subscripts (small numbers) in formulas
2. Only use coefficients to balance
3. Remember that coefficients and subscripts MULTIPLY to count atoms
4. Balance one element at a time
5. If polyatomic ions appear on both sides of the equation, treat them as units
6. Make sure all coefficients are in lowest possible ratio (Ex. 1 : 2 : 2 NOT 2 : 4 : 4)
7. PRACTICE, PRACTICE, PRACTICE!
*Tip for the very difficult equations:
If you have an odd number of atoms of one element on one side of the equation, put
a 2 in front of it, then attempt to balance the other side
See board for examples
Pg. 209: #3
a.) 2 AgNO3
+
H2S →
Ag2S
b.) MnO2 + 4 HCl → MnCl2
+ 2 HNO3
+2 H2O
+
Cl2
4
c.) 3 Zn(OH)2
+ 2 H3PO4
→
Zn3(PO4)2
(HOH)
+ 6 H2O
Pg. 210: #7
+ 6NaOH → 2 Fe(OH)3
a.) 2 FeCl3
b.) CS2
+ 3 Cl2 →
CCl4
+
+ 6 NaCl
S2Cl2
6
c.) CH4
+
Br2
→
Balanced!
CH3Br +
4 2
HBr
II. Types of Chemical Reactions
1. Combination Reaction:
Multiple reactants form ONE PRODUCT
Ex. A: A + B → AB
Ex. B:
2 S + 3 O2 → 2 SO3
2. Decomposition Reaction: ONE REACTANT forms multiple products
Ex. A:
AB
Ex. B:
PbO2
→ A + B
→ Pb + O2
3. Single Replacement Reaction: An element replaces another
element in a compound; ONE ELEMENT and ONE COMPOUND
ON BOTH SIDES
Ex. A: A + BX → AB
Ex. B:
+ X
2 Na + ZnO → Zn
+
Na2O
4. Double Replacement Reaction: an exchange of positive ions
between two compounds; TWO COMPOUNDS ON BOTH SIDES
Ex. A: AB + XY → AX + BY
Ex. B: 3 KOH
+
AlPO4 → Al(OH)3 + K3PO4
5. Combustion Reaction: An element or compound reacts with O2;
O2 IS A REACTANT (on left)
Ex. A:
CH4 + 2 O2 → CO2
Ex. B:
S + O2 → SO2
+ 2 H2O
Predicting Products in a Double Replacement Reaction:
If you remember that in a double replacement reaction, the cations and anions
switch places, you can predict the products for the reaction. Just be careful to
write the formulas correctly on the product side.
(Mg2+ Br1-)
Ex. 1:
MgCl2
+ 2 NaBr
→
MgBr2
(Na1+ SO42-)
Ex. 2: 2 Na3PO4 +
Al2(SO4)3 → 3 Na2SO4
(Na1+ Cl1-)
+ 2 NaCl
(Al3+ PO43-)
+ 2 AlPO4
• Don’t carry the subscripts (little numbers) from the left to
the right side of the arrow UNLESS they are part of a
polyatomic ion formula (ex. PO43-).
• The subscripts are determined the same way here that they
were in Ch 6: criss-cross the ionic charges.