1. Chemical Reactions
Ch. 8

2. Describing Chemical Change
8-1

3. Writing Chemical Equations
Reactant = substance that undergoes a reaction
Product = the new substance formed when reactants undergo a chemical change
Ex: Fe+3 + O-2 Fe2O3 (rust)
reactants
products

4. Equations Word Equations:
Ex: vinegar + baking soda sodium acetate + water + carbon dioxide
Chemical Equations:
Ex: HC2H3O2 (aq) + NaHCO3 (s) NaC2H3O2 (aq) + H2O (l) + CO2 (g)
Shows the state of substance:
aq – aqueous, s – solid, l – liquid, g- gas
aqueous = solid dissolved in water.

5. Balancing Chemical Equations (REVIEW)
Law of Conservation of Mass = matter is neither created nor destroyed…
Thus, mass of products = mass of reactants
Atoms in a chemical reaction don’t change, they rearrange!
The same number of each kind of atom must be on the left side as well as the right!
Ex: H2CO3(aq) H2O(l) + CO2(g)
H = 2, C =1, O = 3
H = 2, C =1, O =3

6. A Balancing Act
To indicate more than one unit of a compound, a coefficient is placed in front
Ex: 2H2O
H = 4, O = 2
NEVER change the subscripts of a compound, that would change its identity!!
H2O is water, H2O2 is hydrogen peroxide
Rule: start by balancing the elements that appear only once of each side of the equation.
Ex: Balance oxygen last!
H2CO3(aq) H2O(l) + CO2(g)

7. Writing Equations! Step 1: Write word equation
Hydrogen + oxygen water + energy
Step 2: Replace words with formulas
H2(g) + O2(g) H2O(l) + energy
Step 3: Balance the atoms on both sides!
2H2(g) + O2(g) 2H2O(l) + energy
Step 4: Check it!
H = 4, O = 2 H = 4, O =2

8. Example Write the balanced equation:
Magnesium chloride + silver nitrate magnesium nitrate + silver chloride
MgCl2 + AgNO3 Mg(NO3)2 + AgCl
Now balance it!
MgCl2 + 2AgNO3 Mg(NO3)2 + 2AgCl

9. You Try!
Magnesium metal + water form solid magnesium hydroxide and hydrogen gas
Mg(s) + H2O(l)  Mg(OH)2(s) + H2(g)
Mg(s) + 2H2O(l)  Mg(OH)2(s) + H2(g)
Potassium chlorate breaks down into potassium chloride and oxygen
KClO3  KCl + O2
2KClO3  2KCl + 3O2
3. Solid potassium reacts with liquid water to produce aqueous potassium hydroxide and hydrogen gas.
K(s) + H2O(l)  KOH(aq) + H2(g)
2K(s) + 2H2O(l)  2KOH(aq) + H2(g)

10. Types of Reactions
8-2

11. Categories of Reactions
There are 5 types of reactions:
Synthesis/Combination = 2 or more substances combine to form a single product
A + B AB
Ex: 2Na + Cl NaCl
Decomposition = a compound breaks down into 2 or more simpler substances
AB A + B
Ex: H2CO H2O + CO2
Sodium + Chlorine --- Sodium chloride

12. 3. Single-Replacement = an element that is not combined replaces an element that is part of a compound
-All compounds are aqueous!
A + BC  AC + B
Ex: Mg(s) + Zn(NO3)2(aq)  Mg(NO3)2(aq) + Zn(s)
Double-Replacement = different atoms in 2 different compounds replace each other
-All reactants are aqueous
-Products will include a gas, a ppt (precipitate), or H2O
AC + BD  AD + BC
Ex: AgNO3 + KCl  AgCl + KNO3

13. Combustion = substance rapidly combines with oxygen producing heat and light.
-Reactants contain C,H,O
-Products are CO2 and H2O vapor
CxHy + O2(g)  CO2(g) + H2O(g)
Ex: 5CH3OH + O2(g)  5CO2(g) + 8H2O(g)

14. You Try! First balance, then put into 1 of 5 categories:
BaCl2 + Na2SO4 BaSO4 + NaCl
BaCl2 + Na2SO4 BaSO4 + 2NaCl
Double Replacement
Mg + CuSO4 Cu + MgSO4
Single Replacement
H2SO4 H2 + SO4
Decomposition
Fe + O FeO
2Fe + O2 2FeO
Synthesis
Al + Cl AlCl3
2Al + 3Cl AlCl3