Chemical Equations
What are they?
• Equations showing chemical change(s).
• Example:
CH4 + 2O2
reactants
CO2 + 2H2O
products
**Law of Conservation of Mass:
In a chemical reaction, mass (atoms)
cannot be created or destroyed.
A closer look…
• Mass is always conserved in a chemical reaction.
CH4 + 2O2
CO2 + 2H2O
Reactants
Products
Carbon
1
1
Hydrogen
4
4
Oxygen
4
4
Most chemical equations give the physical states of
the reactants and products:
•
•
•
•
(s) solid
(l) liquid
(g) gas
(aq) aqueous solution (dissolved in water)
Ex: 2Mg(s) + O2(g)
2HgO(s)
2MgO(s)
2Hg(l) + O2(g)
Common diatomic gases
•
•
•
•
•
•
•
H2
N2
O2
Cl2
Br2
I2
F2
6 Common Types of Chemical Reactions:
1) Synthesis
2) Decomposition
3) Combustion
4) Single Replacement
5) Double Replacement
6) Oxidation Reduction (REDOX) Reaction
Synthesis
Reaction in which 2 or more reactants combine to form a new compound.
A+B
Examples:
• 2K(s) + Cl2(g) 2KCl(s)
• Fe(s) + S(s) FeS(s)
AB
Decomposition
A single compound is broken down into 2 or more products
AB
Examples:
•
•
•
•
CaCO3(s)
CaO(s) + CO2(g)
2H2O(l)
2H2(g) + O2(g)
PbO2(s)
Pb(l) + O2(g)
2HgO(s)
2Hg(l) + O2
A+B
Combustion
(burning in presence of Oxygen)
Oxygen gas (O2) reacts with another substance to produce an oxide*, water,
and heat.
*The oxide is typically carbon dioxide.
Examples:
• CH4(g) + 2 O2(g)
• C3H8(g) + 5 O2(g)
CO2(g) + 2 H2O(g) + heat
3 CO2(g) + 4 H2O(g) + heat
Single Replacement
One element replaces another element in a compound
A + BC
AC + B (a metal replaces a cation)
AB + C
AC + B (a nonmetal replaces an anion)
Examples:
• Mg(s) + Zn(NO3)2(aq)
Mg(NO3)2(aq) + Zn(s)
• 2K(s) + 2H2O(l)
2KOH(aq) + H2(g)
You must follow the rules!
NO REACTION
The Activity Series of Metals
A list of metals in order of decreasing reactivity.
A reactive metal will replace any of the metals listed
below it in the series.
Examples:
• Mg(s) + AgNO3(aq)
• Mg(s) + LiNO3(aq)
Mg(NO3)2(aq) + Ag(s)
No Reaction
Halogen Activity Series
Double Replacement
An exchange of cations between two reacting compounds. This type of
reaction typically occurs between 2 ionic compounds in aqueous solution.
AB + CD
AD + CB
Example:
AgNO3(aq) + KCl(aq)
AgCl(s) + KNO3(aq)
3 Types of Double Replacement Reactions
1. Precipitation Reaction
-produces a precipitate (formation of a solid)
-Example: K2CO3(aq) + BaCl2(aq)
2KCl(aq) + BaCO3(s)
2. Gas Production
-produces a gas
-Example: 2NaCN(aq) + H2SO4(aq)
2HCN(g) + Na2SO4(aq)
3. Neutralization Reaction
-produces water and salt
-Example: Ca(OH)2(aq) + 2HCl(aq)
CaCl2(aq) + 2H2O(l)
Oxidation Reduction
(REDOX) Reactions
Any reaction in which the oxidation states of atoms change
Oxidation=loss of electron(s)
Reduction=gain of electron(s)
0
0
+3 -2
Example: 4Al (s) + 3O2 (g) 2Al2O3 (s)
0
+3
4Al
4Al + 12 e-
6O + 12
e-
0
-2
6O
Al was oxidized
O was reduced
Assigning Oxidation States
(aka Oxidation Number)
Hypothetical charge use to indicate the degree of oxidation (loss of electrons)
Rules in assigning oxidation states:
1) The oxidation state of a free element is zero (0). ex. O2 (g), Ag (s)
2) The oxidation state of a monatomic ion is equal to its ionic charge. (ex. Na+, Cl-3)
3) H has an oxidation of +1 and oxygen has an oxidation state of -2 when they are
present in most compounds. (Exceptions: Hydrogen is -1 in hydrides of metals such as
LiH, and Oxygen is -1 in peroxides such as H2O2.)
4) The sum of all oxidation states in a neutral molecule must be zero, and the sum of
the oxidation states in an ion must be equal to the ion’s overall charge.
Writing and Balancing Chemical
Equations:
Step 1) Determine the reactants, products and the physical states.
Step 2) Write the unbalanced equation to summarize the word equation.
Step 3) Use coefficients to balance each and every element EXCEPT H and O.
Step 4) Use coefficients to balance the Hydrogens.
Step 5) Use coefficients to balance the Oxygens.
Example #1
Chlorine gas and potassium iodide solution react together to form potassium
chloride and iodine gas.
Write the equation:
Balance the equation:
Cl2 (g) + KI(aq)
KCl(s) + I2(g)
Cl2 (g) + 2KI(aq)
2KCl(s) + I2 (g)
Example #2
Aluminum hydroxide solution is heated and decomposed into a solid aluminum
oxide and liquid water.
Write the equation:
Al(OH)3(aq)
Balance the equation 2Al(OH)3(aq)
Al2O3(s) + H2O(l)
Al2O3(s) + 3H2O(l)
Example #3
Zinc and Lead (II) nitrate react to form zinc nitrate and Lead.
Zn(s) + Pb(NO3)2(s)
Zn(NO3)2(s) + Pb(s)
Predicting Products while considering the
Solubility Rules:
• Example:
K2CrO4(s) + Ba(NO3)2(s)
KNO3(s) + BaCrO4(s)