AP Chemistry Chapter Four Outline
Quantities of Reactants and Products
I.
Chemical Equations

A chemical reaction is a process in which reactants are converted into products
Reactants → Products

A balanced chemical equation indicates the relative amounts of reactants and products so that the number of atoms of each element in the reactants equals the number of atoms of the same element in the products.

Symbols are used to identify the physical states of reactants and products: (s) = solid, ( l )= Liquid, (g) = gas, and (aq) = aqueous, which is a substance dissolved in water

Stoichiometry is the relationship between the masses of chemical reactants and products
II.
Patterns of Chemical Reactions
A.
Types of Reactions
1.
Combination, or Synthesis Reactions : Two or more substances react to form a single product
A + B → AB
The halogens and oxygen, due to their high reactivity, undergo synthesis reactions with most elements
2.
Decomposition Reactions : One substance decomposes (breaks apart) to form two or more products
AB → A + B
Many stable compounds will decompose when heated
Many carbonates decompose to give oxides plus carbon dioxide
CaCO
3
(s) + heat → CaO (s) + CO
2
(g)
3.
Displacement Reactions : one element reacts with a compound to form a new compound and release a different element
A + BC → AC + B
In these reactions one element “switches places” with another, so these are also called single replacement reactions
4.
Exchange Reactions : there is an interchange of partners between compounds, a double replacement
AB + CD → AD + CB
5.
Combustion Reactions : an element or compound burns in air or oxygen. The products of the complete combustion of hydrocarbons are always carbon dioxide and water
4CH
3
(g) +7 O
2
(g) → 4CO
2
(g) + 6H
2
O (g)
III.
Balancing Chemical Reactions


Balancing a chemical equation means using coefficients so that the same number of atoms of each element appears on each side of the equation

To balance a reaction do the following:
1.
Write an unbalanced equation containing the correct formulas of the reactants and products
2.
Balance atoms of one of the elements
3.
Balance atoms of the remaining elemens
4.
Verify that the number of atoms of each element is balanced.

How do you know which element to start with?
1. Check for Diatomic Molecules - H
2
- N
2
- O
2
- F
2
- Cl
2
- Br
2
- I
2
If these elements appear by themselves in an equation, they must be written with the subscript
2
2. Balance Metals
3. Balance Nonmetals
Balance equations by changing coefficients never by changing subscripts
4. Balance Oxygen
5. Balance Hydrogen
6. Recount All Atoms
If the atoms are not balanced at this point, there is a problem somewhere. Work your way back up the steps, from bottom to top , until you find the problem, and correct it.
7. If every coefficient will reduce, rewrite in the simplest whole-number ratio.
An equation is not properly balanced if the coefficients are not written in their lowest whole-number ratio.