Formulas and Equations

Chemical Formula - A chemical formula is a shorthand method of describing compounds.

• It uses the atomic symbols in the periodic table to identify the elements in a compound.

• If there is more than one atom of an element in the formula, a subscript is used to show how many atoms are present.

• A subscript placed after a closing parenthesis multiplies everything within the parentheses.

• Parentheses in chemical formulas are used to clarify and to provide additional information. They show that the atoms within the parentheses are chemically connected in a way separately from the rest of the formula. Or the atoms within the parentheses could be a polyatomic ion.

Hydrates - Hydrates are compounds that have a fixed number of water molecules in their crystal lattices.

• Water of Hydration - The water of hydration refers to the “fixed number of water molecules“ within the crystal lattice of a hydrate.

• To show the water of hydration clearly in the chemical formula, it is written after a dot that is placed in the middle of the line. This dot links two separate compounds into one unit.

• The names of hydrates will always contain a prefix in front of “hydrate“ to indicate the number of water molecules in the formula. Common prefixes can be seen below

Empirical Formula - The formula representing the simplest ratio of the atoms in a crystal of a substance is referred to as its empirical formula.

• The formulas for all ionic compounds are empirical formulas.

Molecular Formula - Molecular formulas represent the actual number of each atom present in a single molecule of a compound.

• A formula written this way is called a condensed formula.

Structural Formula - A structural formula shows a chemist the way the atoms are connected with each other and the covalent bonds between the atoms.

Chemical Equation - A chemical equation is written to describe the reaction process of a chemical reaction in which reactants are converted into products.

• The formulas of the reactants are placed on the left side, and the products on the rightsize, of an arrow that indicates that the reactants are being converted into products.

• REACTANTS → PRODUCTS

Substances participating in chemical reactions or physical changes are often in one of the three states of matter, solid (s), liquid (l), or gas (g).

• The symbol (aq) is used to indicate that a substance is dissolved in an aqueous solution.

• These symbols are only used when the state of the substance is significant in the context of the reaction.

Chemical equations must be balanced with the same number of each atom on both sides of the arrow.

• A balanced chemical equation satisfies the law of conservation of matter.

• Equations are balanced by placing the appropriate coefficients in front of the formulas of the reactants and products in order to equalize the atoms on both sides of the arrow.

• Neither the formulas of compounds nor their subscripts are altered to balance an equation.

• There are two methods for determining the coefficients needed to balance an equation: the inspection method and the ion-electron method, the latter of which is used for complex oxidation-reduction equations.

• To balance an equation using the inspection method,

  • Count the number of each atom present in each side of the equation.

  • Balance one atom at a time by adding a coefficient where needed and recount the atoms.

  • Repeat until the same number of each atom is present on both sides of the arrow.

  • The process is easier if one

    1. balances the most complex equation first.

    2. leaves elements that appear in more than one compound on either side till the end.

    3. balances a group of atoms as if they were an individual atom.

• Properly balanced equations have the smallest whole number coefficients possible.

Types of Reactions

Combustion Reactions - In these reactions, an organic (carbon-containing) compound reacts with oxygen to form carbon dioxide and water.

• If the organic compound contains elements than carbon, then it is assumed that those elements end up in the elemental state as products.

Single-Replacement Reactions - In some reactions, an element may react with a compound to produce a different element and a new compound.

• The element essentially replaces one of the elements in the compound.

Double-Replacement Reactions - In these reactions, two compounds react and the cation in one compound replaces the cation in the second compound and vise versa.

Neutralization Reactions - These are a special type of double-replacement reaction in which one reactant is an acid and the other is a base, while the products end up being a salt and water.

Synthesis Reactions - Reactions of two or more elements to form a compound are often called synthesis reactions.

Formation Reactions - A formation reaction is the same as a synthesis reaction except that the product must have a coefficient of one.

Addition Reaction - In these reactions, a simple molecule or an element is just added to another molecule.

Decomposition Reaction - These reactions result when a large molecule decomposes into its elements or into smaller molecules.

Net Ionic Equations - The net ionic equation is the chemical equation that shows only those elements, compounds, and ions that are directly involved in the chemical reaction.

• When ionic compounds react in an aqueous solution, usually only one ion from each compound reacts. The other ions are spectator ions and do not react.

• Writing an equation in ionic form focuses attention on the actual reaction and allows the chemist to find substitute reactants to achieve the same result.

• In ionic reactions the charges must balance as well as the atoms.

Half-reaction Equations - These reactions are used extensively with oxidation-reduction reactions and in describing electrochemical processes.

• The half-reaction is a reduction reaction if electrons are on the reactant side and an oxidation reaction if the electrons are products.

• Half-reactions may be combined to make a complete oxidation-reduction reaction as long as the electrons all cancel.

Oxidation-reduction Reactions - These reactions involve the loss of electrons by one compound or ion and the subsequent gain of the same electrons by another compound or ion.

Bonding

Covalent Bond - When two atoms share valence electrons to form a bond, the bond is known as a covalent bond.

Ionic Bond - When one atom loses electrons and another gains electrons, ions are formed and the attraction between them causes them to form a compound.

Bond formation is dependent on the behavior of the valence electrons.

Polyatomic Ions - Polyatomic ions are unusually stable groups off atoms that tend to act as single units in many chemical reactions.

• Many elements combine with oxygen, and sometimes hydrogen and nitrogen, to form a charged group of atoms called a polyatomic ion.

• All polyatomic ions, except for ammonium, are anions.

• The atoms in a polyatomic ion are bound to each other with covalent bonds.

• When a subscript must be used with a polyatomic ion, it is necessary to place parentheses around the polyatomic ion before adding the subscript.

Law of electroneutrality - the total positive charge of the cations must be exactly canceled by the negative charge of the anions in the chemical formula.

Chemical Driving Forces - Chemists rely on three fundamental principles to make an educated guess about the possibility for a reaction to occur in a double-repalcement reaction.

1. Formation of water

2. Formation of a precipitate

3. Formation of a nonionic (covalent) compound from ionic reactants