Types of Reactions That Occur in Aqueous Solutions

TYPES OF REACTIONS THAT OCCUR IN AQUEOUS SOLUTIONS

1. Precipitation

When two solutions are mixed and an insoluble substance forms that settles out. The solid that forms is called a precipitate. Solubility rules can be used to identify the precipitate.

Example: Aqueous solutions of silver nitrate and potassium chloride are mixed.

AgNO3(aq) + KCl(aq)  AgCl(s) + KNO3(aq)

A white precipitate of silver chloride would settle out of this reaction.

2. Acid-Base

When an acid (which has H+ as its cation) and a base (which has OH- as its anion) react to form a salt (ionic compound) and water. These reactions are also called neutralizations because acids with pH of less than 7 and bases with pH greater than 7 react to form water with a pH of 7.

Example: Sodium hydroxide and hydrochloric acid combine in a neutralization reaction.

NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)

Both precipitation and acid-base are types of double replacement reactions.

Synthesis, decomposition, single replacement and combustion can also be described by a larger category of oxidation-reduction (also known as redox) reactions.

3. OXIDATION-REDUCTION REACTIONS involve electron transfer The atom that is oxidized loses electrons (LEO – losing electrons is oxidation). The atom that is reduced gains electrons (GER – gaining electrons is reduction). For example in the synthesis of magnesium and sulfur to make magnesium sulfide, magnesium is oxidized and sulfur is reduced. Both reactants start off as uncharged atoms, the magnesium loses electrons to become a 2+ ion and the sulfur gains electrons to become a 2- ion. Mg(s) + S(s)  MgS(s)

Rules for Assigning Oxidation States

1. The oxidation state of an atom in an element is ZERO including allotropes [i.e. N2, P4, S8]. 2. The oxidation state of a monatomic ion is the same as its charge. 3. In its compounds, fluorine is always assigned an oxidation state of -1. 4. Oxygen is usually assigned an oxidation state of -2 in its covalent compounds, such

as CO, CO2, SO2, and SO3. Exceptions to this rule include peroxides (compounds 2- containing the O2 group), where each oxygen is assigned an oxidation state of -1,

as in hydrogen peroxide (H2O2), and OF2 in which oxygen is assigned a +2 oxidation state. 5. In its covalent compounds with nonmetals, hydrogen is assigned an oxidation state of +1. Metal hydrides are an exception; H is at the end of the chemical formula since it has an oxidation state of 1-. 6. The sum of the oxidation states must be zero for an electrically neutral compound. For a polyatomic ion, the sum of the oxidation states must equal the charge of the ion. There are many chemical reactions that do not fit neatly into one of these categories. Also, some of the reaction types have been simplified. However, these categories do allow us to explain many of the common chemical reactions that we encounter. WS Types of Reactions

Directions:  Identify the type of reaction: Precipitation (P), Acid-Base (A), or Oxidation-Reduction (O)  Predict the products and balance the equation.  Include physical states (use solubility rules to identify precipitates)  For the oxidation-reduction reactions identify which atom is oxidized and which atom is reduced.

1. Aluminum metal reacts with chlorine gas (the product is a solid)

2. Solutions of acetic acid and potassium hydroxide are mixed.

3. A solution of potassium carbonate is added to a solution of calcium nitrate.

4. A strip of copper is added to a solution of silver nitrate.

5. Methane gas is combusted.

6. Phosphoric acid neutralizes barium hydroxide.

7. Electricity is applied to molten sodium chloride until it decomposes.

8. Solutions of manganese(II) sulfate and ammonium sulfide are mixed.

9. Zinc metal is added to a solution of copper(II) chloride.

10. Solutions of silver nitrate and sodium chromate are mixed.