Addition and Decomposition Reactions
A reaction in which a single compound breaks apart into two or more substances is called a decomposition reaction. The general equation for decomposition reaction is:
An oxidation state, also called an oxidation number, is a hypothetical charge assigned to an atom, ion, or polyatomic ion, indicating how many electrons have been lost (or gained). Chemical reactions all involve electrons. However, in a lot of chemical reactions, the oxidation states of atoms do not change. Chemical reactions in which an oxidation state of one or more atoms change are called oxidation-reduction reactions or redox reactions.Oxidation-reduction reactions form the basis of electrochemistry, the branch of chemistry that studies batteries and other power cells. Combustion, burning a substance (often called fuel) with an oxidizer (often oxygen), is also a type of oxidation-reduction reaction. Consider the burning of methane (CH4) with oxygen gas (O2) to produce carbon dioxide (CO2) and water (H2O).
Oxidation States in
Overall, carbon's oxidation state went from –4 to +4. Hydrogen's oxidation state did not change and stayed +1. Oxygen's oxidation went from zero to –2.
Oxidation is a reaction that involves the removal of an electron from an atom. The carbon in methane in the example is oxidized. The opposite of oxidation is reduction. Reduction is a reaction that involves the addition of an electron to an atom. Oxygen gas in the example is reduced.
An oxidizing agent causes oxidation of another substance by stripping electrons from it, and it is reduced in the process. In the example, oxygen gas is the oxidizing agent. A reducing agent causes the reduction of another substance by giving it electrons, and it is oxidized in the process. In the example, carbon in methane is the reducing agent.
Oxidation always occurs with reduction. When an atom is oxidized, another must be reduced. The name oxidation comes from oxygen because oxygen is a common and strong oxidizing agent. Oxygen typically causes oxidation in other compounds and gets reduced in turn. There are other oxidizing agents, and some are stronger than oxygen. Fluorine, for example, is the most electronegative element and can cause oxidation in oxygen.
Oxidation-reduction reactions, such as combustion, can release energy as heat. It is possible to set up an oxidation-reduction reaction so that it releases energy as electricity. An oxidation-reduction reaction involves a transfer of electrons. If the reaction is separated into two parts, the electron transfer can occur over a wire. A flow of electrons causes electricity. Setting up an oxidation-reduction reaction this way involves setting it up as two half-reactions. A half-reaction is either the oxidation or the reduction part of an oxidation-reduction reaction.Consider the reaction between magnesium (Mg) and copper oxide (CuO):
It is possible to write the two half-reactions as:
Neutralization (Acid-Base) Reactions
The Arrhenius definition of acids and bases, proposed by Swedish chemist Svante Arrhenius, characterizes acids and bases depending on the ions produced by each in aqueous solution. An acid is a substance that produces hydrogen (H+) in solution, whereas a base produces hydroxide (OH–) ions in solution.
A neutralization reaction is a reaction between an acid and a base. Acid-base reactions produce a salt and water.The general equation is:
For example, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), forming sodium chloride (NaCl) and water (H2O).
Another example is the reaction of sulfuric acid (H2SO4) and potassium hydroxide (KOH), forming potassium sulfate (K2SO4) and water.
A third example is the reaction of nitric acid (HNO3) with sodium hydroxide, forming sodium nitrate (NaNO3) and water.
Single- and Double-Displacement Reactions
Another example is when hydrogen iodide (HI) reacts with chlorine gas (Cl2), forming hydrogen chloride (HCl) and iodine (I2)
When two reactants exchange atoms or groups, such a reaction is called a double-displacement reaction. These reactions usually take place in an aqueous state. The general form of this reaction is:
Solubility and Precipitate Formation
Double-displacement reactions do not always result in precipitates. Precipitation reactions are closely connected to solubility, the maximum amount of a substance that can be dissolved in another substance at specific conditions. A substance or dissolved material in a solution is called a solute. A substance that dissolves a material to form a solution is called a solvent. Water is a very common solvent. Solubility depends on the solute, on the solvent, and on conditions such as pressure and temperature.
A substance that has high solubility in a solvent under specific conditions is said to be soluble. Some substances are insoluble, which means they are incapable of being dissolved in another substance.For example, when silver nitrate (AgNO3) and sodium iodide (NaI) react with each other in aqueous forms, insoluble silver iodide (AgI) is formed as a precipitate.
Chemists have come up with a set of rules to determine if a salt is soluble in water or not. To predict if a precipitate will form, consider the solubility rules for salts in water. If two rules appear to contradict each other, the rule with the lower number takes precedence.
1. Group 1 element salts are soluble.
2. Salts with nitrate ions are soluble.
3. Salts of chloride, bromide, and iodide are soluble except for silver, lead, and mercury halides.4. All silver salts are insoluble, except AgNO3 and a few other rare exceptions.
5. Sulfate salts are soluble except for calcium, barium, lead, strontium, and silver salts.
6. Hydroxide salts of Group 1 elements are soluble. Hydroxide salts of Group 2 elements (calcium, strontium, and barium) are partially soluble. Hydroxide salts of transition metals are insoluble.
7. Sulfides of transition metals are insoluble.
8. Carbonates, phosphates, fluorides, and chromates are insoluble.
There are exceptions to these rules. However, one can predict precipitate formation based on the rules with fair success.
Another example is the reaction of calcium (Ca) with fluorine gas (F2) to form calcium fluoride (CaF2).
Zinc (Zn) reacts with hydrochloric acid (HCl) to form zinc chloride (ZnCl2) and hydrogen gas (H2).
Sodium hydroxide (NaOH), a strong base, reacts with hydrochloric acid (HCl) and forms sodium chloride (NaCl) and water (H2O).