Inorganic Chemistry I Reduction-Oxidation (REDOX) Kun Sri Budiasih

Oxidation-Reduction Reactions Reactions that involve the transfer of electrons are called oxidation-reduction or redox reactions Oxidation is the loss of electrons by a reactant Reduction is the gain of electrons by a reactant

Oxidation and reduction always occur together The total number of electrons lost by one substance is the same as the total number of electrons gained by the other For a redox reaction to occur, something must accept the electrons that are lost by another substance The substance that accepts the electrons is called the oxidizing agent

The substance that lost the electrons is called the reduction agent Note that the oxidizing agent is reduced and the reducing agent is oxidized For example: 2 Na + Cl 2  2 NaCl –Na is the reducing agent because it lost electrons and was oxidized –Cl 2 is the oxidizing agent because it gained electrons and was reduced

Oxidation numbers provide a way to keep track of electron transfers : 1)The oxidation number of any free element is zero. 2)The oxidation number of any simple, monoatomic ion is equal to the charge on the ion. 3)The sum of all oxidation numbers of the atoms in a molecule or polyatomic ion must equal the charge on the particle. 4)In its compounds, fluorine has an oxidation number of –1. 5)In its compounds, hydrogen has an oxidation number of +1. 6)In its compounds, oxygen has an oxidation number of –2.

If there is a conflict between two rules apply the rule with the lower number and ignore the conflicting rule In binary ionic compounds with metals, the nonmetals have oxidation numbers equal to the charges on their anions Example: What is the oxidation number of Fe in Fe 2 O 3 ? ANALYSIS: This binary compound is ionic. Apply rule 3 and 6

Fe: 2x O: 3(-2) = -6 0 = 2x + (-6) or x = +3 = ox. number of Fe Note that fractional values of oxidation numbers are allowed In terms of oxidation numbers: –Oxidation is an increase in oxidation number –Reduction is a decrease in oxidation number This provides a simple way to follow redox reactions

Many redox reactions take place in aqueous solution A procedure called the ion-electron method provides a way to balance these equations The oxidation and reduction are divided into equations called half-reactions The half-reactions are balanced separately, then combined into the fully balanced net ionic equation

Both mass and charge must be balanced Charge is balanced by adding electrons to the side of the equation that is more positive or less negative Example: Balance the following skeleton equation

Many reactions occur in either acidic or basic solutions The Ion-Electron Method in Acidic Solution: 1)Divide the equation into two half-reactions. 2)Balance atoms other than H and O. 3)Balance O by adding water. 4)Balance H by adding hydrogen ion. 5)Balance net charge by adding electrons. 6)Make electron gain and loss equal: add half- reactions. 7)Cancel anything that’s the same on both sides of the equation.

The simplest way to balance reactions in basic solution is to first balance them as if they were in acidic solution, then “convert” to basic solution: Additional Steps for Basic Solutions

Example: Balance the following in basic solution:

Metals more active than hydrogen (H 2 ) dissolve in oxidizing acids

Some examples: More active metals will displace a less active metal from its compound This often occurs in solution and is called a single replacement reaction

An activity series arranges metals according to their ease of oxidation They can be used to predict reactions Zinc is a more active metal than copper. Copper ions (blue) collide with zinc metal (gray) picking up electrons. Copper ions become copper atoms (red-brown) and stick to the zinc surface. Zinc ions (yellow) replace the copper ions in solution.

Activity Series for Some Metals and Hydrogen A given element will be displaced from its compounds by any element below it in the table (See Table 6.2 for a more extensive list.)

Oxygen reacts with many substances The products depends, in part, on how much oxygen is available Combustion of hydrocarbons Organic compounds containing O also produce carbon dioxide and water

Organic compounds containing S produce sulfur dioxide Many metals corrode or tarnish when exposed to oxygen Most nonmetals react with oxygen directly Redox reactions are more complicated than most metathesis reactions

In general, it is not possible to balance a redox reaction by inspection This is especially true when acid or bases are involved in the reaction Once balanced, they can be used for stoichiometric calculations Redox titrations are common because they often involve dramatic color changes Mole-to-mole ratios are usually involved

Example: A g sample of tin ore was dissolved in acid solution converting all the tin to tin(II). In a titration, 8.08 mL of M KMnO 4 was required to oxidize the tin(II) to tin(IV). What was the percentage tin in the original sample? ANALYSIS: This is a redox titration in acidic solution. SOLUTION: Form skeleton equation and use the ion-electron method to produce a balanced equation

Use the balanced equation to define equivalence relations and determine the mass of Sn in the original sample Convert to percentage