ASIGNAR NUMEROS DE OXIDACIÓN | Redox

How to Assign Oxidation Numbers in Chemistry

Introduction to Oxidation Numbers

• The lesson begins with an introduction by the instructor, Breim Blado, focusing on the assignment of oxidation numbers within molecules.

• The importance of understanding oxidation states is highlighted as a precursor to studying redox reactions.

Basic Rules for Assigning Oxidation Numbers

• Elements in their elemental form (e.g., Al, Fe, H₂, O₂) always have an oxidation number of zero.

• For monoatomic ions (e.g., Al³⁺, Fe²⁺), the oxidation number equals the charge of the ion.

• Alkali metals (Group 1 elements) have an oxidation number of +1; alkaline earth metals (Group 2 elements) have +2. Hydrogen has special cases: +1 in acids and -1 in hydrides.

• Oxygen typically has an oxidation number of -2 except in peroxides where it is -1.

Example Calculations

Sulfur Dioxide (SO₂)

• In SO₂, oxygen has an oxidation state of -2. To find sulfur's state:

• The equation set up is: S + 2(-2) = 0.

• Solving gives sulfur an oxidation number of +4.

Potassium Dichromate

• For potassium dichromate (K₂Cr₂O₇):

• Potassium has +1 and oxygen has -2.

• Setting up the equation: 2(+1) + 2(Cr) + 7(-2) = 0 leads to Cr having an oxidation state of +6.

Boron Ion Calculation

• In a boron ion with a total negative charge:

• Using similar logic with known values for oxygen (-2), we find boron's state through: B + 3(-2)= -3 leading to B being +3.

Permanganate Ion Calculation

• For permanganate (MnO₄⁻):

• Oxygen contributes a total charge of -8; thus Mn must balance this to achieve a total charge of -1 resulting in Mn having an oxidation state of +7.

Carbonic Acid Calculation

• In carbonic acid (H₂CO₃):

• With hydrogen at +1 and oxygen at -2, setting up the equation allows us to solve for carbon’s oxidation state which balances out to zero overall.

Understanding Oxidation States in Chemistry

Calculation of Oxidation States

• The discussion begins with the calculation of oxidation states, specifically focusing on carbonic acid. The oxidation state of carbon is determined to be +4.

• Moving on to ammonia (NH₃), the speaker explains that nitrogen acts as a hydride with an oxidation state of +3, derived from balancing the equation where nitrogen's charge plus three times hydrogen's charge equals zero.

• The conversation shifts to hydrogen peroxide, where it is noted that oxygen has an unusual oxidation state of -1 instead of the typical -2 found in most compounds.

• To find the total charge in hydrogen peroxide, calculations are performed showing that two times the charge of hydrogen plus two times the charge of oxygen must equal zero. This leads to determining that hydrogen has a +1 charge and oxygen maintains a -1 charge.

Importance of Mastering Oxidation States

• The speaker emphasizes the significance of understanding and practicing oxidation states for fluency in chemistry. Mastery is crucial for advancing in this field and performing calculations automatically.