3.1.2 Oxidation Numbers

• What are oxidation and reduction defined as in chemistry?

  Transfer of electrons
• Oxidation is defined as the loss of electrons
• In the reaction $Fe \rightarrow Fe^{2 + } +$$2e^ -$, iron is oxidized.
• What is reduction defined as in chemistry?

  Gain of electrons
• In the reaction $O +$$2e^ - \rightarrow O^{2 - }$, oxygen is reduced
• In oxidation, the oxidation number increases.
• Match the process with its definition:

  Oxidation ↔️ Loss of electrons

  Reduction ↔️ Gain of electrons
• What does the oxidation number of an atom measure?

  Electron gain or loss
• The oxidation number of an element in its uncombined state is always 0
• In compounds, oxygen always has an oxidation number of -2.

  False
• What is the oxidation number of hydrogen in NaH?

  -1
• The sum of oxidation numbers in a neutral compound is always 0
• What is the sum of oxidation numbers in $SO_{4}^{2 - }$?


  -2
• In peroxides, oxygen has an oxidation number of -1.
• Steps for determining oxidation numbers in compounds or ions

  1️⃣ Identify the oxidation numbers of known atoms

  2️⃣ Use the sum of oxidation numbers rule

  3️⃣ Solve for the unknown oxidation number
• What is the oxidation number of oxygen in $SO_{4}^{2 - }$?


  -2
• In $H_{2}O$, the oxidation number of hydrogen is +1
• The oxidation number of an element in its uncombined state is 0
• What is the oxidation number of $Na^ +$?


  +1
• Oxygen usually has an oxidation number of -2 in compounds.
• Hydrogen usually has an oxidation number of +1
• What is the sum of oxidation numbers in a neutral compound?

  0
• The sum of oxidation numbers in a polyatomic ion equals the charge of the ion.
• Order the compounds from the table based on the oxidation number of sulfur:

  1️⃣ $SO_{4}^{2 - }$

  2️⃣ $H_{2}SO_{3}$

  3️⃣ $Na_{2}SO_{3}$
• Oxidation is defined as the loss of electrons
• What is the change in oxidation number during reduction?

  Decreases
• Oxidation involves the loss of electrons, while reduction involves the gain of electrons.
• Match the process with its definition:

  Oxidation ↔️ Loss of electrons

  Reduction ↔️ Gain of electrons
• The oxidation number of oxygen in a compound is usually -2
• What is the oxidation number of iron in its uncombined state?

  0
• The oxidation number of an element in its uncombined state is always 0.
• In a neutral compound, the sum of oxidation numbers is 0
• What happens to an element's oxidation number during oxidation?

  Increases
• Reduction occurs when an element's oxidation number decreases.
• In the reaction $Fe +$$O_{2} \rightarrow Fe_{2}O_{3}$, iron undergoes oxidation
• What is the change in oxidation number of manganese in the reaction MnO_{4}^ - + Fe^{2 + } \rightarrow Mn^{2 + } + Fe^{3 + }</latex>?

  -5
• Steps to balance redox equations using oxidation numbers:

  1️⃣ Assign oxidation numbers to all atoms

  2️⃣ Identify elements undergoing oxidation and reduction

  3️⃣ Determine the change in oxidation number

  4️⃣ Balance the changes by multiplying compounds

  5️⃣ Balance remaining elements

  6️⃣ Check for balance
• In the balanced redox reaction MnO_{4}^ - + 5Fe^{2 + } \rightarrow Mn^{2 + } + 5Fe^{3 + } + 4H_{2}O</latex>, the oxygen atoms are balanced by adding water
• The balanced redox equation $8H^ +$$+$$MnO_{4}^ - +$$5Fe^{2 + } \rightarrow Mn^{2 + } +$$5Fe^{3 + } +$$4H_{2}O$ is balanced in terms of atoms and charge.