Oxidation-Reduction Reactions (College Board AP® Chemistry)

Identifying Oxidation-Reduction Reactions

• Oxidation-reduction or redox reactions involve the transfer of electrons from one reactant to another

  • When a substance loses electrons is called oxidation

  • When a substance gain electrons is called reduction

• The main principle of redox reactions states that if one reactant lose electrons, another reactant must gain them

• Corrosion, formation of rust or combustion are common oxidation reactions

Formation of oxides

• One of the easiest examples to analyze is the reaction between calcium and oxygen gas to produce calcium oxide (CaO)

• The balanced equation and the full ionic equation for the reaction are shown below

  • Ca and O₂ have a charge of zero because they are in their elemental forms

Balanced equation

2Ca + O₂ → 2CaO

Full ionic equation

2Ca⁰ + O₂⁰ → 2Ca²⁺ + 2O^2–

• Ca⁰ is transformed into Ca²⁺. Since, calcium has acquired a positive charge it means it has lost electrons (electrons are negative particles)

  • Each calcium atom has lost 2 electrons, Ca⁰ was oxidized

• O₂⁰ is transformed into O^2–. Since, oxygen has acquired a negative charge it has gained electrons (electrons are negative particles)

  • Each oxygen atom has gained 2 electrons, O₂⁰ was reduced

• Note that the sum of the charges on the left side of the ionic equation and the right side of the equation is 0

  • Left: 0+0 = 0

  • Right: 2(2+)+2(2-) = 0

Single replacement reactions

• Single replacement reactions occur when one element in its elemental form replaces another element in a compound

• This replacement occurs because elements have different reactivities

• A great example is the reaction between magnesium and hydrochloric acid to form magnesium chloride and hydrogen

  • Mg and H₂ have a charge of zero because they are in their elemental forms

Molecular balanced equation

Mg + 2HCl → MgCl₂ + H₂

Full ionic equation

Mg⁰ + 2H⁺ + 2Cl^– → Mg²⁺ + 2Cl^– + H₂⁰

Net ionic equation

Mg⁰ + 2H⁺  → Mg²⁺ + H₂⁰ 

• Mg⁰ is transformed into Mg²⁺. Since, magnesium has acquired a positive charge it means it has lost electrons (electrons are negative particles)

  • Each magnesium atom have lost 2 electrons, Mg⁰ was oxidized

• 2H⁺ is transformed into H₂⁰. Since, hydrogen had his positive charge reduced to zero. This means it has gained electrons (electrons are negative particles)

  • Each hydrogen atom has gained 2 electrons, H⁺ was reduced

Combustion Reactions

• Another, common redox reaction is combustion

• A combustion occurs when a substance reacts with oxygen gas, releasing energy in the form of light

• A lot of combustion reactions involve the use of a hydrocarbon (compound made from just hydrogen and carbon) as the main source to burn

  • The products of the complete combustion of a hydrocarbon are: water (H₂O) and carbon dioxide (CO₂)

  • A complete combustion occurs when there is excess of oxygen to react with the hydrocarbon

  • E.g. The equation below show the balanced equation for the methane combustion

• Alcohols, such as the one in alcoholic beverages, can also react with oxygen to form water (H₂O) and carbon dioxide (CO₂)

  • E.g. The equation below show the balanced equation for the ethanol combustion