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+ + 2O2–
- 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 O2–. 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
Exam Tip
In ionic equations, charges need to be balanced as well as the number of atoms and molecules
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
Exam Tip
If all the chemical species appear with the same charges on both sides of the full ionic equation, it is NOT a redox reaction