≫3.1 - Oxidation, Reduction and Redox ✔

Question 1

What is the definition of oxidation number?

Answer 1

• The hypothetical charge on an atom assuming that the bonding is ionic.

Question 2

What four things does oxidation [O] mean?

Answer 2

• Gain of O₂
• Loss of H
• Loss of e⁻
• Increase in oxidation number.

Question 3

What four things does reduction [H] mean?

Answer 3

• Loss of O₂
• Gain of H
• Gain of e⁻
• Decrease in oxidation number.

Question 4

Why are reactions called redox reactions?

Answer 4

• Oxidation and reduction occur simultaneously in a reaction because one species loses electrons which can then be donated and gained by the other species.
• Reduction - oxidation.

Question 5

List the rules for oxidation numbers:

Answer 5

• The more electronegative element is always negative.
• Oxidation state of an element is zero.
• Oxidation states in a neutral compound add up to zero.
• Oxidation states in a charged compound add up to the total charge.
• Hydrogen has an oxidation state of +1 (in H⁻ it is -1.)
• Oxygen has an oxidation state of -2 (in H₂O₂ it is -1.)
• All halogens have an oxidation state of -1.
• Metals have a value equivalent to their charge.
• Charge on a non-metal may be equivalent to the rule: 8 - group number.

Question 6

What does an oxidising agent do?

Answer 6

• Accepts electrons from the species that is being oxidised.
• Therefore it gains electrons and is reduced.
• This is seen as a reduction in oxidation number.

Question 7

What does a reducing agent do?

Answer 7

• Donates electrons to the species being reduced.
• Therefore it loses electrons and is oxidised.
• This is seen as an increase in oxidation number.

Question 8

What are half equations used to show?

Answer 8

• Used to show the separate oxidation and reduction reactions that occur in a redox reaction.

Question 9

Give the 4 step method for writing/balancing half equations:

Answer 9

⓵Balance all species excluding oxygen and hydrogen.
⓶Balance oxygen using H₂O (add water to the equation.)
⓷Balance hydrogen using H⁺ ions.
⓸Balance charges using electrons.

Question 10

What do reducing and oxidising agents cause?

Answer 10

• Oxidising agents cause reduction.
  -Oxidising agents are normally a non-metal or positive ion.
• Reducing agents cause oxidation.
  -Reducing agents are normally a metal or negative ion.

Question 11

When combining half equations to determine the overall redox reaction what must be the same and how do you do this?

Answer 11

• The number of electrons must be the same for both half equations.
• This can be done by scaling up the number of moles.

Question 12

What is a disproportionation reaction and give an example:

Answer 12

• Disproportionation reaction is where one molecule acts as both the oxidising and reducing agent.
• Cl₂ + H₂O → HCl + HOCl

Question 13

What side of the half equation do electrons go on for oxidation and reduction equations?

Answer 13

• For oxidation equations the electrons go on the right hand side.
• For reduction equations the electrons go on the left hand side.

Question 14

Write the half equation for the conversion of SO₂ into SO₄²⁻

Answer 14

• SO₂ + 2H₂O → SO₄²⁻ + 2e⁻ + 4H⁺