Topic 3: Redox

Question 1

What does the oxidation number tell you?

Answer 1

• oxidation numbers or oxidation state tell you how many electrons an atom has donated or accepted to form an ion or form part of a compound

Question 2

What are the rules to assign oxidation numbers?

Answer 2

§ All uncombined elements have an oxidation number of 0

• this means they haven’t accepted or donated any electrons
• elements bonded to identical atoms will also have oxidation numbers of 0
• eg. any uncombined element
• O2 or H2

§ The oxidation number of a simple monatomic ion is the same as its charge

• Na+ has an oxidation number of +1
• Mg2+ has oxidation state of +2

§ For molecular ions, the sum of the oxidation numbers is the same as the overall charge of the ion

• each of the constituent atoms will have an o.n of their own and the sum equals overall charge
  e. g. SO4 2- : -8 (for oxygen) +6 = -2

§ For a neutral compound, overall charge is 0

§ Hydrogen always has an oxidation number of +1, except in metal hydrides (MHx) where its -1 and in molecular hydrogen (H2) where it is 0

§ Oxygen nearly away has an oxidation number of -2, except in peroxides (O2 2-) where it is -1, and molecular oxygen where it is 0

Question 3

What do Roman numerals show?

Answer 3

• tells you the oxidation number
• if an element can have multiple oxidation numbers, or it isn’t a normal oxidant number
• the numerals are written after the name of the element they correspond to

Question 4

Explain the roman numerals in ions that contain oxygen, so end with -ate

Answer 4

• the other element in the -ate ion can exist in different oxidation numbers, so it is attached as a Roman numeral after the number of the -ate compound
• correspond to the non-oxygen element

Question 5

What is oxidation?

Answer 5

• a loss of electrons

Question 6

What is reduction?

Answer 6

• a gain in electrons

Question 7

What is a redox reaction?

Answer 7

• reduction and oxidation happen simultaneously

Question 8

What is an oxidising agent?

Answer 8

• accept electrons

- gets reduced

Question 9

What is a reducing agent?

Answer 9

• donates electrons

- gets oxidised

Question 10

What happened if the oxidation number has increased after there reaction?

Answer 10

• element has lost electrons

- been oxidised

Question 11

What happened if the oxidation number has decreased after the reaction?

Answer 11

• element has gained electrons

- been reduced

Question 12

Identify the reducing and oxidising agents in this reaction:

4Fe + 3O2 = 2FeO3

Answer 12

• iron has gone from having an oxidation number of 0 to a number of 3+
• lost electrons and has been oxidised
• reducing agent
• oxygen has gone from an oxidation number of 0 to an oxidation number of -2
• gained electrons and reduced
• oxidising agent

Question 13

What is a disproportionation reaction?

Answer 13

• a special redox reaction

- an element in a single species is simultaneously reduced and oxidised

Question 14

What do ionic half equations show?

Answer 14

• they show oxidation and reduction

- you can show electrons being lost or gained in a half equation

Question 15

What does combining half equations for different oxidising and reducing agents do?

Answer 15

• makes full equations for redox reactions

Question 16

Why do you have to add some H+ ions and H2O to half equations sometimes?

Answer 16

• you can’t balance the equation by just multiplying reactant and products