Redox

Question 1

What is oxidation?

Answer 1

Loss of electrons, increase in oxidation number, gain of oxygen, loss of hydrogen atom

Older definition includes gain of oxygen.

Question 2

What is reduction?

Answer 2

Gain of electrons, decrease in oxidation number, loss of oxygen and gain of hydrogen atom

Question 3

What mnemonic can help remember the definitions of oxidation and reduction?

Answer 3

OIL RIG – Oxidation Is Loss, Reduction Is Gain (of electrons)

Question 4

What is the oxidation number of an elemental form?

Answer 4

0

Question 5

What is the oxidation number of a simple ion?

Answer 5

Equal to its charge

Question 6

What is the oxidation number for Group 1 elements?

Answer 6

+1

Question 7

What is the oxidation number for Group 2 elements?

Answer 7

+2

Question 8

What is the oxidation number of hydrogen in most compounds?

Answer 8

+1 (except metal hydrides: -1)

Question 9

What is the oxidation number of oxygen in most compounds?

Answer 9

-2 (except peroxides: -1, OF₂: +2)

Question 10

What is the oxidation number of fluorine?

Answer 10

-1

Question 11

What must the sum of oxidation numbers equal?

Answer 11

Overall charge

Question 12

What indicates a redox reaction?

Answer 12

Change in oxidation number indicates electron transfer

Question 13

What is the first step in writing half equations?

Answer 13

Balance the atoms (excluding H and O)

Question 14

How do you balance oxygen in half equations?

Answer 14

Using H₂O

Question 15

How do you balance hydrogen in half equations?

Answer 15

Using H⁺

Question 16

How do you balance charge in half equations?

Answer 16

Using e⁻

Question 17

What is an example of a half equation for MnO₄⁻ to Mn²⁺ in acidic solution?

Answer 17

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Question 18

What does combining half equations involve?

Answer 18

Multiplying half-equations to equalise electrons and adding/canceling species

Question 19

What is disproportionation?

Answer 19

A single species is both oxidised and reduced

Question 20

Provide an example of disproportionation.

Answer 20

Cl₂ + H₂O → HCl + HClO

Question 21

What is the key system used in redox titrations?

Answer 21

MnO₄⁻ used to oxidise Fe²⁺

Question 22

What is the end-point indicator in redox titrations with MnO₄⁻?

Answer 22

First permanent pink

Question 23

What is a key feature of KMnO₄ in titrations?

Answer 23

It is self-indicating

Question 24

What are the steps to balance redox equations in alkaline solutions?

Answer 24

Same as acidic, then add OH⁻ to both sides

Question 25

What colour change occurs in the Fe²⁺/Fe³⁺ system?

Answer 25

Green to orange

Question 26

What colour change occurs in the MnO₄⁻/Mn²⁺ system?

Answer 26

Purple to colourless

Question 27

What colour change occurs in the Cr₂O₇²⁻/Cr³⁺ system?

Answer 27

Orange to green

Question 28

What colour change occurs in the I₂/2I⁻ system?

Answer 28

Brown to colourless (or blue-black if starch is added)

Question 29

What should you always state clearly in exam techniques?

Answer 29

Oxidation numbers and changes

Question 30

What is important to use correctly in context during exams?

Answer 30

Oxidised and reduced

Question 31

What type of equations should be written where asked?

Answer 31

Full ionic equations

Question 32

What should be mentioned when calculating concentrations from titrations?

Answer 32

Full mole ratio and show working with units

Question 33

What should be checked in past papers concerning state symbols?

Answer 33

If the mark scheme requires it