Chemistry - Redox Processes

Question 1

What is oxidation?

Answer 1

Gain of oxygen
Loss of hydrogen (oxygen and hydrogen are complementary)
Loss of electrons
Increase in oxidation state

Question 2

What is reduction?

Answer 2

Loss of oxygen
Gain of hydrogen
Gain of electrons
Decrease in oxidation state

Question 3

What are the rules for determining oxidation states for compounds?

Answer 3

• ALL elements will have an oxidation state of 0
• charge of ions is the oxidation state
• elements with fixed O.S include those from Group 1, Group 2, fluorine (-1)
• H is +1 except in metal hydrides (-1)
• O is -2 except in peroxides (-1) and compounds formed with only fluorides (+2)
• O.S of atoms/ions in a compound add up to 0 or its charge

Question 4

What are oxidising and reducing agents?

Answer 4

An oxidising agent can oxidise other substances but itself gets reduced.
A reducing agent can reduce other substances but itself gets oxidised.

Question 5

What is used to test for reducing agents and what is a positive result?

Answer 5

Acidified potassium manganate(VII) is used to test for reducing agents. (manganate(VII) ions are reduced to manganese(II) ions) → manganate(VII) is a purple solution, the latter is colourless (in a positive test)

Question 6

What is used to test for oxidising agents and what is a positive result?

Answer 6

Potassium iodide can be used to test for oxidising agents. Iodide ions are oxidised to iodine by the oxidising agent. Turns from colourless to (usually) brown/pale yellow in a positive test

Question 7

What are rules for redox half-equations and the full balanced equation?

Answer 7

Rules for oxidation and reduction half-equations:
- Balance all elements except O and H
- Balance O by adding H₂O
- Balance H by adding H+ ions
- Balance the ionic charge by adding e-

For a balanced redox equation:
- Combine both half-equations, balancing out the electrons by multiplying either/both sides and then cancelling them out
- Cancel out repeated compounds/elements