PHYSICAL CHEM - REDOX REACTIONS

Question 1

What is reduction?

Answer 1

• loss of O2
• gain of electrons

Question 2

What is oxidation?

Answer 2

• gain of O2
• loss of electrons

Question 3

What does a reducing agent do?

Answer 3

donates electrons

Question 4

What does an oxidising agent do?

Answer 4

accepts electrons

Question 5

What is the oxidation state of an atom?

Answer 5

the total no. of electrons that an atom has donated/accepted

Question 6

What are the rules for working out oxidation states?

Answer 6

• uncombined elements have an oxidation state of 0
• elements only bonded to identical atoms (eg: O2) have an oxidation state of 0
• the oxidation state of a monatomic ion (eg: Na^+) is the same as its charge
• in compound ions the overall oxidation state is its charge
• sum of oxidation states for neutral compounds is 0
• combined O’s oxidation state is nearly always -2 (except for in peroxides)
• combined H’s oxidation state is almost always +1 (except for in metal hydrides where its -1)

Question 7

What are the steps of writing a half equation?

Answer 7

• calculate oxidation states on each side of the equation
• balance the element changing oxidation state
• sort out electrons (if oxidation state becomes more -ve it gains electrons, if it becomes more +ve it loses electrons)
• sort out Os (for every O gained/lost add/remove 1 H2O molecule)
• sort out Hs (for every H gained/lost add/remove 1 H^+ ion)
• check if total charge on the left = the total charge on the right

Question 8

How do you combine half equations?

Answer 8

• add them together
• cancel out electrons
• cancel out any H^+ and/or H2Os