ch4: redox reactions

Question 1

what is the transfer of electrons in redox reactions?

Answer 1

• reactants lose electrons in a process called oxidation
• reactants gain electrons in a process called reduction

Question 2

writing full equations from half equations of redox reactions

Answer 2

KOHES: balance the key element, balance the oxygens with H20, balance the hydrogens with H+ ions, look at the overall charge and balance the charge with electrons and add states.

Question 3

what are oxidising agents (oxidants) and reducing agents (reductants)

Answer 3

oxidising agents
- causes oxidation to occur, but is reduced
reducing agents
- causes reduction to occur, but is oxidised

Question 4

oxidation number rules

Answer 4

1. oxidation number of a free element is always 0 (eg Ox(Na)=0)
2. oxidation number of a simple ion is equal to the charge of the ion (eg Ox(Al3+)=+3)
3. exceptions: metals have an oxidation number equal to their charge on their ions, hydrogen is +1 in compounds with nonmetals but -1 in metal hydrides, oxygen’s ox is -2 except in peroxides when it is -1.
4. sum of oxidation numbers in a neutral compound is 0.
5. sum of the oxidation numbers in a polyatomic ions equal to the charge on the ion.

Question 5

what does a change in oxidation number indicate?

Answer 5

• increase in oxidation number indicates oxidation
• decrease in oxidation number indicates reduction

Question 6

what is a conjugate redox pair?

Answer 6

a conjugate redox pair is a pair that consists of a reducing agent and an oxidising agent. For example:
Zn(s) -> Zn2+ + 2e-
Zn is the reducing agent, and Zn2+ is the oxidising agent. Together, they make a conjugate redox pair.

Question 7

how to balance half equations in alkaline conditions?

Answer 7

balance like normal (KOHES) up until after the hydrogen step. After adding some number of H+ to one side, add the same amount of OH- to both sides. For example, if you add 5H+ to the products side, add 5OH- to both the products and reactants side. Then, combine the 5OH- and the 5H+ to make 5H2O on the side with both H+ and OH- and cancel out any remaining waters.

Question 8

how to balance half equations in acidic conditions?

Answer 8

balance like normal (KOHES)

Question 9

what does H2 and O2 in a full redox equation always produce?

Answer 9

H2O