🟢 TOPIC 3: Redox 1 Flashcards
Define oxidation number:
The total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom
Define redox reaction:
A reaction in which both oxidation and reduction occur - there is no net gain/loss of electrons
Define oxidation:
- loss of electrons
- oxidation number goes up
Define reduction:
- gain of electrons
- oxidation number goes down
What is the oxidation number of uncombined elements?
0
What is the sum of oxidation numbers in compounds?
0
What is the oxidation number of monotomic ions?
The charge on the ion
What is the sum of oxidation numbers in polyatomic/ molecular ions?
The charge on the ion
What is the oxidation numbers of group 1 and group 2 elements?
+1 and +2 (respectively)
What is the oxidation number of aluminium (and usually iron)?
+3
What is usually the oxidation number of lead?
+2
What is the oxidation number of hydrogen?
Always +1, except when in metal hydrides (eg. NaH), when it’s -1
What is the oxidation number of oxygen?
-2, except in peroxides when its -1 (eg. H2O2) or when with F, where it will be +ve
What is the oxidation number of group 7 elements?
-1, except when combined with a more electronegative element, where it will be +ve
Define reducing agent:
A substance that causes another substance to be reduced as it donates electrons easily. It is oxidised itself
Define oxidising agent:
A substance that causes another substance to be oxidised as it accepts electrons easily. It is itself reduced.
Define disproportionation reaction:
Both oxidation and reduction have taken place with the SAME element
(Eg. 2H2O2 —> 2H2O + O2)
[If a reaction is NOT disproportionation then there is not one SPECIES that is being oxidised and reduced - don’t be specific]
In the reaction, NaBr + Cl2 —> NaCl + Br2, which element is being oxidised, and which is being reduced?
ANSWER IN THIS FORMAT
- the oxidation number of sodium has not changed from +1, so sodium has neither been oxidised or reduced
- the oxidation number of bromine has increased from -1 to 0, so bromine has been oxidised
- the oxidation number of chlorine has decreased from 0 to -1, so chlorine has been reduced
How do you write an ionic half equation?
- Work out before and after oxidation states and identify the species that is being oxidised or reduced
- Write a balanced equation for the species that is being oxidised or reduced (other atoms can remain unbalanced)
- Balance oxidation states with e- (remember if there are multiple of the atom, there will have to be multiple e-)
- Work out the new charges (NOT oxidation states) - remember to count ALL electrons
- If charges don’t balance, add H+ ions to one side to balance
- If atoms don’t balance, add enough H2O so it balanaces
(7. If still doesn’t balance, may have to add O2)
Write the half equation for H2O2 to O2
H2O2 —> O2 + 2e- + 2H+
What does in acidic/ basic/alkali conditions mean?
In the prescence of H+ / OH- ions (respectively)
How do you combine half equations?
- Write the two half equations (one oxidation, one reduction) - (for the two separate species)
- Multiply equations so number of electrons in each is the same
- Add the two equations and cancel out electrons and any other species which appear on both sides
What does the roman numeral number indicate in a systematic name?
The oxidation number of the element it comes after (usually a transition metal as they can have multiple oxidation states)
What is the systematic name of NaClO3?
Sodium chlorate (V)
