Topic 3: Redox Flashcards
What does the oxidation number tell you?
- oxidation numbers or oxidation state tell you how many electrons an atom has donated or accepted to form an ion or form part of a compound
What are the rules to assign oxidation numbers?
§ All uncombined elements have an oxidation number of 0
- this means they haven’t accepted or donated any electrons
- elements bonded to identical atoms will also have oxidation numbers of 0
- eg. any uncombined element
- O2 or H2
§ The oxidation number of a simple monatomic ion is the same as its charge
- Na+ has an oxidation number of +1
- Mg2+ has oxidation state of +2
§ For molecular ions, the sum of the oxidation numbers is the same as the overall charge of the ion
- each of the constituent atoms will have an o.n of their own and the sum equals overall charge
e. g. SO4 2- : -8 (for oxygen) +6 = -2
§ For a neutral compound, overall charge is 0
§ Hydrogen always has an oxidation number of +1, except in metal hydrides (MHx) where its -1 and in molecular hydrogen (H2) where it is 0
§ Oxygen nearly away has an oxidation number of -2, except in peroxides (O2 2-) where it is -1, and molecular oxygen where it is 0
What do Roman numerals show?
- tells you the oxidation number
- if an element can have multiple oxidation numbers, or it isn’t a normal oxidant number
- the numerals are written after the name of the element they correspond to
Explain the roman numerals in ions that contain oxygen, so end with -ate
- the other element in the -ate ion can exist in different oxidation numbers, so it is attached as a Roman numeral after the number of the -ate compound
- correspond to the non-oxygen element
What is oxidation?
- a loss of electrons
What is reduction?
- a gain in electrons
What is a redox reaction?
- reduction and oxidation happen simultaneously
What is an oxidising agent?
- accept electrons
- gets reduced
What is a reducing agent?
- donates electrons
- gets oxidised
What happened if the oxidation number has increased after there reaction?
- element has lost electrons
- been oxidised
What happened if the oxidation number has decreased after the reaction?
- element has gained electrons
- been reduced
Identify the reducing and oxidising agents in this reaction:
4Fe + 3O2 = 2FeO3
- iron has gone from having an oxidation number of 0 to a number of 3+
- lost electrons and has been oxidised
- reducing agent
- oxygen has gone from an oxidation number of 0 to an oxidation number of -2
- gained electrons and reduced
- oxidising agent
What is a disproportionation reaction?
- a special redox reaction
- an element in a single species is simultaneously reduced and oxidised
What do ionic half equations show?
- they show oxidation and reduction
- you can show electrons being lost or gained in a half equation
What does combining half equations for different oxidising and reducing agents do?
- makes full equations for redox reactions