DM - redox *03 Flashcards
what is oxidation?
a loss of electrons
What is reduction?
A gain of electrons
What is a redox reaction?
when reduction and oxidation happens simultaneously
What is a reducing agent?
donates its electrons to something
(oxidised itself)
the more powerful a reducing agent the more easily it can be oxidised
What is an oxidising agent?
takes electrons away from something
(reduced itself)
the more powerful the oxidising agent the more easily it can be reduced
What do half equations show?
show whats being reduced and whats been oxidised
eg
Na —-> Na+ + e-
Cl2 + 2e- —-> 2Cl-
Balancing half equations
sometimes cannot balance the equation by just multiplying up the reactants and products and adding electrons
- If oxidising agent contains OXYGEN, need to add some H+ ions (reactants side) and H2O (products side) to make half equations balance
- also need to balance charges
- might need to multiply up and add electrons
Balancing half equations - MnO4- —> Mn2+ by Fe2+ ions
iron is oxidised = Fe2+ (aq) —> Fe3+ (aq) + e-
manganate reduced = MnO4- (aq) —> Mn2+ (aq)
to balance oxygens = MnO4- (aq) —> Mn2+ (aq) + 4H2O (l)
then need to balance hydrogens = MnO4- (aq) + 8H+ (aq) —-> Mn2+ (aq) + 4H2O (l)
finally need to balance charges by adding electrons =
MnO4- (aq) + 8H+ (aq) + 5e- —-> Mn2+ (aq) + 4H2O (l)
overall equation
MnO4- (aq) + 5Fe2+ + 8H+ —-> Mn2+ + 5Fe3+ + 4H2O
Acid-Base titrations - what do they find?
how much acid is needed to neutralise a base
acid-base titrations - how to set up?
- measure out alkali using a pipette, put it in a flask with some indicator, eg. phenolphthalein
- add acid to the burette and do a rough titration to get an idea of where the end point is
- swirl each time acid is added - now do accurate titrations, run the acid within 2cm^3 of the end point then add the acid dropwise
- record the vol. of acid used to neutralise the alkali
- repeat until concordant results (within 0.1cm^3)
Redox titrations - what do they show?
how much oxidising agent is needed to exactly react with a quantity of reducing agent
redox titration - what do you need to know?
concentration of either the oxidising or reducing agent
can use titration results to find the concentration of the other
redox titrations - Manganese (VII) ions, MnO4- method
- measure quantity of reducing agent eg. aq Fe2+ ions using a pipette and put it into a conical flask
- add some dilute sulfuric acid to the flask (in excess)
- so plenty of H+ ions to allow the oxidising agent to be reduced - Add aq MnO4- (oxidising agent) to the reducing agent using a burette, swirling the flask as you do so
- stop when the mixture in the flask becomes tainted with the colour of MnO4- (purple) this is the end point - a rough titration
- then do accurate titrations until concordant results (within 0.1cm^3)
redox titrations - Manganese (VII) ions, MnO4- - colour change
MnO4- in aq potassium manganate (VII), KMnO4, are purple
when added to the reducing agent, they react with it are are reduced to colourless Mn2+ ions
the reaction continues until all the reducing agent has reacted
the next drop into the flask will give the mixture the pink colour of the oxidising agent
need to spot exactly where this happens
redox titrations - what can results be used for?
to find the conc. of the reagent
redox titrations - Manganese (VII) ions, MnO4- and Fe2+ equation
MnO4- (aq) + 8H+ (aq) + 5Fe2+ (aq) —–> Mn2+ (aq) + 2H2O (l) + 5Fe3+ (aq)
5 moles of Fe2+ react with 1 mole of MnO4-
electrochemical cells - what are they?
made from two different metals dipped in salt solution of their own ions and connected by a wire
always two reactions - redox (one reduction and one oxidation)
- oxidation at anode (-ve electrode)
- reduction at cathode (+ve electrode)
CHARGES ARE THE OTHER WAY ROUND
so electrons flow through the wire from the most reactive metal to the least
electrochemical cells - zinc/copper
zinc loses electrons more easily than copper, so in the half cell zinc (s) (from the zinc electrode) loses electrons and is oxidised to Zn2+ (aq) ions
Zn (s) —-> Zn2+ (aq) + 2e-
this releases electrons into the external circuit
in the other half cell the same number of electrons are taken from the external circuit, reducing the Cu2+ ions to copper atoms
Cu2+ (aq) + 2e- ——> Cu (s)
electrochemical cells - how can you measure the cell potential, Ecell?
put a voltmeter between the two half cells to measure the voltage between the two half cells
this is the cell potential