5.2.3 - Redox And Electrode Potentials Flashcards
What is an oxidising agent?
Causes something to be oxidised but is in itself reduced.
What is a reducing agent?
Causes something to be reduced but is in itself oxidised.
When assigning oxidation numbers what is first?
Elements and then metals.
What is the order of preference for non-metals?
Hydrogen, fluorine, oxygen, chlorine, bromine, iodine.
When balancing half equations what is used in acidic conditions?
H+ and H2O
In alkali conditions what is used to balance equations?
OH- and H2O
What is the overall equation of manganate and iron?
MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ +4H2O
What colour are manganate ions in aqueous potassium manganate?
Purple
How do you calculate error on a pipette?
(0.06/25) x 100
How do you calculate the error on a burette?
(0.05x2/titre) x 100
What are the rows down on a titration table?
Final reading (cm3), initial reading (cm3), volume used (cm3)
What are the columns across on a titration table?
Rough, 1, 2, 3
What dp are titration values recorded to?
2dp ending in 5 or 0
What are concordant results?
Results within 0.1 of one another.
What dp is the average titre recorded to?
1dp
What is the standard electrode potential of a half cell?
The emf of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentration 1moldm-3 and a gas pressure of 1atm.
Describe a metal/metal ion half cell?
Metal dipping into a solution of metal ions. (the same metal)
Describe a metal ion / metal ion half cell?
Platinum electrode dipping into solution of both metal ions.
Describe a non metal gas / non metal ions half cell?
Gas enters through container which leads to platinum electrode. All in non-metal ions in solution.
What connects the two cells?
High resistance voltmeter.
Why is a high resistance voltmeter used?
So no current can flow.
What connects the two solutions?
Salt bridge.
What does the salt bridge do?
Completes the circuit, allowing ions to flow but not electrons.
What is the salt bridge made of?
A strip of filter paper soaked in potassium nitrate solution. KNO3 is used as it does not react with any of the half cell solutions.
When 2 cells are joined together which is there negative terminal and what does this mean?
The one with the more negative electropotential value, the reaction goes backwards, releasing more electrons.
When 2 cells are joined together which is the positive terminal, and what does that mean?
The one with the more positive electropotential value, the reaction goes forwards gaining electrons.
How do you calculate the the cell potential of 2 half cells?
Ecell = most positive - most negative
When calculating the overall equation which half equation do you flip?
The one with the more negative electropotential value.
Which way do the electrons flow in the cell?
From the negative terminal to positive terminal.
Why does I- react with MnO4-?
I2/I- reaction has the more negative E value so this equilibrium will shift to the left, releasing electrons. MnO4/Mn2+ reaction has the more positive E value so this equilibrium will move to the right and gain electrons.
What are the limitations of using the electrochemical series to predict reactions?
- They are based on values under standard conditions.
- They predict equilibrium position not rate which may be too slow because of high activation energy.
What are the 3 types of electrochemical cells?
Non-rechargable, rechargeable and fuel cells.
What is a fuel cell?
A fuel cell uses the energy from the reaction of a fuel (H2 or hydrogen rich fuels such as methanol) with an oxidant (O2) to create a voltage.
How does a fuel cell work?
The reactant flows in and the product flows out while the electrolyte remains in the cell so they can operate continuously as long as there is a supply of fuel and oxidant.
What are the two half equations for the hydrogen-oxygen fuel cell?
2H2O + 2e- H2 + 2OH-
1/2O2 + H2O + 2e- 2OH-
What is the overall equation for the hydrogen-oxygen fuel cell?
H2 + 1/2O2 -> H2O
What are the benefits of the fuel cell?
Works continuously as long as there is a constant supply of oxidant and fuel. Doesn’t have to be recharged.
What are the risks of fuel cells?
Toxic if ingested (e.g lithium cells). Rapid discharge can cause fires/explosions.
What are the disadvantages of fuel cells?
A lot of energy is needed to make fuel cells. Difficulty in disposing of them. Making hydrogen uses electricity and storing H2 under pressure as liquid is flammable and explosive.
