23 Redox and Electrode Potentials Flashcards
What is a reducing agent? (1 mark)
The species in a reaction which adds electrons to another species.
What is an oxidising agent? (1 mark)
A species in a reaction which takes away electrons from another species.
Does an oxidising agent get oxidised or reduced? (1 mark)
Reduced
Does a reducing agent get oxidised or reduced? (1 mark)
Oxidised
What does an increase in oxidation number indicate? (1 mark)
Oxidatipn
What does a decrease in oxidation number indicate? (1 mark)
Reduction
When writing half equation, what would you do if there was oxygen on the left but not on the right hand side of the equation? (2 marks)
Add water to the right hand side and then balance the equation by adding the correct number of H+ ions to the left.
How do you combine half equations? (2 marks)
Make the number of electrons the same in both.
Combine them.
Cancel out any species which are the same on both sides.
What colour is potassium manganate, how does this affect reading the level on the Burettes during titrations? (2 marks)
Dark pink/purple
Meniscus is read at the top, as it is too dark to see the bottom
How is the end point indicated during a potassium manganate titration? (1 mark)
First permanent pink colour
Is the thiosulfate or iodine reduced in a iodine/thiosulfate titration? (1 mark)
Iodine
Is the thiosulfate or iodine oxidised in a iodine/thiosulfate titration? (1 mark)
Thiosulfate
What is the equation for the oxidation of a thiosulfate ion? (2 marks)
2S2O3 2- —> S4O6 2- + 2e-
How is the end point detected in a iodine/thiosulfate titration? (3 marks)
Add the thiosulfate solution to the iodine solution until there is a colour change of orange to straw colour.
Then add a small amount of starch (this forms a black colour).
Continue adding the thiosulfate, until the solution is clear.
This is the end point.
Why do starch used in an iodine/thiosulfate titration? (1 mark)
It is hard to see the end point of the reaction, as it changes from a pale yellow to clear.
What oxidising agents can be used to titrate a thiosulfate solution? (3 marks)
I2
ClO-
Cu2+
How are samples of copper analysed? (3 marks)
The alloy is dissolved in HNO3, and then neutralised.
The Cu2+ is then reacted with I- to form I2 and CuI.
This brown mixture is titrated with Na2S2O3.
What colour is a mixture of I2 and CuI? (1 mark)
Brown
What is MnO4- reduced to? (1 mark)
Mn2+
What is Cr2O72- reduced to? (1 mark)
Cr3+
What does a voltaic cell do? (1 mark)
Converts chemical energy into electrical energy.
How do voltaic cells convert chemical energy into electrical energy? (1 mark)
Redox reactions, the movement of electrons from this.
What do half cells contain? (1 mark)
Chemical species present in a redox half equation.
How many half cells make up a voltaic cell? (1 mark)
Two
In a cell, why can chemicals not mix? (1 mark)
Electrons would flow in an uncontrolled way and heat energy would be released.
What is a metal/metal ion half-cell? (1 mark)
A metal rod dipped into solution of its aqueous metal ion.
What is the phase boundary in a metal/metal ion half-cell? (1 mark)
Where are the metal is in contact with its ions.
What is set up at the phase boundary? (1 mark)
An equilibrium.
What does an ion/ion half-cell contain? (1 mark)
Ions of the same element in different oxidation states.
A platinum electrode.
Why is a platinum electrode used in an ion/ion half-cell? (1 mark)
Because there is no metal to transport electrons.
Does the electrode with more or less reactive metal lose electrons and what is this electrode called? (2 marks)
The electrode with more reactive metal loses electrons, this is the negative electrode.
Does the electrode with more or less reactive metal gain electrons and what is this electrode called? (2 marks)
The electrode with less reactive metal gains electrons, this is the positive electrode.
How is a standard electrode potential found? (1 mark)
An electrode is connected to a standard hydrogen half-cell, under standard conditions:
What are the standard conditions used when finding a standard electrode potential? (1 mark)
Concentrations of all solutions are 1moldm-3.
Temperature is 298K.
Pressure is 100kPa
Define the term standard electrode potential. (2 marks)
Standard electrode potential is the e.m.f of a half-cell connected to a standard hydrogen half-cell under standard conditions.
What does e.m.f mean? (2 marks)
Electromotive force: the maximum potential difference between two electrodes.
What is the standard electrode potential of hydrogen? (1 mark)
0V
If the standard electrode potential is more negative, then is it oxidised or reduced? (1 mark)
Oxidised
What does a salt bridge allow? (1 mark)
Ions to flow
If the standard electrode potential is more positive, then is it oxidised or reduced? (1 mark)
Reduced
How do you prepare a salt bridge? (2 marks)
Soak a strip of filter paper in a saturated aqueous solution of potassium nitrate.
How do you calculate the standard cell potential from standard electrode potentials? (1 mark)
Cell= positive electrode — negative electrode
In terms of electrode potential, how can the reaction between two systems be predicted (Will it happen?)? (2 marks)
A reaction should take place if the redox system of the oxidising agent has a more positive value than the redox system of the reducing agent.
Why is concentration a limitation of using standard electrode potentials to predict the feasibility of reactions? (2 marks)
Standard use 1moldm-3.
If the concentration used is different from this, the electrode potential will be different from this value.
State 2 limitations of using standard electrode potentials to predict the feasibility of reactions? (2 marks)
The predictions will be wrong if :
The activation energy separating reactants and products is too high.
The conditions are not standard (conc, pressure, temp)
Explain what the statement ‘the activation energy separating reactants and products is too high’ means when explaining the limitations of using electrode potentials to predict feasibility of reactions. (2 marks)
The reaction is feasible but not spontaneous.
Are primary cells rechargeable? (1 mark)
No
Are secondary cells rechargeable? (1 mark)
Yes
How do fuel cells create a voltage? (1 mark)
Uses energy from the reaction of a fuel with oxygen.
Fuel cells can operate continuously if supplied with…… (2 marks)
Oxygen and fuel
What is the most common type of fuel cell? (1 mark)
Hydrogen
Explain why hydrogen fuel cells maintain a constant voltage. (1 mark)
They are continuously fed with hydrogen and oxygen.
