Topic 14 - Redox II Flashcards
What is the definition of a primary standard?
Chemicals weighed out and made to a standard solution. They react exactly and rapidly as described by their chemical equation.
What are the three characteristics of a primary standard?
- They must be very pure
- They cannot lose or gain mass when exposed to air
- They will usually have high molar masses so that weighing errors can be minimised
What is potassium manganate (when talking in terms of redox)?
oxidising agent (it is reduced)
What is the half equation for when potassium manganate acts as an oxidising agent under acidic conditions?
MnO4- (aq) + 8H+ -> Mn2+ (aq) + 4H2O (l)
Why do you not need to add indicator when completing a potassium manganate titration?
As potassium manganate (VII) is self indicating
What is the disadvantage to doing a potassium manganate titration (colour)?
Due to the strong purple colour of the MnO4- ions it is difficult to read the burette when performing the titration
What is the colour change as the MnO4- ions in potassium manganate are reduced to Mn2+?
purple/pink to colourless
What is the primary standard in potassium manganate?
MnO4- ions
What is the definition of a standard solution?
A solution that has been prepared from a primary standard that you know the exact concentration of
What does a potassium manganate titration measure?
A reducing agent
What does a Iodine/ thiosulphate titration measure?
An oxidising agent
How is a standard solution prepared in an Iodone/thiosulphate titration?
- Excess KI is added to the oxidising agent. All the oxidising reagent reacts forming a brown solution of iodine.
- The iodine produced is titrated with sodium thiosulphate which oxidises it back to colourless iodide ions.
What does the overall ratio between thiosulphate ions and oxidising agent depend on in Iodine/thiosulphate titrations?
The reacting ratio of the iodide and the oxidising agent
What is the indicator used in Iodine/thiosulphate titrations?
starch
At what point is the starch indicator added in an Iodine/thiosulphate titration?
When the brown solution -> pale yellow solution
What is the colour change in an Iodine/thiosulphate titration?
brown -> colourless
NB - we add starch as it is a blue/black colour making it easier to observe the point at which the solution is colourless
What species is the reducing agent in an Iodine/thiosulphate titration?
S2O3(2-)
What is the equation for the titration of Iodine against thiosulphate ions?
2S2O3 (2-)(aq) + I2 (aq) -> 2I- (aq) + S4O6 (2-)
What are metal displacement reactions more accurately known as?
metal/metal ion redox reactions
What type of reactions are metal ion redox reactions?
equilibrium/reversible
Do metal atoms always act as reducing agents?
no - it depends what other reactant is present in the redox
Half equations of metal ions losing or gaining electrons should be treated as…?
equilibria
Write the half equation of Zinc
Zn(2+) (aq) + 2e- <=> Zn (s)
Do you have to do metal ion redox reactions in one beaker?
no
What is the definition of a half cell?
A metal strip placed in a solution of its own ions and left to reach equilibrium
What does 1M mean?
1 moldm^-3
What is used to show the equilibrium position of a metal ion half equation?
a +ve or -ve sign in front of the E (standard) value
What does a +ve sign in front of the E (standard) value of a half equation tell you?
That the forwards reaction is favoured
What does a ive sign in front of the E (standard) value of a half equation tell you?
That the backwards reaction is favoured
Describe what happens in a zinc half cell
Zn(2+) (aq) + 2e- <=> Zn (s) E = -0.76V
- The E value is negative meaning that metal zinc atoms show a tendency to form Zn2+ ions
- This LEAVES ELECTRONS ON THE METAL STRIP
- A dynamic equilibrium is set up lying to the LHS
- The liberated e- are delocalised across the metal strip giving it a -ve charge
- The solution is +ve as it contains Zn(2+) ions
Describe what happens in a copper half cell
Cu(2+) (aq) + 2e- <=> Cu (s) E = +0.34V
- The E value is positive meaning that Cu2+ ions show a tendency to be reduced, TAKING ELECTRONS FROM THE METAL STRIP and form Cu (s)
- A dynamic equilibrium is set up lying to the RHS
- The metal strip is +ve as Cu2+ ions have TAKEN ELECTRONS FROM IT
- The solution is negative as there is a deficit of Cu2+ ions
What is the symbol for electrode potential?
E (standard)
What is an electrode potential?
A potential set up in half cells between the metal strip and the solution due to the separation of positive and negative charges
What dies the size of the electrode potential depend on?
- The type of metal (whether it shows a tendency to lose or gain electrons)
- Concentration of the solution
What are electrode potentials measured under?
Standard conditions:
- 298K
- 101kPa
- 1 moldm^-3
What is an electrochemical cell?
A combination of half cells, one releasing electrons and other gaining electrons, in order to turn chemical energy into electrical energy
Describe the set up of a metal electrode in a solution of its ions half cell
RIGHT:
- Reduction favoured half cell (+ve metal strip)
LEFT
- Oxidation favoured half cell (-ve metal strip)
- Wires attached to the metal strips with a high resistance voltmeter in the centre
- A salt bridge between the 2 solution
What is the electrode in an electrochemical cell?
The metal strip
What does a salt bridge do in electrochemical cells?
Allows ions to flow between the solutions to complete the circuit, keeping the balance of positive and negative ions in each half cell without mixing them together
What is a salt bridge usually made out of in electrochemical cells
A piece of filter paper or agar gel soaked in POTASSIUM NITRATE (KNO3)
Why are salt bridges made out of the substances that they are made out of?
So that they do not react with either of the solutions in the half cells
If you placed a bulb in place of a voltmeter in an electrochemical cell, what would happen? Why?
The bulb would light up - There is a potential difference between the two electrodes, therefore, current would flow (chemical energy -> electrical energy)
What is the E cell value?
The maximum possible potential difference between the two half cells
How is the E (standard) cell value measured?
High resistance voltmeter - Allows no/zero current through, therefore, maximum potential difference can be measured
Do half cells only ever consist of a metal electrode and its ions?
no
What are the 3 ways that half cells can be set up?
- A metal strip and its ions in solution
- 2 ions in solution
- A molecule and its ions
What are 2 examples of a half cell comprised of 2 ions in solution?
- Fe3+/Fe2+ half cell
- MnO4-/MN2+ half cell
What are 2 examples of a half cell comprised of a molecule and its ions?
- Cl2/Cl- half cell
- H2/H+ half cell
Describe the set up of a half cell made from 2 solutions of ions
- 1 moldm^-3 of both solutions of ions
- Platinum electrode
Describe the set up of a half cell made from a molecule and its ions
- Beaker containing 1 moldm^-3 of ions
- platinum coated PLATINUM BLACK electrode
- Downturned glass tube placed in solution (not touching the bottom) with gas coming in through a tube in the top and holes in the part of the glass tube submerged in the solution to allow the gas to escape
Why is a platinum black electrode used in a hydrogen half cell?
- High surface area for adsorption of ions
- Acts as a catalyst
What is platinum black?
Finely divided platinum
What is the definition of electromotive force?
The potential difference between two electrodes when no current is flowing = E celll value
Can we measure electrode potentials on their own?
No
What is the definition of a standard electrode potential?
The emf of a half cell compared with a standard hydrogen half cell under the conditions:
- stated temperature (usually 298K)
- concentration of all aqueous ions = 1M
- pressure of any gases is 1 atm
What is the half cell that is used when determining the standard electrode potentials of other half cells?
Standard hydrogen half cell
Standard cell potential definition
The emf between the 2 half cells that make up the electrochemical cell under standard conditions/ difference between the standard electrode potentials of each half cell
What is the equation for standard cell potential, E cell?
E cell = E (reduction/more positive) - E (oxidation/more negative)
A more negative E (standard) value for a half cell means…?
stronger reducing agent (more electrons on metal electrode)
A more positive E (standard) value for a half cell means…?
stronger oxidising agent (more electrons taken up on metal electrode)
Which way are the half cells normally arranged in an electrochemical cell?
LEFT: NEGATIVE
RIGHT: POSITIVE
What is required for a reaction to be feasible?
E cell value > 0V
What is required for a reaction to go to completion?
E cell value > 0.6V
What are 2 reasons that a reaction that was predicted to be feasible may not happen in practice?
- reaction has a high activation energy (not kinetically feasible under standard conditions)
- Conditions are different from standard conditions
What are the 3 changing conditions that affect electrode potentials?
- cell temperature
- cell concentration
- cell pressure (if there is a gas electrode)
What will happen if you increase the concentration of ions in a half cell?
The POE will shift to favour the reduction reaction, therefore, the electrode potential will become more positive/less negative
- opposite applies
What will happen if you increase the temperature of a half cell?
Higher temperatures -> increased tendency for metals to dissolve and form ions in solution. Therefore, the POE will shift to favour the reduction reaction and the electrode potential will become more positive/less negative
- opposite applies
What 2 values is E cell proportional to?
(delta) S total
lnKc
State how to draw and all the notation used in conventional cell diagrams
- Write the species from L(negative) -> R(positive) as that is how the electrons move
NOTATION:
1 vertical line - phase boundary
2 vertical lines - salt bridge
comma - different species but no phase boundary
What is the definition of an electrolyte?
A compound which dissolves in water to form aqueous ions that can conduct heat and electricity
What is the definition of a battery?
2 or more electrochemical cells connected in series
What is the definition of a storage cell?
An electrochemical cell that is based on reversible chemical changes so that it can be recharged by an external electricity supply
What happens when a storage cell is being recharged?
Electrolysis occurs in the cell to reform the chemicals that make up the cell (that form the equilibrium)
What is the definition of electrolysis?
The breaking down of an ionic compound by the passage of electricity in order to separate the ions and cause a new reaction
What are the 2 types of storage cells that you need to know?
- Lead - acid cells
- Lithium cells
What are the 2 main uses of lead-acid cells?
- Motor vehicles to power the starter motor and lights
- Back up power supply in hospitals in case of power cuts
What is the electrolyte used in lead - acid cells?
6M (fairly concentrated) sulphuric acid
What are the 2 electrodes in lead - acid cells?
- lead
- lead oxide
Write the equation for what happens at the negative electrode in lead acid cells
Pb (s) + SO4(2-) (aq) -> PbSO4 (s) + 2e-
0 -> +2 oxidation state
Describe what happens at the negative electrode in lead acid cells
When the cell is working the lead solid is being oxidised, giving up 2 negative electrons per atom, therefore discharging. The ppt. lead (II) sulphate is formed.
Write the equation for what happens at the positive electrode in lead acid cells
PbO2 (s) + SO4(2-) (aq) + 2e- + 4H+ -> PbSO4 (s) + 2H2O
+4 -> +2 oxidation state
Describe what happens at the positive electrode in lead acid cells
When the cell is working the lead (IV) oxide reacts with H+ ions in the sulphuric acid electrolyte and takes electrons (2 per ion). The ppt. lead (II) sulphate is formed.
What happens when a lead acid cell is discharged for long periods of time?
The insoluble solid lead (II) sulphate builds up becoming coarser and thicker. This means that the process CANNOT BE REVERSED when the cells are recharged
What happens when lead acid cells are recharged?
The current is reversed -> reactions are reversed to reform the original reactants
Where are lithium cells mainly used?
mobile phones and laptops
What is the negative electrode in lithium cells?
Carbon with a lattice layer containing lithium atoms
What is the positive electrode in lithium cells?
Lattice layer of MnO2 into which Li+ ions can move
What is the equation at the negative electrode in lithium cells?
2Li -> 2Li(+) + 2e-
electrons flow around the external circuit
What is the equation at the positive electrode in lithium cells?
2MnO2 (s) + 2e- + 2Li+ -> Mn2O3 (s) + Li2O (s)
What is the electrolyte in lithium cells?
A polymer material
What is a physical positive of using lithium in lithium cells?
Lithium is a low density metal, therefore, the cells are relatively light
What is a characteristic of lithium that is both an advantage and disadvantage in lithium cells?
Lithium is very reactive
- High electrode potential -> a large cell emf
- Reacts with oxygen in the air to form Li2O which is non-conductive, therefore lithium cells use electrodes that are made of materials with lithium inserted into their lattice structure
What is a fuel cell?
An electrochemical cell that produces electrical energy continuously due to continuous supply of the fuel and oxygen
What fuels are commonly used in fuel cells?
Hydrogen, hydrocarbons, alcohols
What is the fuel cell you need to know about?
Hydrogen - oxygen fuel cell
What conditions are required in a hydrogen - oxygen fuel cell?
Acidic (H+)
What is the electrolyte used in hydrogen - oxygen fuel cells?
solid polymer electrolyte that is permeable to protons
What is the equation for the negative terminal of a hydrogen - oxygen fuel cell in acidic conditions?
H2 (g) -> 2H+ + 2e-
What is the equation for the positive terminal of a hydrogen - oxygen fuel cell in acidic conditions?
1/2O2 (g) + 2H+ + 2e- -> H2O (l)
What is the main use of an alkaline hydrogen - oxygen fuel cell?
Space missions - to provide electricity and drinking water
What is the equation for the negative terminal of a hydrogen - oxygen fuel cell in alkaline conditions?
2H2O (l) + 2e- -> H2 (g) + 2OH- (aq)
What is the equation for the positive terminal of a hydrogen - oxygen fuel cell in alkaline conditions?
1/2O2 + H2O(l) + 2e- -> 2OH- (aq)
Are the overall reaction equations the same for hydrogen - oxygen fuel cells?
Yes
What is the overall equation for hydrogen-oxygen fuel cells?
1/2O2 (g) + H2 (g) -> H2O (l)
What is the equation for the positive terminal of a fuel cell when methanol is used as the fuel?
CH3OH (l) + H2O -> CO2 + 6H+ + 6e-
What is the equation for the negative terminal of a fuel cell when methanol is used as the fuel?
3/2O2 (g) + 6H+ + 6e- -> 3H2O (l)
What is the overall equation of a fuel cell when methanol is used as the fuel?
CH3OH (l) + 3/2O2 (g) -> CO2 (g) + 2H2O (l)
What is the advantage to using fuel cells over fossil fuels?
- Turns chemical energy to electric energy with 70% efficiency over 40% by fossil fuels
- Quieter in operation
- Fewer moving parts
- Eliminates greenhouse gases
What is the positive electrode (cathode) coated in, in a hydrogen-oxygen fuel cell?
nickel oxide or platinum
What are some disadvantages of using fuel cells in cars as motors comparatively to fossil fuels?
- Production of energy is lower
- Catalyst needed
- Fuel cells a lot heavier
- Driving range shorter
When doing redox titration calculations what is the first thing you must do?
WRITE HALF EQUATIONS -> FULL IONIC EQUATION
- as you need reacting moles
