1. H2 gas is pumped in at a pressure of 100kPa 2. The electrolyte contains H+ ions of concentration 1moldm-3 3. There must be a platinum electrode 4. The whole system must have a temperature of 298K

Electrode Potential Flashcards by Evie Dorrington

How to tell the difference between a reducing agent and an oxidising agent

Reducing agents are oxidised, they donate and lose electrons
Oxidising agents are reduced, they accept and gain electrons

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How would you define a half cell

A strip of metal dipped into a solution of its own ions can be called an electrode or a half cell- when the strip of metal is dipped in solution of its an ions an equilibrium is set up

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Using Cu+ as an example, explore how the equilibrium is reached

the Cu2+ ions dissolve into the solution, giving the solution a positive charge
the electrons collect in the copper strip, giving it a negative charge
-this means a potential difference is established between the two

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How does a voltage reading identify where the equilibrium lies

If there is a large voltage, the equilibrium is to the right
If there is a small voltage, the equilibrium is to the left

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Describe how you would connect two half cells

The two metal rods are connected with wires and a high resistance voltmeter
The two beakers of electrolyte are connected with a salt bridge to complete the circuit. This is a glass tube or porous material soaked in potassium nitrate solution

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Why is KNO3 a suitable solution for a salt bridge

KNO3 is unreactive with the electrodes and the ions are free to move

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What is the role of the voltmeter and what happens if it was removed or replaced

Prevents electrons flowing and enables the voltage to be measured, if the voltage meter was removed electrons can flow from the left electrode to the right
If the voltmeter is replaced with an ammeter or a bulb, the electrons can flow and a current is produced

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Why might the current produced by a cell fall to 0 after some time

All the reactants are used up

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What will happen to a cell once the reactants are used up

Stops working or starts to leak

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Why is platinum a suitable electrode

Pt is unreactive and conducts electricity

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Key points for SHE

H2 gas is pumped in at a pressure of 100kPa
The electrolyte contains H+ ions of concentration 1moldm-3
There must be a platinum electrode
The whole system must have a temperature of 298K

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What does it mean that the SHE is defined as 0

If another electrode is connected up to the standard hydrogen electrode and voltage between them is measured

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The standard electrode potential of Cu2+/Cu is 0.37 V. Why might the electrode potential of the following cell not be 0.37V

The concentration of the CuSO4 solution is not 1moldm-3

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Give an example of how to write conventional cell representation using Zinc and Copper

Zn(s)I Zn2+(aq)II Cu2+(aq)I Cu(s)

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what des the conventional cell representation show?

That at the left electrode, Zinc is oxidised to Zn2+ and at the right electrode Cu2+ is reduced to Cu

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Key facts for electrode potential

the right electrode is the positive electrode
the right electrode has a more positive standard electrode potential than the left electrode
the left electrode is the negative electrode
the left electrode has a less positive standard electrode potential than the right electrode
vertical solid lines indicate a phase boundary
a double vertical line represents the salt bridge
the species with the highest oxidation state should be written closest to the salt bridge

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on which side is the SHE always written on

the left

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using a table, how can you identify which is the strongest oxidising agent and which is the weakest reducing agent

the substance with the most positive E° is the weakest reducing agent on the right and the strongest oxidising agent on the left

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Why will Ag+ ions react with Li

the E° Ag+ is greater than E° for Ag+ and Ag+ is a stronger oxidizing agent

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what will be observed when Ag+ reacts with Li

Solid Ag forms

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What happens when Li+ is added to Ag

E° Ag is greater than E° Li+ and Li+ cannot oxidise Ag

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Identify a chemical that reacts with Cu

Ag+ because E° Ag+ is greater than E°Cu

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how to calculate EMF

Right-left/ reduction-oxidation/ more positive-least positive

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Write an equation for the cell discharge of Silver and Lithium

Ag+(aq) + Li(s) –> Ag(s) + Li+ (s)

write an equation for the cell recharge of silver and lithium

Ag(s) + Li+(s) --> Ag+(aq) + Li(s)

give an environmental advantage of using rechargeable cells

metals are reused

give an environmental disadvantage of using rechargeable cells

mains electricity is used to recharge, which may come from combusting fossil fuels, which releases Co2

the cell made of the systems Mg/Mg2+ and Fe2+/Fe under standard conditions has an emf of 1.93 V deduce how the emf changes when the conc of Mg2+ is decreased Mg2+ + 2e- --> Mg Fe2+(aq) + 2e- --> Fe

Mg2+ electrode equilibrium shifts to the left so E° of Mg2+/Mg gets more negative as more electrons are made and released. Overall EMF increases

a diagram shows that the amount of copper in the electrodes is much greater than the amount of copper ions in the copper sulfate solutions- in which direction would the electrons flow in the above cell

from right to left the Cu2+ is more concentrated on the left, so reduction on Cu2+ is more likely to happen on the left the left electrode is the positive electrode, so the right electrode is the negative electrode

single use batteries

the electrochemical reaction taking place inside the cell is irreversible- once the reactants have been used up they can't be used again

how is lithium ion battery a rechargeable battery

the positive electrode is made of lithium cobalt oxide Li(CoO2) and the negative electrode is made of graphite, when it is discharged, a layer of Li on the negative electrode dissociate to form Li+ and release an electron: Li—> Li+ + e- The Li has been oxidised, so this is the negative hand electrode or the left At the positive electrode a lithium ion and electron combine with CoO2 from the electrolyte to form Li+(CoO2) CoO2 + Li+ + e- —> Li(CoO2) Oxidation state is+4 so the Co has been reduced- this is the positive or right hand electrode Overall equation: CoO2 + Li —> Li(CoO2)

What happens when the battery runs out and why does it? Give the equation for recharging

Because all the battery has CoO2 or Li has been used up. It can be connected to a stronger electrical current which forces the reaction back in the reverse direction to recharge the battery Equation for recharging: Li(CoO2)—> CoO2 + Li

What is a fuel cell

Uses the energy from the reaction of a fuel with oxygen to create a voltage Modern fuel cells are based off on methanol, natural gas and petrol Combusting a fuel can can generate a flow of electrons

Advantages of using fuel cells for energy instead of fossil fuels

Greater efficiency than burning hydrogen in a combustion engine Less polluting as water is the only product

Disadvantages of using fuel cells for energy instead of fossil fuels

H2 is difficult to store Fossil fuels are combusted to produce the hydrogen, which releases CO2

Advantages of fuel cells compared to other type of cells

Voltage is constant, as fuel and oxygen is supplied constantly so concentrations of reactants remain constant

What is an electrolyte

An acid or alkaline membrane which allows ions to move from one compartment of the cell to another, almost like a salt bridge

Hydrogen oxygen fuel in acidic conditions

At the negative electrode: H2 —> 2H+ + 2e- At the positive electrode: O2 + 4H+ + 4e- —> 2H2O Overall equation: 2H2 + O2 —> 2H2O

Hydrogen oxygen duel cells in alkaline conditions

Negative electrode: H2 + 2OH- —> 2H2O + 2e- Positive electrode: O2 + 2H2O + 4e- —-> 4OH- Overall equation: 2H2 + O2 —>2H2O

Hydrogen oxygen cells in both conditions

- same overall equation, therefore both cells have the same EMF - the voltage can remain constant - reagents can be continuously supplied - no environmental issues / damage as water is the only product - to generate H2 fossil fuels are burnt which produces O2

Give one reason why the emf of this cell changes when the electrodes are connected and a current flows

The concentration of ions change

A student determined the concentration in a solution of iron chloride by titration with acidified potassium dichromate solution. A second student titrated the same solution. Explain why the second student obtained a greater concentration for the iron ions

MNO4- will oxidise the chloride ions so a larger volume will be needed

Give two reasons why KNO3 is a good reagent for a salt bridge

1- doesn’t react 2- ions can move

Explain why the ammeter would fall to zero after a time

Fe3+ ions would be used up

How to identify the weakest oxidising agent

It will have the most negative EO value

A rechargeable nickel;-cadmium cell is an alternative cell. Deduce the oxidation state of nickel in the cell after recharging is complete Write an equation for this recharge

2Ni(OH)2 + Cd(OH)2 —> 2NiO(OH) + Cd + 2H2O +3

One environmental advantage of rechargeable cells

Metal compounds are reused

Potassium manganate is an oxidising agent Suggest why 0.0200 moles-3 of potassium manganate does not need to be kept away from flammable material

The solution is very dilute

Explain why rechargeable cells are connected to these solar cells

Solar cells do not supply electrical energy all of the time- rechargeable cells can store electrical energy for use when the solar cells are not working

Suggest one reason why many waste disposal centres contain a separate section for cells and batteries

Prevent pollution of dangerous substances

You are provided with the Daniel cell including a zinc electrode of a known mass Briefly outline how you would carry out an experiment to confirm your answer to part b

Weigh mass of Zinc before and after Allow cell to discharge until [Cu2+]—> 0.5 Confirm using colorimeter

Suggest the main advantage of using hydrogen in a fuel cell rather than in an internal combustion engine

Less energy wasted

Suggest two reasons why the concentratation of chlorine in the vase decreases with time Chlorine used to kill bacteria

1- reacts with water to from HCl and HClO 2- used up killing bacteria

Give two reasons why the value of the cell emf that you calculated in part b would not normally be achieved when the cell operates in sea water

Non-standard conditions and aluminium may react in an alkaline solution

Explain briefly the results obtained for the iron foil (constant mass)

It is protected by the operation of a sacrificial anode

Suggest briefly why the aluminium foil stopped decreasing in mass over time

Coating developed which prevents further erosion

Explain the function of the salt bridge (2marks)

Allow the transfer of ions between electrodes Maintain charge

Suggest the major advantage of using the fuel cell compared to internal combustion engine

Internal combustion wastes more heat energy