A - Electrode Potentials Flashcards
How is a simple electrochemical cell made?
From two different metals dipped in salt solutions of their own ions, connect by a wire (the external circuit).
A voltmeter is located in the external circuit.
The solutions are connected by a salt broche which allows ions to flow through.
What is the simplest form of salt bridge?
Filter paper soaked in potassium nitrate solution - KNO3(aq).
What type of reactions occur in an electrochemical cell?
Redox reactions, one oxidation and one reduction reaction.
What is an electrode?
What is another name for an electrode?
A rod of metal dipped into a solution of its own ions.
Also called a half-cell.
If you could measure the potential of a half-cell, what would it tell you?
How readily electrons are released by the metal, that is, how good a reducing agent the metal is.
Can electrical potential be measured directly?
If not, what is measured instead?
No it cannot.
Potential difference (voltage).
Why is a salt bridge used rather than a wire?
Because a wire only transfers electrons, whereas a salt bridge transfers ions.
How do electrons flow in an electrochemical cell?
From the more reactive metal to the less reactive metal through the external circuit.
What is EMF?
Electromotive force (the cell potential) - shows the voltage between the two half-cells.
What is unique about a platinum electrode?
It’s an inert metal so won’t react with the ions.
What are electrode potentials measured against?
The standard hydrogen electrode.
What conditions can affect the cell potential?
Temperature, pressure and concentration.
Define the standard electrode potential of a half-cell.
The voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.
What are the standard conditions for the standard hydrogen electrode?
- Any solutions must have a concentration of 1.00 mol dm-3.
- Temperature must be 298 K.
- Pressure must be 100 kPa.
If standard conditions are maintained, what will the reading on the voltmeter be when the half-cell is connected to the standard hydrogen electrode?
The standard electrode potential of that half-cell.
Describe the standard hydrogen electrode.
Hydrogen gas is bubbled into a solution of H+ (aq) ions.
Since hydrogen doesn’t conduct, electrical contact is made via a piece of unreactive platinum metal (coated with finely divided platinum to increase the surface area and allow any reaction to proceed rapidly).
The electrode is under standard conditions.
What is the potential of the standard hydrogen electrode defined as?
0.
What must the salt chosen for the salt bridge not do?
It must not react with either of the solutions in the half cells.
What reactions occur at the negative and positive electrodes.
Oxidation occurs at the negative electrode.
Reduction occurs at the positive electrode.
What is an electrochemical series?
A list of standard electrode potentials for different electrochemical half-cells.
What do more positive electrode potentials mean in an electrochemical series?
- The left-hand substances are more easily reduced.
2. The right-hand substances are more stable.
What do more negative electrode potentials on and electrochemical series mean?
- The right-hand substances are more easily oxidised.
2. The left-hand substances are more stable.
What can you calculate using standard electrode potentials?
What formula allows you to do this?
The standard cell potential (EMF) when two half-cells are joined together.
EMF(cell) = EMF(reduced) - EMF(oxidised)
What directions will the equilibrium equations go in for a half-cell that has a more negative electrode potential, and one that has a more positive electrode potential?
The half equation for a more negative electrode potential will go in the direction of oxidation.
The half equation for a more positive electrode potential will go in the direction of reduction.
What are the conventions for the shorthand representation of electrochemical cells?
- A vertical solid line indicates a phase boundary.
- A double vertical line shows a salt bridge.
- The species with the highest oxidation state for each species is written next to the salt bridge.
- When giving the EMF value, give the polarity (whether it’s positive or negative) of the right hand electrode, as the cell representation is written.
How does an electrode potential tell you if a reaction is feasible?
If it is positive.
Batteries are based on the principles of what?
Electrochemical cells.
What are the three types of battery?
- Non-rechargeable (irreversible)
- Rechargeable
- Fuel cells
Discuss the cost comparison between rechargeable and non-rechargeable batteries.
Non-rechargeable batteries are cheaper initially compared to rechargeable batteries. But as you can re-charge and reuse rechargeable batteries, they last longer and work out cheaper in the long run.
Are lithium batteries rechargeable or non-rechargeable?
Rechargeable.
Name some examples of things lithium batteries are found in.
Devices such as mobile phones and laptops. I
Describe a lithium cell.
Made up of a lithium cobalt oxide (LiCoO2) electrode and a graphite electrode. The electrolyte is a lithium salt in an organic solvent.
What are the half-equations for the lithium cell?
At the positive electrode:
Li+ + CoO2 + e- —> Li+[CoO2]-
At the negative electrode:
Li —> Li+ + e-
How are lithium batteries recharged?
Why can’t non-rechargeable batteries be recharged in this way?
A current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions occurring at each electrode (as they are equilibrium reactions).
Reactions taking place in a non-rechargeable cell are difficult or impossible to reverse in this way.
What are fuel cells used to generate?
Do they need to be electrically recharged?
An electric current.
No.
How are the chemicals that are used to generate the electricity stored in most cells?
How does this differ with a fuel cell?
In most cells, they are contained in the electrodes and the electrolyte that form the cell.
In a fuel cell, the chemicals are stored separately outside the cell and fed in when electricity is required.
Give an example of a fuel cell. What is it used in?
The alkaline hydrogen-oxygen fuel cell, can be used to lower electric vehicles.
Describe briefly what occurs in the alkaline hydrogen-oxygen fuel cell.
Hydrogen and oxygen gases are fed into two separate platinum-containing electrode. The electrodes are separated by an anion-exchange membrane that allows anions (OH-) and water to pass through it, but not hydrogen and oxygen gas. The electrolyte is an aqueous alkaline (KOH) solution.
What are the electrode reactions occurring in the alkaline hydrogen-oxygen fuel cell?
Hydrogen is fed into the negative electrode. The reaction is:
2H2(g) + 4OH-(aq) —> 4H2O(l) + 4e-
Oxygen is fed into the positive electrode. The reaction is:
O2(g) + 2H2O(l) + 4e- —> 4OH-(aq)
What is an electrolyte?
An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water.
What are the advantages of using a fuel cell?
- When used in cars, as opposed to internal combustion engines, fuel cells are more efficient - they convert more of their available energy into kinetic energy to get the car moving. Internal combustion engines waste a lot of their energy producing heat.
- The only waste product is water, so no toxic chemicals need to be disposed of and no CO2 emissions from the cell itself.
- Don’t need to be recharged - just need supply of hydrogen and oxygen.
What are the disadvantages of using a fuel cell?
- You need energy to produce a supply of hydrogen and oxygen. They can be produced from the electrolysis of water (I.e by reusing the water product from the cell), but this required electricity, and this electricity is normally generated by burning fossil fuels.
So the whole process isn’t usually carbon neutral.
- Hydrogen is also highly flammable so it needs to be handled carefully when it is stored or transported.
