Electrode potentials and electrochemical cells Flashcards
What is a half cell
A rod of metal dipped in a solution of its own ions where an equilibrium is established
What is a salt bridge and what is its purpose
- A salt bridge completes the electrical circuit.
- it is a piece of filter paper soaked in a solution of a salt (this is usually saturated potassium nitrate)
Why is a salt bridge used instead of a metal wire to complete the circuit in an electrochemical cell
To avoid further metal ion potentials in the circuit.
What does having the more negative e.m.f make a chemical
- A better reducing agent
- This means it will be oxidised in an electrochemical cell.
What two things happen when two solutions with different emf values are connected in an electrochemical cell. Compound X has a more negative e.m.f value than compound Y
- Compound X is a better reducing agent so is oxidised.
- This oxidation occurs at the negative anode.
- Compound Y is reduced at the positive cathode.
What happens to the concentration of metal ions in the half cell where the metal is oxidised
The solid metal dissolves to form positive cations and this increases the concentration of positive metal cations.
What two types of change should
you mention if asked changes in an electrochemical cell
- How the concentrations of the various metal ions in the solution change.
- If the metal being reduced is deposited on the rod.
- The direction of electron flow.
What do we use the hydrogen electrode for and why
- Comparing the tendency of different metals to release electrons.
- It is a standard electrode and we define its e.m.f value as zero.
Describe the reagents and conditions for the standard hydrogen electrode
- Hydrogen gas is bubbled into a solution of aqueous H+ ions.
- Platinum metal makes the electrical contact as hydrogen does not conduct.
- The electrode is used under standard conditions.
- This is a 1.00 moldm^-3 solution of H+ ions.
- A temperature of 298K
- A pressure of 100kPa
How does the standard hydrogen electrode act as a control
The potential of the standard hydrogen electrode is defined as zero so if it is connected to another electrode, the measured voltage is the electrode potential of that cell.
What is the electromotive force E (emf) of a cell
The measured voltage (against a standard hydrogen electrode)
What does the emf value being positive tell us
That the reaction is feasible
What are the conventions for representing an electrochemical cell with a phase diagram
- A vertical solid line represents a phase boundary
- A double vertical line shows a salt bridge
- The species with the highest oxidation state is written next to the salt bridge.
- When you have the emf value for the cell, write it to the right of the cell diagram as you would enthalpy change of a reaction.
What is the equation for emf
Emf= Electrode potential (R)- Electrode potential (L)
What is a Daniell cell
- A zinc/copper cell
- Zn—> Zn2+ + 2e-
- Cu 2+ 2e- —> Cu (s)
Describe the uses of a Daniell cell, it’s advantages and it’s disadvantages
- The Daniell cell provides an emf of 1.1V
- It was used to provide the energy for old fashioned telegraphs but is not practical for portable devices.
- This is because of the solutions of liquids it contains.
What is a Lechanché cell
- The cell is the basis of most ordinary disposable batteries and the electrolysed is a paste rather than a liquid.
- In it’s most common commercial form, Zn is oxidised and ammonium ions are reduced.
What is a problem of many commercial batteries
- As the cell discharges, the zinc is used up and the walls of the zinc canister become thin and prone to leakage.
- Electrolytes such as ammonium chloride are acidic and corrosive.
How are rechargeable batteries ‘recharged’
An external voltage is applied which drives the electrons in the opposite direction to the feasible reaction and hence resets the cell.
What do the rechargeable lead-acid batteries used in cars consist of
- Six 2V cells connected in series.
- Each cell consists of two plates dipped in a solution of sulfuric acid.
- The positive plate is made of lead coated with lead IV oxide PbO2 and the negative plate is made of lead.
- On discharging, the battery drives electrons from the lead plate to the lead IV oxide coated one.
What are the advantages and disadvantages of a nickel-cadmium battery
- They are more expensive to buy
- They can be recharged up to 500 times, reducing the effective cost significantly.
When drawing cell representations which side of the cell has the species being oxidised and which has the one being reduced
- The right hand side has the compound being reduced
- The left hand side has the compound being oxidised.
At which electrode does reduction occur
The positive electrode which is the cathode
At which electrode does oxidation occur
At the negative electrode which is the anode
What does the left hand side of a fuel cell diagram show us
- The species being oxidised
- The anode (negative electrode)
What does the right hand side of a diagram of a fuel cell show us
- The compound being reduced
- The cathode (positive electrode)
What is different about the structure of a lithium ion cell
- The electrolyte is a solid polymer rather than a liquid or paste so it cannot leak
- Thus also means it’s charge can be topped up at any point without the memory effect of some other rechargeable batteries which can only be recharged efficiently when they have been fully discharged.
- The cell can be bent or folded without leaking.
Describe how the electrolyte and electrodes are arranged in a lithium ion cell
- The positive electrode is made up of lithium cobalt oxide and the negative electrode is made up of carbon.
- On charging, electrons are forced through the external circuit from positive to negative electrode and at the same time lithium ions move through the electrolyte towards the positive electrode to maintain the balance of charge.
What is the overall reaction that takes place in a hydrogen-oxygen fuel cell
2H2 (g) + O2 (g) —> 2H2O (l)
What are the two half equations for what occurs in a hydrogen-oxygen fuel cell
1) 2H2 (g) + 4OH- —> 4 H20 (l) + 4e-
2) O2 (g) + 2H2O + 4e- —> 4OH- (aq)
What are the advantages of hydrogen fuel cells
- Hydrogen fuel cells do not need recharging
- Hydrogen fuel cells produce a voltage as long as a fuel is supplied
- The only by-product is water which is safe and is not a greenhouse gas.
What are the disadvantages of hydrogen fuel cells
- They are very expensive
- The fuel for hydrogen fuel cells is not readily available.
- hydrogen gas is hard to store safely as it is incredible flammable
- Hydrogen fuel cells use fuels which are gained from burning hydrocarbons (methane +steam) which produces carbon dioxide which is a greenhouse gas.
What are the advantage of rechargeable cells and batteries
They are cheap
What are the disadvantages of rechargeable cells and batteries
- They need recharging which takes time
- They contain harmful toxic chemicals which can be dangerous and pollute the environment is not recycled (and they require a separate recycling process).
State the meaning of the term electrochemical series
List of electrode potentials in numerical order
