Electrochemistry Flashcards
Define electrolysis
The decomposition of an ionic compound when molten or in aqueous solution by an electric current.
During electrolysis, reduction occurs at the cathode because ions gain electrons from the cathode, and oxidation occurs at the anode because ions lose electrons to the anode.
Electrolysis is often used to extract metals that are high in the reactivity series and cannot be extracted by heating their ores with carbon.
Electrolysis is generally carried out in an electrolysis cell.
Describe an electrolysis cell
In the electrolysis cell:
⚪️ The electrolyte is the compound that is decomposed.
It is either a molten ionic compound or a concentrated aqueous solution of ions.
⚪️ The electrodes are rods made from either carbon (graphite) or metal which conducts electricity to and from the electrolyte.
*The anode is positive electrode (where oxidation occurs) and the cathode is negative electrode (where reduction occurs).
⚪️The power supply must be direct current.
What will happen if metal atoms are formed?
If metal atoms are formed, they may be deposited as a layer of metal on the cathode. Alternatively they may form a molten layer in the cell. If hydrogen gas is formed, it bubbles off. 🫧
What is the mass of a substance produced at an electrode during electrolysis proportional to?
⚪️ The time over which a constant electric current passes
⚪️ The strength of the electric current
Combining current and time we get the relationship:
Q=I x t
Q=charge in coulombs, C
I= current in amperes, A
t=time in seconds, s
The mass of a substance produced at or removed from an electrode during electrolysis is proportional to the quantity of electricity (in coulombs) which passes through the electrolyte.
What is a Faraday?
The quantity of an electric charge (in coulombs) carried by one mole of electrons or one mole of singly charged ions. (Symbol F).
One Faraday is the quantity of electric charge carried by one mole of electrons or one mole of singly charged ions. Its value is 96,500 C mol-1.
What is the relationship between the Faraday and the Avagadro constant?
F=Le
F= Faraday
L=Avagadro’s constant
e=charge on electron
How can we use the value of Faraday F to calculate the mass of substance deposited at an electrode?
Step 1: write the half equation for the reaction
Step 2: find the number of columns required to deposit one mole of product at the electrode. (Multiply F with electrons)
Step 3: calculate the charge transferred during electrolysis using Q=It
Step 4: calculate the mass by simple proportion using the relative atomic mass.
How can we use the value of F to calculate the volume of gas produced at an electrode?
Step one: write the half equation for the reaction
Step two: find the number of coulombs required to produce one mole of gas . (Multiply electrons with F)
Step three: calculate the charge transferred during the electrolysis using Q=It.
Step four: calculate the volume by simple proportion using the relationship one mole of gas occupies 24.0 dm^3 at r.t.p
What is the value for the charge on an electron?
1.60x10^-19 C
What is an electrode potential?
Electrode potential, E: the voltage measured for a half-cell compared with another half-cell. Electrode potential is measured in volts.
When we place a metal into a solution of its ions, we establish an electron potential (voltage) between the metal and the metal ions in solution. We cannot measure this potential directly but we can measure the difference in potential between the metal/metal ion system and another system. We call this value the electrode potential. The system we use for comparison is the standard hydrogen electrode.
Describe the standard hydrogen electrode
The standard hydrogen electrode is a type of half cell that can be used as reference electrode.
The electrode consists of:
⚪️ hydrogen gas at 101 KPa pressure, in equilibrium with H plus ions of concentration 1.00 mol dm-3.
⚪️ a platinum electrode covered with platinum black in contact with the hydrogen gas and the H+ ions.
The platinum black is finely divided platinum. This allows close contact of hydrogen gas and H+ ions in solution so that equilibrium between H2 gas and H+ ions is established quickly.
The platinum electrode is inert so it does not take part in the reaction.
When is a redox equilibrium established?
The redox equilibrium is established when the rate of electron gain equals the rate of electron loss.
The position of equilibrium reaction is affected by changes in concentration, temperature and pressure.
What can be understood from the value of electrode potential?
Electrode potential values give us an indication of how easy it is to reduce a substance.
By convention, electrode potential refers to the reduction reaction. So the electrons appear on the left-hand side of the half equation.
⚪️ the more positive or less negative the electrode potential the easier it is to reduce the ions on the left. So the metal on the right is relatively unreactive and it is a relatively poor reducing agent.
⚪️ the more negative or less positive the electrode potential the more difficult it is to reduce the ions on the left. So the metal on the right is relatively reactive and it is a relatively good reducing agent.
What is a half cell?
One half of an electrochemical cell which either donates electrons to or receives electrons from an external circuit when connected to another half cell.
We use the following standard conditions to make the half cell:
⚪️ the aqueous ions have a concentration of 1.0 mol dm ^-3
⚪️ the temperature is 25°C (298K)
⚪️ the rod must be pure metal.
A half cell does not have to be a metal/metal ion system.
We can construct half cells for any half equation written.
What is an electrochemical cell?
If we connect to half cells together, we have made an electrochemical cell. We can measure the voltage between these two half cells.
Half cells are connected together using:
⚪️ wires connecting the metal rods in each half cell to a higher resistant voltmeter. The electrons flow round this external circuit from the metal with the less positive electrode potential to the metal with more positive electrode potential.
⚪️ a salt bridge to complete the electric circuit allowing the movement of ions between the two half-cells so that ionic balance is maintained. A salt bridge does not allow the movement of electrons.
What is a salt Bridge made out of?
A salt Bridge can be made from a strip of filter paper or other inert porous material soaked in a saturated solution of potassium nitrate.
What is standard electrode potential?
The voltage of an electrochemical cell will depend on these factors so we should use standard conditions when comparing electrode potential. These are:
⚪️ concentration of ions at 1.00 mol dm^-3
⚪️ a temperature of 25°C (298K)
⚪️ any gases should be at a pressure of 1 atm (101kPa)
⚪️ the value of the electrode potential of the half cell is measured relative to the standard hydrogen electrode.
Under these conditions, the electrode potential we measure is called the standard electrode potential. This has the symbol, E^o (E standard)
What are the three main types of half cell whose E standard value can be obtained when connected to a standard hydrogen electrode?
⚪️ metal/ metal ion half cell
⚪️ non-metal/non-metal ion half cell
⚪️ ion/ ion half cell
Describe the set up of a half-cell containing metals and metal ions
The metal/metal ion is connected to a standard hydrogen electrode and the voltage is measured.
By looking at the E standard values we can determine the direction of electron transfer.
Describe the set up standard electrode potential of half cells containing non metals and non metal ions?
In half cells that do not contain a metal, electrical contact with the solution is made by using platinum wire/foil as an electrode. The redox equilibrium is established at the surface of the platinum. The platinum electrode is inert so plays no part in the reaction.
The platinum must be in contact with both the element and the aqueous solution of its ions.
Describe the set up of half-cells containing ions of the same element in different oxidation states.
Half cells can contain two ions of different oxidation states derived from the same element for example Fe2+ and Fe3+.
A platinum electrode is used. The concentration of each ion (1.00 mol dm^-3) is present in the same half cell containing the platinum electrode.
Some reactions involve several ionic species:
For example:
MnO4- + 8H+ + 5e- ⇌ Mn2+ + 4H2O
So the half cell contains 1.00 mol dm^-3 of MnO4-, Mn2+, H+ ions.
What is standard cell potential?
The difference in standard electrode potential between two specified half cells.
This is the voltage of the electrochemical cell.
In order to calculate the cell voltage, we always subtract the less positive E standard value from the more positive E standard value.
Describe the direction of electron flow across an electrochemical cell.
- The positive pole of a cell is the half-cell which has the less negative or more positive E standard value.
- The direction of electron flow through the external circuit is from the half cell with the more negative (less positive) value of E to the half cell with the less negative value of E standard.
The electrons move through the wires in an external circuit. They do not travel through the electrolyte solution.
When is a reaction said to be feasible?
Feasible: likely to take place
A reaction is likely to be feasible if standard cell potential is positive value.
Describe the oxidising ability group 17 (halogens)?
As we go down group 17 from F2 and I2 the oxidising ability of the halogen descreases and ability of halide ions to act as reducing agents increases.
How can we predict whether or not a given reaction will occur under non-standard conditions?
If the E standard values of the two half reactions involved differ by more than 0.30 V than the reaction predicted by values is highly likely to occur.
If the E standard values differ by less than 0.30 V non-standard conditions may result in an unexpected outcome.
What is the Nernst equation when R, T and F are constant?
E=E standard + 0.059/Z multiplied by log10 [oxidised species/reduced species]
Z is the number of electrons transferred in the reaction.
What happens when pure molten ionic compounds containing two simple ions are electrolysed?
A metal is formed at the cathode and a non-metal at the anode.
Define discharged?
Ions changed into atoms or molecules.
