Electrochemistry Flashcards
Purpose of Electrolysis
It is often used to extract metals that are high in the reactivity series. These metals cannot be extracted by heating their ores with carbon. Electrolysis is also used to produce non-metals such as chlorine and purify some metals.
Electrolysis is what type of reaction?
Redox
The mass of the substance produced at the electrode during electrolysis is proportional to
- the time over which a constant electric current (d.c) passes.
- the strength of the electric current.
Faraday
F = charge/1 mole of e-
One Faraday is the quantity of electric charge carried by 1 mole of electrons or 1 mole of singly charged ions. Its value is 96500 C/mol. (Relationship between faraday and the Avogadro constant is F=Le)
i.e. Ag+ + e- —-> Ag (One Faraday of electricity 96500C is required to deposit 1 mole of Silver)
i.e. 2Cl- —-> Cl2 + 2e- (Two Faraday of electricity 2x96500 C to deposit 1 mole of Copper)
Value of F (Faraday) can be used to calculate?
- the mass of substance deposited at an electrode
- the volume of gas produced at an electrode
(Worked example 1 & 2 pg no. 408)
STEPS TO FIND MASS OR VOLUME USING FARADAY’S CoNSTANT:
1. Write the half-equation.
2. Find the number of coulombs required to deposit 1 mole of product at the electrode (F x no. of electrons in half equation).
3. Calculate the charge transferred during the electrolysis(Q = I x t).
4. Calculate the Mass (mass = Charge calculated in step 3 divided by amount of Faraday in C mol-1 in step 2 and finally multiply by Mr) or Volume (volume = Charge calculated in step 3 divided by amount of Faraday in C mol-1 in step 2 and finally multiply by 24.0 dm3) by simple proportions.
Value of L (Avogadro constant) can be found using
L= (charge on 1 mole of electrons) ÷ (charge on 1 electron)
(Practical Activity 20.1 pg no. 409 + Texbook Qs 8, 9.) —->Difficult so review from time to time.
Electrode potential (E) vs Standard cell potential
Electrode potential, E: the voltage measured for a half-cell compared with another half-cell under standard conditions.
Standard cell potential: the difference in standard electrode potential between two specified half cells under standard conditions.
When is redox equilibrium established
when the rate of electron gain equals the rate of electron loss
Standard conditions for electrolysis
101 KPa/ 1 atm, 1.00 mol/dm3 and 298K
Why is platinum black used as an electrode?
The platinum black is finely divided platinum. This allows close contact of hydrogen gas and H+ ions is established quickly. The platinum electrode is inert so it does not take part in the reaction.
What do Electrode potential values indicate?
Electrode potential values give us an indication of how easy it is to reduce a substance.
» 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 poor reducing agent. e.g. Ag+ + e- <——> Ag
» 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 is relatively good reducing agent. e.g. Zn2+ + 2e- <—-> Zn
Electron flow in the electrolytic circuit
Electrons flow round the external circuit from the metal with the more negative (or less positive) electrode potential to the metal with less negative (or more positive) electrode potential
Half-equations for reactions taking place at the half-cells.
⁍ Br2/2Br-
⁍ O2 + H20/OH-
⁍ VO2+ + H2O/VO2 + —>whole charge of Plus 1
- 2Br–(aq) → Br2(l) + 2e–
- O2(g) + 4H+(aq) + 4e- —> 2H2O(l)
- VO2+(aq) + 2H+(aq) + e- —> VO2+(aq) + H2O(l).
Purpose of salt bridge
a salt bridge completes the electrical circuit allowing the movement of ions between the two half-cells so that the ionic balance is maintained. It does not allow the movement of electrons. 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.
Why aqueous silver nitrate is not used in a salt bridge when connecting half-cell containing Zn and 1.00 mol ZnCl2(aq) to another half-cell?
Silver ions (Ag+) will react with the Chloride ions (Cl-) to form a precipitate therefore, consuming the free ions of Silver ions, impairing the flow of current
What do the positive and negative terminal indicate?
Reduction takes place at the positive terminal and Oxidation takes place at the negative terminal.
Calculating standard cell potential
Sum of both the cell potential (or more positive − less positive)
The tendency of the species on left and right side of the equation to reduce and oxidise, determined by their respective electrode potentials in a standard cell
For each half-equation, the more oxidised form is on the left and the more reduced form is on the right.
» The more positive the value of Eo, the greater tendency for the half-equation to proceed in the forward direction.
» The less positive the value of Eo, the greater the tendency for the half-equation to proceed in the reverse direction.
Feasible
A reaction is likely to take place if E○cell has a positive value.
When is the redox reaction in electrolysis likely to occur?
A redox reaction occurs in the direction where the better oxidising agent reacts with the better reducing agent.
Two ways to predict if a reaction is likely to occur
- Using E○ values (Identify the stronger reducing and oxidising agent and predict if the reaction takes place)
- Relative voltage of the half-cells (Write the equation for reduction first and add the electrode potential which is more positive. Then, write the oxidation equation by reversing it with e- released on right side and add the electrode potential by switching the sign to negative as it’s oxidation)
- Worked example 3, 4 & 5 pg. 420,421& 422.
- Worked example 6 & 7 pg. 422
As the values of E○ for reduction reactions gets more negative
◘ The species on the left of the equation becomes weaker oxidising agents. They accept electrons less readily.
◘ The species on the right of the equation become stronger reducing agents. They release electrons more readily.
What trend does Group 17 from F2 to I2 follow based on their E○ values?
As we go down the group 17, the oxidising ability of the halogen decreases and the ability of the halide ions to act as reducing agents increases.