Redox Equilibria - 14 Flashcards
what does an electrochemical cell consist of
*A cell has two half–cells.
*The two half cells have to be connected with a salt bridge.
*Simple half cells will consist of a metal (acts an electrode) and a solution of a compound containing that metal (eg Cu and CuSO4).
*These two half cells will produce a small voltage if connected into a circuit. (i.e. become a Battery or cell).
Why use a high resistance voltmeter?
- The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit.
- In this state it is possible to measure the maximum possible potential difference (E).
- The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing.
Why does a voltage form when a zinc half-cell is connected to a copper half-cell?
- When connected together the zinc half-cell has more of a tendency to oxidise to the Zn2+ ion and release electrons than the copper half-cell. (Zn —–> Zn2+ + 2e-)
- More electrons will therefore build up on the zinc electrode than the copper electrode.
- A potential difference is created between the two electrodes.
- The zinc strip is the negative terminal and the copper strip is the positive terminal.
- This potential difference is measured with a high resistance voltmeter, and is given the symbol E.
- this same concept applies to any other cell
what is a salt bridge
A salt bridge is a piece of filter paper soaked in potassium nitrate
what is the function of a salt bridge
- Completes the circuit by allowing the flow of ions between the half-cells.
- Maintains electrical neutrality by preventing charge buildup in each half-cell.
Without a salt bridge, the cell would stop working as charge would build up and prevent electron flow.
why is a wire not used instead of a salt bridge
A wire is not used because the metal wire would set up its own electrode system with the solutions and wires do not allow the flow of ions
if a system does not contain an electrode that acts as a metal what can be used instead and why
- Platinum
- As: It provides a conducting surface for electron transfer
- it is unreactive and can conduct electricity
what is the standard hydrogen electrode (SHE)
is an electrode that assigned the potential of 0 volts which used to measure the potential of all electrodes by comparing their potential to that of the standard hydrogen electrode
what is the hydrogen electrode equilibrium
H2 (g) <——> 2H+ (aq) + 2e-
what is the hydrogen electrode represented by in a standard electrode
Pt |H2 (g) | H+ (aq)
conditions of standard hydrogen electrode
- Hydrogen gas at pressure of 100 kPa
- Solution containing the hydrogen ion at 1.00 mol dm-3 (solution is usually 1 mol dm-3 HCl)
- Temperature at 298K
what does a SHE half cell consist of
a platinum wire is used which is coated in finely divided platinum (platinum black) which acts as a catalyst, because it is porous and can absorb the hydrogen gas
Why are secondary standard electrodes used, and what are some common examples?
- The Standard Hydrogen Electrode (SHE) is difficult to use, so easier secondary standards are used instead.
- These are calibrated (measured in comparison) against the SHE to ensure accuracy.
- Common secondary standards:
- Silver/Silver Chloride Electrode (Ag/AgCl)
- Calomel Electrode (Hg₂Cl₂)
what is the standard electrode potential
the potential difference measured between an electrode system and hydrogen electrode system under standard conditions
what are standard conditions
- All ion solutions at 1.00 mol dm-3
- temperature 298K
- gases at 100 kPa pressure
- No current flowing
how to calculate the e cell
Ecell = Ered – Eox
how to identify which half cell is being oxidised and reduced
- The more negative half cell will always oxidise
- The more positive half cell will always reduce
Which half cell always oxidises in a redox reaction?
the more negative half cell (so if putting in an equation write it backwards)
Which half cell always reduces in a redox reaction?
The more positive half cell (so if putting in an equation write it forwards)
Where are the strongest reducing agents found in the electrochemical series?
At the most negative end (right side)
Where are the strongest oxidising agents found in the electrochemical series?
At the most positive end (left side)
on the electrochemical series which substance is more likely to reduce and which is more likely to oxidise
the more positive electrochemical will reduce (write forwards) and more negative electrochemical will oxidise (write backwards)
What does E°cell measure in a redox cell?
How far the reaction is from equilibrium; the more positive the E°cell, the more likely the reaction is to occur.
What happens to E°cell as current flows through a cell?
The reaction proceeds, reactant concentrations drop, and E°cell falls to zero as the reaction occurs
