Module 5- standard electrode potentials Flashcards
What do electrode potentials measure??
They are measuring how easily an element is able to give up its electrons (be oxidised/ be a reducing agent)
Why is magnesium a better reducing agent than copper??
It is better at giving up its electrons than copper
In equilibrium of the equations
Mg2+ + 2e- Mg and Cu2+ + 2e- Cu,
equilibrium lies more to the left for the Mg equation and more to the right for the Cu equation.
Both metals went to lose electrons, but Mg is better, forcing Cu to accept them and be reduced.
How do electrode potentials link to creating a voltage??
Electrons will flow from high concentrations to low concentrations, using two half cells with different electrode potentials will create a difference in the build up of electrons on each electrode, causing a flow of electrons which creates a voltage.
What 8 things must be included in a standard hydrogen electrode cell diagram
Hydrogen always goes on left In diagram Conditions: 298K 1 bar of pressure 1 mol/dm3 solutions (or make sure they are equimolar)
Cell:
WHICH WAY ELECTRONS FLOW
Magnesium electrode in contact with 1mol/dm solution of Mg ions
Platinum (inert) electrode in contact with 1 mol/dm H+ ions
H2 gas pumped into platinum electrode at one bar
Salt bridge
High resistance voltmeter
What must each electrode be in to make the cell work, and what if no solid metal/non metal is involved in the equation?
Each metal/ non metal must be in contact with its ions in AQEOUS solution, and ions of the same element with different oxidation states must be in contact with inert platinum electrodes eg Cr2+ and Cr3+ ions in solution
What is the definition of standard electrode potential/hydrogen potential??
voltage/ potential difference of the half cell combined with standard hydrogen potential under standard conditions of 298k, 1 bar of pressure and 1mol/dm
How do we use standard hydrogen potentials for comparison??
All values of standard electrode potentials are compared to a value for hydrogen of 0.00v, which basically measures how good elements are at giving up their electrons in comparison to hydrogen.
Anything with a value more negative than 0.00v is better at giving up its electrons/being oxidised than hydrogen (opposite applies for anything more positive)
eg Mg has standard value of -2.36v, so is better at giving up electrons than hydrogen, and would force them to accept them in a reaction.
How does standard electrode values of hydrogen and magnesium explain the voltage and flow of electrons in a half cell
mg has value of -2.36v compared to 0 for hydrogen, so Mg’s electrode has a higher build up of electrons than hydrogen because it is better at giving them up. This potential difference in electron build up means electrons flow from Mg electrode to the platinum hydrogen electrode, creating a voltage.
How does changing the conditions of standard half cells affect voltage/potential difference??
- Because the two half equations at the cells are in equilibrium, changing the concentrations can change the potential difference because of a change in the electron build up.
- if you increase the concentration of the better reducing agent ie the one giving up electrons, equilibrium shifts further left, providing a bigger electron difference, constituting to a bigger voltage when the electrons flow (equally you could reduce the concentration of the oxidising agent)
A reactant with a more negative electrode potential will…
reduce something with a less negative electrode
Whats MPBOA
More positive (electrode potential) better oxidising agent
What to do when constructing a cell diagram
-Half cell with most negative E.P goes on the left ( unless its hydrogen which always goes on the left)This will be the species being oxidised
- short hand circuit diagram shows oxides forms in middle
eg Zn/Zn2+//Cu2+/Cu
/=phase boundary of cell
//= external circuit (electrons flow from most negative to least negative.
overall equation would be
How do you work out the electrode potential of a cell??
1) draw out the two half equations, putting the most negative valued equation on top
2) use the anti clockwise rule, with the top equation going right to left and the bottom going left to right, making sure the correct reactants are being turned into the correct products
3) if this all lines up, do the bottom value minus the top value and you should get a positive value, indicating feasibility.
Limitations of predicting feasibility using standard electrode potentials:
Reaction rate- some reactions may be feasible but have huge activation energies that can’t be achieved under standard conditions ie happen very slowly. Standard values give indication of thermodynamic feasibility but no indication of reaction rate
concentration- changing concentrations will change equilibrium (le chatelliers principle)
Only applies to aqueous solutions
