Module 5: Redox + electrode potentials Flashcards
What does an oxidising agent do?
Causes another reactant to be oxidised
-receives electrons
What does a reducing agent do?
Causes another reactant to be reduced
-donates/loses electrons
Balancing half equations in acidic conditions
In half equations:
- balance species (balance oxygen using H2O and hydrogen using H+)
- balance charges by adding electrons
-combine half equations to create overall equation
Balancing half equations in alkaline conditions
In half equations:
- balance species (balance oxygen using H2O and hydrogen using H+)
- balance charges by adding electrons
- convert back to alkaline conditions: add OH- to neutralise any H+ ions [must add the same number to both sides to maintain the balance]. Creates H2O molecules.
-cancel and combine half equations to create overall equation
What is a spectator ion?
Ions that are present in the reaction but unchanged by the end of it
What is the oxidation number of all elements in their natural state?
0
What do the oxidation numbers of the atoms of any molecule add up to?
0
What do the oxidation numbers of the components of any ion add up to?
The charge of that ion
Rules for applying oxidation numbers in order:
1) elements in groups 1,2,3 always have +1,+2,or +3
2) fluorine is always -1
3) hydrogen is usually +1
4) oxygen is usually -2
5) chlorine is usually -1
If oxidation number increases, are electrons lost or gained?
oxidation number increases = lost electrons (oxidised)
If oxidation number decreases, are electrons lost or gained?
oxidation number decreases = gained electrons (reduced)
Chemical that is oxidised is called the…
reducing agent
Chemical that is reduced is called the…
oxidising agent
What is the purpose of a salt bridge?
- prevents a charge gradient as salt cancels out the charge forming (cell needs an electron gradient, not an ion gradient)
- completes circuit
How can a salt bridge be made?
Filter paper soaked in a salt that wont react with electrodes/form a precipitate
Both ends of salt bridge need to be in the ion solution of the half cells
What does the high resistance voltmeter measure?
Stops the flow of electrons (which would be current) to measure the potential difference
Is the anode negative or positive?
Negative- it is where oxidation occurs, which is what provides the cell’s electrons
Is the cathode positive or negative?
Positive- electrons travel to it
Which way do electrons flow?
From - anode to +cathode
Which cell is drawn on the left in cell diagrams?
The cell with the most negative electrode potential (this becomes the anode)
Standard Electrode Potential
Voltage/Potential Difference of the half cell compared with standard hydrogen electrode
Under Standard Conditions
What are the standard conditions?
298K (25 C), 1 atm (100kPa), 1 moldm-3
How is a hydrogen electrode created?
Hydrogen is bubbled over a platinum electrode
Which side is the hydrogen electrode drawn in diagrams?
left
What concentration of H2SO4 is used in a hydrogen half cell?
- 5 moldm-3
- when dissociates into ions, [H+]= 1 moldm-3
More negative standard electrode potential =
better reducing agent (better at giving up electrons)
More positive standard electrode potential =
better oxdising agent (better at receiving electrons)
MPBOA
Drawing cell shorthand
- most negative electrode potential on left (species that is oxidised)
- oxidised forms go in middle
- / = phase boundary within cell
- // = external circuit
e.g Zn / Zn2+ // Cu2+ / Cu
How can standard electrode potentials be used?
To predict cell potential of combinations of half cells
Anti-clockwise rule
- write two half equations (more negative electrode potential on the top)
- draw anti-clockwise arrows to give reaction directions
- cell potential = E more positive equation - E more negative (top) equation
- if potential is positive, reaction is feasible
How to prove a reaction is not feasible
If arrows of anti clockwise rule do not go the correct way for the reaction you’re looking at (doesn’t go from your starting chemical to product)
Top equation - Bottom equation
This will give a negative number (proves unfeasibility)
How do you calculate overall cell potential using anti clockwise rule?
E bottom equation (most positive)- E top equation (most negative)
Why might a reaction not happen even if cell potential is positive?
- Activation energy is too high
- Conditions are not standard (influence equilibrium)
Fuel Cells
use the energy from the chemical reaction of a fuel with oxygen to create a voltage
Main difference between primary and secondary cells
Secondary- rechargeable as reaction can be reversed
How long do fuel cells last?
Should continuously produce electricity as long as fuel is supplied
What is the difference between primary and secondary cells
Primary cells are non rechargeable where as secondary cells are rechargeable.
In a primary cell the reaction can not be reversed, secondary cells can be reversed during charging
What is the oxidation equation in an alkali hydrogen fuel cell
H2(g) + 2OH-(aq) -> 2H2O(l) + 2e-
What is the reduction equation in an alkali hydrogen fuel cell
1/2O2(g) + H2O(l) + 2e- -> 2OH-(aq)
What is the overall equation in an alkali hydrogen fuel cell
H2(g) + 1/2O2(g) -> H2O (l)
What is the oxidation equation in an acid hydrogen fuel cell
H2(g) -> 2H+(aq) +2e-
What is the reduction equation in an acid hydrogen fuel cell
1/2O2(g) +2H+(aq) +2e- -> H2O(l)
What is the overall equation in an acid hydrogen fuel cell
H2(g) + 1/2O2(g) -> H2O(l)
Difference between storage and fuel cells
In a storage cell the chemical reactions that produce the electrical current are produced from materials that are already in the cell where as the fuel cell the reactions are almost always hydrogen and oxygen and are fed to the cell. Fuel cell have higher energy density
