Electrochemistry Flashcards
What are the four rules for determining the number and side on which to put the electrons in a redox half reaction?
- The electrons are always on the left hand side of a reduction half-reaction
- The electrons are always on the right hand side on oxidation half reaction
- For a reduction half reaction, the number of electrons required = initial oxidation # - final oxidation #
- The number of electrons required for an oxidation reaction = final oxidation number - initial oxidation number
How should half reactions be balanced?
Each half reaction should be balanced so that if they were put together the overall reaction would balance out (the individual half reactions don’t need to be balanced separately)
How are the oxidation/reduction capabilities of substances measured?
What are these referenced to?
The standard half-cell potentials (E°) under standard conditions reveal redox capabilities of substances
Referenced to the following reaction:
2H+ + 2e- → H2
where E° = 0
The strongest reducing agents have large negative E° values.
The strongest oxidizing agents have large positive E° values.
How does the standard half-cell potential (E°) of substances effect redox reaction spontaneity?
How is E° related to reducing/oxidizing capability?
The more positive the E° value, the more likely the reaction will occur spontaneously as written.
The strongest reducing agents have large negative E° values.
The strongest oxidizing agents have large positive E° values.
What are batteries?
Self sustained galvanic cells that use spontaneous (large positive E°) redox reactions to produce electricity.
There are oxidation and reduction reactions occurring at either end of the battery to produce electron flow out of the compartment where the oxidation occurs to the compartment where the reduction takes place.
Electrons flow from what electrode to what electrode?
What is happening at each of these electrodes in galvanic cells?
Anode to cathode
Anode: oxidation
Cathode: reduction
What is the voltage difference between the anode and cathode in a galvanic cell called?
The electromotive force (emf) of the cell
What is a salt bridge (electrochemistry)? And what are its two important functions?
The salt bridge connects the two compartments of a galvanic cell chemically (with ions like Na+ and Cl-)
- Maintenance of neutrality. Eg. as Zn(s) becomes Zn2+ (aq), Cl- can come into the anode compartment to maintain neutrality.
- Completing the circuit. Negative charge leaves the anode compartment via electrons in a wire and then returns via chemicals (ie. Cl-) in the salt bridge. Thus the galvanic cell is an electrochemical cell.
How can you use the same substance in each compartment of a galvanic cell and still generate electricity?
What would the electromotive force (emf) be in this case?
By varying the molar concentration, as this effects the E (potential) in the compartment.
In this case, the emf would be equal to the difference between the two potentials (E)
These cells are called concentration cells.
The cathode is the electrode with the largest positive E
What is an electrolytic cell?
A current is imposed on the system to drive a non-spontaneous redox reaction
- Generally a battery is used to produce the current imposed on the electrolytic cell
- The battery acts as an electron pump: electrons flow into the electrolytic cell at the cathode and flow out of it at the anode
- The half-reaction occurring at the cathode is a reduction since it requires electrons
- The half-reaction occurring at the anode is an oxidation since it produces electrons
What is Faraday’s law?
Faraday’s law relates the amount of elements deposited, or gas liberated, at an electrode due to current.
Faraday’s law allows us to calculate the amount of ions that will ‘plate’ onto the cathode in a galvanic cell
The weight of product formed at an electrode is proportional to the amount of electricity transferred at the electrode and to the equivalent weight of the material.
The standard potential for the reaction:
Zn+2 + 2e- ⇋ Zn
equals -0.763. When a strip of Zn is placed in a .1 M solution of HCl, does the zinc stip begin to dissolve?
Yes! H2 gas and ZnCl2 (aq) are produced.
The equation shows the reduction of Zn+2 to Zn. The negative reduction potential for Zn+2 means that Zn has a positive oxidation potential (ie. Zn is easily oxidized). Zinc will displace the hydrogen, which has a zero standard reduction or oxidation potential (ie. hydrogen is the standard against which other substances are measured). As given in the problem, ionic zinc is Zn+2, therefore zinc metal liberates hydrogen gas and produces ZnCl2 (aq), effectively dissolving.
Will a substance with a high positive oxidation potential dissolve in an acid? Why or why not?
Yes, because the material is easily oxidized it will displace hydrogen (which has a zero oxidation potential) to attach to whatever the H is attached to (eg. chlorine in hydrochloric acid) becoming aqueous (soluble) and liberating H as hydrogen gas
What happens when a substance with a highly positive oxidation potential encounters a substances with a lesser oxidation potential?
The high oxidation potential substance will be oxidized and the other will be reduced. Often this results in the high oxidation potential substance dissolving in solutions.
By how much does the oxidation number of manganese in MnO4- differ from Mn2+?
5
The oxidation number is an electron book keeping device that provides a measure of whether the atom is neutral, electron-rich or electron poor. It can be thought of as a fictitious charge that would be obtained if the elctrons in a compound are assigned to the more EN atoms.
Oxidation numbers are obtained by application of a set of empirical rules. The sum of all oxidation states in a species ALWAYS equals the charge on the species.
Ox# of Mn in MnO4- is 7
Ox# of Mn2+ is 2
