GenChem11 (Redox Rxns and Electrochemistry) Flashcards
The study of relationships between chemical reactions and electrical energy
Electrochemistry
Spontaneous reactions that produce electrical energy and non-spontaneous reactions that use electrical energy to produce a chemical change
Electrochemical Reactions
States that electrical charge can neither be created or destroyed
Law of conservation of charge
A loss of electrons is
A gain of electrons is
NOTE: These processes must occur simultaneously
Oxidation
Reduction
Something that causes another atom to undergo oxidation in a redox reaction and is itself reduced
Oxidizing Agent
Something that causes another atom to be reduced and is itself oxidized
Reducing Agent
What is the oxidation number of free elements? (ex: N2, P4, S8, He)
Zero.
True/False: Is the oxidation number for a monatomic ion is equal to the charge of the ion
True
The oxidation number of each group IA element in a compound is
The oxidation number of each group IIA element in a compound is
+1
+2
The oxidation number of each VIIA element is ______
Unless…
-1
Unless paired with an element of higher electronegativity (ex: in HOCl, Cl is +1)
The oxidation number of hydrogen is ___ in compounds with less electronegative elements than hydrogen
Give examples of these compounds
The normal oxidation number of H is
-1
NaH, CaH2
+1
In most compounds, the oxidation number of oxygen is..
This is not true when oxygen is paired with..
In peroxides, like BaO2, the oxidation number of oxygen is ____ because of…
-2
more electronegative atoms (ex: +2 in OF2)
In peroxides, like BaO2, the oxidation number of oxygen is -1 because of the structure of the peroxide ion [O-O]2-
The sum of the oxidation numbers of all the atoms present in a neutral compound is
Zero
Assign oxidation numbers to the following reaction and determine what is oxidized/reduced and what is the oxidizing/reducing agent:
SnCl2 +PbCl4 —> SnCl4 + PbCl2
Sn is oxidize
Pb is reduced
SnCl2 is the reducing agent (reductant)
PbCl4 is the oxidizing agent (oxidant)
Balance the following redox reaction:
MnO4(-) + I(-) —–> I2 + Mn(2+)
1) Balancing Atoms:
8H(+) + MnO4(-) —> 4H2O + Mn(2+)
2I(-) —> I2
2) Balancing charges 2I(-) --->I2 + 2e(-) (there must be -2 on both sides) 8H(+) + MnO4(-) + 5e(-) ---> Mn(2+) +4H20 (there must be +2 charge on both sides)
3) Bringing it all together
10 is common denominator for electrons
16H(+) + 10I(-) + 2MnO4(-) —> 5I2 + 2Mn(2+) +8H2O
4) Confirm that mass and charge are balanced (+4 on each side)
Contained systems in which a redox reaction occurs
Name the two types
Electrochemical cells
Galvanic (voltaic)
Electrolytic
Spontaneous reactions occur in ____ cells
Non-spontaneous reactions occur in ____ cells
Galvanic (voltaic) cells
Electrolytic cells
In all electrochemical cells, oxidation occurs at the
Reduction occurs at the
Anode
Cathode
(AN OX, RED CAT)
What is ΔG for galvanic (voltaic) cells?
This makes the reaction spontaneous or non-spontaneous?
ΔG<0 (negative)
Spontaneous
In a galvanic cell, electrons flow from….
Anode to cathode
In a galvanic cell, cations from the salt bridge flow towards the
Anions flow towards the
Anode (negatively charged)
Cathode (positively charged)
anode I anode solution II cathode I cathode solution
Cell diagram!
A single vertical line in a cell diagram indicates
A double vertical line indicates
A phase boundary
The presence of a salt bridge or some type of barrier
What is ΔG for electrolytic cells?
Therefore, it is spontaneous or non-spontaneous?
ΔG>0
Non-spontaneous
This type of reaction in a cell requires electrical energy to be induced.
Electrolysis
True/False: The oxidation and reduction half reactions are usually placed in one container in electrolytic cells
True
The amount of chemical change induced in an electrolytic cell is directly proportional to the number of ________ that are exchanged during a redox reaction
moles of electrons
One electron carries how much charge?
One mole of electrons contains how much charge (Faraday’s constant)?
1.6*10^-19
96,500 C
In an electrolytic cell, what is positively charged?
What is negatively charged?
Anode (negative salt ions flow towards it)
Cathode (positive salt ions flow towards it)
What is the source of the negative charge on the anode in a galvanic cell?
What is the source of the positive charge on the anode in an electrolytic cell?
The oxidation reaction that takes place there
The anode in an electrolytic cell is attached to the positive pole of a battery
No matter what type of cell, oxidation always occurs at the
Electrons flow from ___ to ____
Anode
Anode to the cathode
The cathode, regardless of cell type, attract the…
Cations (for reduction)
The tendency of a species to acquire electrons and be reduced, which can be used to determine what reactions occur during electrolysis in aqueous solution
Reduction potential
A more positive reduction potential means that a species is less/more likely to be reduced
More likely to be reduced
A reduction potential is measured in volts and is defined relative to the standard hydrogen electrode, which is arbitrarily give the a potential of
0.00 volts.
Standard reduction potential, or E°, is measured under standard conditions, which are..
25°C, 1atm, and 1M concentration of each ion that is part of the reaction
The difference in potential between two half cells, it is determined by adding the standard reduction potential of the reduced species and the standard oxidation potential of the oxidized species.
Should you multiply the volts by the number of moles being oxidized or reduced?
Standard electromotive force, E°cell
No!!
The standard emf of a galvanic cell is
The standard emf of an electrolytic cell is
positive
negative
What is the equation for Gibbs free energy, useful for determining spontaneity based upon the number of coulombs and the energy available
ΔG = -nFEcell
n=moles of electrons exchanged, F=faraday’s constant, E= Emf of the cell
NOTE: if Faraday’s constant is expressed in coulombs (J/V), then ΔG must be expressed in
J, NOT kJ
If the reaction takes place under standard conditions, what is the equation for gibbs free energy
ΔG° = -nFE°cell
What is the Nernst equation, useful for varying concentrations of reactants in a cell that aren’t 1M.
Ecell = E°cell - (RT/nF)(lnQ)
Q= [products]/[reactants]
Q= [C^c][D^d]/[A^a][B^b]`
The emf of a cell can be measured by a
A volmeter that drawas no current and gives a more accurate reading of the difference in potential between two electrodes is
voltmeter
potentiometer
Another manner of determining ΔG°, using the equilibrium constant
relate this equation to E°cell
ΔG° = -RT * lnKeq
nFE°cell = RT * lnKeq
What is Chad’s equation for non-standard cell potentials at 25°C?
E= E°- (0.0592/n) * logQ
If the current of a cell is 10A for 1.5 hours, how much charge is that?
How many moles of Na are produced from this charge?
10A = 10C/S
10C/S *5400s = 54,000 C
54,000C *(1 mol electrons/96500C) * (1mol Na/1 mol electrons) about equal to 0.55 mol Na
Note that if it’s Al3+, the number of moles of electrons would change to 3 and you would divide everything by 3
What is Chad’s equation for moles of product produced from amps?
(I)(T)/(n)(F)
