MCAT Review - Electrochemistry
Faraday's Law
- The amount of chemical change induced in an electrolytic cell is directly proportional to the number of moles of electrons that are exchanged during the oxidation reduction reaction. - the number of moles exchanged can be determined from the balanced half reaction - States that the liberation of gas and deposition of elements when electrode is directly proportional to the number of electrons being transferred during the oxidation reduction reaction. Here, normality, or gram equivalent weight as used. These observations or proxy measurements of the amount of current flowing in a circuit. - In general, for reaction that involves the transfer of n electrons per atom M: M^n+ + n e- —> M (s)
salt bridge function
- permits the exchange of cations and anions - contains an inert electrolyte (usually KCl or NH4NO3), which contains ions that will not react with the electrodes, or with the ions in solution
Galvanic Cells (voltaic)
- spontaneous, meaning that the reactions free energy is decreasing (delta G <0) as the cell releases energy to the environment - If the free energy changes negative for these cells, they are electromotive force must be positive
One electron carries what charge?
1.6 x10^-19 coulombs (C)
How much current is required to produce 0.23 kg Na from a molten sodium chloride electrolytic cell that runs for 30 hours? Assume the cell is 100% efficient.
10 A
Faraday constant (value)
96,485 C/mol e- ** can be rounded to 10^5 C/mol e- **
Nernst equation
Ecell= E°cell - (RT/nF) (lnQ) Ecell is the emf of the cell under non-standard conditions E°cell is the emf of the cell under standard conditions R is the ideal gas constant T is the temperature in kelvin n is the number of moles of electrons F is the faraday constant Q is the reaction quotient for the reaction to give a point of time - used when conditions deviate from standard conditions
How are concentration and the emf related?
Emf varies with the changing concentrations of the species in the cell
Circle which electrode each of the following statements describes in a galvanic cell: Electrons flow toward it Current flows toward it Has (-) designation Attracts cations
In a galvanic cell, the anode is the site of oxidation, has current flowing toward it, and has a (-) designation. The cathode has electrons flowing toward it, and attracts cations.
Circle which electrode each of the following statements describes an electrolytic cell: Electrons flow toward it Current flows toward it Has (-) designation Attracts cations
In an electrolytic cell, the anode is the side of oxidation and has current flowing toward it. The cathode has electrons flowing toward it, and has a (-) designation, and attracts cations.
Reaction quotient equation
Q = [C]^c [D]^d / [A]^a [B]^b
Redox reaction in a galvanic cell
Spontaneous, therefore, the change in Gibbs free energy for the reaction is negative (delta G < 0).
In galvanic cells, the current is
Spontaneously generated as electrons are released by the oxidize species at the anode and travel through the conductive material to the cathode, where reduction takes place.
electrodeposition equation
Summarizes this process and helps determine the number of moles of element being deposited on a plate. Equation can also be used to determine the amount of gas liberated during electrolysis mol M = It/nF mol M is the amount of metal ion being deposited at a specific electrode I is the current T is the time n is the number of electron equivalents for specific metal ion F is the faraday constant ** moles of metal, it is not fun **
Electrolyte
Surrounding each of the electrodes is an aqueous electrolyte solution, composed of cations and anions
isoelectric focusing
Technic used to separate amino acids are polypeptides based on their isoelectric point - The positively charged amino acids will migrate towards the cathode - The negatively charged amino acids will migrate toward the anode
What occurs as a spontaneous reaction proceeds toward equilibrium?
The movement of electrons results in a conversion of electrical, potential energy into kinetic energy
Because the anode of a galvanic cell is the source of electrons, it is considered
The negative electrode, while the cathode is considered, the positive electrode Electrons, therefore, move from negative (Lowe electrical potential) to positive (high electrical potential) while the current - the flow of positive charge - is from positive to negative.
Plating or galvanization
The precipitation process onto the cathode itself.
If only a wire were provided for this electron flow?
The reaction would soon stop because an excess positive charge would build up on the anode, and an XX negative charge would build up on the cathode. Eventually, the excessive charge accumulation would provide a counter voltage large enough to prevent the oxidation reduction reaction from taking place, and the current with cease. This charge gradient is dissipated by the presence of a salt bridge
Cathode always attracts
cations
Electrochemical cells
contained systems in which oxidation-reduction reactions occur
Which type of cell has a positive delta G? A positive Ecell?
electrolytic cells are non spontaneous and have a positive deltaG. Galvanic cells are spontaneous and have a negative deltaG; therefore, they have a positive Ecell.
Three fundamental types of electrochemical cells
galvanic (voltaic cells), electrolytic cells, and concentration cells
Galvanic cells and concentration cells
house spontaneous reactions
surge current
periods of large current (amperage) early in the discharge cycle - This is preferable in appliances, such as remote controls that demand rapid responses
lead-acid battery (lead storage battery)
specific type of rechargeable battery - voltaic cell that consist of 2/2 cells when fully charged
Movement of current (I)
the current (I) runs from cathode to anode
Electrolysis
the decomposition of a substance by an electric current - Because electro lysis is nonspontaneous, the electrode can consist of any material as long as it can resist a high temperatures in corrosion of the process
Anode
the electrode at which oxidation occurs
Cathode
the electrode at which reduction occurs
electromotive force (emf)
the force that results in the motion of electrons due to a difference in potential Corresponds to the voltage or electrical, potential difference of the cell
standard hydrogen electrode
the standard electrode against which the reduction potential of all electrodes can be measured (volts)
reduction potential
the tendency of a substance to gain electrons and to be reduced - tells us how likely a compound is to be reduced. The more positive the value, the more likely it is to be reduced, and the more it wants to be reduced.
How is the faraday constant expressed in coulombs?
{J/V}
In an electrochemical cell, the work done
Is dependent on the number of coulombs of charge transferred in the energy available.
Oxidation potential and reduction half reaction
It should be noted that reduction and oxidation are opposite processes. Therefore, to obtain the oxidation potential of a given half reaction, both the reduction, half reaction and the sign of the reduction potential reversed.
Voltmeter
A device used to measure voltage, or electrical potential energy difference
Positive E°red vs Less positive E°red
A greater relative tendency for reduction to occur, while a less positive E°red means a greater relative tendency for oxidation joker
energy density
A measure of a batteries ability to produce power as a function of its weight
electrode
A metal part of an electrochemical cell, which gains or loses electrons - the site of oxidation and reduction
If a cell's electromotive force (emf) is denoted as a positive value, what does that mean? What if it is negative?
A positive emf means the cell is spontaneous (galvanic); a negative emf means the cell is nonspontaneous (electrolytic).
All types of electrochemical cells have
A reduction reaction occurring at the cathode, an oxidation reaction occurring at the anode, a current, flowing from catheter and out, and electron flow from anode a cathode
salt bridge
A tube that allows the slow transfer of ions and maintains the neutrality of the electrolyte solutions. Contains an inert salt Purpose of the salt bridge is to exchange and ions and cations to balance, or dissipate, newly generated charges
nickel-cadmium battery
A type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. - AA and AAA batteries
Cations in salt bridge
Flow into the solution on the cathode side to balance out the charge of the sulfate ion left, and solution with the copper ions are reduced to Cu and precipitate onto the electrode
Each species has its own intrinsic reduction, potential: the more positive the potential,
The greater the tendency to be reduced
How is the charge carried by one more of electrons calculated?
By multiplying 1.6x10^-19 by Avogadro's number: [1.6x10^-19/electron][6.02x10^23 electrons/1 mol e-] = 96,485 C/mol e-
Currents flow
Cathode to anode
Cathode in an electrolytic cell
Considered negative because it is attached to the negative power of the external voltage source, and attracts cations from the solution
Anode in an electrolytic cell
Considered positive because it is attached to the positive pole of the external voltage source, and attracts anions from the solution.
Note the significance of the negative sign on the right side of the equation. Why is this? Delta G° = -nEF°cell
Delta G and E°cell will always have opposite sides; therefore, galvanic cells have negative Delta G° and positive E°cell values. Electrolytic cells have positive delta G and negative Ecell values.
Equation to determine delta G°
Delta G° = -RTlnKeq R is the ideal gas constant T is the absolute temperature Keq is the equilibrium constant for the reaction
How are delta G° and emf related?
Delta G° = -nEF°cell Delta G is the standard change in free energy n is the number of moles of electrons exchange F is the faraday constant E°cell is the standard emf of the cell
Combining the two expressions that sulfur standard for energy change
Delta G° = -nFE°cell = -RTlnKeq or nFE°cell = RTlnKeq
Anions from the salt bridge
Diffuse into the solution on the Anna outside to balance out the charge of the newly created zinc ions
Relative reactivities of different have cells can be compared to predict what?
Direction of electron flow
How is the emf determined?
E°cell = E°red,cathode- E°red,anode
standard reduction potential
E˚ - measured under standard conditions (25° C/298 K, 1 atm, 1 M) a higher E˚ means a greater tendency for reduction to occur a lower E˚ means a greater tendency for oxidation to occur
movement of electrons
For all electrochemical cells, the movement of electrons is from anode to cathode Electrons move through an electrochemical cell opposite to the flow of current
For electrolytic cells, define cathode and anode
For electrolytic cells, the electrode with a more positive reduction potential is forced by the external voltage source to be oxidized and his, therefore, the anode. The electrode with the less positive reduction potential is forced to be reduced. It is, therefore, the cathode. Because the movement of electrons in the direction against the tendency or desires of the respective electrochemical species, the reaction is nonspontaneous, and delta G is positive.
Direction of spontaneous movement of charge for a galvanic cell
For galvanic cells, the direction of spontaneous movement of charges from the anode, the side of oxidation, to the cathode, the side of reduction.
Spontaneity is indicated by the change in
Gibbs free energy, delta G
How are standard reduction potential's measured?
Hey sample is measured by setting up a cell relative to a standard hydrogen electrode, which is given a reduction potential of 0V by convention.
With electrolytic cells, should the half reactions be separated?
No, because the desired reaction is not spontaneous
Electrolytic cells contain
Nonspontaneous reactions
According to the faraday equation, when will one mole of metal be produced?
One mole of metal M (s) well logically be produced if n moles of electrons are supplied to one mole of M^n+
Rechargeable cell or rechargeable battery
One that can function as both a galvanic and electrolytic cell
Electrolytic cells
Opposite of galvanic cells - House of nonspontaneous reaction to that require the input of energy to proceed; the change in free energy for an electrolytic cell is positive, called electrolysis
Write the cell diagram for the discharging state of a lead acid battery.
Pb (s) | H2SO4 (4M) || H2SO4 (4M) | PbO2 (s)
If the equilibrium constant is equal to one
The E°cell = 0V for any concentration cell with equal molar concentrations in both half cells because there's no net ionic equation
For redox reactions with the equilibrium constant less than one
The E°cell will be negative because the natural logarithm of any number between zero and one is negative These properties are characteristics of electrolytic cells, which house, non-spontaneous, oxidation reduction reactions
If the equilibrium constant for the reaction is greater than one
The E°cell will be positive, because the natural logarithm of any number greater than one is positive. These properties are characteristics of galvanic cells which house, spontaneous, oxidation reduction reactions.
What is the faraday constant equivalent to?
The amount of charge contained in one mole of electrons or one equivalent
Positive electromotive force
The cell is able to release energy (delta G < 0), which means it is spontaneous
What is an example of a concentration cell in a biological system?
The cell membrane of a neuron
Negative electromotive force
The cell must absorb energy (delta G > 0), which means it is nonspontaneous
Defined the change in Gibbs free energy, delta G
The change in the amount of energy available in a chemical system to do work.
When the electrodes are connected to each other by conductive material, what happens?
The charge will begin to Flow as a result of an oxidation reduction reaction that is taking place between the two have sales.
For the concentration, so, what must the concentrations of the ions be?
The concentrations of the ions in the two compartments must be different for there to be a miserable voltage and current
Because concentration cells have chemically identical electrodes, how is the current generated?
The current is generated as a function of a concentration gradient, establish between the two solution surrounding the electrodes. The concentration gradient result in a potential difference between the two compartments, and drives the movement of electrons in the direction that result in equilibrium of the ion gradient.
When will the current stop?
The current will stop when the concentration of ionic species in the half cells are equal. This implies that the voltage (V) or electromotive force of a concentration cell is zero when the concentrations are equal; the voltage as a function of concentrations can be calculated using the Nernst equation.
Standard electromotive force
The difference in standard reduction potential between the two half-cells under standard conditions
For galvanic cells, define cathode and anode
The electrode with the more positive reduction potential is the cathode, and the electrode with the less positive reduction potential is the anode Because the species with a stronger tendency to gain electrons is actually doing so, the reaction of spontaneous and delta G is negative.
What is the distinguishing characteristic of a concentration cell?
The electrodes are chemically identical
What does the external voltage source supply?
The external voltage source, battery, supplies, energy sufficient to drive the oxidation reduction reaction in the direction that is thermodynamically unfavorable (nonspontaneous)
When will the standard reduction potential of an electrode change?
The standard reduction potential of an electrode will not change UNLESS the chemical identity of the electronics changed
Because the solutions and electrodes are physically separated, ...
They must be connected by conductive material to complete the circuit
half-cells of voltaic cells
Two electrodes of distinct chemical identity are placed in separate compartments, which are called have sales. The two electrodes are are connected to each other by conductive material, such as a copper wire
Why do we separate the reduction and oxidation half reaction to the two compartments?
We are able to harness this energy and use it to do work by connecting various electrical devices into the circuit between the two electrodes
Voltmeter
a device used to measure voltage
concentration cell
a galvanic cell - it contains to have cells connected by conductive material, allowing a SPONTANEOUS oxidation reduction reaction to proceed, which generates a current and delivers energy
Potentiometer
a kind of voltmeter that draws no current and gives a more accurate reading of the difference in potential between two electrodes
Nernst equation
a mathematical relationship used to calculate an ionic equilibrium potential
cell diagram
a shorthand notation representing the reactions in an electrochemical cell 1) the reactants and products are always listed from left to right in this form: Anode | anode solution || cathode solution | cathode 2) a vertical line indicates a Phase boundary 3) a double vertical line indicates the presence of a salt bridge or some other type of barrier
Where does reduction always take place?
cathode
Daniell cell
an electrochemical cell in which the anode is the site of Zn metal oxidation and the cathode is the site of Cu2+ ion reduction
Anode always attracts
anions
Where does oxidation always take place?
anode
Electrons always flow from
anode to cathode
