Flashcards in General Chemistry- Electrochemistry Deck (86)
What is the primary purpose of the mitochondrion?
To manufacture a deliverable and usable form of energy.
What is the average daily turnover of ATP in humans.
More than 50 kilograms
What is the voltage?
An electrical potential difference
What is the voltage called in a concentration cell?
What is the voltage called in the mitrochondria?
Proton-motive force in mitochondria
In a battery, what reaction takes place?
An oxidation reduction reaction takes place and the electrons move in the direction that causes the concentration gradient to be dissipated.
In the mitochondria, how is charge harnessed?
The charge builds up in the form of a hydrogen ion (proton) gradient between the intermembrane space and the matrix. Embedded in the iner membrane is ATP synthase, which serves a dual role as a prton channel and a catalyst for the formation of the high-energy phosphate bond in ATP, As the hydrogen ions flow down their electrochemical gradient, energy is dissipated, and this energy is harnessed by ATP synthase to form ATP
What are electrochemical cells?
Contained systems in which redox reactions occur
What are the three fundamental types of electrochemical cells?
What batteries can help understand the electrochemical cells?
Which two electrochemical cells house spontaneous reactions?
Galvanic cells and Concentration cells
Which cells house nonspontaneous reactions?
Spontaneity is indicated by the change in what?
Gibbs Free Energy
All three electrochemical cells contain what?
Electrodes where oxidation and reduction take place
For all electrochemical cells, the electrode where oxidation occurs is called what? Reduction?
What is electromotive force?
(emf) corresponds to the voltage or electrical potential difference of the cell.
If the emf is positive what does it mean for the cell? Negative?
Positive: The cell is able to release energy (/\G 0), wo it is nonspontaneous
For all electrochemical cells, the movement of electrons is from ___________ to __________.
Anode to cathode
The current runs from __________ to ________.
Cathode to anode
In chemistry current represents what?
A theoretical flow of positive charge
What are always equal in magnitude but opposite in direction?
Current and flow equal in magnitude, but opposite in direction
Can batteries and cells be influenced by temperature change?
What type of electrochemical cells are nonrechargeable batteries?
Galvanic cells, also called voltaic cells
In galvanic cells, the free energy is what?
Decreasing, as the cell reases energy to the environment
If free energy is negative then what is positive?
Their electromotive force (Ecell) must be positive. They always have opposite signs
What does the inside of a galvanic cell look like?
Two electrodes of distinct chemical identity are placed in separate compartments, which are called half-cells. The two electrodes are connected to each other by a conductive material, such as a copper wire. Along the wire, there may be other various components of a circuit, such as resistors or capacitors. Surrounding each of the electrodes is an aqueous electrolyte solution composed of cations and anions. The cations in the two half-cell colutions can be the same element as the respective metal electrode. Connecting the two solution is a structure called a salt bridge. When the electrodes are connected to each other by a conductive material, charge will begin to flow as the result of redox reaction that takes place in the two half-cells. The redox reaction in a galvanic cell is spontanteous. As the reaction procees toward equilibrium, the movement of electrons results in a conversion of electrical potential energy into kinetic energy. By separating the reduction and oxidation half-reactions, 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.
What does a salt bridge consist of?
An Inert salt
What does the inner workings of a Daniell cell look like?
A zinc electrode is placed in an aqueous ZnSO4 solution and a copper electrode is placed in an aqueous CuSO4 solution. The anode of this cell is the zinc bar where Zn (s) is oxidized to Zn 2+ (aq). The cathode is the copper bar, and it is the site of the reduction of Cu 2+ (aq) to Cu (s)
What are the half-cell reactions for the Daniell cell?
Zn (s) --> Zn 2+ (aq) + 2 e- Ered= -0.762V (Anode)
Cu 2+ (aq) + 2e- --> Cu (s) Ered= +0.340V (cathode)
What is the net reaction of a Daniell cell?
Zn (s) + Cu 2+ (aq) --> Zn 2+ (aq) + Cu (s) Ecell=+1.102V
If the Daniell cell was set up wrong what would happen?
The Cu 2+ ions would react directly with the zinc bar, and no useful electrical work would be done. Because the solution and electrodes are physically separated, they must be connected by a conductive material to complete the circuit.
However, 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 excess negative charge would build up on the cathode. Eventually, the excessive charge accumulation would provide a countervoltage large enough to preven the redox reaction from taking placec. The charge gradient is dissipated by the presence of a salt bridge, which permits the exchange of cations and anions.
How does the salt bridge work?
It contains an inert electrolyte, usually KCl or NH4NO3, which contains ions that will not react with the electrodes or with the ions in solution. While the anions from the salt bridge diffuse into the solution on the anode side to balance out the charge of the newly created Zn 2+ ions, the cations of the salt bridge (K+) flow into the solution on the cathode side to balance out the charge of the sulfate ions left in solution when the Cu 2+ ions are reduced to Cu and precipitate onto the electrode.
What is plating?
The precipitation process onto the cathode
What's another name for plating?
Why is cell life short in batteries?
The flow depletes the salt bridge and along with the finite quantity of Cu 2+ in the solution, accounts for the relatively short lifespan of the cell.
What is a cell diagram?
A shorthand notation representing the reactions in an electrochemical cell.
What is a cell diagram for the Daniell cell?
Zn (s) | Zn 2+ (1M) || Cu 2+ (1M) | Cu (s)
What are the rules used to construct a cell diagram?
1. The reactants and products are always listed from left to right in this form: anode | anode solution || cathode solution | cathode
2. A single vertical line indicates a phase boundary
3. a double vertical line indicates the presence of a salt bridge or some other type of barrier.
What is similar and different about electrolytic cells and galvanic cells?
All types of electrochemical cells have a reduction reaction occuring at the cathod, and oxidation reaction at the anode, a current from from cathode to anode, and electron flow from anode to cathode.
Electrolytic cells are opposite of galvanic cells. Electrolytic cells house nonspontaneous reactions that require the input of energy to proceed.
What is electrolysis?
Redox reaction driven by an external voltage source, in which chemical compounds are decomposed.
In the electrolytic cell of molten NaCl, what occurs?
NaCl is decomposed into Cl2 (g) and Na (l). The external voltage source--a battery-- supplies energy sufficient to drive the redox reaction in the direction that is thermodynamically unfavorable. Na+ ions migrate toward the cathod, where they are reduced to Na (l). At the same time, Cl- ions migrate toward the anion, where they are oxidized to Cl2 (g). Notice that the half-reactions do not need to be separated into different compartments. Sodium is a liquid and less dense than the molten salt, and thus easily removed as it floats to the top of the reaction vessel.
Who was the 1st to define certain quantitative principle of electrolytic cells?
What did Michael Faraday theorize?
He theorized that 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 redox reaction. The number of moles exchanged can be determined from the balanced half-reaction. In general, for a reaction that infoved the transfer of n electrons per atom M,
(M^n+) + ne- --> M (s)
What does Michael Faraday's equation suggest?
One mole of metal M (s) will logically be produced if n moles of electrongs are supplied to one mole of (M^n+). Additionally, the number of moles of electrons needed to produce a certain amount of M (s) can now be related to the measureable electrical property of charge. One electron carries a charge of 1.6 x 10^-19 coulombs (C). The charge carried by one mole of electrons can be calculated by multiplying this number by Avogadro's number.
What is Faraday constant?
One Faraday is equivalent to the amount of charge contained in one mole of electrons (1 F= 96,485 C) or one equivalent. On the MCAT round up to 10^5 C/mol e-
What is the electrodeposition equation?
mol M= It/nF
What do the letters represent in the electrodeposition equation?
mol M= It/nF
mol M: amount of metal ion being deposited at a specific electrode
n: number of electron equivalents for a specific metal ion
F: Faraday constant
What is the difference between a galvanic cell and a concentration cell?
The design. The electrodes are chemically indentical. For example, if both electrodes are copper metal, they have the same reduction potential. Therefore, current is generated as a function of a concentration gradient established between the two solutions surrounding the electrodes.
How does concentration cells work?
The concentration gradient results in a potential difference between the two compartments and drives the movement of electrons in the direction that results in equilibration of the ion gradient. 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.
How can the voltage be calculated, as a function of concentration?
The Nerst Equation
Biologically, how can a concentration cell best be represented?
By the cell membrane of a neuron
Biologically, what occurs in a concentration cell?
Sodium and potassium cations, and chlorine anions are exchanged as needed to produce an electrical potential. The actual value depends on both the concentrations and charges of the ions. In this way, a resting membrane potential (Vm) can be maintained. Disturbances of the resting membrane potential, if sufficiently large, may stimulate the firing of an action potential.
What's special about rechargeable cells or rechargeable batteries?
One that can function as both a galvanic and electrolytic cell.
What is an example of a rechargeable battery?
A Lead-acid battery or Lead storage battery.
What does a lead-acid battery consist of?
As a voltaic cell, when fully charged, it consists of two half-cells, a Ph anode and a porous PbO2 cathode, connected by a conductive material (concentrated 4M H2SO4). When fully discharged, it consists of two PbSO4 electroplated lead electrodes with a dilute concentration of H2SO4.
What is the oxidation half reacction of the lead anode of a lead-acid battery?
Pb (s) + H2SO4- (aq) --> PbSO4 (s) + H+ (aq) + 2e-
E degree red= -0.356V
What is the reduction half-reaction of the lead (IV) oxide cathode of a lead-based battery?
PBO2 (s) + SO4 2+ (aq) + 4H+ + 2e- --> PBSO4 (s) + 2 H2O
E degree red= 1.685V
Both half reaction cause what in a lead-acid battery?
The electrodes to plate with the lead sulfate and dilute the acid electrolyte when discharging. The lead anode is negatively charged and attracts the anionic bisulfate. the Lead (IV) oxide cathode is bit more complicated. This electrode is porous, which allows the electrolyte (sulfuric acid) to solvate the cathode into lead and oxide ions. Then, the hydrogen ions in solution react with the oxide ions to produce water, and the remaining sulfate ions react with the lead to produce the electroplated lead sulfate.
What is the net equation for a discharging lead-acid battery?
Pb (s) + PBO2 (s) + 2 H2SO4- (aq) --> 2 PBSO4 (s) + 2 H2O
E degree cell= 2.041V
When charging, the lead-acid cell is part of what?
An electrolytic circuit.
When charging, how is the Lead-aid cell different?
The equations and electrode charge designations are the opposite because an external source reverses the electroplating process and concetrated the acid solution
What is energy density?
A measure of a battery's ability to produce power as a function of its weight.
Which cell has the lowest energy density? Why?
Lead-acid batteries have the lowest energy-to-weight ratios. Lead-acid batteries, therefore, require a heavier amount of battery material to produce a certain output as compared to other batteries.
What's another example of a rechargeable cell?
What do Nickel-Cadmium batteries consist of ?
Two half-cells made of solid cadmium (the anode) and Nickel (III) oxide-hydroxide (the Cathode) connected by a conductive material, usually potassium hydroxide (KOH)
What is the oxidation half reaction at the cadmium anode?
Cd (s) + 2 OH- (aq) --> Cd(OH)2 (s) + 2e-
E degree red= -0.86V
What is the reduction half-reaction at the nickel (III) oxide-hydroxide cathode?
2 NiO(OH) (s) + 2 H20 + 2e- --> 2 Ni(OH)2 (s) + 2OH-
E degree red= 0.49
What is the net reaction equation for Ni-Cd batteries?
2 NiO(OH) (s) + Cd (s) + 2 H20 --> 2 Ni(OH)2 (s) + Cd(OH)2 (s)
E degree cell= 1.35V
Why are some Ni-Cd batteries vented?
Because charging reverses the electrolytic cell potentials and so the vents allow for the release of built up hydrogen and oxygen gas during electrolysis.
Ni-Cd have higher ________ compared to lead-acid batteries?
Energy density and surge current.
What is surge current?
Period of large current (amperage) early in the discharge cycle.
What is important to note about Ni-Cd batteries?
Been replaced by nickel-metal hydride (NiMH) batteries.
How does Nickel-metal hydride batteries compare to Ni-Cd batteries?
More energy density, are more cost effective, and are significantly less toxic. In lieu of a pure metal anode, a metal hydride is used instead.
What is considered the negative electrode in a galvanic cell? Why?
Because it is the source of electrons
What is considered positive in a electrolytic cell? Why?
It is attached to the positive pole of the external voltage source and attracts anions from the solution.
What is considered negative in an electrolytic cell? Why?
It is attached to the negative pole of the external voltage source and attracts cations from the solution.
Despite the different charge signs in the cell. What always takes place at the anode? Cathode?
Electrons flow from the anode to the cathode
Current from the cathode to the anode.
Now matter the charge designation, cathode always attracts _______ and the anode always attracts ______.
Cathode attracts cations
What is isoelectric focusing?
A technique used to separate amino acids or polypeptides based on their isoelectric points (pI)
In isoelectric focusing, the amino acids will migrate where?
The positively charged amino acids will migrate toward the cathode, negatively charged amino acids will migrate toward the anode.
How have the relative tendencies of different species to be reduced been determined?
Experimentally, using the tendency of the hydrogen ion (H+) to be reduced as an arbitrary zero reference point.
The reduction potential is measured in what?
How is reduction potential defined, relative to what?
The standard hydrogen electrode (SHE) which is given a potential of 0 V by convention.
How can the species in a reaction that will be oxidized or reduced by determined?
From the reduction potential of each species.
What is the reduction potential of a species?
The tendency of a species to gain electrons and to be reduced.