General Chemistry- Electrochemistry
Mnemonic for electrodes in an Electrochemical Cell
AN OX and a RED CAT The anode is the site of oxidation; reduction occurs at the cathode.
How is the Ni-Cd battery different when charging?
charging reverses the electrolytic cell potentials. Some Ni-Cd designs are vented for this reason to allow for the release of built up hydrogen and oxygen gas during electrolysis.
How is the lead battery different when charging?
When charging, the lead-acid cell is part of an electrolytic circuit. The discharging equations and electrode charge designations are the opposite because an external source reverses the electroplating process and concentrates the acid solution—this external source is very evident when one uses jumper cables to restart a car.
How are electrolytic cells different from galvanic/voltaic cells?
Whereas galvanic cells house spontaneous oxidation-reduction reactions that generate electrical energy, electrolytic cells house nonspontaneous reactions that require the input of energy to proceed.
electromotive force (emf),
corresponds to the voltage or electrical potential difference of the cell.
It is important to note that modern Ni-Cd batteries have largely been replaced by
more efficient nickel-metal hydride (NiMH) batteries. These newer batteries have more energy density, are more cost effective, and are significantly less toxic. As the name suggests, in lieu of a pure metal anode, a metal hydride is used instead.
In an electrolytic cell, the anode is...
positive and the cathode is negative. This is because an external source is used to reverse the charge of an electrolytic cell. However, in both types of cells, reduction occurs at the cathode, and oxidation occurs at the anode; cations are attracted to the cathode, and anions are attracted to the anode.
All of the nonrechargeable batteries you own are galvanic cells, also called voltaic cells. Because household batteries are used to supply energy to a flashlight or remote control, the reactions in these cells must be
spontaneous. This means that the reaction's free energy is decreasing (ΔG < 0) as the cell releases energy to the environment. If the free energy change is negative for these cells, their electromotive force (Ecell) must be positive; the free energy change and electromotive force always have opposite signs.
If the emf is positive
the electrochemical cell is able to release energy (ΔG < 0), which means it is spontaneous.
If the emf is negative
the electrochemical cell must absorb energy (ΔG > 0), which means it is nonspontaneous.
Compared to the energy density in other cells, Ni-Cd batteries...
Ni-Cd batteries have a higher energy density than lead-acid batteries. The electrochemistry of the Ni-Cd half-reactions also tends to provide higher surge current. This is preferable in appliances such as remote controls that demand rapid responses.
What physics formula for work is similar to ΔG° = -nFE°cell?
Notice the similarity of this relationship to that expressed in the physics formula W = qΔV for the amount of work available or needed in the transport of a charge q across a potential difference ΔV: n × F is a charge, and E° cell is a voltage.
Faraday's Constant (1 Faraday)
One faraday (F) is equivalent to the amount of charge contained in one mole of electrons (1 F = 96,485 C). On the test round up to 10⁵.
rechargeable cell or rechargeable battery
is one that can function as both a galvanic and electrolytic cell.
Anode
Always the site of oxidation. It attracts anions.
Nernst equation and equilibria
ΔG° can also be determined in another manner: ΔG° = -RT ln Keq where R is the ideal gas constant, T is the absolute temperature, and Keq is the equilibrium constant for the reaction. Combining the two expressions that solve for standard free energy change, we see that ΔG° = -nFE°cell = -RT ln Keq or nFE°cell = RT ln Keq By extension, if the values for n, T, and Keq are known, then E°cell for the reaction is easily calculated. On the MCAT, you will not be expected to calculate natural logarithm values in your head. That being said, these equations can still be tested but in a conceptual way When Keq > 1, then +𝐸°cell (products are favored) When Keq < 1, then -𝐸°cell (reactants are favored) When Keq = 1, then 𝐸°cell= 0 (concentrations are equal)
How do galvanic (voltaic) cell work?
*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. *Surrounding each of the electrodes is an aqueous electrolyte solution composed of cations and anions. *Connecting the two solutions is a structure called a salt bridge, which consists of an inert salt. *When the electrodes are connected to each other by a conductive material, charge will begin to flow as the result of an oxidation-reduction reaction that is taking place between the two half-cells. *The redox reaction in a galvanic cell is spontaneous, (ΔG < 0). *the movement of electrons results in a conversion of electrical potential energy into kinetic energy. *By separating the reduction and oxidation half-reactions into 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.
Concentration cell
A concentration cell is a special type of galvanic cell. The distinguishing characteristic is the electrodes are chemically identical. 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. 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 concentrations 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.
Remember that the reaction quotient, Q, for a general reaction aA + bB → cC + dD has the form:
Although the expression for the reaction quotient Q has two terms for the concentrations of reactants and two terms for the concentrations of products, remember that only the species in solution are included- do not include pure substances (s, l, g). When considering the case of the Daniell cell, for example, only the concentrations of zinc and copper ions are considered: Zn(s)+Cu²⁺(aq)→Zn²⁺(aq)+Cu(s) Q=[Zn²⁺]/[Cu²⁺]
Cathode
Always the site of reduction. It attracts cations
What is true of anions and cations and their relationship with anodes and cathodes regardless of the type of cell (galvanic, electrolytic, or concentration cells).
Anions are attracted to the anode. Cations are attracted to the cathode.
Simplified version of the Nernst equation to use on the MCAT
Ecell=E⁰cell-(0.0592/n)*logQ This simplified version of the equation brings together R, T (298 K), and F, and converts the natural logarithm to the base-ten logarithm to make calculations easier. E° cell is the emf of the cell under standard conditions, n is the number of moles of electrons, and Q is the reaction quotient for the reaction at a given point in time.
Describe how are electrolytic cells work
Electrolytic cells house nonspontaneous reactions that require the input of energy to proceed: ∆G is positive. This type of oxidation-reduction reaction driven by an external voltage source is called electrolysis, in which chemical compounds are decomposed. For example, molten NaCl is decomposed into Cl₂(g) and Na(l). *The external voltage source—a battery—supplies energy sufficient to drive the oxidation-reduction reaction in the direction that is thermodynamically unfavorable (nonspontaneous). *In this example, Na+ ions migrate toward the cathode, where they are reduced to Na(l). At the same time, Cl- ions migrate toward the anode, where they are oxidized to Cl₂(g).
Mnemonic for remembering electron flow in all electrochemical cells.
Electron flow in an electrochemical cell happens in alphabetical order: A → C Electrons flow from anode to cathode in all types of electrochemical cells.
How can you determine which electrode will be the cathode and which will be the anode in an electrolyic cell?
For electrolytic cells, the electrode with the more positive reduction potential is forced by the external voltage source to be oxidized and is, therefore, the anode. The electrode with the less positive reduction potential is forced to be reduced and is, therefore, the cathode. Because the movement of electrons is in the direction against the tendency or desires of the respective electrochemical species, the reaction is nonspontaneous and ΔG is positive.
How can you determine which electrode will be the cathode and which will be the anode in a galvanic cell?
For galvanic cells, 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 (that wants to gain electrons more) is actually doing so, the reaction is spontaneous and ΔG is negative.
What is an example of a Galvanic (Voltaic) cell half reaction? And Net reaction?
Half Reaction: Zn(s)→Zn²⁺(aq)+2e⁻ E⁰red=-0.726 (anode) Cu²⁺(aq) +2e⁻→Cu(s) E⁰red=0.340 (cathode) Net Reaction Zn(s)+Cu²⁺(aq)→Zn²⁺(aq)+Cu(s) Ecell=+1.102V
Lead-Acid Batteries/Lead Storage Battery
Is a specific type of rechargeable battery. As a voltaic cell, when fully charged, it consists of two half-cells—a Pb anode and a porous PbO₂ cathode, connected by a conductive material (concentrated 4 M H₂SO₄). When fully discharged, it consists of two PbSO₄ electroplated lead electrodes with a dilute concentration of H₂SO₄.
Reduction Potential:
Quantifies the tendency for a species to gain e⁻ and be reduced. More positive E⁰red = greater tendency to be reduced.
Summary of Batteries electrode charge designations of batteries
Rechargeable Batteries: Can experience charging (electrolytic) and discharging (galvanic) states. Lead-Acid: Discharging: Pb anode, PbO₂ cathode in a concentrated sulfuric acid solution. Low energy density. Ni-Cd: Discharging: Cd anode, NiO(OH) cathode in a concentrated KOH solution. Higher energy density than lead-acid batteries. NiMH: More common than Ni-Cd because they have higher energy density.
Surge Currents
Surge currents are periods of large current (amperage) early in the discharge cycle.
What are the half-reactions and net reactions for Ni-Cd batteries when discharging?
The oxidation half-reaction at the cadmium (negative) anode is: Cd(s)+2OH⁻(aq)→Cd(OH)₂(s)+2e⁻ E⁰red=-0.86 The reduction half-reaction at the nickel oxide-hydroxide (positive) cathode is: 2Ni(OH)(s)+2H₂O+2e⁻→2Ni(OH)₂(s)+2OH⁻ E⁰red=0.49 Both half-reactions cause the electrodes to plate with their respective products. Overall, the net equation for a Ni-Cd battery is: 2Ni(OH)(s)+Cd+2H₂O→2Ni(OH)(s)₂+Cd(OH)₂(s) E⁰Cell=0.49V-(-0.86V)-=1.35V
What are the oxidation and reduction half-reactions for lead batteries when discharging? What is the Net Reaction when dischahrging
The oxidation half-reaction at the lead (negative) anode is: Pb(s) + HSO₄⁻(aq)→ PbSO₄(s)+H⁺(aq)+2e⁻ E⁰red=-0.356V The reduction half-reaction at the lead(IV) oxide (positive) cathode is: PbO₂(s)+SO₄²⁻(aq)+4H⁺+2e⁻→PbSO₄(s)+2H₂O E⁰red=1.685V Net Reaction: Pb(s) +PbO₂(s)+2H₂SO₄⁻(aq)→2PbSO₄(s)+2H₂O E⁰Cell=1.685V-(-0.356V)-=2.041V Remember Lead-Acid: Discharging: Pb anode, PbO2 cathode in a concentrated sulfuric acid solution. Low energy density.
What is the purpose of the salt bridge in a galvanic or voltaic cell?
The purpose of the salt bridge is to exchange anions and cations to balance, or dissipate, newly generated charges. If only a wire were provided for this electron flow, an excess positive charge would build up on the anode, and an excess negative charge would build up on the cathode causing a countervoltage large enough to prevent the oxidation-reduction reaction and the current. The salt bridge contains an inert electrolyte, usually KCl or NH₄NO₃, which contains ions that will not react with the electrodes or with the ions in solution but will help balance out the charge.
E° red indicates?
The reduction potential, or the likelihood of a compound to be reduced via a given reaction.
Describe how to write a cell diagram, which is shorthand notation representing the reactions in an electrochemical cell.
Zn (s) | Zn²⁺ (1 M) || Cu2+ (1 M) | Cu (s) The following rules are used in constructing a cell diagram: 1. The reactants and products are always listed from left to right in this form: anode | anode solution (concentration) || cathode solution (concentration) | 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.
Nickel-cadmium batteries
are also rechargeable cells. They consist of two half-cells made of solid cadmium (the anode) and nickel(III) oxide-hydroxide (the cathode) connected by a conductive material, typically potassium hydroxide (KOH). Most of us are familiar with AA and AAA cells made of Ni-Cd materials, inside of which the electrodes are layered and wrapped around in a cylinder.
Michael Faraday was the first to define certain quantitative principles governing the behavior of electrolytic cells. Faraday's laws state that the liberation of gas and deposition of elements on electrodes is:
directly proportional to the number of electrons being transferred during the oxidation-reduction reaction. Here, normality or gram equivalent weight is used. These observations are proxy measurements of the amount of current flowing in a circuit. In general, for a reaction that involves the transfer of n electrons per atom M, Mⁿ⁺ + n e⁻ → M (s) According to this equation, one mole of metal M (s) will logically be produced if n moles of electrons are supplied to one mole of Mⁿ⁺. Additionally, the number of moles of electrons needed to produce a certain amount of M (s) can now be related to the measurable electrical property of charge.
For all electrochemical cells, the movement of electrons is? The movement of the current is?
electrons from anode to cathode the current (I) runs from cathode to anode The current and the flow of electrons are always of equal magnitude but in opposite directions.
If E°cell is positive, ln Keq is positive. This means that Keq must be
greater than one and that the equilibrium lies to the right (products are favored). Knowing
Compared to the energy density in other cells, Lead-acid batteries
have some of the lowest energy-to-weight ratios (otherwise known as energy density). Lead-acid batteries, therefore, require a heavier amount of battery material to produce a certain output as compared to other batteries.
Energy density
is a measure of a battery's ability to produce power as a function of its weight.
Because the anode of a galvanic cell is the source of electrons, it is considered...
the negative electrode; the cathode is considered the positive electrode. Electrons, therefore, move from negative (low electrical potential) to positive (high electrical potential), while the current—the flow of positive charge—is from positive (high electrical potential) to negative (low electrical potential). MNEMONIC In a galvanic cell, the anode is negative.
Concentration and the emf of a cell are related: emf varies with the changing concentrations of the species in the cell. When conditions deviate from standard conditions, one can use the Nernst equation:
where Ecell is the emf of the cell under nonstandard conditions, E° cell is the emf of the cell under standard conditions, R is the ideal gas constant, T is the temperature in kelvins, n is the number of moles of electrons, F is the Faraday constant, and Q is the reaction quotient for the reaction at a given point in time.
The electrodeposition equation summarizes this process and helps determine the number of moles of element being deposited on a plate:
where mol M is the amount of metal ion being deposited at a specific electrode, I is current, t is time, n is the number of electron equivalents for a specific metal ion, and F is the Faraday constant. This equation can also be used to determine the amount of gas liberated during electrolysis. Mnemonic for the Electrodeposition equation: Calculating Moles of Metal, It is Not Fun.
standard electromotive force (emf or E° cell) of a reaction
which is the difference in potential (voltage) between two half-cells under standard conditions. The emf of a reaction is determined by calculating the difference in reduction potentials between the two half-cells: E° cell = E°red,cathode − E° red,anode When subtracting standard potentials, do not multiply them by the number of moles oxidized or reduced. This is because the potential of each electrode does not depend on the size of the electrode (the amount of material), but rather the identity of the material. The standard reduction potential of an electrode will not change unless the chemical identity of that electrode is changed.
In an electrochemical cell, the work done is dependent on the number of coulombs of charge transferred and the energy available. Thus, ΔG° and emf are related as follows:
ΔG° = -nFE°cell where ΔG° is the standard change in free energy, n is the number of moles of electrons exchanged, F is the Faraday constant, and E°cell is the standard emf of the cell. Keep in mind that, if the Faraday constant is expressed in coulombs (J/V) then ΔG° must be expressed in J, not kJ.
What is the significance of the negative sign on the right side of the equation ΔG° = -nFE°cell?
ΔG° and E°cell will always have opposite signs. Therefore, galvanic cells have negative ΔG° and positive E°cell values; electrolytic cells have positive ΔG° and negative E°cell values.
