Electrochemistry

what are the similarities between galvanic cells and electrolytic cells?

- oxidation always takes place at the anode and reduction always takes place at the cathode - electrons always flow through the wire from the anode to the cathode -current always flows from cathode to anode -the cathode always attracts cations and the anode always attracts anions (regardless of charge)

NiMH batteries

- replaced modern Ni-Cd batteries - more efficient - nickel-metal hydride (NiMH) batteries - have more energy density, and are more cost effective and are significantly less toxic - in lieu of a pure metal anode, a metal hydride is used instead

Michael Faraday's principles governing the behavior of electrolytic cells

- the amount of chemical change induced in an electrolytic cell is directly proportional to the number of moles of electrons that are exchanged during oxidation-reduction reactions - the number of moles exchanged can be determined from the balanced half reaction.

plating

-aka galvanization -the precipitation process that occurs in the cathode

what type of electrochemical cell is the cell membrane

-concentration cell -sodium and potassium cations, and chlorine anions are exchanged as needed to produce an electrical potential -the actual value depends on the concentrations and charges of the ions -a resting membrane potential (Vm) is maintained -disturbances of the resting membrane potential, may stimulate the firing of an action potential

Electrode Charge Designations in Galvanic Cell

-current is spontaneously generated as electrons are released by the oxidized species at the anode and travel through the conductive material to the cathode, where reduction takes place -the anode of galvanic cells is the source of electrons, it is considered the negative electrode: the cathode is considered the positive electrode -electrons move from negative (low electrical potential) to positive (high electrical potential) - the current- the flow of positive charge- is from positive (high electrical potential) to negative (low electrical potential)

rules for constructing a cell diagram

1. the reactants and products are always listed from right to left using 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 barier

Nernst equation at 298K

Ecell=E'cell- (0.0592/n)logQ

Reaction Quotient for: aA + bB ⇌ cC + dD

Q= [C]^c[D]^d/ [A]^a[B]^b only species in solution are included

what direction does current always flow

cathode to anode

cathode attracts which type of ion

cations

Daniel Cell

cations in the two half-cell solutions can be of the same element as the respective metal electrode in this galvanic cell, zinc is the anode and copper is the cathode; each electrode is bathed in an electrolyte solution containing its cation and sulfate

electrochemical cells

contained systems in which oxidation-reduction reactions occur

electromotive force (emf)

corresponds to the voltage or electrical potential difference of the cell. if the emf is positive, the cell is able to release energy (deltaG<0), which means it is spontaenous if the emf is negative, the cell must absorb energy (deltaG>0) which means it is nonspontaneous

standard change in free energy from standard emf equation

deltaG'=-nFE'cell -->deltaG' is the standard change in free energy (J) -->n is the number of moles of electrons exchanged exchanged --> F is the Faraday constant (J/V) --> E'cell is the standard emf of the cell notice the significance of the negative charge- deltaG and emf will always be positive galvanic cells have negative deltaG' and positive E'cell values. electrolytic cells have positive deltaG' and negative E'cell values

free energy change nonstandard equation

deltaG=deltaG' + RTlnQ

flow of electrons in galvanic cell

during the course of the reaction, electrons flow from the zinc anode through the wire and to the copper cathode. A voltmeter can be connected to measure the electromotive force. anions flow externally from the salt bridge into the ZnSO4 and the cations flow externally from the salt bridge into the CuSO4. This flow depletes the salt bridge and along with the finite quantity of Cu2+ in the solution, accounts for the relatively short lifespan of the cell

mnemonic for electron flow in electrochemical cell

electron flow in an electrochemical cell: A--> C (order in the alphabet) electrons frlow from anode to cathode in all types of electrochemical cells

voltmeter

emf of a cell can be measured with a voltmeter

electron flow in galvanic cell

from negative to positive

current flow in galvanic cell

from positive to negative

three fundamental types of electrochemical cells

galvanic cells (aka voltaic cells), electrolytic cells, and concentration cells in addition, there are specific commercial cells such as Ni-Cd batters through which we can understand these fundamental models Galvanic cells and concentration cells house spontaneous reactions, whereas electrolytic cells contain non spontaneous reactions

electrodeposition equation

helps determine the number of moles of element being deposited on a plate molM=It/nF -mol M is the amount of metal ion being deposited at a specific electrode - I is the current - t is time - n is the number of electron equivalents for a specific metal ion - F is the Faraday constant

negative emf

if the emf is negative, the cell must absorb energy (deltaG>0) which means it is nonspontaneous

positive emf

if the emf is positive, the cell is able to release energy (deltaG<0), which means it is spontaenous

compare and contrast electrolytic cells and galvanic cells

in a galvanic cell, the anode is negative and the cathode is positive. 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.

equation for moles of electrons transfered during half reaction

in general, for a reaction that involves the transfer of n electrons per atom M, M^n+ + ne --> M (s) one mole of metal M(s) will logically be produced if n moles of electrons are supplied to one mole of M^n+ the number of moles of electrons needed to produce a certain number of M(s) can now be related to the measurable electrical property of charge one electron carries a charge of 1.6x10^-10 coulombs (C). The charge carried by one mole of electrons can be calculated by mutiplying this number by Avogadro's number (6.02*10^23)

Standard Reduction Potential (E'red)

measured under standard conditions: 25 degrees celsius (298K), 1 atm, and 1 M concentrations. the relative reactivities of different half cells can be compared to predict the direction of electron flow. A more positive E'red means a greater relative tendency for reduction occur, while a less positive E'red means a greater tendency for oxidation to occur

charge of anode in galvanic cell

negative

charge of cathode in electrolytic cell

negative

electrolysis

occurs in electrolytic cells a type of oxidation-reduction reaction driven by an external voltage source, in which chemical compounds are decomposed because electrolysis is non-spontaneous, the electrode (anode or cathode) can consist of any material so long as it can resist the high temperatures and corrosion of the process

Faraday's Constant

one faraday (F) is equivalent to the amount of charge contained in one mole of electrons (1F= 96,485 C) or more equivalent. On the MCAT, you should round up this number to 10^5 C/mol*e- to make it more managable

surge currents

periods of large current (amperage) early in the discharge cycle. This is preferable in appliances such as remote controls that demand rapid responses.

salt bridge

permits the exchange of cations and anions to balance, or dissipate, newly generated charges 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 while anions from the salt bridge (Cl-) diffuse into the solution on the anode side (ZnSO4) to balance out the charge of the newly created Zn2+ ions, the cations of the salt bridge (K+) flow into the solution on the cathode side (CuSO4) to balance out the charge of the sulfate ions left in solution when the Cu2+ ions are reduced to Cu and precipitate onto the electrode precipitation onto the cathode itself can be called plating or galvanization

charge of anode in electrolytic cell

positive

charge of cathode in galvanic cell

positive

Nickel-cadmium batteries

rechargable cells 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). ex. AA and AAA cells made of Ni-Cd charging reverses the electrolytic cell potetials some Ni-Cd designs are vented for this reason to allow for the release of built up hydrogen and oxygen gas during electrolysis higher energy density than lead-acid batteries tend to provide higher surge current these have been replaced by nickel-metal hydride (NiMH) batteries which have more energy density, are cost effective, and are significatly less toxic

rechargeable cell

rechargeable battery is one that function as both a galvanic and electrolytic cell

how to obtain oxidation potential

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 are reversed

Gibbs free energy

spontaneity is indicated by Gibbs free energy, deltaG

standard electromotive force (emf or E'cell)

standard reduction potentials are also used to calculate standard electromotive force (emf or E'cell) of a reaction , which is the difference in potential (voltage) between two half-cells under standard conditions.

direction of spontaneous movement of charge in galvanic cells

the direction of spontaneous movement of charge is from the anode, the site of oxidation, to the cathode, the site of reduction

anode

the electrode where oxidation occurs

cathode

the electrode where reduction occurs

in emf problems which will be the cathode

the higher reduction potential

more positive reduction potential

the more positive the value, the more likely it is to be reduced- the more it wants to be reduced

direction of current and electrons for all electrochemical cells

the movement of electrons is from anode to cathode, and the current (I) runs from cathode to anode. the current and flow of electrons are always of equal magnitude but opposite direction electrons move opposite the flow current (I) because current is defined as the flow of positive charge

reduction potential

the species in a reaction that will be oxidized or reduced can be determined from the reduction potential of each species, defined as the tendency of a species to gain electrons and to be reduced. Each species has its own intrinsic oxidation potential; the more positive the potential, the greater the tendency to be reduced

Nernst equation

used when conditions deviate from standard conditions Ecell=E'cell- (RT)/(nF)lnQ 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 number of moles of electrons -F is the Faraday constant -Q is the reaction quotient for the given reaction

does potential of electrode depend on size?

when subtracting standard potentials, do not multiple by 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 is changed

how to figure out the sign of a logarithm

whether it is log or ln, remember that a logarithm will be positive when equilibrium constants are greater than 1, negative when equilibrium constants are less than 1, and 0 when equilibrium constants are equal to 1

electrolytic cells

-opposite of Galvanic cells -non-spontaneous reactions that require the input of energy to proceed -change in free energy is positive -undergoes an electrolysis -molten NaCl is decomposed into Cl2 (g) and Na(l). The external voltage source (battery) supplies energy sufficient to drive the oxidation-reduction reaction in the direction that is thermodynamically unfavorable (nonspontaneous) -half reactions do not need to be separated into different compartments, that is because the desired reaction is nonspontaneous

concentration cell

-special type of galvanic cell -contains two half-cells connected by a conductive material, allowing a spontaneous oxidation-reduction reaction to proceed, which generates a current and delivers energy - its distinction is in its design: the electrodes are chemically identical (they have the same reduction potential) -current is therefore 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 equilibrium of ion gradient -the current will stop when the concentration of ionic species in the half-cells is equal - the voltage or electromotive force of a concentration cell is zero when the concentrations are equal -voltage can be calculated using the nernst equation

Electrode Charge Designations in Electrolytic cells

-the anode of an electrolytic cell is considered positive because it is attached to the positive pole of the external voltage source and attracts anions from the solution -the cathode of the electrolytic cell is considered negative because it is attached to the negative pole of the external voltage source and attracts cations in solution

reduction potentials in 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 desired of the respective electrochemical species, the reaction is non spontaneous

reduction potentials in 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 tendancy to gain electrons (that wants to gain electrons more) is actually doing so, the reaction is spontaneous and delta G is negative

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 to calculate emf of a cell

E'cell= E'red,cathode- E'red,anode

mnemonic for electrodeposition equation

calculating Moles of Metal, it is not fun

A more positive E'red means

a greater relative tendency for reduction occur

a less positive E'red means

a greater tendency for oxidation to occur

potentiometer

a kind of voltmeter that draws no current and gives more accurate reading of the difference in potential between the two electrodes

Energy density

a measure of a battery's ability to produce power as a function of its weight.

standard hydrogen electrode (SHE)

a reduction potential is measured in volts (V) and defined relative to the standard hydrogen electrode (SHE), which is given a potential of 0V by convention

A cell diagram

a short hand notation representing the reactions in an electrochemical cell

isoelectric focusing

a technique used to separate amino acids or polypeptides based on their isoelectric points (pI) the positively cahrge amino acids (protonated at the solution's pH) will migrate toward the cathode; negatively charged amino acids (deprotonated at the solution's pH) will migrate toward the anode

where does reduction always occur

cathode

lead-acid battery

aka lead storage battery specific type of rechargeable battery a voltaic cell, when fully charged consists of two half-cells a Pb anode and a porous PbO2 cathode connected by a conductive material (concentrated 4 M H2SO4) when fully charged, it consists of two PbSO4 electroplated lead electrodes with a dilute concentration of H2SO4 both half-reactions cause the electrodes to plate with lead sulfate (PbSO4) and dilute the acid electrolyte when discharging. The lead is negatively charged and attracts anionic bisulfate. The lead (IV) oxide cathode has a porous electrode which allows the electrolyte (suluric acid) to solvate the cathode into lead and oxide ions. Then, the hydrogen ions in soluion react with the oxide ion to produce water, and the remaining sulfate ions react with the lead to produce the electroplated lead sulfate. When charging, the lead-acid cell is part of a electrolytic circuit. lead-acid batteries have the lowest energy-to-weight ratios (otherwise known as energy density). require a heavier amount of battery material to produce a certain output as compared to other batteries.

gavlanic cells

all of non-rechargeable batteries you own are galvanic cells, also called voltaic cells. these reactions are spontaneous, that means deltaG<0 as the cell releases energy to the environment by extension, emf must be positive two electrodes of distinct chemical identity are placed in separate compartments (half-cells) which are connected by a conductive material such as a copper wire. Along the wire there may be other various components such as resistors or capacitors surrounding each of the electrodes is an aqueous solution composed of cations and anions. in a galvanic cell, connecting the two solutions is 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 galvonic cell is spontaneous, and therefore change in gibbs free energy for the reaction is negative as the spontaneous reaction moves towards equilibrium the movement of electrons results in 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

electrodes

all three types of electrochemical cells contain electrodes where oxidation and reduction take place for all electrochemical cells, the electrode where oxidation occurs is the anode and the electrode where reduction occurs in the cathode

Similarities between Galvanic Cells and Electrolytic cells

all types of electrochemical cells have a reduction reaction occurring at the cathode, an oxidation reaction occuring at the anode, and electron flow from anode to cathode

anode attracts which type of ion

anions

where does oxidation always occur

anode

what direction does electron flow always occur

anode to cathode

effect of temperature on batteries

batteries in cars, like most galvanic cells, tend to fail most in cold whether