Unit 3 - Electrochemistry and Potentiometry

Question 1

Counterions

Answer 1

salts that shield ions from interacting electrostatically. This is because of the opposite charges of the counterions being attracted to the ions trying to interact electrostatically

Ex. Na+ being attracted to SO42- and Cl- being attracted to Ca2+

This improves solubility because it increases the distance of the electrostatic compound, thus, is better at separating. Ksp increases with added salt

Question 2

What would a graph look like if its eq’m constant was truly constant? Why wouldn’t a line be straight?

Answer 2

It would be a horizontal line. A line would not be straight because high salt concentrations change the “constants” to shift more towards the ion products

Question 3

4 properties of Ionic strength

Answer 3

1) ionic strength increases as concentration and ion charge increases
2) strength of effect: monovalent < divalent < trivalent < etc.
3) For 1:1 salts (monovalent), ionic strength equals molarity
4) For other salts, ionic strength is greater than molarity

Question 4

Activity

Answer 4

The effective concentration of each species in an eq’m. It is related through concentration through the activity coefficient, gamma

Question 5

Activity in neutral molecules vs. ionic molecules

Answer 5

For neutral molecules, the activity coefficient is generally unity (gamma = 1)

For ions, the activity coefficient can be calculated from ionic strength

Question 6

Redox reaction

Answer 6

change in the oxidation states of the reactants

Question 7

Reduction

Answer 7

gain of electrons (oxidation state decreases)

Question 8

Oxidation

Answer 8

loss of electrons (oxidation state increases)

Question 9

Salt bridge

Answer 9

feeds ions that make the solution electrically neutral. It completes the circuit by allow ions to conduct current across the salt bridge

usually a tube with a porous frit at each end and filled with electrolyte (ex. KCl). It prevents direct mixing of the two electrolyte solutions. It avoids direct reaction of Cu2+ at Zn(s) electrode. Forces electrons through external circuit

Question 10

Anode

Answer 10

electrode where oxidation occurs

Question 11

Cathode

Answer 11

electrode where reduction occurs

Question 12

Cell potentials

Answer 12

As the cell operates, the potential will gradually decrease to zero (at eq’m). The potential difference (voltage) measured at any point reflects the driving force of the associated redox reaction towards eq’m

Question 13

Positive cell potential

Answer 13

galvanic cell (spontaneous reaction; can perform electrical work)

Question 14

Negative cell potential

Answer 14

electrolytic cell (non-spontaneous; electrical work must be done to drive the chemical reaction)

Question 15

Standard half-cell potentials are measured at standard conditions. Name standard the 4 conditions

Answer 15

1) activity of all reactants and products is unity, a = 1
2) for solutes, a concentration of approx. 1.0 M
3) For gases, a pressure of 1.0 atm
4) Temperature = 25 degrees Celsius

Question 16

Standard potential

Answer 16

a measure of the driving force for a reaction from a state of unit activity for reactant(s) and product(s) to their eq’m concentrations when coupled with the SHE half-cell. That is, how far the rxn is from eq’m

Question 17

Standard half-cell potentials are _____ for reductions reactions

Answer 17

positive when the reduction rxn is spontaneous relative to the SHE

negative when the reduction rxn is non-spontaneous relative to the SHE

independent of the # of moles of reactants or products in the balanced half-rxn

Question 18

Standard Hydrogen Electrode (SHE)

Answer 18

the reference point to which other half-cells are compared. It is assigned a zero potential, E0 = 0.000V (at all temps)

Bubble 1 atm of H2(g) to maintain saturated solution
Serves as either cathode or anode
Reversible rxn

Question 19

Saturated Calomel Electrode (SCE)

Answer 19

SHE is not convenient in lab experiments, so SCE is used. It consists of a paste of Hg metal and calomel (Hg2Cl2), has KCl filling solution. Salt bridges are built into these electrodes

Question 20

Ag/AgCl reference electrode

Answer 20

more convenient to use than SCE
consists of a silver wire with layer of silver chloride
KCl filling solution
Salt bridge is built in

Question 21

Potentiometry

Answer 21

use of electrode potentials to determine analyte concentrations

Question 22

Inert electrodes

Answer 22

Respond to redox couples without participating directly in the reaction. It serves as an interface for transferring electrons from one half-cell to the other. Very useful for redox titrations

Ex.
Platinum, gold, palladium, carbon

Question 23

Electrodes of the first kind - Metal electrodes

Answer 23

not widely used, but can be used for precipitation and complex formation titrations

Not all metal/metal cation systems establish equilibrium quickly

Poor selectivity (prone to reaction with other metals)

Some metals dissolve at low pH or are easily oxidized

Ex. Ag/Ag+, Hg/Hg2+ (neutral solutions) and Cu/Cu2+, Zn/Zn2+ (deaerated solutions)

Question 24

Electrodes of the second kind

Answer 24

Respond to ions that form a sparingly soluble salt with the metal

ex. Silver electrode with chloride
AgCl(s) Ag(s) + Cl-

can be used for precipitation and complex formation titrations, but not usually used

Electrodes of the second kind is just an electrode of the first kind coupled to a Ksp eq’m

Question 25

Redox Titrations

Answer 25

An electrochemical analog of an acid-base titration. A redox active analyte can be titrated with a strong oxidizing or reducing agent to determine the quantity of analyte

Question 26

Standard oxidizing and reducing agents definition

Answer 26

Strong, standardized oxidizing or reducing agents

Redox reactions go nearly to completion

Question 27

Equivalence point

Answer 27

Amount of oxidant/reductant added is equal to the amount of the analyte

Question 28

Endpoint

Answer 28

Observable change that (approximately) signals the equivalence point

Question 29

2 techniques that can determine the endpoint of a titration

Answer 29

1) tracking changes in potential

2) redox indicator dyes

Question 30

2 purposes of auxiliary oxidizing and reducing agents

Answer 30

1) used for pre-oxidation or pre-reduction of samples

2) excess auxiliary agent is relatively easy to quench or remove

Question 31

Auxiliary reducing agents examples

Answer 31

solid metal (ex. Zn, Al, Ag, etc.)
A very large negative E0 means it is a very strong reducing agent

Question 32

Auxiliary oxidizing agents examples

Answer 32

ex. Bismuthate, peroxydisulfate, hydrogen peroxide

A very large positive E0 means it is a very strong oxidizing agent

Question 33

Standard reducing agents examples

Answer 33

ex. Ferrous ion (Fe2+), Iodine/Sodium thiosulfate (S2O3 2-)

The lower the E0, the better. It makes the cell potential high, making the reaction more spontaneous

Question 34

Standard oxidizing agents examples

Answer 34

ex. Permanganate ion (MnO-4), Ceric ion (Ce4+), Dichromate ion (Cr2O7 2-)

Have positive E0

Question 35

When will the potential of a cell be 0V? How can there be potential?

Answer 35

Potential of a cell is 0 when any reaction is at equilibrium.
ex. Ce4+, Ce3+, Fe3+, Fe2+ are mixed in the same beaker with nothing to prevent them from reacting, therefore, any reaction between these species will come to an eq’m

For a cell to have potential, there must be at least one redox reaction that is never at equilibrium
ex. the reference electrode is immersed in the beaker with Ce4+, Ce3+, Fe3+, Fe2+, but its filling solution is separated from the beaker solution by a salt bridge. The redox rxns between the beaker and reference electrode do not come to an eq’m

Question 36

Redox indicators

Answer 36

Electrochemical cell is not necessary, so colour changes can be used as indicators in redox titrations. A colour change is observed when the system changes from a large excess of the oxidized form to a large excess of the reduced form (or vice versa)

Colour changes are observed within +/- 59.2/n mV of E0 for the indicator (n=2 for many indicators)

Question 37

Common redox indicators

Answer 37

1,10-Phenanthrolines, Starch/Iodine, Diphenylamine derivatives

Question 38

When is there electric potential? How are they like in potentiometry?

Answer 38

Anytime there is a separation of charge over a distance. In potentiometry, these potentials usually appear as liquid junction potentials or membrane potentials

Question 39

Liquid junction potential (E_j)

Answer 39

a potential difference that develops across an ion permeable boundary between different electrolyte solutions

Caused by the diffusion of cation and anion at different rates (make sure ions have similar mobility)

Resultant separation of charge generates a voltage (that can be measured)

Can reach voltages on the order of approx. 10^-2 V

Question 40

Limitation of liquid junction potential

Answer 40

it limits the accuracy of potential measurements (their contribution to the net potential is unknown). Minimize liquid jxn potentials (few mV) by using concentrated electrolytes where the cation and anion have similar mobility (ex. KCl instead of NaCl)

Question 41

Membrane potentials

Answer 41

ex. glass interface, bulk SiO2 terminates in SiOH groups

Depending on the pH, the membrane will acquire an amount of negative charge (for visual look on review sheet near the end, or part 3, slide 7). The negative charges on the glass interface cannot diffuse. The anionic interface is screened by cations in solution forming an electric double layer

The separation of charge at the double-layer generates a membrane potential, E_m. The magnitude of E_m will depend on the charge on the membrane (ie. interface)

Question 42

Boundary potentials

Answer 42

If a membrane is thin and conductive, the difference between two membrane potentials (each side of the interface) can be measured as a boundary potential, E_b. The boundary of a thin glass membrane is pH-sensitive

Question 43

Glass as a membrane and its physical properties

Answer 43

it is amorphous SiO2. No long range structure, and is cooled to rigidity without crystallization

ex. Soda-lime silicate glass. Soda (Na2O) is used a “flux” to lower melting point in manufacture. Lime (CaCO3) is used to prevent glass from dissolving

Consists of irregular arrangement of SiO4 tetrahedra
Incomplete bonding, Si–O- groups associated with cations
Can hydrate surface layer; exchange monovalent ions for protons

***binding of hydrogen ions is more strongly favoured than alkali metal cations

Question 44

Glass as a pH sensitive membrane

Answer 44

Na+ and H+ ions conduct electricity in the hydrated outer layers of the glass membrane. Na+ conducts electricity in the dry interior

Question 45

Ion selective electrodes

Answer 45

Pressure difference is coming from ion-selective membrane, such as the different concentration of protons in the internal solution and external solution. This is why we need 2 reference electrodes, because they give constant potential

Question 46

Combination glass electrode

Answer 46

internal Ag/AgCl reference electrode. Junction to maintain electrical contact

Question 47

Alkaline error

Answer 47

At high pH, the concentration of H3O+ is very low

Sodium (and some other monovalent metals) can associate with the glass membrane
Generate a change in potential
Measure a pH between 0 and -1 units lower than the true value (positive bias)
Modern glass membranes are relatively immune to alkaline error up to approx. pH=12

Question 48

Two methods used to determine the selectivity coefficient (of the Nikolsky equation)

Answer 48

1) separate solution method - measure the analyte and interferent ion(s) separately
2) fixed interference method - prepare a calibration curve with different activities of analyte and a fixed activity of interference

Question 49

Total Ionic Strength Adjustment Buffer (TISAB)

Answer 49

A buffer solution that increases the total ionic strength to a high level (to make them similar)

The ionic strength of the buffer overwhelms the contribution from the sample, such that the sample and calibration standards can be measured at approximately the same ionic strength, even though the ionic strength of the sample is unknown

This is important because we are using activity (which is directly related to ionic strength) to be measured, which can be correlated to the analyte concentration

Question 50

Sources of error in pH measurements

Answer 50

Chemical errors:
Junction potential different between standard and sample
Junction potential drifts over time (ex. material in salt bridge could leak out)
Alkaline error (aka sodium error)
Acid error (electrode response decreases at pH<1)

User errors: (there are more listed)
Standard solutions used for calibration are inaccurate
Glass membrane is not clean
Calibration and sample temp not matched

Question 51

Types of ion-selective electrodes

Answer 51

Glass
Solid-state
Liquid membranes

Question 52

Ionophore

Answer 52

a hydrophobic organic molecule that serves as an ion exchanger (ie. reversibly binds an analyte ion)

Question 53

Liquid membrane ion-selective electrodes

Answer 53

function is analogous to glass membrane. Glass membrane is replaced with an ionophore in a hydrophobic matrix. A membrane potential develops when the activity of the analyte ion is different between the two sides of the membrane

Question 54

Solid-state ion-selective electrodes

Answer 54

Inorganic crystals
Mobile monovalent ions conduct electricity through crystal vacancies
Vacancies created by doping