Unit 3 Flashcards
Activity
measure of the effective concentration of a species under non-ideal (e.g., concentrated) conditions. This determines the real chemical potential for a real solution rather than an ideal one.
Ionic strength
net effect of the dissolved electrolyte
Formula for ionic strength and define all terms
ionic strength (mu) = (1/2) E c*(z^2)
where E= sigma
c = concentration of ions (make sure to multiply by molar coefficients!)
z = charge (+ or - doesn’t end up mattering)
According to the extended dubeye-huckel equation, as ionic strength increases, the activity coefficient ___
decreases
Formula for activity and define all terms
a = gamma * c
gamma = activity coefficient c = concentration
How does increasing ionic strength affect activity?
decrease
Effective concentration
??? Maybe the actual concentration of species that actually partakes in the chemical reaction
Fill this in:
increase ionic strength = ___ pKa = ___ Ka = ___ solubility
increase ionic strength = decrease pKa = increase Ka = increase solubility
Debye-Huckel Limiting Law and define all terms
log (gamma) = - 0.51 (z^2) sqrt(ionic strength)
gamma = activity coefficient z = charge of species
Extended Debye-Huckel equation and define all terms
log (gamma) = (- A (z^2) sqrt (ionic strength))/(1 + a B sqrt(ionic strength))
gamma =activity coefficient
A and B = temperature dependent constants (should be given)
a = effective diameter of ion (dw about, should be given)
In a galvanic cell, where does oxidation and reduction occur (anode or cathode)
Anode=oxidation, cathode=reduction
What is the purpose of a salt bridge?
- prevent direct mixing of electrolyte solutions
- forces electrons through external circuit
- completes circuit when ions conduct current across the salt bridge
What direction do electrons flow in a galvanic cell?
anode to cathode
True or false: the potential of a cell should never reach zero
False. Reaches zero at equilibrium
In the table of standard potentials, will the values be for oxidation or reduction reactions?
reduction
A positive cell potential corresponds to what type of cell?
Galvanic cell (spontaneous reaction)
A negative cell potential corresponds to what type of cell?
Electrolytic cell (electrical work must be done to drive the chemical reaction)
If you’re only given standard cell potentials for reduction reactions, how do you find the cell potential for oxidation reactions
Just flip the sign (magnitude of cell potential stays the same)
True or false: a standard potential is a measure of the driving force for a reaction from a state of unit activity for reactants and products to their equilibrium concentrations
True. Basically how far away a reaction is from its equilibrium
What does SHE stand for and what is it?
Standard hydrogen electrode. Basically a reference point to compare all other half-cells potentials
What is the SHE standard cell potential?
Assigned cell potential of 0.0
Formula for Nernst equation (at standard condition) and all terms
E=E(naught) - (0.0592/n) ln ([C]^c[D]^d/[A]^a[B]^b)
E(naught)= standard cell potential
n=number of moles of electrons that are transferred
What happens to cell potential when temperature increases?
Cell potential decreases (by looking at full Nernst equation):
E=E(naught) - (RT/nF) ln ([C]^c[D]^d/[A]^a[B]^b)
What is the thermodynamic potential?
Difference between the cathode and anode potential when both written as reductions (look at slides POT #1 slide 40)
What is SCE?
Saturated calomel electrode. Paste of Hg metal and calomel (Hg2Cl2) and KCl filling solution. Has salt bridge (ceramic frit) built inside it.
Describe and draw the Ag/AgCl reference electrode
See slides POT #1 slide 44
Potentiometry
use of electrode potentials to determine the concentration of analytes
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When the measurement circuit draws non-trivial current and the voltage suddenly drops. Can be minimized with high resistance measurement circuits
Draw the setup for potentiometric measurement
See POT #2 slide 10
Indicators of the first kind
metal electrode in direct equilibrium with its metal cation in solution
Why are indicators of the first kind not really used
- Not all metal/metal cation systems establish equilibrium quickly
- poor selectivity because it tends to react with other metals
- some metals dissolve at low pH or are easily oxidized
Indicators of the second kind
- responds to ions that for a sparingly soluble salt with the metal (ex: silver electrode with chloride in solution
- not really widely used (reason not really given lol)
In a redox titration, what happens when there is a large difference in the reduction potential between titrant and analyte?
- redox reaction goes further toward completion (K increases)
- sharper end-point
What is 1,10-phenanthrolines?
Redox indicator for Fe2+ and other cations. Reacts rapidly and reversibly and has a pronounce colour change
What is starch/iodine?
Redox indicator. Forms a blue complex with triiodide ions (oxidizing agent) or colourless with reducing agent
What is diphenylamine derivatives?
Redox indicator. Oxidized = purple, reduced = colourless
In membrane potentials, what does an increase in charge density do to potential?
Increase potential
Liquid junction potential
potential difference that develops across an ion permeable boundary between different electrolyte solutions because cations/anions diffuse at different rates (not a good thing, want to minimize this)
How do you minimize the liquid junction potential?
Use concentrated electrolytes where the cation and anions have similar mobilities (KCl instead of NaCl, Na is much smaller than Cl)
Boundary potential
difference between two membrane potentials on each side of the interface
Formula for membrane potential
Eb = E1 - E2
What affects the membrane potential of a thin glass membrane?
pH
Name the three most common glass membrane materials
Quartz, silica glass, soda-lime silicate glass T
The interior of a glass membrane is exposed to pH 1.0 and the exterior to pH 5.0. Which side is more negatively charged?
Exterior
Draw setup for potentiometric measurements with an ISE
See POT #3 slide 16
Draw a combination glass electrode
See POT #3 slide 17
Formula for Ecell with pH sensitive glass electrode
Ecell = K - 0.0592 pH
Draw the graph of Ecell versus pH
See POT #3 slide 20
Formula for Nikolsky equation and define all terms
See POT #3 slide 23
Formula for Nikolsky equation (with alkaline error) and define all terms
See POT #3 slide 22
TISAB
Total ionic strength adjustment buffer. Increases ionic strength to a very high level so that the ionic strength of the sample and calibration standards are roughly the same (even though the sample ionic strength is unknown).
Possible errors with glass electrodes (2 main categories)
Chemical and user errors
Give three examples of chemical errors
- junction potential different between standard and sample
- junction potential drifts over time
- alkaline error
- acid error (electrode response decreases at pH less than 1)
Give three examples of user errors
- standards used for calibration curves aren’t accurate
- glass membrane not clean
- glass not fully hydrated
- insufficient time to reach equilibrium (but really shouldn’t take that long)
3 types of ion selective electrodes
- glass
- solid-state (crystalline)
- liquid membranes
