Chemistry Exam 2 - Chem 255

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Questions and Answers

What is the equation used to calculate the Gibbs free energy change in relation to equilibrium?

DG = nRT ln Keq

DG = RT ln Keq

DG = -nFE (correct)

DG = -nF - E

Which statement accurately describes the signal in Atomic Emission Spectroscopy?

Signal results from electrons moving from lower to higher energy levels, so temperature of the flame need not be stable.

Signal results from electrons moving from lower to higher energy levels, so temperature of the flame must be stable.

Signal results from electrons moving from higher to lower energy levels, so temperature of the flame need not be stable.

Signal results from electrons moving from higher to lower energy levels, so temperature of the flame must be stable. (correct)

In reduction reactions, which of the following species has the highest standard reduction potential?

Ni2+ + 2 e-

Ag+ + e-

Cu2+ + 2 e-

Au+ + e- (correct)

What is the [Ca2+] concentration in the original unknown sample after a 25.00 mL sample is diluted to 50.00 mL and measured at -22.5 mV?

4.85 x 10-3 M Signup and view all the answers

Which relationship expresses the relationship between concentration and absorbed light for a solution?

A = ebC Signup and view all the answers

In which order would hexane, benzene, and dichlorobenzene elute from a normal phase liquid chromatography column?

hexane, benzene, dichlorobenzene Signup and view all the answers

What does the term 'n' represent in the context of the equation DG = -nFE?

Number of electrons transferred Signup and view all the answers

Why must the flame temperature be stable in Atomic Emission Spectroscopy?

To ensure consistent electron transitions. Signup and view all the answers

What is the correct unit for the gas constant R when calculating Gibbs energy in J/K·mole?

J/K·mole Signup and view all the answers

Given the calibration equation E = -28.14(-log[Ca2+]) + 51.1, what is the relationship between the potential and the concentration of Ca2+?

Inverse, with a negative slope. Signup and view all the answers

If the equilibrium constant K is greater than 1, what can be inferred about the reaction?

The products are favored. Signup and view all the answers

Which of the following formulas represents the relationship between reaction rates and equilibrium constants?

k = K * k1 Signup and view all the answers

Which of the following is NOT a property of normal phase liquid chromatography?

Requires a high temperature for separation. Signup and view all the answers

What technique is used to analyze the concentration of ions in a solution using a selective electrode?

Potentiometry. Signup and view all the answers

What does the symbol E° represent in the context of electrochemistry?

Standard cell potential Signup and view all the answers

In the equation t' = tr + k, what does 'k' typically represent?

Rate constant Signup and view all the answers

What effect does dilution have on the concentration of a solution?

Decreases concentration. Signup and view all the answers

Which statement is true regarding the stoichiometry of electrochemical cells?

Electrons flow from anode to cathode. Signup and view all the answers

What is the correct representation of the anode reaction in the given spontaneous battery?

Pb(s) → Pb2+(aq) + 2 e− Signup and view all the answers

Which reaction correctly describes the cathode reaction for the battery?

2H+(aq) + 2 e− → H2(g) Signup and view all the answers

What is the expected volume at the equivalence point when titrating 15 mL of 0.15 M S2O8^2− with 0.15 M I−?

15.0 mL Signup and view all the answers

What is the potential with respect to a silver-silver chloride electrode if a cell potential is 0.344 V with respect to SHE?

-0.147 V Signup and view all the answers

Which organic solvent is more efficient for extracting aspirin and salicylic acid from water, based on their solubility data?

Chloroform, because it has a lower volume required for aspirin than ethyl ether Signup and view all the answers

What is the oxidation state change for lead in the anode reaction?

Decreases from 0 to +2 Signup and view all the answers

When comparing the solvent capacities of chloroform and ethyl ether for salicylic acid, which statement is accurate?

Ethyl ether dissolves salicylic acid in less volume than chloroform. Signup and view all the answers

Which of the following best describes the reaction products of the redox reaction between iodide and peroxodisulfate?

Iodine (I2) and sulfate (SO4^2−) Signup and view all the answers

Which solvent has larger partition coefficients for both aspirin and salicylic acid compared to chloroform?

Ethyl ether Signup and view all the answers

What phenomenon of band broadening in chromatography involves solute molecules taking different paths through the stationary phase?

Mass transfer Signup and view all the answers

In chromatography, what does the stationary phase consist of in ion exchange chromatography?

Cross-linked polymer resin with ionic functional groups Signup and view all the answers

Which type of chromatography is also known as gel permeation chromatography?

Size exclusion chromatography Signup and view all the answers

What is likely the cause of band broadening that relates to solute molecules diffusing to lower concentrations?

Mass transfer Signup and view all the answers

Which type of chromatography utilizes open tubular columns?

Gas chromatography (GC) Signup and view all the answers

Which statement regarding partition chromatography is correct?

It separates based on the solute's partitioning between two immiscible liquids. Signup and view all the answers

What kind of chromatography is best described as utilizing isocratic or gradient elution strategies?

High Performance Liquid Chromatography (HPLC) Signup and view all the answers

What mass of copper is contained in a 0.1248 gram sample of bronze, given that it was electrolyzed with a current of 12.5 mA for 1.5 hours?

0.0754 g Signup and view all the answers

In a gas chromatography analysis, if the peak area for methyl benzoate is 471 and for butyl benzoate is 397, what is the expected concentration of methyl benzoate in the plant stream based on the internal standard?

1.50 mg/mL Signup and view all the answers

What is the role of aqua regia in the digestion of the bronze sample?

To oxidize copper to Cu2+ Signup and view all the answers

What defines the electrical double layer at a metal-electrolyte interface?

The double layer consisting of cations and anions adjacent to the electrode Signup and view all the answers

If the total charge that passed during the copper reduction step is known, how should it be measured?

In Coulombs using a current meter Signup and view all the answers

What substance is used as an internal standard to quantify methyl benzoate in the gas chromatography experiment?

Butyl benzoate Signup and view all the answers

Why are ion selective electrodes potentially responsive to solutes other than the target analyte?

Because solutes with similar ionic sizes can replace the analyte Signup and view all the answers

When digesting bronze in aqua regia, what component is primarily responsible for the dissolution of copper?

Nitric acid Signup and view all the answers

What is the value of the selectivity coefficient 𝐾K+,H+ for the potassium ion-selective electrode?

0.00432 Signup and view all the answers

What is the potential of the potassium electrode when placed in a 6.53×10−5 M K+ solution at pH 6.61?

-0.168 V Signup and view all the answers

What does the equation for measured potential take into account?

Both K+ and H+ ion concentrations Signup and view all the answers

What happens to the potential of the K+ ion-selective electrode when the pH is lowered to 1.22?

It becomes more negative Signup and view all the answers

How many nonpolar compounds are observed in the chromatogram?

Seven Signup and view all the answers

What is the retention factor associated with analyte 1 based on the chromatogram provided?

0.5 Signup and view all the answers

Which analytes co-elute in the provided chromatogram?

2 and 3 Signup and view all the answers

What is one method to improve the separation of peaks in a chromatogram?

Adjust the ratio of the mobile phase components Signup and view all the answers

Study Notes

Exam 2 - Chem 255

Important Instructions: Show all work, report answers with correct significant figures and units, scientific calculator permitted.

Useful Information

Free Energy: ΔG = -nFE = -RTlnK; E = E° - (0.05916/n)logQ
Rate Constant: k = m/t, t = t – t_m, k = k_vt_m
Other Formulae: A = εbc, E = E° - (RT/nF)lnQ, N= kv² +v₁ , D=- or D = K_a0, α= 12tk/r1, A₁[A] , [X]=( ε_xλ1ε_xλ2) (A_λ1A_λ2)/ (ε_xλ1ε_xλ2)(A_λ1A_λ2).

Reduction Reactions and Standard Potentials (E°)

Table of reduction reactions and corresponding standard potentials (E°) is provided. Values are given for various half-reactions.

Multiple Choice Questions (Exam 2)

Atomic Emission Spectroscopy: Signal results from electrons moving from higher to lower energy levels; temperature stability of the flame is not required.
Ion-Selective Electrode Calibration: Data on a Ca2+ ion-selective electrode calibration curve is present, allowing calculation of Ca2+ concentration in an unknown sample.
Liquid Chromatography: The elution order for hexane, benzene, and dichlorobenzene in a normal-phase liquid chromatography column is hexane, benzene, and dichlorobenzene.
Electrochemical Reactions: Half-reactions for a battery are described in anode and cathode forms.

Redox Reaction Kinetics

Iodide (I¯) and peroxodisulfate (S2O3¯) react to produce iodine (I2) and sulfate (SO4¯).
The equivalence point of 15 mL of 0.15 M S2O3¯ titrated with 0.15 M I− should occur at ~15.0 mL

Electrode Potential in Relation to SHE

Silver-silver chloride electrode potential is +0.197 V vs. the Standard Hydrogen Electrode(SHE).
Cell potential of 0.344 V vs. SHE.
Potential vs. silver-silver chloride electrode is -0.147 V.

Solubility in Different Solvents

Data on solubility of aspirin and salicylic acid in chloroform and ethyl ether is given.
Chloroform offers better extraction due to larger differences in partition coefficients.

Band Broadening (Chromatography)

Mass transfer is one mechanism that results in band broadening in chromatography.

Chromatography Concepts and Techniques

Table summarizing chromatography concepts and their applicability to different techniques.

Electrochemical Analysis of Bronze

Bronze sample (0.1248 g) was electrolyzed to determine copper content.
Electrolysis time (1.5 hours) and current (12.5 mA) were used to calculate copper percentage.

Gas Chromatography Analysis

Quantitative analysis of methyl benzoate in a manufacturing plant waste sample using gas chromatography with butyl benzoate as an internal standard.
Chromatographic data (areas and concentrations) was calculated to determine the concentration of methyl benzoate in the plant stream.

Ion Selective Electrode

Potassium electrode interacts with the membrane but also responds to H+ concentration through a selectivity coefficient
The potential of an electrode placed in a 6.53×10 −5 M K+ solution at pH 6.61 (with a given potential) was calculated, changing pH while keeping ionic strength constant.

Reverse-Phase HPLC Chromatogram

A reverse-phase HPLC chromatogram of seven nonpolar compounds was given, showing that 2 and 3 co-eluted.
Retention factors (k) for analytes were determined and the resolution for analytes 6 and 7 was calculated.

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Description

Prepare for your Chem 255 Exam 2 with this quiz that covers essential concepts such as free energy, rate constants, and reduction reactions. Ensure that you can apply formulas correctly and interpret data from the provided tables of standard potentials. Answer multiple-choice questions on topics like atomic emission spectroscopy to test your knowledge.