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CAS CH 110 Lecture Notes - Lecture 25: Chemical Equation, Standard Tuning

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maroonhare647
25 Apr 2017
School
Boston University
Department
Chemistry
Course
CAS CH 110
Professor
Pinghua Liu

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Document Summary

F = ena = (1. 602177 10-19c) {6. 0223 1023(mol e-)-1} = 96,485 c mol-1. The electrical charge of one electron is e, therefore the amount of charge for one mole of electrons (avogadro"s constant) is -en. A; potential e, the work will be nen. For n mole of electrons travel along the electrical. The definition of reversibility requires the pushing force balance against an equal and opposite force. Under these conditions the maximum work occurs for a reversible process. e of the charge per mole of electrons (the product of the elementary charge -e and. For standard e or standard emf we can write g = -nfe . Meaning the gas will be 1 bar of pressure and each solution will be 1 m. The value of e is the same, regardless of how we write the equation, but the value of g depends on the stoichiometric coefficients in the chemical equation.

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Related Questions

1)

How are the equilibrium constant and the standard cell potential of an oxidation-reduction reaction related?

a) They are not related. The equilibrium constant derives from enthalpy while reduction potential only deals with charge.
b) Both are a function of the standard change in Gibbs free energy.
c) Both are related to the amount of work that can be done by a reaction on its surroundings.

d) Both are a function of the number of electrons transferred in the reaction.

2)

Calculate the standard cell potential (E∘) for the reaction

X(s)+Y+(aq)→X+(aq)+Y(s)

if K = 2.30×10−4.

Express your answer to three significant figures and include the appropriate units.

3) Consider the reaction

Mg(s)+Fe2+(aq)→Mg2+(aq)+Fe(s)

at 77 ∘C , where [Fe2+]= 3.80 M and [Mg2+]= 0.310 M .

A)

What is the value for the temperature, T, in kelvins?

Express your answer to three significant figures and include the appropriate units.

B)

What is the value for n?

Express your answer as an integer and include the appropriate units (i.e. enter mol for moles)

MC8/20

± The Nernst Equation and pH

Sulfuric acid is a very strong acid that can act as an oxidizing agent at high concentrations (very low pH, or even negative pH values). Under standard conditions, sulfuric acid has a low reduction potential,

SO42−(aq)+4H+(aq)+2e−⇌SO2(g)+2H2O(l), +0.20 V

which means it cannot oxidize any of the halides F2, Cl2, Br2, or I2. If the H+ ion concentration is increased, however, the driving force for the sulfuric acid reduction is also increased according to Le Châtelier's principle. Sulfuric acid cannot oxidize the fluoride or chloride anions, but it can oxidize bromide and iodide anions when there are enough H+ ions present. The standard reduction potentials of the halogens are as follows:

F2+2e−Cl2+2e−Br2+2e−I2+2e−→→→→2F−,2Cl−,2Br−,2I−,+2.87 V+1.36 V+1.07 V+0.54 V

The Nernst equation allows us to determine what nonstandard conditions allow the reaction to occur (have a positive E value).

The Nernst equation relates a nonstandard potential, E, to the standard potential, E∘, and the reaction quotient, Q, by

E=E∘−2.303RTnFlogQ=E∘−0.0592 VnlogQ

where R=8.314 J/(mol⋠K), T is the Kelvin temperature, n is the number of moles of electrons transferred in the reaction, and F=96,485 C/mol e−.

Part A

At 75.0 ∘C , what is the maximum value of the reaction quotient, Q, needed to produce a non-negative E value for the reaction

SO42−(aq)+4H+(aq)+2Br−(aq)⇌Br2(aq)+SO2(g)+2H2O(l)

In other words, what is Q when E=0 at this temperature?

Express your answer numerically to two significant figures.

Hints

Q =

6.4×10−26

SubmitMy AnswersGive Up

Correct

The reaction quotient is Q=6.4×10−26.

For any Q value less than this, E will be positive and therefore the reaction will favor products.

Part B

What pH is needed to produce this value of Q if the concentration and pressure values are

[Br2]=2.50×10−4M,

[Br−]=11.90M,

[SO42−]=9.40M, and

PSO2=2.75×10−5atm?

Express your answer numerically to two decimal places.

Hints
pH =