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CHEM 1050 Lecture 9: Guided Reading 9 - Electrochem Part3 - Week 9

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School
University of Guelph
Department
Chemistry
Course
CHEM 1050
Professor
De Laat

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Document Summary

# learn the half-reactions for water undergoing oxidation and reduction. # predict the half-reactions in an aqueous electrolysis. # calculate the amount of charge from the amount of product in an electrolysis. # calculate the amount of product from the amount of charge in an electrolysis. The big difference with electrolysis is that we know the reactants (but have to predict the products)! One form of electrolysis involves a molten salt. Molten means the salt is heated to the point where the solid is converted to a liquid state. For molten sodium chloride, nacl(l) (melting point = 800 c) we only have na+ (l) so the only two reactions that can take place are: (l) and cl ( oxidation or reduction ) at anode: ( oxidation or reduction ) at cathode: Ref: ebbing and gammon, general chemistry, 10th ed. , 2013, p. 807. Problem 1: which of the following are true for an electrolytic cell: g < 0.

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Related Questions

I am studying an iron chloride redox battery cell (not a flow battery). It is constructed with an iron negative electrode, a carbon positive electrode with an iron (II) chloride electrolyte. The electrodes are physically isolated from each other in a beaker of iron(II) chloride. This cell can be charged and discharged and will run a motor. The output of the battery is 1.2V and can deliver 0.2A.

Unlike most redox batteries, the positive carbon electrode does not take part in the chemistry. The positive carbon electrode, as far as I understand it, simply provides a reaction site for the positive half reaction.

We are told that the half reaction at the positive electrode converts Fe2+ ions to Fe3+ ions (charge) and back again (discharge).

The half reaction at the negative electrode converts Fe2+ ions to metallic iron (charge) and back again (discharge).

The task is to fully describe the iron chloride redox chemistry at both electrodes during charge and discharge.

Where I am:

I am starting with an assumption that during charge and the application of external electrical energy, the dissociated ions in the electrolyte, Fe2+ and Cl-, must migrate to the oppositely charged electrodes. This would be independent of the electrode material and determined simply by the external EMF applied to the battery during charge. The Fe2+ ions must migrate to the negative electrode while the Cl- ions must migrate to the positive electrode.

The result is that the negative electrode will be surrounded by Fe2+ ions and the positive electrode will be surrounded by Cl- ions.

It seems clear that Fe2+ ions will react with incoming electrons, at the negative electrode, to form metallic iron.

However, I do not understand how any reaction can occur at the positive electrode with respect to any variety of "iron chloride". There should be no Fe2+ ions at the positive electrode. Therefore, no reaction involving Fe2+ ions can occur at the positive electrode.

The positive electrode is carbon surrounded by Cl- ions. It is under the influence of a positive electric potential by the external charge source. The only possible reaction I can see occurring is that the Cl- ions may liberate an electron at the positive electrode to form chlorine gas. However, this is not observed under the charging voltage applied, at 25OC and 1 atm. There are no bubbles or chlorine smell.

I have found various references such as:

https://courses.lumenlearning.com/boundless-chemistry/chapter/electrolysis/

That state:

Oxidation of ions or neutral molecules occurs at the anode, and reduction of ions or neutral molecules occurs at the cathode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode:

(aq)+e−

This is exactly what we are told is happening at the positive electrode but I am unable to see how this can occur.

How can a positive ion be in proximity to the positive electrode in order to react with it?? How can any reaction involving Fe2+ ions happen at the positive electrode??

 

1.) When balancing oxidation–reduction reactions in acidic solution by the half-reaction method, the
addition of the reactant H+ is required to balance the product
a. O2. b. OH–. c. O. d. H2O. e. none of these


2.) Which of the following statements is true for a voltaic (galvanic) cell?
a. The electron flow is from the positive electrode to the negative electrode.
b. The electron flow is from the anode to the cathode.
c. The electron flow is from the negative cathode to the positive anode.
d. The electron flow is through the salt bridge.


3.) The anode in a voltaic cell and in an electrolytic cell is
a. the site of oxidation and of reduction, respectively.
b. the site of reduction and of oxidation, respectively.
c. the site of oxidation in both cells.
d. positive in both cells.

1. (1 point) What are transferred in an oxidation-reduction reaction? *

• atoms

• electrons

• ions

• protons

2. (1 point) In the reaction of beryllium with fluorine, which atom is oxidized? *

• fluorine

• beryllium

• neither of them

• both of them

3. (1 point) Ag - - > Ag ^+ +1 e^- This equation represents the type of reaction called _____. *

• hydrolysis

• oxidation

• redox

• reduction

4. (1 point) What is the reducing agent in the following reaction? 2K + S - - > K2S *

• K

• S

• K2S

• None of the above

5. (1 point) The oxidation number of oxygen when it is in a compound other than a peroxide is ____. *

• -2

• -1

• 0

• +2

6. (1 point) What is the oxidation number for each atom in NH4F? *

• N=+3, H=-4, F=+1

• N=-3, H=+1, F=-1

• N=+3, H+1, F=-1

• N=-3, H=-1, F=+1

7. (1 point) Which atom has a change in oxidation number of +4 in the following redox reaction?K2Cr2O7 + H2O + S - - > KOH + Cr2O3 + SO2 *

• K

• Cr

• O

• S

Metal activity series for question 8

8. (1 point) Of the following metals, which ions are most easily reduced? Use the above table to answer the question. *

• magnesium

• lead

• copper

• sodium

9. (1 point) In a zinc-copper cell, Zn(s) | Zn^+2(1M) || Cu^+2(1M) | Cu(s), which electrode is positive? *

• Cu^+2

• Cu(s)

• Zn(s)

• Zn^+2

10. (1 point) How must a redox reaction that is to be used as a source of electrical energy be set up? *

• One half reaction must involve two metals.

• Two half reactions must use a metal wire electrodes.

• One half reactions must involve more than one electron.

• Two half reactions must be physically separated.

11. (1 point) In a voltaic cell at which electrode does reduction occur? *

• anode only

• cathode only

• both anode and cathode

• Either anode or cathode , depending on the metal

12. (1 point) What assures that there is no charge build up in a voltaic cell as oxidation and reduction occur? *

• one of the half cells

• the electrolyte solutions

• the moving electrons

• the salt bridge

13. (1 point) In the most common dry cell what material is the electrode that is the center rod made of? *

• copper

• graphite

• lead

• zinc

14. (1 point) To charge a lead storage battery you must ________. *

• apply a direct current to it.

• hold a magnet close to it.

• Use alternating current through it.

• increase the oxygen in it.

15. (1 point) In a hydrogen- oxygen fuel cell ______________. *

• hydrogen diffuses through the cathode.

• hydrogen and oxygen are mixed before entering the anode.

• oxygen diffuses through the anode.

• oxygen diffuses through the cathode.

16. (1 point) When electrical energy is used to cause a chemical reaction we call this process _____. *

• electronation

• electrolysis

• hydration

• reduction

17. (1 point) Which of the following is true about an electrolytic cell? *

• It changes chemical energy into electrical energy.

• It is the type of cell used in electrocution

• It uses an electric current to make a nonspontaneous reaction go.

• All of the above

18. (1 point) The products formed in the net reaction of the electrolysis of water are ____. *

• aqueous hydrogen ion and hydroxyl ion

• hydrogen gas and oxygen gas

• hydrogen gas and liquid oxygen

• water vapor

19. (1 point) What occurs in electroplating? *

• decomposition of an ionic compound

• decomposition of a salt layer

• deposition of a metal layer on a material

• deposition of a salt layer on a metal

Open response Questions : Answer questions 20 through 22 in complete sentences

20. ( 5 points) Write the equation for the reaction of calcium and oxygen ( 1 point). Write the oxidation numbers for both the reactants and the products ( 2 points) What is the reducing agent and why? ( 2 points)

21. ( 5 points) Tell how a voltaic cell works. Be sure to mention the various parts of the voltaic cell and what they have to do with how it works.

22. (4 points) What are the signs given to the electrodes in both the voltaic and electrolytic cells? What reaction (oxidation or reduction) is occurring at each?