CHEM-C 127 Lecture Notes - Lecture 3: Electrolytic Cell, Electrical Energy, Electrochemistry

MC15/20 Animation—Analysis of Copper-Zinc Voltaic Cell

Some oxidation-reduction reactions are spontaneous, and the energy released by them can be used for electrical work. This principle is used in the working of a voltaic cell. Voltaic cells, or electrochemical cells, make use of the electrons that are transferred from the species that releases electrons and undergoes oxidation to the species that accepts electrons and undergoes reduction. If it is possible by any means to separate the oxidation reaction and the reduction reaction and then connect them externally so that the electrons flow from one compartment to the other, you can construct an electrochemical cell.

Watch the video that describes the cell reaction and the cell components of a voltaic cell.

In a copper-zinc voltaic cell, one half-cell consists of a Zn electrode inserted in a solution of zinc sulfate and the other half-cell consists of a Cu electrode inserted in a copper sulfate solution. These two half-cells are separated by a salt bridge.

At the zinc electrode (anode), Zn metal undergoes oxidation by losing two electrons and enters the solution as Zn2+ ions. The oxidation half-cell reaction that takes place at the anode is

Zn(s)→Zn2+(aq)+2e−

The Cu ions undergo reduction by accepting two electrons from the copper electrode (cathode) and depositing on the electrode as Cu(s). The reduction half-cell reaction that takes place at the cathode is

Cu2+(aq)+2e−→Cu(s)

The electrons lost by the Zn metal are gained by the Cu ion. The transfer of electrons between Zn metal and Cu ions is made possible by connecting the wire between the Zn electrode and the Cu electrode. Thus, in the voltaic cell, the electrons flow through an external circuit from the anode to the cathode. For a voltaic cell to work, the solution in the two half-cells must remain electrically neutral. This can happen only if the flow of ions is countered with the flow of electrons. The flow of ions is made possible with the use of a salt bridge. A salt bridge is a solution of some other metal that has common ions. If a copper-zinc voltaic cell utilizes ZnSO4 and CuSO4solution, you will use a saturated Na2SO4 solution in the salt bridge. Thus, the salt bridge will help the migration of ions across the two compartments, or two half-cells.

Part A

Label the diagram according to the components and processes of a voltaic cell.

Drag the appropriate labels to their respective targets.

Hints

SubmitMy AnswersGive Up

Correct

The cell reaction for the voltaic cell is

Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)

Here, Zn undergoes oxidation by losing 2e−, and the Cu2+ ions accept 2e− to form metallic Cu, which is deposited at the copper electrode. At the zinc electrode, oxidation occurs, and hence it is known as the anode. Reduction occurs at the copper electrode, and hence it is called the cathode.

The electrochemical cell notation for this cell is Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s).

Construction of a voltaic cell

Consider a chromium-silver voltaic cell that is constructed such that one half-cell consists of the chromium, Cr, electrode immersed in a Cr(NO3)3 solution, and the other half-cell consists of the silver, Ag, electrode immersed in a AgNO3 solution. The two electrodes are connected by a copper wire. The Cr electrode acts as the anode, and the Ag electrode acts as the cathode. To maintain electric neutrality, you add a KNO3 salt bridge separating the two half-cells. Use this information to solve Parts B, C, and D.

Part B

The half-cell is a chamber in the voltaic cell where one half-cell is the site of the oxidation reaction and the other half-cell is the site of the reduction reaction.

Type the half-cell reaction that takes place at the anode for the chromium-silver voltaic cell. Indicate the physical states using the abbreviation (s), (l), or (g) for solid, liquid, or gas, respectively. Use (aq) for an aqueous solution. Do not forget to add electrons in your reaction.

Express your answer as a chemical equation.

Hints

SubmitMy AnswersGive Up

Part C

The half-cell is a chamber in the voltaic cell where one half-cell is the site of an oxidation reaction and the other half-cell is the site of a reduction reaction.

Type the half-cell reaction that takes place at the cathode for the chromium-silver voltaic cell. Indicate physical states using the abbreviation (s), (l), or (g) for solid, liquid, or gas, respectively. Use (aq) for an aqueous solution. Do not forget to add electrons in your reaction.

Express your answer as a chemical equation.

Hints

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?

Test 1-Multiple Choice: Choose and underline the best answer in each of the following statement/question.

_____1. Which of the following best differentiates an aldehyde from a ketone?

a. In aldehydes, carbon is bonded to a hydroxyl group.

b. In aldehydes, hydrogen is bonded to an oxygen group.

c. In aldehydes, hydrogen is bonded to a carbonyl group.

d. In aldehydes, oxygen is bonded to an oxygen group.

_____2. Which of statement is not true about petroleum and its products?

a. Petroleum is fossil fuel.

b. Petroleum is a mixture of hydrocarbons with sulfur as impurity.

c. There is no harm that comes from the use of petroleum products.

d. Petroleum comes from decomposition of materials of living origin.

_____3. Cooking gas or liquefied petroleum gas(LPG) is a mixture of C₃H₈ and C₄H₁₀. Which are these compounds?

a. pentane and butane b. methane and butane

c. propane and butane d. propane and methane

_____4. Which of the following compounds ionizes in water and, thus conducts electricity poorly?

a. acetic acid b. hydrochloric acid c. distilled water d. sodium hydroxide

_____5. A lead storage cell can function both as an electrolytic cell and as a voltaic cell. When a lead storage battery is being recharged, it converts ________ energy to_____--energy

a. chemical : electrical b. chemical to heat c. electrical to mechanical

_____6. The acid electrolyte in a lead storage battery is

a. HCl b. H₂SO₄ c. HNO₃ d. CH₃COOH

____7. The anode and cathode in a dry cell respectively are

a. Zn and C b. C and Zn c. Pb and PbO₂ d. PbO₂ and Pb

____8. What is the proper name for the compound CH₃CH₂CH₃?

a. ethane b. propane c.butane d. pentane

____9. Which of the following is the simplest hydrocarbon?

a. CH₂ b. C₂H₄ c. CH₃ d. CH₄

____10. Hydrocarbons such as CH₂ =CH₂ are said t be

a. ionic b. aromatic c. saturated d. unsaturated

_____11. Which of the following best describes hydrocarbons?

a. flammable ionic compounds that are insoluble in water and are denser than water

b. flammable ionic compounds that are insoluble in water and are denser than water

c. flammable covalent compound that are insoluble in water and are less dense than water

d. flammable covalent compounds that are soluble in water and less dense than water

_____12. Which of the following will result if hexane and water are mixed?

a. an explosion

b. clear solution of hexane dissolved in water

c. a layer of hexane on top of a layer of water

d. a layer of water on top of a layer of hexane

_____13.Which of the following is not a gas?

a. ethene b. octane c. methane d. acetylene

_____14. Which of the following is another name for wood alcohol?

a. methanol b. propanol c. grain alcohol d. rubbing alcohol

_____15. Which of the following alcohols groups are found in beverages?

a. ethyl b. methyl c. denatured d. wood

_____16. Which of the following is the only element present in organic compounds?

a. hydrogen b. oxygen c. carbon d. nitrogen

_____17. Which of the following is chlorofluorocarbon?

a. C₃F₈ b. CHF₃ c. CFCl₃ d. CHCl₃

_____18.What percent of alcohol is present in vodka which is 90 proof?

a. 9 b. 45 c. 90 d. 180

____20. Which of the following is formed when carbohydrates undergo fermentation process?

a. ethanol b. methanol c. glucose d. formaldehyde

_____21. In ethene, the two carbon atoms are joined by a

a.single bond b. double bond c. triple d. ionic

____22. The IUPAC name of CH₃CH₂CHCH₂CH₃ is

│

CH₂CH₃

a. 3-methylpentane b. 3-ethylpentane c. 2-ehtylpentane d. 2-methylpentane

____23. The IUPAC name of CH₃-O-CH₃ is

a. methoxymethane b. ethylether c. dimethylether d. dimethyloxide

____24. Which is an alcohol?

a. C₆H₆

b. CH₃-C-CH₃

│

O

c. CH₃-O-CH₃

d. CH₃OH

____25. A group which both aldehydes and ketones have in common is

a.-C=O b. –OH c. –O- d. -COOH

___26. The aroma associated with this class of compounds is regarded as pleasant.

a. amides b. esters c. carboxylic acids d. ketones

____27. The IUPAC name of CH₃CH₂CHO is

a. propanone b. propanol c. propanal d. propionic acid

____28.The –COOH group is called

a. aldehyde group b. hydroxyl group c. carbonyl group d. carboxylic group

____29.The alkyl group of butane is

a. C₂H₃ b. C₃H₅ c.C₄H₇ d. C₄H₉

____30. The simplest alkene is

a. CH=CH b. CH₂=CH₃ c. CH₂=CH₂ d. CH₃CH=CH₂

Test 2. Answer the following questions completely.(3 points each)

1.Why are alcohols of lower molecular weight more soluble in water than those with higher molecular weights?

2. Which of the following pairs of compounds has

a. higher bolilng point:1-bromopentane or 1-chloropentane _____________________

b. the greater density: heptanes or octane ______________________
c. the higher boiling point :pentyl alcohol or pentylamine ______________________

d. the higher boiling point: hexylamine or dipropylamine ______________________

3. Explain why each of the following names are incorrect.Give the correct IUPAC name.

a. 3-Butanone

b. 1-Butanone

4. What is unique about carbon atom that makes it form numerous compounds?

5. Acetone or propanone is completely soluble in water,but 4-heptanone is completely insoluble.Explain.

Test 3.. Give the name and chemical formula of the functional group of each of the following organic groups. The first one has been done for you.