7/28/2019 Chem_Eng

1/23

Table of Contents

Middle School Chemistry WorkshopDujiangyan, China

Preface 2

Activity 1: Observing a Simple Redox Reaction 3Discussion: Activity 1: Laboratory Report 4Activity 2: Measuring Conductivity of Solutions 6Discussion: Activity 2: Laboratory Report 7Activity 3: Making Electrochemical Cells 9Discussion: Activity 3: Laboratory Report 11Activity 4: Determining the Activity Series of Metals 13Discussion: Activity 4: Laboratory Report 15Activity 5: Electrolysis of Water 17Discussion: Activity 5: Laboratory Report 19Discussion: A Test on Redox Reactions 21

Part 1 Lab Practical Test 21Part 2 Multiple Choice 21Part 3 Balancing Equations 22Part 4 Essay Questions 22

7/28/2019 Chem_Eng

2/23

PrefaceOver the years I have found that students who dont like science usually are those whohave not had successful experiences in science. For the last 10 years of my teaching I havedeveloped a way to help them learn the work that they thought they could not understand. Idid not water down the contents, and they appreciated that when they found out they coulddo as well as any of their classmates. What I did was to teach them how to learn science. Idid not teach them science. They learned it on their own. To begin a unit of study I wouldintroduce a topic by asking them to do an investigation in the lab. They would makecritical observations, ask about apparent contradictions in their work, and draw conclusionsfrom their observations. Inevitably, their work led them to ask more questions in other areas of studies, and they had to read books, search the Internet, or go to the lab to find theanswers. I acted as a facilitator or advisor and not the teacher who was the ultimate sourceof all knowledge. When they finished the unit of work, they learned a lot more than I couldteach them by lecturing. What they have done is much more than learning the facts. Theyhave learned how to learn. In preparing for this workshop I have recreated some of mylessons on OxidationReduction and would like to share them with you. Keep in mind thatI have not explained what kind of work I expected from them and, most importantly, how Igraded them. As they are an important part of learning, I will discuss them with you after each lesson.

Oxidation and Reduction Reactions

Concepts to be learned:1. OxidationReduction Reactions2. Oxidation half reaction3. Reduction half reaction

Skills to be learned:1. Making electrolytic cells2. Electrolysis of water 3. Electroplating4. Identifying anode and cathode5. Balancing redox reactions by half reactions

2

7/28/2019 Chem_Eng

3/23

Activity 1

Problem: To learn to identify an oxidation from a reduction in a Redox reaction

Materials: 150ml beaker 1 M HCl (hydrochloric acid)Mossy Zinc

Method : 1. Add about 100 ml of HCl solution to a beaker. Feel the outside of the beaker. Doesit feel warm? Record your observations in Data/Observation section.

2. Add the pieces of zinc into the acid. Do you see any changes when the zinc isadded to the acid? Feel the outside of the beaker. Does the temperature change? Record them in your Observations.

3. Light a long wooden splint and put the flame over the acid in the beaker. Recordwhat you see.

Data/Observations:

Conclusions:1. You have observed an OxidationReduction reaction in this activity. Is it an

exothermic or endothermicreaction? What evidence do you have to back up your explanation?

2. In this chemical reaction between HCl and Zn, two products are formed. Can youidentify one product based onwhat you did in Step 3 in the Method?

3. Write the balanced equation for the reaction between zinc (Zn) and hydrochloricacid (HCl)

4. HCl at room conditions is a gas. When it dissolves in water, it forms ions withwater. Write a balanced equation

showing the dissociation of HCl molecules into ions. These ions are responsible for the conduction of electric current in Activity 1.

5. The reaction between zinc and hydrochloric acid may be written as follows:

Zn (s) + 2 H +1 (aq) + 2 Cl 1 (aq) Zn +2 (aq) + 2 Cl 1 (aq) + H 2 (g)

(a) Define oxidation and write the balanced oxidation half reaction of the abovereaction.

(b) Define reduction and write the balanced reduction half reaction of the zincHClreaction.

(c) Add the oxidation half reaction to the reduction half reaction as you would withtwo algebraic equations. Doyou now have a balanced chemical reaction between zinc and hydrochloric acid? Why or why not? Explain.

3

7/28/2019 Chem_Eng

4/23

A Sample ReportChemistry Activity 1 Problem: To learn to identify an oxidation from a reduction in a Redox reaction

Materials: 150ml beaker 1 M HCl (hydrochloric acid)Mossy Zinc

Method: 1. I poured about 100 ml of HCl solution into a beaker and felt the outside of the beaker. I recorded my

observations in the Data/Observation section.2. I added a few pieces of zinc into the acid. I noted the change in the color of the zinc

pieces and felt the change in the temperature of the beaker. All the observationswere recorded in the

data/observations.3. I lit a long wooden splint and put the flame over but not into the acid in the beaker.

Observations wererecorded.

Data/Observations:

1. The beaker felt normal. It was not warmer or cooler than the room temperature.2. The shinny piece of zinc turned dark almost immediately after they were put into

the acid. The zinc pieces became tiny flakes. The beaker felt a little warmer than before.

3. When the flame of the wooden splint was placed over the acid, I heard popping noises. When I looked more carefully, I saw little puffs of fire over the

acid.

Conclusions:

1. The reaction between zinc and hydrochloric acid is exothermic because the beaker felt a littlewarmer than before after the reaction. The energy must have come from the heat released by thereaction. 2. One of the products is hydrogen as it burns with a pop in the air. (A test for hydrogen)

3. Zn + 2HCl ZnCl 2 + H 2 (gas)

4. HCl (g) H +1 (aq) + Cl 1 (aq)

5. (a) Oxidation is chemical reaction in which electrons are lost or are a product.

Zn Zn +2 (aq) + 2 e -1

(b) Reduction is a chemical reaction in which electrons are gained.

2 H +1 (aq) + 2 e -1 H2 (g)

(c) Zn Zn +2 (aq) + 2 e -1

4

7/28/2019 Chem_Eng

5/23

Add) 2 H +1 (aq) + 2 e -1 H2 (g)

Zn + 2 H +1 (aq) + 2 e -1 Zn +2 (aq) + 2 e -1 + H 2 (g)

Canceling the 2 e -1 on both sides of the equation

Zn + 2 H +1 (aq) Zn +2 (aq) + H 2 (g)

This equation shows only those ions/atoms that were involved in the chemical change. Thechloride ions (Cl 1 ) did not changed and therefore were not shown. The above equation isoften referred to as the net ionic equation.

5

7/28/2019 Chem_Eng

6/23

Activity 2

Problem: To determine the electrical conductivity of some solutions

Materials: 150ml beakersDistilled water Rubbing alcohol0.5 M NaCl solution0.5 M HCl hydrochloric acid solution0.5 M KNO 3 solution

Method: 1. Fill a beaker half full with one of the five liquids listed above.2. Use the Conductivity Indicator to test the conductivity of the liquids one at a time.

Be sure that you wash and rinse the probes of the Indicator with distilled water between uses. Record your observations in your data table.

Data/Observations

Solutions Distilled Rubbing 0.5 M NaCl 0.5M HCl 0.5MKNO 3Water Alcohol

Conductivity

Conclusions:1. Explain how the Conductivity Indicator works.2. Explain what are ionic substances at the atomic level?

3. Explain what are covalent substances at the molecular level.4. According to your results some solutions are conductors. Explain, at the atomic

level, how they conductelectricity.

5. What are electrolytes and nonelectrolytes? How are they similar or different fromionic and covalent

substances? Explain.6. Go to the lab and do the following:

(a) Put about 150 ml of 0.5 M BaCl 2 solution in a beaker and leave the probes of theConductivity Indicator in the solution. Is the solution a conductor? Record your

observations.

(b) Use a dropper to add one drop of 0.5 M Ag 2(SO 4) solution to the solution in the beaker. Use glass stirrer to

mix the solutions gently. What do you notice about the brightness of thelight bulb of the Conductivity Indicator? Do you see any changes in the beaker?Record your observations.

(c) Continue to add, one drop at a time, the Ag 2(SO 4) solution to the beaker and stir it gently and continuously.

What do you notice about the solution in the beaker and the brightness of thelight bulb? Record your observations.

Explain, at the molecular level, what you see in doing steps (a) to (c). Does it help6

7/28/2019 Chem_Eng

7/23

you better understand why asolution is a conductor?

A Sample ReportChemistry: Activity 2

Problem: To determine the electrical conductivity of some solutions

Materials: 150ml beakersDistilled water Rubbing alcohol0.5 M NaCl solution0.5 M HCl hydrochloric acid solution0.5 M KNO 3 solution

Method: 1. I filled a beaker half full with one of the five liquids listed above.2. I used the Conductivity Indicator to test the conductivity of the liquids one at a

time, and I washed and rinsed the probes of the Indicator with distilled water between uses. Observations were recorded in my data table.

Data/Observations

Solutions Distilled Rubbing 0.5 M NaCl 0.5M HCl 0.5MKNO 3 Water Alcohol

Conductivity No No Yes Yes Yes

Conclusions: Power Light BulbSource

1. The schematic diagram of the Indicator may be as shown:When the two probes are connected by a wire, the circuit is closed andthe electric current is able to go through the light bulb causing it to light up.When the probes are placed in a beaker of solution, if the bulb glows, Probesthe solution is a condcutor. If the bulb does not glow, the solution is anonconductor. (When a student gives this explanation, it may be

satisfactory but it also opens to more questions. For example, a metal wireis different from a liquid. Electrons can run through a wire, but how do theypass through a solution?)

2. In a chemical reaction when some electrons of one atom are lost to another atom, ions areformed. The atom that loses the electron becomes positively charged and the atom that gains theelectrons becomes negatively charged. The charged atoms are called ions. These ions are heldtogether by their electrical charges, and the new product is called an ionic substance. For example, when Na atoms react with Cl atoms, each Na atom loses an electron to form Na +1 ion,and a Cl atom gains the electrons lost by a Na atom to become Cl 1 ion. The new product is

7

7/28/2019 Chem_Eng

8/23

NaCl, an ionic compound. A similar reaction can take place between two groups of atoms. For example, the group of atoms NH 4 can lose an electron to the group of atom NO # to form NH 4+1 ion and (NO 3) 1 ions to form an ionic compound of NH 4 NO #. ( Are ionic substances likely to besolids? Why? Are their melting and boiling points likely to be high? Why? )

3. A covalent compound is formed when two atoms react chemically by sharing each other electrons to form a new compound. The coattraction for each other electrons holds themtogether in a group called a molecule. For example, a hydrogen atom shares a pair of electronswith a chlorine atom to form the molecule HCl. (More questions about electronssharing can beasked. For example, is the electronsharing even between the two atoms? What if the sharing isnot even, meaning that atom has a stronger hold the electrons than the other, what would happento the molecules?)

4. When an ionic substance is dissolved in water, the ions are free from each other and free tomove about in the solution. The presence of freemoving ions makes the solution a conductor.Sodium chloride is an ion substance, and its solution is a conductor. HCl is not an ionicsubstance, but its solution also conducts electric current. It means that HCl solution must containions. (How do the ions actually conduct electric current? Why does a solution of HCl haveions?)

5. Chemical compounds whose water solutions conduct electric current are called electrolytes.Therefore, all ionic substances are electrolytes. Some covalent substances such as HCl do formions in water solution, and these compounds are also called electrolytes.

6. Go to the lab and do the following:(a) Put about 150 ml of 0.5 M BaCl 2 solution in a beaker and leave the probes of theConductivity Indicator in the solution. Is the solution a conductor? Record your observations.(b) Use a dropper to add one drop of 0.5 M Ag 2(SO 4) solution to the solution in the beaker. Usea glass stirrer to mix the solutions gently. What do you notice about the brightness of the light

bulb of the Conductivity Indicator? Do you see any changes in the beaker? Record your

observations.(c) Continue to add, one drop at a time, the Ag 2(SO 4) solution to the beaker and stir it gentlyand continuously. What do you notice about the solution in the beaker and the brightness of thelight bulb? Record your observations.

Explain, at the ionic level, what you have observed in doing steps (a) to (c). Does it help youunderstand better why a solution is a conductor?

(Do all ionic solids dissolve in water?)

8

7/28/2019 Chem_Eng

9/23

Activity 3

Problem: To make an electrochemical cell (battery)

Materials: A carbon (graphite) electrode C (s)A zinc electrode Zn (s)An aluminum electrode Al (s)An iron electrode Fe (s)0.5 M HCl (aq) hydrochloric acid solutionA voltmeter 350 ml beaker

Method: 1. Obtain a zinc and a carbon electrode. Use a piece of fine sand paper to clean the Znuntil its color is even. Record its colors in you data table.

2. Using the wire with alligator clips to connect the Zn electrode to the positiveterminal of the voltmeter

and another wire to connect the carbon electrode to the negative terminalof the voltmeter. Now place both electrodes into the beaker of HCl (aq) to makean electrochemical cell. Dont let the electrodes touch each other at any time.

3. What does the voltmeter read? Record you reading in you data table. If you cannotread the meter, reverse the

connections from the electrodes to the meter. Note which terminal is

connected to the Zn electrode for the meter to show a reading. Record all your reading and observations in your data table.

4. Leave the electrodes in the solution for about 2 minutes. Describe any visiblechanges to the electrodes

and the solution. Record them in your data table.5. Repeat steps 1 4 with aluminum/carbon electrodes and with iron / carbon

electrodes. Record all youobservations in the data section

6. Disconnect the electrodes from the voltmeter. Rinse them under water, dry themand leave them on the

table. Discard the solution and clean the beaker.

Data/Observations:

Voltage of the cell Voltage of the cell Voltage of the cellWith Zn and C With Al and C with Fe and C

Conclusions:1. Graphite will not react with the hydrochloric acid and it is used because it is a good

9

7/28/2019 Chem_Eng

10/23

conductor of electricity, but all the metals reacted with the acid. Write a balancedequation for each metal

reacted with HCl.2. For each of the above redox reaction, write the balanced equation of the oxidation

half reaction and thereduction half reaction. Which half reactions show that electrons are a

product?3. The voltmeter gives a reading when a flow of electrons passes through it. Why did

you have to reverse theelectrode connections to the meter before you get a reading?

4. When the voltmeter gave a reading with Zn and C electrodes connecting to it, itmeant that a flow of

electrons had passed through it . Where did the electrons come from and why did theymove through the voltmeter? Explain.

5. Did you get the same voltage reading in all three cells? Why or why not? Explain.6. Why were you asked not to allow the electrodes in a cell to touch each other in

Step 2 in the Method?7. If an electrochemical cell is made with Zn and Iron electrode, will the voltmeter

show any reading? Go to

the lab and make a cell with Zn and Fe, and explain your observations.8. If you were to make a cell with Fe and Al, which electrode would you connect to

the positive terminal of thevoltmeter to get a reading? What would be the reading based on the resultsyou got from this activity? Go to the lab and make a cell with Fe and Al and seeif your answers are correct.

9. Having made the cell with Fe and Al predict the voltage of a cell made with Al andZn. Explain how you made such a prediction.

10. Can you make an electrochemical cell by replacing the Hydrochloric acid withdistilled water? Why or

why not? Explain your answer. Now go to the lab and make a cell with

distilled water. Is your answer correct? Explain.11. If you were to use a solution of NaCl instead of HCl (aq), do you think the cell

would produce electriccurrent? Explain your answer. Now, go to the lab and make a cell with NaClsolution. Does the cell work? In general, what kind of solutions would you use tomake an electrochemical cell?

10

7/28/2019 Chem_Eng

11/23

A Sample Report

Chemistry Activity 3

Problem: To make an electrochemical cell (battery)

Materials: A carbon (graphite) electrode C (s)A zinc electrode Zn (s)An aluminum electrode Al (s)An iron electrode Fe (s)0.5 M HCl (aq) hydrochloric acid solutionA voltmeter 350 ml beaker

Method:1. I obtained a zinc and a carbon electrode and sanded the Zn electrode until it was a

bright silver color. Irecorded my observations.

2. I connected the Zn electrode to the positive terminal of the voltmeter with a wireand the carbon electrode to

the negative terminal of the voltmeter. I then placed both electrodes intothe beaker containing about 200 ml of HCl (aq) to make an electrochemicalcell. Care was taken not to allow the electrode to touch each other. 3. The voltmeter needle went off to the side of the dial that had no calibrations. After connecting

the Zn electrode to the negative terminal and the carbon electrode to the positive terminal,the voltmeter gave a reading and I recorded in the data table. 4. The zinc electrodethat was in the HCl solution turned black, and the color was noted in theObservation.

5. Steps 1 4 were repeated with aluminum/carbon electrodes and with iron / carbonelectrodes. All observations

and voltage readings were recorded6. All electrodes were washed in water and dry.

Data/Observations:11

7/28/2019 Chem_Eng

12/23

Voltage of the cell Voltage of the cell Voltage of the cellWith Zn and C With Al and C with Fe and C

0.81 v 1.66 v 0.41 v

Conclusions:

1. Zn (s) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (g)2 Al (s) + 6 HCl (aq) 2 AlCl 3 (aq) + 3 H 2 (g)Fe (s) + 2 HCl (aq) FeCl 2 (aq) + H 2 (g)

2 Electrons are a product in Oxidation half reactions

Zn(s) Zn +2(aq) + 2 e 1

Al (s) Al +3(aq) + 3 e 1

Fe(s) Fe +2(aq) + 2 e 1

Reduction half reactions for all three redox are the same.

2 H +1(aq) + 2 e 1 H2(g)

3. The voltmeter gives a reading only if the electronflow is from the negative terminal throughthe meter to the positive terminal. It appears that electrons flow from the Zinc electrode throughthe meter to the carbon electrode. (The question about the right hand rule on the current flow andthe magnetic field direction may be asked here.)

4. According to the oxidation half reaction in Question 2 , the zinc atoms loses electrons to become water soluble Zn 2 ions. Left on the zinc electrode are electrons. Since all the electrons

are negatively charged, they repel each other. When a wire connects the zinc electrode to thecarbon electrode, the electrons will move through the wire where the resistance is less to thecarbon electrode. This flow of electrons is the electric current provided by a battery. 5. The readings are different. It could mean that some metals lose electrons more readily than theother. A metal that loses electrons easily will build up a higher concentration of them on theelectrode, and they will repel each other with a greater force. Electrons being pushed by agreater force has are said to have a greater voltage. (Questions about emf may be asked here.)

6. If the electrodes are in direct contact with each other, electrons will follow the path of least

resistance to flow from one electrode to the other without going through the external wire. The battery is said to be short-circuited.

7. The zinc metal appears to give up their electrons more readily than the iron metal. When thetwo metal is put in beaker of electrolyte solution, the concentration of electrons will be higher onthe zinc electrode than on the iron electrode. Electrons will flow from Zn to Fe through theexternal wire. Because zincs electrons push with a force of 0.81v and the irons electrons push

back with a force of 0.41v, my prediction is that the electrons will flow from zinc to iron with anet force of 0.40v. (Students will go to the lab to make a Fe/Zn cell to verify their predictionhere.)

12

7/28/2019 Chem_Eng

13/23

8. The Voltmeter reading of a Fe/Al cell will probably be 1.66v 0.41v = 1.25 v. To get areading the Al electrode will be connected to negative terminal.(Students will go to the lab to make a Fe/Al cell to verify their prediction here.)

9. The voltmeter reading of a Al/Zn cell will probably be 1.66v 0.81v -= o.85v. (Why?)

10. Hydrochloric acid is a conductor and will allow the electrons to complete the circuit.However, water in not a conductor. The circuit will be open and the cell will not be able to

provide an electric current through the external circuit.(Students will make a cell with distilled water to check their answers.)

11. NaCl solution can be used to replace HCl because NaCl solution is a conductor. In generalany electricity conducting solutions can be used.(Students will make a cell with distilled water to check their answers.)

Activity 4

Problem: To determine the metal activity series of five metals

Materials: Iron, zinc, aluminum, copper, and lead electrodes Voltmeter 350 ml beakersFe(NO 3)2 (aq), A Voltmeter Zn(NO 3)2 (aq)

Al(NO 3)3 (aq)Cu(NO 3)2, (aq) Zn FePb(NO 3)2 (aq) Electrode Electrode

NH 4(NO 3) (aq)Salt

BridgeWith Fe(NO 3)2

Zn(NO 3)2 NH 4 NO 3 Solution Solution

Cotton Plug

Method: Make a cell using all possible combinations of two different metal electrodes andtheir respective solutions as shown in the diagram above. You will use the same salt

bridge for all your cells. In each case record in your data table the voltage given bythe voltmeter and the direction of the electron flow.

Data/Observations:

Battery with Voltage Electron flow through the external wire

Zn & Fe From to13

7/28/2019 Chem_Eng

14/23

Zn & Al From to

Zn & Cu From to

Zn & Pb From to

Fe & Al From to

Fe & Cu From to

Fe & Pb From to

Al & Cu From to

Al & Pb From to

Cu & Pb From to

Conclusions:1. Oxidation is a chemical reaction in which electrons are lost. Another way of

stating it is that oxidation is a reaction in which electrons are a product. Write the balancedoxidations for all the cells.

2. Reduction is the chemical reaction in which electrons are gained. Write the balanced reductions for all the cells.

3. The chemical that loses electron in oxidation is called a reducing agent, and thechemical that gains electron in reduction is called an oxidizing agent. Explain why electrons

always flow from the reducing agent to the oxidizing agent through the external circuit,4. Explain the function of the salt bridge. (Hints: When Zn(NO 3)2 is dissolved in

water, it forms Zn +2 ions and (NO 3) 1 ions. These ions are free to move about in solutions. Unlessa barrier is solid and impermeable, the ions are small enough to pass through porous barrier suchas cotton or clay. )

5. Based on the data you have collected, rank the metals from the most active to theleast active. Compare your results with those given in your textbook. Are they the same?

14

7/28/2019 Chem_Eng

15/23

Sample ReportChemistry Activity 4

Problem: To determine the metal activity series of five metals

Materials: Iron, zinc, aluminum, copper, and lead electrodes350 ml beakersFe(NO 3)2 (aq),

Zn(NO 3)2 (aq)Al(NO 3)3 (aq)Cu(NO 3)2, (aq)Pb(NO 3)2 (aq)A voltmeter A salt bridge with NH 4 NO 3 solution

Method: I followed the diagram and made a cell with Zn and Fe electrodes. I measured andrecorded voltage of the cell with my voltmeter. As the electrons flew from the

positive terminal of the voltmeter to the negative, I also recorded the electron flowdirection in the table below.

I repeated the above steps with all possible pairs of given electrodes

Data/Observations:

Battery with Voltage Electron flow through the external wire

1 Zn & Fe 0.32 v From Zn to Fe

2 Zn & Al 0.92 v From Al to Zn

15

7/28/2019 Chem_Eng

16/23

3 Zn & Cu 0.89 v From Zn to Cu

4 Zn & Pb 1.10 v From Zn to Pb

5 Fe & Al 1.24 v From Al to Fe

6 Fe & Cu 0.58 v From Fe to Cu

7 Fe & Pb 0.31 v From Fe to Pb

8 Al & Cu 2.02 v From Al to Cu

9 Al & Pb 1.34 v From Al to Pb

10 Cu & Pb 0.26 v From Pb to Cu

Conclusions:

Questions 1 and 2:

Oxidation half reaction Reduction half reaction

Cell 1 Zn (s) Zn +2(aq) + 2 e Fe+2(aq) + 2 e 1 Fe(s)

Cell 2 Al(s) Al +3(aq) + 3 e 1 Zn +2(aq) + 2 e Zn(s)

Cell 3 Zn (s) Zn +2(aq) + 2 e 1 Cu +2(aq) + 2 e 1 Cu(s)

Cell 4 Zn (s) Zn +2(aq) + 2 e 1 Pb+2(aq) + 2 e 1 Pb(s)

Cell 5 Al(s) Al +3(aq) + 3 e Fe+2(aq) + 2 e 1 Fe(s)

Cell 6 Fe (s) Fe +2(aq) + 2 e 1 Cu +2(aq) + 2 e 1 Cu(s)

Cell 7 Fe (s) Fe+2

(aq) + 2 e 1

Pb+2

(aq) + 2 e 1

Pb(s)

Cell 8 Al(s) Al +3(aq) + 3 e 1 Cu +2(aq) + 2 e 1 Cu(s)

Cell 9 Al(s) Al +3(aq) + 3 e 1 Pb+2(aq) + 2 e 1 Pb(s)

Cell 10 Pb (s) Pb +2(aq) + 2 e 1 Cu +2(aq) + 2 e 1 Cu(s)

3, Since the anode produces electrons in the reaction, it will always have a greater concentration16

7/28/2019 Chem_Eng

17/23

of electrons than the cathode. The negative charges the electrons carry will repel them away fromeach other through the external circuit to the cathode.

4. When the electrons move from the anode to the cathode, the solution in which the anode issubmersed will become positively charged. To maintain neutrality negative ions in the solutionwhere the cathode is must move through the salt bridge to balance the electron loss. At the sametime the electrons in the cathode are being gained by the metal ions to form neutral atoms, thesolution where the cathode is will become more negatively charged. Positives from the anodesolution must move through the bridge neutralize the solution.

5. From the most active to the least active metal:AlZnFePbCu

(Why?)

Activity 5

In making an electrochemical cell, a redox reaction, more accurately the oxidation part of it, isused to generate an electric current. This process can be reversed. Electrons can be used forcedinto a cell to produce a chemical reaction. This reverse process is called electrolysis. In this

activity, you will electrolyzed water to make hydrogen and oxygen.

Problem: To produce hydrogen and oxygen by electrolysis of water

Materials: Steel electrodesConnecting wires with alligator clips BatteriesBeaker Test tubes

Na 2CO 3 (s) Connecting Wire Connecting Wire

Steel electrode Steel Electrode

Inverted test tubesFilled with water

Method: 1 Set up your equipment as shown in17

7/28/2019 Chem_Eng

18/23

the diagram. Fill the beaker & test tubes with water.2.Connect the electrodes to the battery. If no bubbles are seen

to escape from the electrodes into the test tubes,add solid Na 2CO 3 to the water in the beaker and stir

the water with a stirring rod until all the solid is dissolvedConnect the electrodes to the battery and record your observations.

3.When one test tube is filled with a gas, disconnect the battery. Insert a stopper into the mouths of the test tubes while the mouths are till

under water.4.Test the gas in the full test tube for hydrogen. Record you observations.5.Test the gas in the half full test tube for oxygen. Record your observations

Data/Observations:

Conclusions:1. What gas was produced by the negative electrode? Explain why? Write the

balanced equation for this reaction. Is the negativeelectrode an anode or a cathode?

2. What gas was produced by the positive electrode? Explain why? Write the balanced equation for this reaction. Is the positive

electrode an anode or a cathode?3. Use the equations of the two half reaction to write the equation of the complete

redox reaction.4. Explain what is electrolysis.5. Why was Na 2CO 3 needed in the electrolysis of water? Explain. (Hints: Is water a

good conductor of electricity?)6. Explain why electrolysis of solid sodium does not work while the electrolysis of

molten NaCl produces sodium and chlorine gas?

18

7/28/2019 Chem_Eng

19/23

A Sample ReportChemistry: Activity 5Activity 5

Problem: To produce hydrogen and oxygen by electrolysis of water

Materials: Steel electrodesConnecting wires with alligator clips BatteriesBatteryBeaker Test tubes Connecting Wire Connecting Wire

Na 2CO 3 (s)

Steel electrode Steel electrode

Inverted test tubes filled withwater

Method: 1 I set up the electrolysis equipment as shown in the diagram and I filled the beaker &test tubes with water.

2.I connected the electrodes to the terminals of the battery, but I did not see any bubbles rising from the electrodes. I added solid

19

7/28/2019 Chem_Eng

20/23

Na 2CO 3 to the water in the beaker and stirred the water with astirring rod until all the solid was dissolved.

I connected the electrodes to the battery and recorded my observations.3.When one test tube was filled with a gas, I disconnected the electrodes from the

battery. I insert stoppers into the mouths of the test tubes while the mouths were tillunder water before I removed them from the water.

4. I tested the gas in the full test tube for hydrogen and record my observations.5. I tested the gas in the half full test tube for oxygen and recorded my observations

Data/Observation1. When the beaker and the test tubes were filled with distilled water, nothinghappened when the electrodes were connected to the battery.2. When Na 2CO 3 (s) was added to the water, the water turned cloudy at first and

became clear after much stirring.3. Noting seemed to happen after the addition of Na 2CO 3 (s) until the test tubes werelifted up so that the mouths were about 1 cm above the tips of the electrodes. Bubblesrose from the tips of the electrodes into the test tubes.4. The test tube with the negative electrode seemed to fill up more quickly.5. When the test tube with the negative electrode is complete filled, I disconnected the

electrodes form the battery and stoppered both test tubes with rubber stopper beforeremoving them out of water. The test tube over the positive electrode appeared to beless than half filled with a gas.6. The gas in the full test tube burned with a popping sound when a flaming woodensplint was placed over the mouth of the test tube. The gas in the test tube reignited theglowing splint when it was inserted into the test tube.

Conclusions:1. According to the test results, the gas produced by the negative electrode is

hydrogen. Water dissociates into H + ions and OH ions. The negative electrode has a high

concentration of e

, which attract the H+

ions toward it. When The H+

ions accept the e

, they become H 2 gas.

2 H + + 2 e H2

The equation shows that electrons are gained by the hydrogen ions. By definition thisis a reduction. The electrode is called cathode.

2. According to the test results, the gas collected in the test tube with the positiveelectrode is oxygen. Since OH ions are negative, they are attracted to the positive electrode,where the ions gives up an e to become oxygen.

4 OH

O2 + 2 H 2O + 4 e

In this reaction because electrons are lost, the reaction is an oxidation. The electrodeat which oxidation takes place is called an anode.

3. In a redox reaction the loss and gain of electrons must be equal. Since the reductionin Question 1 shows a gain of 2 electrons while the oxidation in Question two shows a loss of 4electrons, I need to multiply first equation by 2 before adding it to the second equation.

2 ( 2 H + + 2 e H2 )20

7/28/2019 Chem_Eng

21/23

4 H + + 4 e 2 H 24 OH O2 + 2 H 2O + 4 e

4 H + + 4 OH 2 H 2 + O 2 + 2 H 2O(Why is there water on the left side of the equation?)

4. The process, where electrical energy is used to produce a chemical change, iscalled electrolysis.

5. Distilled water was not a conductor as I found in Activity 2. For electrolysis to take place the solution must be a conductor. Since Na 2CO 3 is an ionic compound, its solution is aconductor. Water molecules are changed when electricity passes through the solution.(When Na 2CO 3 is dissolved in water, there will be Na + and CO 3 2 ions, why are they not attractedto the electrodes to form new products?)

6. In solid NaCl, the ions are not free to move. When electricity is passed into thesolid, the ions cannot move to the electrodes to be oxidized and reduced. Once it is melted theions are free to move and electrolysis can take place.

Test on OxidationReduction Reaction

Part 1 Lab Practical Test

1. Follow the directions given below to electroplate a piece of copper with zinc..

Pour about 400 ml of 0.5M Zn(SO 4) solution into a 600 ml beaker. Use the nonmetallicscouring pad to clean the two copper electrodes until they shine. Place the two electrodes into thezinc sulfate solution, but be sure that they dont touch each. Record all your observation on a

piece of paper and then explain the reasons for all the changes that occur where the electrons pass through

2. Go to the lab and do the following:(a) Put about 150 ml of 0.5 M BaCl 2 solution in a beaker and leave the probes of the

Conductivity Indicator in the solution. Is the solution a conductor? Record your observations.(b) Use a dropper to add one drop of 0.5 M Ag 2(SO 4) solution to the solution in the beaker. Use aglass stirrer to mix the solutions gently. What do you notice about the brightness of the light bulbof the Conductivity Indicator? Do you see any changes in the beaker? Record your observations.(c) Continue to add, one drop at a time, the Ag 2(SO 4) solution to the beaker and stir it gently and

continuously. What do you notice about the solution in the beaker and the brightnessof the light bulb? Record your observations.Explain, at the molecular level, what you see in doing steps (a) to (c). What is the necessaryrequirement for a solution to be a conductor?

21

7/28/2019 Chem_Eng

22/23

3. What is the function of a salt bridge? Can you substitute the bridge with a copper wire? Go tothe lab and make a cell with a wire. Does the cell function? Why or why not? Explain.

4. In an ionic solid, the ions are held together by their opposite charges. To break the bonds,attractive forces, between the ions, energy is needed. Heat energy is often used to beak the

bonds. You can see the bonds being broken when the solid changes into a liquid because theions are no longer held together. Ammonium nitrate NH 4 NO 3 is an ionic solid. If you added it towater, you will notice the solid dissolves in water. The bonds between the ions must have been

broken, but where does the energy come from? Before answering the question, go to the lab anddissolve a teaspoon of solid NH 4 NO 3 in about 100 ml of water. What observations lead you toyour answer?

Part 2 Multiple Choice Questions

1. Which of the following is an electrolyte?(a) NaNO 3 (b) ethanol (c) vinegar (d) Salt solution (e) Peanut oil

2. Which of the following gases supports combustion but will not burn?

(a) Hydrogen (b) oxygen (c) nitrogen (d) carbon dioxide (e) carbon monoxide

3. In the reaction between magnesium and hydrochloric acid, the oxidizing agent is(a) hydrogen. (b) magnesium chloride. (d) hydrochloric acid. (e) magnesium.

4. In the electrochemical cells made with zinc in zinc nitrate and aluminum in aluminum nitrate,the reducing agent is:

(a) zinc (b) zinc nitrate (c) aluminum (d) aluminum nitrate

5. Which of the following reactions is NOT a redox reaction?(a) 2 H 2 (g) + O 2 (g) 2 H 2O (g)

(b) 2 Al (s) + 3 Cl 2 (g) 2 AlCl 3 (s)

(c) NaCl (s) Na +1 (aq) + Cl 1 (aq)

(d) 2 CuO (s) + C (s) + heat CO 2 (g) + 2 Cu (s)

Part 3 Balancing Equations

1.The equation for the reaction between aluminum and hydrochloric acid is as follows:Al (s) + 3 HCl (aq) AlCl 3 (aq) + 3 H 2

Write the balanced oxidation half reaction below:

2. The two half equations of a reactions are:Cu +2 + 2e Cu 0

Al 0 Al+3 + 3e Write a balanced equation for the redox reaction.

Part 4 Essay Questions

1. Why does an electrochemical cell need an electrolytic solution? Explain.22

7/28/2019 Chem_Eng

23/23

2. In an electrochemical cell, is one electrode positively charged and the other negativelycharged? Explain.

3. In an electrochemical cell, electrons always flow from anode to cathode. Why? Explain.

4. Based on their atomic structure why do you think that one metal is more active than the other?

5. The Conductivity Indicator that you used in Activity 2 is run with a low voltage current. Why?

6. Will there be a chemical reaction between Cu metal and Zn(NO 3)2 solution? Based on your understanding of the Metal Activity Series explain your answer.

7. In the electrolysis of a water solution of NaCl, what would expect to be the product at thenegative electrode? Explain your answer.

8. Two carbon electrodes are put into a beaker of CuSO 4 solution. If the two electrodes areconnected to a battery, describe what may see at the negative electrode. Explain your answer.