Electrolysis

EXPERIMENT Electrolysis

Using electrolysis, visitors separate hydrogen and oxygen from water molecules in a solution. A pH indicator in the water allows visitors to observe how the pH of the solution changes as the gases are removed.

OBJECTIVES: Visitors learn how industry uses electricity to separate certain chemical compounds into desired products.

SCIENCE TOPICS PROCESS SKILLS VOCABULARY

Electrolysis Observing Acid pH Investigating Atom Inferring Base Electrolysis Indicator Molecule Solution

Electrolysis

Procedure: 1. Always wear safety goggles. 2. Rinse the two test tubes in the sink. 3. Use a wooden stick to stir the green solution in the plastic tub. (If the solution is yellow, add a drop of sodium hydroxide and stir. If the solution is blue, add a drop of hydrochloric acid and stir.) 4. Use the plastic dropper to fill a test tube to the rim with the green solution. 5. Seal the end of the full test tube with your thumb and turn it upside down. 6.  Place the upside-down test tube (and your thumb) into the solution in the tub.  Keep the test tube in the solution as you remove your thumb from the opening.  Immediately slide the test tube over a metal spike in the bottom of the tub. 7. Repeat steps 4 through 6 with the other test tube. 8. CLOSE the switch. Observe the test tubes for about one minute. Which test tube has more bubbles? What color is the solution in each test tube? 9. OPEN the switch. Lift the test tubes and empty them into the tub. Rinse the test tubes and your hands in the sink.

What products are made using electrolysis?

A Closer Look: In this experiment. electricity passing through the solution separated water molecules (H2O) into hydrogen (H) and oxygen (0) bubbles in a process called electrolysis. The test tube on the left produced hydrogen gas (H2). The test tube on the right produced oxygen gas(O2). Twice as much hydrogen was produced because there are two hydrogen atoms (H) and one oxygen atom (0) in each water molecule (H2O).

Bromthymol blue, an indicator, colors the solution. It becomes yellow in acidic solutions, blue in basic solutions, and green in neutral solutions, like water. The solution in the left test tube turns blue because removing hydrogen from water creates a basic solution. The right tube turns yellow because removing oxygen from water creates an acidic solution.

Electrolysis is used in industry to separate the elements of a compound. Its applications include making chlorine gas (Cl2) or pure sodium (Na) metal. Pure sodium is used in street lamps to give off a bright yellow light.

Experiment: Electrolysis Operating Guide

See Materials Prep (with amounts to have on hand) for more details  One plastic electrolysis tub (from Central Scientific catalog: “compact electrolysis apparatus”)  Two small (13- by 100-mm) test tubes (keep four on hand)  One plastic pipette (keep two on hand)  One wooden stirring stick (keep six on hand)  Two 6V lantern batteries (keep two fresh batteries on hand)  Two wire leads (each 8 to 12 inches long) with alligator clips on each end (one red wire, one white wire)  Two black leads (each 8 to 12 inches long) with an alligator clip on one end only  Three 30-ml dropper bottles  0.1M HCl (hydrochloric acid) (keep 100 ml on hand) OR — 1M HCl (keep 1 l on hand) OR — concentrated (12M) HCl (keep 100 ml on hand)  0.1M NaOH (sodium hydroxide) (keep 100 ml on hand) OR — 1M NaOH (sodium hydroxide) (keep 25 ml on hand) OR — solid NaOH (keep 500 g on hand)  Two 125-ml plastic storage bottles  One 1000-ml plastic storage bottle

 Na2SO4  10H2O (sodium sulfate) (keep 250 g on hand)  Bromthymol blue (keep 25 ml on hand)  One knife switch  Two small screws  One wooden block approximately 3 inches wide and 8 inches long  One 600-ml beaker or flask

Setup/Takedown Procedures

 Label the electrolysis tub “Water (H2O) with Sodium Sulfate (Na2SO4) and Bromthymol Blue.” With a permanent pen, draw a line 2 cm from the top edge.  Label the 30-ml dropper bottles “Hydrochloric Acid (HCl),” “Sodium Hydroxide (NaOH),” and “Bromthymol Blue.” To prepare the battery-wire-switch setup:  Strip ½ in. of insulation to leave bare wire on one end of each of the two black wire leads.

Experiment: Electrolysis Operating Guide

 Attach the knife switch to the center of the wooden block with screws. Label the open position “Open” and the closed position “Closed.”  Attach the loose (bare) end of one black wire to the “Open” position of the knife switch.  Attach the loose (bare) end of the second black wire to the “Closed” position of the knife switch.

 At the start of the day, check each battery with a voltmeter (the batteries should be close to 6V).  On a tray lined with a white mat, set out the following:  Labeled dropper bottles of HCl (hydrochloric acid), NaOH (sodium hydroxide), and bromthymol blue  Two test tubes  One stirring stick  Batteries, electrolysis tub, knife-switch setup, and wires.  Attach the clips on the ends of the red wire to the negative terminal of one battery and the positive terminal of the second battery.  Connect one of the two clips on the white wire to the positive terminal of the first battery. Attach the other clip of the white wire to the right- hand spike on the bottom of the electrolysis tub.  Connect the free clip of one of the black wires to the left-hand spike on the bottom of the electrolysis tub.  Attach the free clip of the other black wire to the negative terminal of the second battery.

 Fill the tub to the line (2 cm from the top) with the Na2SO4 (sodium sulfate) solution. Prepare more as needed (see Materials Prep).

 Make sure the Na2SO4 (sodium sulfate) solution is green. If the solution is yellow, add one drop of NaOH (sodium hydroxide) solution. If the solution is blue, add one drop of HCl (hydrochloric acid) solution.  Refill the dropper bottles of HCl (hydrochloric acid), NaOH (sodium hydroxide), and bromthymol blue. Prepare new solutions as needed (see Materials Prep).

 At the end of the day, pour the green Na2SO4 (sodium sulfate) solution back into the stock bottle.  Disconnect the wires.  Rinse the tub and the test tubes.  Return all equipment to the tub.

Experiment: Electrolysis Operating Guide

 Clean the tray and leave it at the station.  Clean the white mat and return it to general storage.

◊ Check the solution periodically to make sure that it is full and that it is green.

◊ If the solution in the test tubes turns color when visitors first add the green solution, the tubes were not rinsed well before the start of the activity.

Connect this experiment to the larger display in Unit 2 if the visitor has been to the lab before.

Na2SO4 (sodium sulfate), HCl (hydrochloric acid), and NaOH (sodium hydroxide) are hazardous substances; follow the handling and disposal instructions.

Consult the Material Safety Data Sheets (MSDS) for additional information.

To prepare 0.1M HCl:

 In a graduated cylinder, measure 90 ml of dH2O (deionized water). Pour into a 125-ml plastic bottle.  In a graduated cylinder, measure 10 ml of 1.0M HCl (hydrochloric acid). (Prepare more 1.0M HCl as needed. See below.)  Add the acid to the plastic bottle.

Experiment: Electrolysis Operating Guide

To prepare 1.0M HCl (hydrochloric acid): CAUTION: Concentrated HCl (hydrochloric acid) is extremely corrosive. Handle it with care. Wear protective gloves, an apron, and eyewear. Use HCl only in the fume hood. Avoid contact with skin or clothing. Neutralize any spills with baking soda and clean them up with copious amounts of water. CAUTION: A large amount of heat will be evolved after you mix HCl with water. Do not handle the beaker until it has cooled.  Turn on the fume hood.  Wear protective eyewear, chemical safety gloves, and an apron or lab jacket.  In the fume hood, measure 83 ml of concentrated (12.1M) HCl (hydrochloric acid).

 Add the acid to 917 ml of dH2O (deionized water) to make 1 l.  Store any extra solution in labeled/dated bottles in the acid cabinet.  Turn off the fume hood.

To prepare 0.1M NaOH (sodium hydroxide):

 In a graduated cylinder, measure 90 ml of dH2O (deionized water). Pour it into a 125-ml plastic bottle.  In a graduated cylinder, measure 10 ml of NaOH (sodium hydroxide). (Prepare more 1.0M NaOH as needed. See below.)  Add the NaOH to the plastic bottle.

To prepare 1.0M NaOH (sodium hydroxide): CAUTION: Concentrated or solid NaOH (sodium hydroxide) is extremely corrosive. Handle it with care. Wear protective gloves, an apron, and eyewear. Avoid contact with skin or clothing. CAUTION: A large amount of heat will be evolved after you mix NaOH with water. Do not handle the beaker until it has cooled.  Weigh 40 g solid NaOH (sodium hydroxide).

 Add 1000 ml dH2O (deionized water).  Mix to dissolve the contents.  Store the solution in a labeled and dated 1000-ml plastic bottle in the hydroxide cabinet.

To prepare Na2SO4 (sodium sulfate) solution with bromthymol blue:

 Weigh 32.2 g Na2SO4  10H2O (sodium sulfate).  Add the above to a 600-ml beaker or flask.

 Add dH2O (deionized water) to bring the volume to 500 ml,  Mix the solution well.

Experiment: Electrolysis Operating Guide

 Add a few drops of bromthymol blue solution to make a green solution.  Store the solution in a labeled/dated plastic storage bottle.

To prepare bromthymol blue solution:  Weigh 0.1 g bromthymol blue sodium salt.

 Add 100 ml H2O (water) and mix.  Store the solution in a labeled/dated bottle.  Keep at least 50 ml on hand in the indicator cabinet.