Identifying Chemical Reactions

Introduction

Review the five types of chemical reactions in this colorful and dramatic collection of five demonstrations.
The set of five demonstrations includes:

1. Single Replacement Reaction—Aluminum foil is placed in a solution of copper(II) chloride. The solution rapidly heats up as the aluminum metal dissolves and a dark reddish–brown precipitate of copper metal forms.
2. Double Replacement Reaction—Solutions of blue-green copper chloride and clear sodium phosphate are mixed. The solution fades to a pale green as dark-turquoise copper phosphate precipitates out of solution.
3. Combination Reaction—Water is placed in an aluminum pan and solid calcium oxide is added. The two chemicals combine to form calcium hydroxide and the heat liberated is great enough to fry an egg!
4. Decomposition Reaction—A Petri dish, filled with a salt solution containing universal indicator, is placed on an overhead projector. Positive and negative leads from a 9-volt battery are placed in the solution on opposite sides of the Petri dish. Water is decomposed at both electrodes, causing pH changes that produce a rainbow of colors in the solution.
5. Combustion Reaction—Add a little isopropyl alcohol to a 2-L soda bottle, ignite the vapors and “whoosh!” The combution of isopropyl alcohol and oxygen produces a rush of gases and a spectacular blue flame.

The series of demonstrations may be presented in a variety of ways. Each demonstration may be used to introduce a specific chemical reaction, or all the demonstrations can be performed together as a review of reaction types. A student worksheet is included as an optional assessment tool for the instructor.

Concepts

• Oxidation–reduction
• Single replacement reaction
• Double replacement reaction
• Precipitation reaction
• Combination reaction
• Exothermic reaction
• Decomposition reaction
• pH
• Electrolysis
• Exothermic reactions
• Activation energy
• Combustion

Materials Included In Kit

Additional Materials Required

Background

Single Replacement Reaction—Aluminum and Copper(II) Chloride
Watch aluminum foil disappear as it is added to a green-blue solution of copper(II) chloride. Observe color changes, production of a gas, formation of elemental copper, and a drastic change in temperature from this single-replacement, oxidation–reduction reaction.

Double Replacement Reaction—Copper(II) Chloride and Sodium Phosphate
A blue-green solution of copper(II) chloride is combined with a colorless solution of sodium phosphate. The initial blue color of the solution fades and the final products consist of a turquoise solid and a pale green liquid in this example of a double-replacement, precipitation reaction.

Combination Reaction—Calcium Oxide and Water
When water is added to calcium oxide, the resulting combination reaction produces calcium hydroxide and enough heat to fry an egg.

Decomposition Reaction—Electrolysis of Water
Demonstrate the decomposition reaction of water in a very colorful and dramatic way on an overhead projector.

Combustion Reactions—Isopropyl Alcohol and Oxygen
Wow the students with a whoosh! Students will love to see the blue alcohol flame shoot out the mouth of the bottle in this highly exothermic combustion reaction.

Materials

Safety Precautions

Copper(II) chloride solution is toxic by ingestion. Small volumes of hydrogen gas are produced from the reaction. Hydrogen is a highly flammable gas; keep flames away from the reaction mixture. Calcium oxide is a corrosive solid and a severe body tissue irritant. Avoid all body tissue contact. Reaction of calcium oxide and water will produce large amounts of heat—skin burns are possible. A lump of calcium oxide may disintegrate violently and splatter when water is added. This should be a teacher demonstration only. Do not allow students to perform this procedure. Food-grade items that are brought into the lab are considered laboratory chemicals and should not be consumed. Do not eat the egg after it is cooked. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. 

Please read all safety precautions before proceeding with the Combustion Reactions—Isopropyl Alcohol and Oxygen demonstration.

• Do not perform demonstration under sprinkler head or smoke detector.
• Isopropyl alcohol is a flammable liquid and a fire hazard. It is slightly toxic by ingestion and inhalation. Use in a well-ventilated room.
• Always recap the alcohol bottle and move it far from the demonstration area. Never leave an open bottle of alcohol in the vicinity of the demonstration.
• A safety shield is highly recommended for explosive reactions. Even the mildest explosion creates some chance of shattering and flying objects. Protective eyewear must be worn by the demonstrator as well as by anyone viewing the demonstration.
• Never perform alcohol explosions in glass bottles. The large quantities of gases (H₂O and CO₂) produced during the rapid combustion will easily shatter a glass container. Serious accidents have occurred performing this demonstration in a glass container—do not use glass. Use a plastic bottle made of PETE (polyethylene teraphthalate).
• Always pour out any excess liquid alcohol from the plastic bottle before igniting the vapor. If any liquid alcohol is left, it will increase the amount of gaseous afterburning. The liquid could also ignite, which may cause the plastic bottle to melt. Always keep a lid or some sort of cover nearby to place over the mouth of the bottle and extinguish the flame if it continues so long as to begin melting the plastic. Excess alcohol on the outside of the bottle should be wiped off in order to avoid its igniting and softening the plastic bottle.
• Never, ever use a pure oxygen environment as the potential for an extremely violent and deadly explosion is possible.
• Never use methyl alcohol for this demonstration. The high volatility of methyl alcohol means that it has the potential for the most violent combustion of any alcohol.
• Replace the plastic bottle after every demonstration. The heat of the reaction will cause the bottle to contract and deform.
• Do not perform this demonstration directly underneath smoke/heat detectors or sprinkler systems.
• Make sure the ceiling is at least 4 feet above the whoosh bottle to prevent possible scorching and fire.
• Always wear protective eyewear when performing this demonstration.
• Please consult current Safety Data Sheets for additional safety information on isopropyl alcohol.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. For the Single Replacement Reaction demonstration, allow the solid material in the beakers to settle. Decant the copper(II) chloride solution down the drain according to Flinn Suggested Disposal Method #26b. Dispose of the solid copper and leftover aluminum foil in the solid waste according to Flinn Suggested Disposal Method #26a. The product mixture from the Double Replacement Reaction demonstration consisting of solid copper(II) phosphate and sodium chloride solution may be filtered and the copper(II) phosphate solid disposed of according to Flinn Suggested Disposal Method #26a. The remaining sodium chloride solution may be disposed of according to Flinn Suggested Disposal Method #26b. The solid calcium hydroxide remaining after the Combination Reaction demonstration may be neutralized with hydrochloric acid and rinsed down the drain with excess water according to Flinn Suggested Disposal Method #10. All materials from the Decomposition Reaction demonstration may be disposed of according to Flinn Suggested Disposal Method #26b. Excess isopropyl alcohol from the Combustion Reactions demonstration may be disposed of by allowing it to evaporate in a fume hood according to Flinn Suggested Disposal Method #18a. The bottle may be disposed of according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

Combustion Reactions—Isopropyl Alcohol and Oxygen

Before each demonstration, inspect the plastic bottle for grazing, frosting, cracking or any small flaws. Replace the bottle if it shows signs of fatigue.

Procedure

Single Replacement Reaction—Aluminum and Copper(II) Chloride

1. Place a 500-mL borosilicate glass graduated cylinder on the demonstration table.
2. Use a 250-mL graduated cylinder to measure 140 mL of 1.0 M CuCl₂ solution. Pour this solution into the 500-mL graduated cylinder.
3. Measure and add 140 mL of distilled or deionized water to the 500-mL cylinder. The solution is now 0.5 M CuCl₂.
4. Cut a piece of aluminum foil approximately 6" x 12". Loosely roll the foil into a cylinder that will fit into the 500-mL graduated cylinder. (Note: Do not wad up the aluminum foil tightly as this decreases the surface area and slows any reaction that may occur.)
5. Measure the temperature of the solution before adding the foil.
6. Place the aluminum foil into the 500-mL graduated cylinder, using a stirring rod to push it down completely into the solution. Measure the temperature of the reaction mixture again. Notice the large increase in temperature, indicating a very exothermic reaction.
7. (Optional) Have students record the evidence for a chemical reaction and the properties of the product(s) on the Data Table.

Double Replacement Reaction—Copper(II) Chloride and Sodium Phosphate

1. Using a 100-mL graduated cylinder, transfer 70 mL of the 0.05 M copper(II) chloride solution to the 250-mL cylinder.
2. Using a clean 100-mL graduated cylinder, measure out 50‑mL of 0.05 M sodium phosphate solution.
3. Have students note the initial colors of the two reactant solutions.
4. Add the sodium phosphate solution to the copper(II) chloride solution. Stir with stirring rod.
5. (Optional) Have students record the evidence for a chemical reaction and the properties of the product(s) on the data table.

Combination Reaction—Calcium Oxide and Water

1. Measure 100 g of calcium oxide into a large weighing dish.
2. Place one of the aluminum pie pans onto a heat-resistant surface or pad. Using a graduated cylinder, measure and add 40 mL of distilled water to the pan.
3. Spray the second aluminum pan with cooking spray or add a small amount of cooking oil to the pan.
4. Quickly but carefully add the calcium oxide to the water in the first aluminum pan. Distribute the calcium oxide evenly across the pan to get efficient contact between the solid calcium oxide and water. Caution: Splattering may occur.
5. Immediately place the second aluminum pan directly on top of the calcium oxide.
6. Wait about one minute—steam should be evident emanating from the bottom pan, and the cooking oil or spray may begin to sizzle.
7. Quickly break a small egg into the top pan. Caution: The pan will be hot. Wear a heat-resistant glove or use an oven mitt to hold the top pan with one hand while gently using the spatula to check the egg with the other hand.
8. Cook the egg to order! Continue cooking for several minutes—the egg white will continue to cook after the steam has subsided. (The egg white will “fry” but the yolk will remain runny.)
9. (Optional) Have students record the evidence for a chemical reaction and the properties of the products on the data table.

Decomposition Reaction—Electrolysis of Water

1. Place the two halves of a Petri dish on the projection stage of an overhead projector.
2. Measure out about 25 mL of the sodium chloride/universal indicator solution into a 25-mL graduated cylinder.
3. Pour enough sodium chloride/universal indicator solution into each half of the Petri dish to just cover the bottom of each half dish. Adjust the overhead projector so that the dishes are in clear focus. Each dish should appear to be a rich, transparent green color.
4. Break a pencil lead in half. Attach the leads to opposite sides of the Petri dish with the alligator clips. Make sure the tip of each lead is submerged in the green solution and the alligator clips remain out of the solution.
5. To start the demonstration, clip the 9-volt battery into the snaps on the battery clip (see Figure 1).
   {13939_Procedure_Figure_1_Demonstration setup}
6. Let the demonstration run for 5–10 minutes and note the changing colors over time. (A purple color will appear at the cathode very quickly. A red–orange color at the anode will appear more slowly. Over time, the entire spectrum of universal indicator colors will appear.)
7. (Optional) Have students record the evidence for a chemical reaction and the properties of the product(s) on the data table.

Combustion Reactions—Isopropyl Alcohol and Oxygen

1. Add about 5–6 mL of isopropyl alcohol to the 2-L plastic bottle. Do not add more than 8 mL of alcohol. Recap the bottle of alcohol tightly and move it far from the demonstration area.
2. Lay the bottle sideways on a flat surface allowing the alcohol to flow from base to mouth. Slowly swirl the bottle for about 30 seconds, trying to spread alcohol liquid completely over the entire interior surface. This allows the liquid alcohol to volatilize and makes the vapor concentration uniform throughout the bottle. If a lot of liquid alcohol is still visible, swirl the bottle for another 30 seconds.
3. Pour out any excess liquid alcohol.
4. Stand the bottle on the demonstration table or lab bench, placing it in the front of the room and behind a safety shield.
5. Dim the lights in the room.
6. Light a match or wood splint that is taped to a meter stick or other long stick.
7. Stand back and, at arm’s length, bring the burning match or wood splint over or slightly down into the mouth of the bottle. (Note: Be sure you are on the safe side of the safety shield as well.)
8. Observe the explosive “whoosh” that results.
9. After the reaction has subsided and all the flames are out, wait for a minute or two until the bottle has cooled slightly. Pour out the water droplets from the bottle into a 10-mL graduated cylinder using a small funnel. As much as 1–2 mL of water may result, showing that water is one of the products of the combustion of alcohol.
10. (Optional) Have students record evidence for a chemical reaction and the properties of the products in the Data Table.
11. Repeating the Demonstration: The demonstration cannot be repeated with the same bottle. Use a new 2-L plastic bottle to repeat the demonstration.

Student Worksheet PDF

13939_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the Single Replacement Reaction—Aluminum and Copper(II) Chloride demonstration seven times using the amounts indicated in the procedure.
• The reaction temperature increases from room temperature (25 °C) to nearly 60 °C when the aluminum has completely reacted with the copper(II) chloride. Therefore, be sure to perform the demonstration in borosilicate glassware. Check the glassware for any cracks or chips—do not use any damaged glassware.
• The foil will begin to slowly rise up in the cylinder from the gas bubbles attaching to the foil piece. Most of the reaction will occur at the top of the solution (which will turn gray or colorless) and there will be unreacted green-blue copper solution at the bottom of the cylinder. A long stirring rod will be useful in pushing the foil down to the bottom.
• A large borosilicate hydrometer cylinder or large beaker may also be used.
• Exact amount of reactants are not critical.
• The use of a digital thermometer allows for quick temperature measurements.
• Add more aluminum foil, if needed, to get the final solution to colorless.
• This kit contains enough chemicals to perform the Double Replacement Reaction—Copper(II) Chloride and Sodium Phosphate demonstration seven times using the amounts indicated in the procedure.
• The final solution color depends on the concentration of unreacted copper(II) ions. The fewer ions, the more pale the final solution color.
• Add more 0.05 M sodium phosphate to completely clear the solution of copper(II) ions.
• This kit contains 700 g of CaO, enough to perform the Combination Reaction—Calcium Oxide and Water demonstration at least seven times. The aluminum pans and spatula are reusable. Order more calcium oxide from Flinn Scientific to repeat this demonstration. The shelf life for calcium oxide is poor—always use fresh calcium oxide for best results.
• Use an oven mitt or hot pad to hold the pans when cooking the egg. Place the aluminum pans on a heat-resistant surface—the bottom pan will get very hot. This reaction generates a lot of heat; use proper care handling the pans. Steam burns are possible—the temperature of the solid mixture may reach a maximum of about 120 °C under the conditions used in this demonstration.
• This kit contains enough chemicals to perform the Decomposition Reaction—Electrolysis of Water demonstration at least seven times: 9-V battery, battery leads with alligator clips, tube of 12 pencil leads, 200 mL of sodium chloride/universal indicator solution and seven disposable Petri dishes.
• Concepts of pH and electrolysis should be discussed prior to this demonstration. Universal indicator colors, as they relate to pH values, should also be discussed. The Flinn Overhead Color Chart (Catalog No. AP5367) is a nice visual supplement during this discussion. Note that some chlorine is produced at the anode, along with oxygen gas.
• The demonstration can be repeated using a pH 7 buffer and the salt and universal indicator solution to show the effects of buffering a solution. (When the solution is buffered, color changes occur much slower, if at all.)
• Enough isopropyl alcohol (50 mL) is provided to perform the Combustion Reactions—Isopropyl Alcohol and Oxygen demonstration as written at least eight times.
• Depending on how much alcohol vapor is in the bottle, you may have to place the flame slightly inside the lip of the soda bottle before it ignites.
• The demonstration works best if the alcohol vapor is prepared immediately before the demonstration. If the bottle with the vapor sits for a while, the vapor tends to settle and is harder to light.
• The first effect that may be observed is the usual “whoosh,” which involves a moderately violent thrust of blue flames and gas out of the mouth of the bottle. Some afterburning or dancing flames of burning vapor in the body of the bottle may also result.
• The second effect is a slower burn of gas down the inside surface of the bottle, producing a ring, plate, or cone of fire, which may be accompanied by an upward thrust or ball of yellow flames in the center of the bottle. The sound accompanying these slower burns is actually more of a “whomp.” This effect can also be observed by using 70% isopropyl alcohol, illustrating reduced vapor pressure due to dilution.
• Use only a plastic bottle made of PETE (polyethylene teraphthalate).

Sample Data

See Teacher PDF.

Discussion

Single Replacement Reaction—Aluminum and Copper(II) Chloride
Aluminum foil reacts with an aqueous solution of copper(II) chloride according to Equation 1. The reaction may be classified as a single replacement, oxidation–reduction reaction.

The oxidation of aluminum metal to aluminum(III) (Al⁰ to Al³⁺) is inferred from the dissolving of the aluminum foil and is represented by the oxidation half-reaction that follows. The simultaneous reduction of copper(II) or copper(II) ions to copper metal (Cu²⁺ to Cu⁰) will occur and solid copper metal precipitates from solution according to the reduction half-reaction that follows. As the copper(II) ions are reduced to copper, the green-blue solution color will fade until the solution is completely colorless—the indication that the reaction is complete and all of the copper(II) ions have been reduced. It is observed that hydrogen gas is simultaneously released from the reaction when aluminum metal foil is added to copper(II) chloride solution. If the pH of the copper(II) chloride solution is measured, it is found to be slightly acidic. Hence there are free hydrogen ions in solution, that cause the side reaction of hydrogen ions with the aluminum surface to form hydrogen gas and aluminum ions (see Equation 2). Due to the limited concentration of hydrogen ions, this reaction consumes only a small amount of the aluminum. An interesting note is that the reaction will not proceed if copper sulfate solution is used in place of copper chloride solution. Chloride ions are needed to penetrate the aluminum oxide coating (Al₂O₃) to expose the aluminum metal to oxidation.

Double Replacement Reaction—Copper(II) Chloride and Sodium Phosphate
A solution of copper(II) chloride reacts with a solution of sodium phosphate according to Equation 3. The reaction may be classified as a double-replacement precipitation reaction. Insoluble copper(II) phosphate, Cu₃(PO₄)₂, precipitates out of solution as a torquoise-colored solid. The removal of most of the blue-green copper(II) ions, Cu²⁺(aq), from solution reduces the solution color to a very pale green.

Combination Reaction—Calcium Oxide and Water
Calcium oxide is also known as lime or quicklime. Calcium oxide is produced by heating limestone (calcium carbonate) in air. However, calcium oxide readily absorbs and reacts with carbon dioxide and water to form calcium carbonate (CaCO₃) and calcium hydroxide [Ca(OH)₂], respectively. When water is added to calcium oxide, a combination reaction occurs, producing calcium hydroxide and a large amount of heat. Calcium hydroxide is used to treat acidic soils, soften water and prepare building materials such as plaster, mortar and bricks. The solubility of calcium hydroxide in water is very low, about 1.6 g/L. Therefore, the product of the reaction of CaO and H₂O is Ca(OH)₂(s), not Ca(OH)₂(aq). The reaction is highly exothermic, producing enough heat to fry the egg.

CaO(s) + H₂O(l) → Ca(OH)₂(s) + heat
ΔH = ΔH_f(products) – ΔH_f(reactants)
ΔH = ΔH_f[Ca(OH)₂(s)] – {ΔH_f[CaO(s)] + ΔH_f[H₂O(l)]}
ΔH = –986.1 kJ/mole – [–635.1 kJ/mole + (–285.8 kJ/mole)] = –65.2 kJ/mole

Decomposition Reaction—Electrolysis of Water
When an electric current is passed through an aqueous solution containing an electrolyte (such as NaCl), the water molecules decompose into their constituent elements, hydrogen, H₂(g), and oxygen, O₂(g). The overall reaction occurs as two separate, independent half-reactions. Reduction of the hydrogen atoms to elemental hydrogen (H₂) occurs at the cathode (–), while oxidation of the oxygen atoms in water to elemental oxygen (O₂) occurs at the anode (+). Each half-reaction is accompanied by the production of OH^– or H⁺ ions as shown:

Cathode: 4e^– + 4H₂O(l) → 2H₂(g) + 4OH^–
Anode: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e^–

At the cathode, the excess OH^– will cause the pH to increase, resulting in a color change of the universal indicator solution from green (neutral, pH 7) to purple (basic, pH ≥ 10).

At the anode, the excess H⁺ will cause the pH to decrease, resulting in a color change of the universal indicator solution from green to an orange/red color (acidic, pH ≤ 4). The electrolysis half-reactions can also be followed by observing the production of gas bubbles at the cathode (H₂) and anode (O₂). More gas bubbles will be generated at the cathode.

Universal indicator is an acid–base indicator that is different colors at different pH values. All colors will be visible in the Petri dish as electrolysis progresses and as the pH conditions continually change due to diffusion and neutralization. Combustion Reactions—Isopropyl Alcohol and Oxygen
Low-boiling alcohols vaporize readily. When isopropyl alcohol is placed in a 2-L, small-mouthed plastic bottle, it forms a volatile mixture with the air. A simple match held by the mouth of the bottle provides the activation energy needed for the combustion of the alcohol/air mixture.

Only a small amount of alcohol is used and it quickly vaporizes to a heavier-than-air vapor. The alcohol vapor and air are all that remain in the bottle. Alcohol molecules in the vapor phase are farther apart than in the liquid phase and present far more surface area for reaction; therefore the combustion reaction that occurs is very fast.

Since the burning is so rapid and occurs in the confined space of a 2-L bottle with a small neck, the sound produced is very interesting, like a “whoosh.” The equation for the combustion reaction of isopropyl alcohol is shown below. One mole of isopropyl alcohol reacts with 4.5 moles of oxygen to produce 3 moles of carbon dioxide and 4 moles of water:

(CH₃)₂CHOH(g) + ⁹⁄₂O₂(g) → 3CO₂(g) + 4H₂O(g)                        ΔH = –1,235 kJ/mol

References

Special thanks to the late Cliff Schrader, Summit County ESC, Cuyahoga Falls OH, for providing Flinn with the idea for the Single Replacement Reaction—Aluminum and Copper(II) Chloride activity. Andy Cherkas, Walter Rohr and Pat Funk also provided insight and advice on this activity.

Special thanks to DeWayne Lieneman, retired, Glenbard South High School, Glen Ellyn, IL, for providing us with the instructions for the Combination Reaction—Calcium Oxide and Water activity.

Special thanks to Mike Shaw, West Stokes High School, King, NC, for the Decomposition Reaction—Electrolysis of Water demonstration idea.

Flinn Scientific would like to thank John Fortman, Dept. of Chemistry, Wright State University, Dayton OH, for all of his research, safety notes and variations on the classic Combustion Reactions—Isopropyl Alcohol and Oxygen demonstration. John has written an excellent article on this demonstration; see reference listed below. Lee Marek, Naperville North H.S., Naperville, IL, and Bill Deese have also popularized this demonstration.

Fortman, J. J.; Rush, A. C.; Stamper, J. E. J. Chem. Ed. 1999, 76, 1092–1093.