Synthesis of Pigments and Types of Reactions

 

            Many pigments are directly mined and refined. Iron (III) oxide, Fe2O3, is mined and produces a range of red to yellow pigments depending on how it is treated. This compound can be produced by the synthesis reaction:

4 Fe(s)   +   3 O2 (g)   ®   2 Fe2O3 (s)

 

Titanium (IV) oxide, TiO2, is the most commonly used white pigment and is produced by the following sequence of reactions:

Titanium ore + HCl ®  TiCl4                               TiCl4 (l) + 2 H2O(l) ® TiO2 (s)  + 4 HCl (g)

 

Black pigment uses carbon produced by the combustion of hydrocarbons or wood.  In this experiment, you will synthesize three pigments to illustrate some of the different types of reactions.

                Combustion reactions involve the reaction of elements or compounds with oxygen to produce the oxygen containing compounds of the elements. Complete combustion of hydrocarbons, such as the paraffin used in candles, produces CO2 and H2O by the following reaction:

2 C20H42 (s)  + 61 O2 (g)   ®  40 CO2 (g)    +   42 H2O (g)

 

If insufficient oxygen is present, carbon is produced as a side product and is called lamp black or soot and is used as a black pigment.

                Double displacement reactions often involve the combination of two aqueous solutions of soluble compounds to form a slightly soluble compound, which appears as a solid in the equation and a second soluble product. The solid, which is formed in this reaction, is called a precipitate and the double displacement reaction is called a precipitation reaction. Calcium carbonate can be used as a white pigment and is formed by the following reaction:

CaCl2 (aq)  +  Na2CO3 (aq)    ®    CaCO3 (s)  +  2 NaCl(aq)

 

                Decomposition reactions involve heating a compound to break up the compound to form new products. The metal oxide, carbon dioxide gas, and water vapor are common products. Some examples of decomposition reactions are:

CaCO3 (s)   +   heat  ®  CaO(s)  +  CO2 (g)

Sn(OH)4 (s)   +   heat  ®  SnO2 (s)  +  2 H2O (g)

2 NaHCO3 (s) )   +   heat  ®  Na2O (s)   +   2 CO2 (g)   +   H2O (g)

 

Iron (III) oxide can be used as a red pigment and is formed by the following sequence of a double displacement and a decomposition reaction:

FeCl3 (aq)   +   3 NaOH(aq)  ®  Fe(OH)3 (s)   +   3 NaCl(aq)

2 Fe(OH)3 (s)   +   heat  ®  Fe2O3 (s)  +  3 H2O (g)

 

 

Prelab Questions

1.        Write a balanced chemical reaction for the complete combustion of liquid octane, C8H18, to form carbon dioxide gas and water vapor.

2.        Write a balanced chemical reaction for the reaction of tin (IV) nitrate and sodium carbonate.

3.        Write a balanced chemical reaction for the decomposition of calcium hydroxide.

4.        Write a balanced chemical reaction for the decomposition of titanium (IV) carbonate.

5.        Write a balanced chemical reaction for the decomposition of aluminum hydrogen carbonate to form aluminum oxide, carbon dioxide gas, and water vapor.

6.        Write a balanced chemical reaction for the synthesis of titanium (IV) oxide from the elements.

 

 

Procedure:

Part 1: Black Pigment

1.        Support an evaporating dish on an iron ring. Do not use a wire-gauze on the ring and be sure that the dish will not fall through the ring.

2.        Light the candle and drip a small amount of paraffin (wax) on a square piece of aluminum foil and stick the base in the molten wax so that the candle stands upright.

3.        Place the candle under the evaporating dish and lower the dish as close to the candle flame as possible without extinguishing the flame.

4.        Let the candle burn for 5 minutes.

5.        Extinguish the flame and let the dish cool to room temperature.

6.        Mass a small beaker or weighing boat.

7.        Carefully scrape the carbon off the dish into the beaker.

8.        Re-mass the beaker and carbon. Calculate the mass of the carbon produced. Add your carbon to that generated by the other groups.

 

 

Part 2: White Pigment

 

1.        Add 25 mL of 1.00M Na2CO3 into a 100 mL beaker.

2.        Add 25 mL of 1.00 M CaCl2 into the beaker containing the Na2CO3. Stir for a few minutes.

3.        Set up and iron ring to support a funnel. Place a funnel in the ring and adjust the height of the funnel so the tip of the funnel is just inside the rim of the waste beaker.

4.        Write your name in pencil on the edge of a dry piece of filter paper. Mass a piece of filter paper. Fold the paper and place it in the funnel. You can wet the paper with a little water to hold it in place.

5.        Carefully pour the contents of the reaction into the filter paper. Be careful not to overfill the paper or to tear the paper. Do not stir the solid CaCO3 in the paper since this might tear the paper.

6.        Use a wash bottle to help you completely transfer the solid to the paper. Let the liquid drain through the paper.

7.        Rinse the product twice with water. Let the water drain from the solid between each rinsing.

8.        Carefully remove the filter paper from the funnel and open it on a watch glass. Your instructor will tell you whether to let the product air dry or to place it in an oven.

9.        After the product is dry. Re-mass the filter paper and product. Calculate the mass of the CaCO3 produced. Add your CaCO3 to that generated by the other groups.

 

Part 3: Red Pigment

1.        Add 25 ml of 3.00M NaOH into a 100 mL beaker.

2.        Add 25 mL of 1.00M FeCl3 into the beaker containing the NaOH. Stir for a few minutes.

3.        Set up and iron ring to support a funnel. Place a funnel in the ring and adjust the height of the funnel so the tip of the funnel is just inside the rim of the waste beaker.

4.        Write your name in pencil on the edge of a dry piece of filter paper. Mass a piece of filter paper. Fold the paper and place it in the funnel. You can wet the paper with a little water to hold it in place.

5.        Carefully pour the contents of the reaction into the filter paper. Be careful not to overfill the paper or to tear the paper. Do not stir the solid Fe(OH)3 in the paper since this might tear the paper.

6.        Use a wash bottle to help you completely transfer the solid to the paper. Let the liquid drain through the paper.

7.        Rinse the product twice with water. Let the water drain from the solid between each rinsing.

8.        Carefully remove the filter paper from the funnel and open it on a watch glass. Your instructor will tell you whether to let the product air dry or to place it in an oven.

9.        After the product is dry. Re-mass the filter paper and product. Calculate the mass of the Fe(OH)3 produced.

10.     Mass a large test tube. Place the dried Fe(OH)3 in the test tube.

11.     Heat the compound gently at first and then more vigorously to drive off all the water. Be sure that the mouth of the test tube does not point towards anyone. Carefully heat the upper part of the tube to drive off any water that may have condensed there.

12.     When water is no longer being released, let the test tube cool to room temperature. Re-mass the test tube. Calculate the mass of Fe2O3 produced. Add your Fe2O3 to that generated by the other groups.

 

 

Synthesis of Pigments and Types of Reactions

 

Data Tables

 

Part 1: Black Pigment

Mass of beaker

 

Mass of beaker and carbon

 

Mass of carbon

 

 

Part 2: White Pigment

Mass of filter paper

 

Mass of filter paper and CaCO3

 

Mass of CaCO3

 

 

 

Part 3: Red Pigment

Mass of filter paper

 

Mass of filter paper and Fe(OH)3

 

Mass of Fe(OH)3

 

Mass of test tube

 

Mass of test tube and Fe2O3

 

Mass of Fe2O3

 

 

 

 

Conclusion.

 

 

1. Complete and balance the following reactions that describe the production of the three pigments produced in this activity.

 

·         C20H42(s)  +  O2(g)    à

 

 

·         CaCl2(aq)  +  Na2CO3(aq) à

 

·         a. FeCl2(aq)  +  NaOH9aq)  à

 

 

b. Fe(OH)3(aq)  + heat  à

 

 

2. Identify the type of reaction for each equation above.

1.       Synthesis of Pigments and Types of Reactions

 

Setup Sheet

 

For a class of 20 students working in sets of 2. It is assumed that some equipment will be reused for the different parts. If different groups are doing only one part, less equipment will be needed.

 

Part 1: Black Pigment

10            Ring stands

10            Bunsen burners and sparkers

10            Iron rings that fit the evaporating dishes

10                  Evaporating dishes

10            Candles

10            25mL graduated cylinders (if solutions are used instead of solids)

10            Scoopulas

10            Small beakers or weighing boats

Matches

Squares of Aluminum foil

 

Part 2: White Pigment

10            Wash bottles with distilled water

10            Boxes Kimwipes

10            Funnels to fit the filter paper

10            Iron rings that fit the funnels

10            Large watch glasses

10            Scoopulas

20            50mL beakers

10            250ml beakers

10            250mL beakers labeled waste

Plastic weighing boats

15cm Whatman No.4 fast filtering qualitative filter paper

 

Part 3: Red Pigment

10            Large watch glasses

10            Large 20x150mm test tubes or 25x150mm test tubes

10            Test tube clamps

 

 

Synthesis of Pigments and Types of Reactions

 

Instructor’s Notes

 

Part 1: Black Pigment

1. Inexpensive candles can be bought at dollar stores. Five minutes of burning produced about 0.03g of carbon. Additional carbon must be purchased for the next lab.

 

Part 2: White Pigment

1. Instead of having the students mass the solid, 25.0mL of 1.00M CaCl2 and 1.00M Na2CO3 can be used. The solid or the solutions contain 0.0250 mol of each compound and should produce 0.0250 mol of CaCO3. CaCO3 has a molar mass off 100 so 2.50g of CaCO3 should be produced. 1.98g of CaCO3 was obtained after drying. The molar mass of Na2CO3 is 106. The amount of solid CaCl2 massed depends on whether the anhydrous CaCl2 (molar mass 111) or the di-hydrate CaCl2.2H2O (molar mass 147) is used.

2. Drying the samples overnight in a 110oC oven is recommended.

 

Part 3: Red Pigment

1. Instead of having the students mass the solid, 25.0mL of 1.00M FeCl3 and 3.00M NaOH can be used. The solid or the solutions contain 0.0250 mol of FeCl3 and 0.075 moles of NaOH. FeCl3 and NaOH react in a 1:3 mole ratio and should produce 0.0250 mol of Fe(OH)3. If 0.0250 mol of Fe(OH)3 are decomposed, 0.125mol of Fe2O3 should be produced. Fe2O3 has a molar mass of 159.6 so 2.00g of Fe2O3 should be produced. 1.80g of Fe2O3 was obtained after the decomposition. The molar mass of NaOH is 40.0. The molar mass of FeCl3.6H2O is 270.

2. Drying the samples overnight in a 110oC oven is recommended. This will remove the water from the wet solid but will not decompose the Fe(OH)3. It will greatly reduce the amount of solid that must be transferred to the test tube and heated.

3.Clean the test tubes by putting 6M HCl or H2SO4 into the tubes. Warming the tubes with the acid in them will speed up the reaction.

 

The solids will have to be ground to a fine powder before they can be used as a pigment.