Purpose

To produce alum ([latex]\text{KAl}(\text{SO}_4)_2\cdot 12\text{H}_2\text{O}[/latex]) from scrap pieces of aluminum.

Expected Learning Outcomes

After performing this experiment, students should be able to

• Synthesize and characterize compounds using typical chemical procedures (LO 3).
• Determine the theoretical and percent yield for a reaction. (LO 4)
• Identify ways to improve synthesis procedures. (LO 6)

Textbook Reference

Tro, Chemistry: Structures and Properties, 2nd Ed., Ch. 7.3-5.

Introduction

Alum

Many products are produced through various chemical reactions. Examples include soap (from fats), plastics (from crude oil components), and drugs (from various precursors).  It is important, of course, to ensure that as much of the raw materials comes out as product as possible. Clearly, we want to obtain as much iron from iron ore, for instance – or we’d waste money on raw materials! (or, you know, the owner of the business won’t be particularly happy).

In this experiment, we will start with scrap aluminum metal from cans and, by reacting this in turn with potassium hydroxide and sulfuric acid, produce two useful products:

• Hydrogen gas (H₂): This is used in the Haber process for the production of ammonia, the production of various other organic compounds (e.g. methanol and hydrogenation of soaps and fats) as well as hydrogen fuel cells.
• Alum ([latex]\text{KAl}(\text{SO}_4)_2\cdot 12\text{H}_2\text{O}[/latex]): This compound is used in water purification as well as pickling and fire extinguishers.

The Chemistry of the Synthesis Process

There are several steps to this reaction:

1. Potassium hydroxide is added to aluminum to form potassium aluminum hydroxide in an oxidation-reduction (redox) reaction:

[latex]2\text{Al}(s) + 2\text{KOH} (aq) + 6\text{H}_2\text{O}(l) \to 2\text{KAl}(\text{OH})_4 (aq) + 3\text{H}_2(g)[/latex]

The complex ion [latex]\text{Al}(\text{OH})_4^-[/latex] is called the aluminate ion. As bases will react with aluminum in this reaction, we cannot place basic cleaning agents (e.g. bleach and detergent) in aluminum cans.

2. Sulfuric acid is added to the potassium aluminum hydroxide to forum aluminum sulfate in a two-step reaction:

\begin{eqnarray}
2\ce{KAl(OH)4}(aq) + \ce{H2SO4} (aq) &\to& 2\ce{Al(OH)3}(s) + 2\ce{H2O}(l) + \ce{K2SO4} (aq) \\
2\ce{Al(OH)3}(s) + 3\ce{H2SO4} (aq) &\to& \ce{Al2(SO4)3} (aq) + 6\ce{H2O}(l)
\end{eqnarray}

3. As the mixture cools, alum is formed through crystallization

\begin{equation}
\ce{Al2(SO4)3} (aq) + \ce{K2SO4} (aq) + 24\ce{H2O}(l) \to 2\ce{KAl(SO4)2}\cdot 12\ce{H2O} (s)
\end{equation}

The overall reaction is therefore

[latex]2\ce{Al}(s) + 2\ce{KOH}(aq) + 4\ce{H2SO4}(aq) + 22\ce{H2O}(l) \to 2\ce{KAl(SO4)2}\cdot 12\ce{H2O}(s) + 3\ce{H2}(g)[/latex]

The crystals of alum are removed from solution by gravity filtration and are then washed in an ethanol:water mixture. This helps remove any contaminants from the crystals, and also helps dry the crystals quickly.

Procedures: Week 1 – Preparation of Alum

Chemicals Required

Obtain approximately the following amounts of chemicals:

• 1 g aluminum.
• 50 mL of 1.5 M potassium hydroxide
• 30 mL 6 M sulfuric acid.  You need to handle this in the fume hood.
• 50 mL ethanol:water mixture. This is provided as tubes in ice and you need to take one tube.

You will also need filter paper; this can be obtained as you need it.

Preparing the Sample

1. Obtain approximately 1 g aluminum, being sure to ensure that you have thin strips of the material. Record accurately the final mass of aluminum used and place the aluminum into a 250 mL beaker.

First Reaction: Oxidation of Aluminum to Form KAl(OH)₄

In a fume hood

2. Slowly add 50 mL 1.5 M potassium hydroxide into the beaker.
3. Using a very gentle temperature, place the mixture on a hot plate and heat (without boiling) the mixture until all of the aluminum has reacted (no hydrogen gas bubbles are given off, and the aluminum cannot be seen in solid form). This process will take about 20 minutes. Make sure that all of the aluminum is in the liquid, otherwise it won’t come into contact with the hydroxide and thus won’t be able to react.

At your lab bench:

4. While the mixture is being heated, set up the gravity filtration setup at your lab bench as described in Using Laboratory Equipment. A 250 mL Erlenmeyer flask with a magnetic stir bar should be placed under the gravity filter (funnel + filter paper).
5. After the reaction from step 3 has completed, gravity filter the solution into a 250 mL Erlenmeyer flask.  To do this, decant the reaction mixture into the filter using a glass rod to block solid particles from the reaction mixture from entering the filter, such that larger insoluble particles won’t enter the filter while allowing the liquid (and smaller particles) through. The filtrate in the Erlenmeyer flask at the bottom should be clear and colorless. If the liquid doesn’t flow through the filter paper, try vacuum filtering the solution into a vacuum/side-arm flask and then transferring the clear and colorless filtrate into the 250 Erlenmeyer flask that has the stir bar.
6. Allow the Erlenmeyer flask to cool to room temperature.

Subsequent Reactions: Addition of Sulfuric Acid

7.  Put the Erlenmeyer flask containing a stir bar and the solution of KAl(OH)₄ from the previous part on a stir/hot plate and set the stir bar to stir at a reasonable rate consistently on the stir/hot plate.  Do not turn on the heating element on the hot plate.
8. Add 30 mL of 6 M sulfuric acid quickly, but carefully, in 2-3 mL portions.

9. Check to see if the Erlenmeyer flask has any white specks of insoluble Al(OH)₃ left in it. If so, gently heat the mixture while continuing the stirring until the solution becomes clear again.
10. Turn off all heating (if necessary) and allow the flask to cool back to close to room temperature. In the meantime, create an ice bath with a 600 mL beaker and after it has cooled to near room temperature, place the Erlenmeyer flask into the ice bath. Wait for crystals to form.
    If, after five minutes, no crystals are observed, scratch the bottom of the flask with a glass rod. This would hopefully cause nucleation of crystals to occur.
11. When crystal start forming, swirl the flask, and allow the flask to cool in the ice bath for another 10 minutes.  Keep the ice bath after this – you will need it for the next part.

While waiting for the crystals to form:

12. Set up a vacuum filtration setup following Using Laboratory Equipment. Pour some deionized water into the filter. Be sure to put the Buchner funnel into a side-arm flask, NOT an Erlenmeyer flask.

Vacuum Filtration and Recovery of Crystals

13. Label a clean, dry evaporating dish^[1] with you and your partner’s names. Dry the evaporating dish. Weigh the empty, dry evaporating dish.
14. Run the water aspirator to get the vacuum filtration setup going. This needs to continue for the duration of the vacuum filtration.
15. Swirl the Erlenmeyer flask so that as many of the crystals are dislodged as possible, and pour this quickly onto the Büchner funnel (hold a 2nd magnetic stir bar against the side of the Erlenmeyer flask where the first magnetic stir bar is so that the magnetic stir bar isn’t transferred into the Büchner funnel. Use a glass stir rod to scrape as many crystals out of the Erlenmeyer flask as possible as spread all of the crystals as evenly as possible across the entire filter paper.
16. Collect 50 mL of 50:50 ethanol:water mixture in a new Erlenmeyer flask and place it onto your ice bath.
17. After you have transferred as many crystals as you can from the Erlenmeyer flask to the Büchner funnel, pour about 10 mL of the ethanol:water mixture into the original Erlenmeyer flask and swirl to try and dislodge any loose crystals.  Pour this onto the vacuum filter, making sure to pour the ethanol:water mixture that’s in the Erlenmeyer flask over ALL of the solid on the filter paper.
18. Repeat this step until all of your tube of the ethanol:water mixture is used up. In addition to increasing our yield by getting as much solid out of the Erlenmeyer flask as possible, this also washes the crystals – dissolving impurities and have them filtered off.
19. Let the filter paper and the crystals dry by letting them sit on the Büchner funnel with the vacuum on (water on) so that air is being sucked through the crystals for at least 5 minutes.
20. Transfer the crystals to the labeled evaporating dish and place the evaporating dish into the drying oven. Label the crystals appropriately (with chemical name, manufacturer name (you), today’s date, and hazard information (Health = 2, Fire = 0, Reactivity = 0)

Procedurally, you are required to have your notebook signed at the end of this laboratory period, but the notebook grade will be assigned at the conclusion of next week’s lab period.

Procedures: Week 2

This week, you will observe and determine the mass of the dried crystals.

1. Record the appearance of the dried crystals.
2. Determine the mass of the evaporating dish containing the dried crystals.
3. Glass vials will be provided for holding samples of the crystals. Place some of these in a vial, and label these properly using the labels provided. Turn in these vials to your instructor. It is not essential to submit all of the crystals formed.

Waste Disposal (for both weeks)

All liquid waste must be disposed of in the designated beaker. Excess alum can be rinsed down the drain with plenty of water.

Calculations

Given the initial amount of aluminum used (determined in step 1), and by determining the formula mass of alum ([latex]\ce{KAl(SO4)2}\cdot 12\ce{H2O}[/latex]), determine the theoretical yield of alum using the balanced chemical equation to provide the stoichiometric ratio.

From this, and given the actual yield of alum determined in this reaction, determine the percent yield of alum in this reaction using the equation

\begin{equation}
\mbox{percent yield} = \frac{\mbox{actual yield of product}}{\mbox{theoretical yield of product}} \times 100\%
\end{equation}