Experiment 2 Online Tutorial >> Synthesis of Alum >> Introduction

Experiment 2 - Synthesis of Alum

Introduction

The goal of this experiment is to convert the Al in a beverage can into Alum KAl(SO₄)₂ ^. 12H₂O(s). The reaction is done in two stages.

Stage 1: Addition of KOH to dissolve the aluminum sample

The KOH dissolves the beverage can by converting the aluminum from the metallic form, Al(s), to an ionic form, Al(OH)₄^-_(aq), that is dissolved in the solution. Note that this is a redox reaction in which the Al loses three electrons to form an atom with a formal charge of 3: Al(OH)₄^-. This can be viewed as an Al³⁺ ion surrounded by 4 (OH^-)'s, as in Al³⁺(OH^-)₄.

- Tutor 1 helps you balance the redox reaction that occurs when KOH dissolves aluminum.

- Tutor 2 helps you determine the amount of KOH needed to fully dissolve the can.

Stage 2: Addition of H₂SO₄.

Stage 1 converted the Al in the can from the metallic form to the oxidized Al³⁺ form. However, the Al³⁺ is complexed to 4 OH^- ions to form Al(OH)₄^-_(aq). We next want to strip away the OH^- ions, which we can do by adding a strong acid (H₂SO₄). [The two H⁺'s in H₂SO₄ react with the OH^- ions to form H₂O.] An interesting thing happens here. The first OH^- we strip away leads to Al(OH)₃, which is an insoluble material and so forms a white sludge. If we continue to add H₂SO₄, and heat, we can dissolve this solid by stripping away the remaining OH^- ions, giving us a solution with bare Al³⁺ ions. When we cool the solution, we get solid alum by the reaction:

     Al³⁺_(aq) + K⁺_(aq) + 2SO₄^2-_(aq) + 12H₂O_(l) => KAl(SO₄)₂ • 12H₂O(s)

Our goal was to convert the Al metal in the can to alum. The % yield is a measure of how well we succeeded at this goal: How much alum did we get compared to the amount we would get if the reaction had gone completely?

- Tutor 3 helps you determine the % yield, based on

• The amount of Al you started with (the Al in the alum product must come from the Al in the beverage can you started with)
• The amount of K⁺ you added in stage 1 (the K⁺ in the alum product must come from the KOH you added in stage 1)