Synthesis of Alum Salt Crystals, Essays (university) of Chemistry

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Synthesis of Alum:

KAl(SO 4 ) 2 .12H 2 O

Joshua K. Reid July 8, 2021

Greenville Technical College

Chemistry 110-W

Synthesis of Alum: Abstract – In this experiment, aluminum foil was converted into the chemical compound potassium aluminum sulfate dodecahydrate, commonly called Alum. Dissolving Aluminum in a KOH (Potassium Hydroxide) solution, with the addition of Sulfuric Acid (H 2 SO 4 ) forms precipitate. Heat was used to dissolve the precipitate, and then cooled to form the Alum crystals which can be filtered out using a vacuum filter. Once dry, the mass in grams of Alum can be calculated. Introduction – Potassium alum is also known as potash alum or Tawas. It is aluminum potassium sulfate. Potassium alum is considered by the FDA as a generally recognized safe substance. This is the type of alum that you find in the grocery store for pickling and in baking powder. It is also used in leather tanning, as a flocculant in water purification, as an ingredient in aftershave and deodorant, and as a fire proofing treatment in textiles (1). Usually, when you hear about alum it is in reference to potassium alum, which is the hydrated form of potassium aluminum sulfate and has the chemical formula KAl(SO 4 ) 2 ·12H 2 O. One of the interesting properties of aluminum is that it is amphoteric, meaning it will dissolve in both strong, aqueous acids and strong, aqueous bases. In both instances, hydrogen gas is released as part of the reaction. Alums are ionic compounds that crystallize from solutions containing sulfate ion. In an article titled “Synthesis of Alum”, published by Messa Community College, it describes a trivalent cation such as Al 3 +, Cr 3 +, or Fe 3 +^ and a monovalent cation such as K+^ , Na+^ , or NH 4 +, in which six of the water molecules bind tightly to the trivalent metal ion; the remaining six molecules bind more loosely to the monovalent cation and the sulfate anion (2). Purpose – In this experiment, aluminum foil was converted into the chemical compound potassium aluminum sulfate, KAl(SO 4 ) 2 ·12H 2 O. Skills learned in this lab included data recording, measurements of lab equipment / materials and solutions, calculations of theoretical and actual yields, and overall lab safety.

watch-glass and filter paper were subtracted from the final measurement to give the actual yield of Alum. Lastly, the Alum created was heated, to verify its melting point. A small amount of Alum was placed in a capillary, and the capillary was placed in the melting point apparatus, and the meting point temperature was recorded, at 92°C in this instance. Observations – During this experiment, significant heat, and gaseous discharge was noticed. There was a quite a reaction by the aluminum to the potassium hydroxide solution. The aluminum appeared to turn black as the process went on, then the solution went clear as all the aluminum dissolved into the solution. The addition of the sulfuric acid created a white , hazy cloud in the solution, resembling water that is on the cusp of freezing completely but not quite there. Once the final solution was heated, the cloudiness disappeared and all that was left was a clear solution. Cooling in the ice bath brought on the haziness again as the Alum crystals formed in the bottom of the beaker. Data – Mass of Aluminum (Al) 1.009 g Volume and Concentration of KOH Solution 25mL - 4 M Volume and Concentration of H 2 SO 4 Solution 25mL - 6 M Mass of watch-glass 52.579 g Mass of filter paper .701 g Mass of Alum, Watch-glass, and Filter Paper 61.541g Mass of Alum 8.261g Calculations – Reaction to form alum from Aluminum is given as:

2Al(s)+2KOH( aq )+2H 2 O+4H 2 SO 4 ( aq )→2KAl(SO 4 ) 2 (s)∙12H 2 O(l)

The first reaction that needs to be carried out is that of aluminum and potassium hydroxide, KOH: 2 Al (s) + 2 KOH (aq) + 6 H 2 O (l) → 2 Al(OH) 4 – (aq) + 2 K+ (aq) + 3 H 2 (g)

Adding sulfuric acid, H 2 SO 4 , to the resulting solution will cause the compound Al(OH) to precipitate: 2K+^ (aq) + 2Al(OH) 4 –^ (aq) + H 2 SO 4 (aq) →2K+^ (aq) + 2Al(OH) 3 (s) + 2 H 2 O(l) + SO 4 2–(aq) Crystals of the double salt KAl(SO 4 ) 2 ⋅12 H 2 O (s), or alum, will form upon cooling this final solution since the solubility of alum in water decreases as the temperature is lowered: 2K+ (aq) + 2Al3+^ (aq) + 4SO 4 2–^ (aq) + 24H 2 O(l) → 2KAl(SO 4 ) 2 •12 H 2 O (s) Molar mass of Aluminum = 26.98 g/mol Molar Mass of Alum = 474.33 g/mol K = 39.09830 g/mol Al = 26.98154 g/mol S = 32.06 g/mol * O = 15.9994 g/mol * H = 1.00794 g/mol * O = 15.9994 g/mol * 12 Mols of Aluminum =

mass of Al (g)

molar mass of Al (

g

mol

1.009 g Al

g

mol

Al

=0.037 mols (Al)

Mass of 2 moles of Aluminum = ¿

Mass of 2 moles of Alum = ( 2 × 474.33) g / mol =948.67 g / mol

Theoretical yield of alum = .

2 mols Alum

2 Mols Al

∗.037 mol Al =0.037 mols Alum

0.037 mols

Alum ∗474.

g

mol

Alum

1 mol Alum

=17.55 g Alum

Actual Yield of Alum = 61.54 g ( total )−( 52.58 g ( watchglass )+.701 g ( Filterpaper ) (^) )=8.26 g Alum Percent Yield of Alum =

8.26 g Alum

17.55 g Alum

∗ 100 =47.1% yield

Works Cited –

1. Helmenstine, Anne Marie, Ph.D. "What Is Alum and How Is It Used?" ThoughtCo, Feb. 16, 2021, thoughtco.com/what-is-alum-608508.
2. “Synthesis of Alum.” Mesa Community College , www.mesacc.edu/~paudy84101/CHM151LL/6B%20Alum.pdf. Accessed 8 July 2021.
3. Dorf, Richard. “Kinetics and Reaction Engineering.” The Engineering Handbook - Second Edition, Amsterdam, Netherlands, Amsterdam University Press, 1995, pp. 793–796.