Lab 8: Synthesis Of Alum From Aluminum Foil
- Page ID
- 514170
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)PURPOSE
- Synthesize alum crystals from aluminum foil through a multi-step reaction process.
- Apply stoichiometric principles to calculate theoretical and percent yields.
INTRODUCTION
Alum is a common name for potassium aluminum sulfate dodecahydrate, with the chemical formula K[Al(SO4)2]∙12 H2O. It has various applications, including water purification, dyeing, and medicine.
This experiment involves the synthesis of alum from aluminum foil through a series of reactions:
- Reaction with Potassium Hydroxide: Aluminum reacts with potassium hydroxide (KOH) to form a potassium aluminum hydroxide complex and hydrogen gas.
- Neutralization with Sulfuric Acid: Sulfuric acid (H2SO4) is added to neutralize the potassium hydroxide, forming potassium aluminum sulfate.
- Crystallization: The alum is crystallized from the solution by cooling in an ice bath.
The overall reaction for the synthesis of alum is:
2 Al (s) + 2 KOH (aq) + 4 H2SO4 (aq) + 22 H2O (l) → 2 K[Al(SO4)2]∙12 H2O (s) + 3 H2 (g)
This experiment also highlights a green application of chemistry, as it utilizes recycled aluminum foil as the starting material.
SAFETY PRECAUTIONS
- Wear appropriate personal protective equipment, including safety goggles, gloves, and a lab coat.
- Potassium hydroxide and sulfuric acid are corrosive. Avoid contact with skin and eyes.
- The reaction with potassium hydroxide produces hydrogen gas, which is flammable. Perform this step in a fume hood to avoid the accumulation of combustible gas.
- Sulfuric acid should be added slowly and carefully to the reaction mixture to control the reaction and avoid splattering.
- Use caution when working with hot plates and glassware.
- Dispose of chemicals properly according to the instructions provided.
EQUIPMENT AND CHEMICALS NEEDED
| Equipment | Equipment | Chemicals |
|---|---|---|
| 250 mL beaker | Ice Bath | 1.4 M Potassium Hydroxide (KOH) |
| Hot plate | Spatula | 9.0 M Sulfuric Acid (H2SO4) |
| Stirring rod | Filter Paper | Ethanol |
| Vacuum filtration assembly (Buchner funnel, rubber collar, filtration flask, filtration flask trap, vacuum line, vacuum hose) | Aluminum Foil | Deionized Water |
EXPERIMENTAL PROCEDURE
Preparation of Aluminum Foil
1) Accurately weigh between 0.9 and 1.2 grams of aluminum foil. Record the weight in the Data Table.
2) Cut the aluminum foil into small pieces (approximately 5 mm square) and place them in a 250 mL beaker.
Reaction with Potassium Hydroxide
3) Place the beaker containing the aluminum foil in a fume hood.
4) Carefully add 50 mL of 1.4 M potassium hydroxide (KOH) solution to the beaker. Record any changes you observe after adding KOH in the data and observations.
5) Once the initial bubbling subsides, place the beaker on a hot plate in the fume hood and heat the mixture to continue the reaction.
6) If the volume of the solution decreases to below 25 mL, add deionized water to maintain a volume of approximately 25 mL.
7) Continue heating until the evolution of hydrogen gas ceases (approximately 30 minutes). The solution will turn cloudy gray or black.
8) Allow the beaker to cool to room temperature.
Neutralization with Sulfuric Acid
9) Slowly and carefully add 20 mL of 9.0 M sulfuric acid (H2SO4) to the cooled solution. This should be done in a fume hood.
10) Observe the formation of a white aluminum hydroxide precipitate, which should dissolve as sulfuric acid is added.
11) If the white precipitate does not entirely dissolve, gently stir and heat the solution to aid dissolution.
Crystallization of Alum and Vacuum Filtration
12) Place the beaker containing the potassium aluminum solution in an ice bath to chill and induce crystallization of alum. This should take approximately 15 minutes.
13) Meanwhile, prepare a chilled wash solution mixture by adding 15 mL of ethanol and 15 mL of deionized water to a 100 mL beaker and place it in the ice bath until you are ready to filter your crystals.
14) If crystals do not form in the potassium aluminum solution beaker, gently scratch the sides and bottom with a stirring rod to promote crystallization.
15) If crystallization is still not observed, boil the solution to reduce its volume and return it to the ice bath.
16) Set up a vacuum filtration apparatus.
17) Place a filter paper in the Buchner funnel and filter the chilled alum solution, ensuring all crystals are transferred to the funnel.
18) Rinse the beaker with half of the chilled ethanol/water mixture to remove any remaining crystals.
19) Repeat the rinsing with the second half of the ethanol/water mixture.
20) Continue vacuum suction until the recovered alum is dry.
Collection of Alum and Calculations
21) Accurately weigh and record the mass of a clean, dry beaker.
22) Carefully transfer the dried alum crystals to the weighed beaker.
23) Accurately weigh the beaker containing the alum and record its mass.
24) Calculate the mass of the recovered alum.
25) Determine the limiting reactant.
26) Calculate the theoretical yield of alum.
27) Calculate the percent yield of alum.
Chemical Disposal and Clean-Up
28) Dispose of the filtrate containing ethanol rinse in the designated non-halogenated waste container.
29) Place the recovered alum in a recovered alum jar or container.
30) Clean and return all equipment to its designated areas.
Calculating the Percent Yield
The theoretical yield is calculated using the overall chemical equation:
2 Al (s) + 2 KOH (aq) + 4 H2SO4 (aq) + 22 H2O (l) → 2 K[Al(SO4)2]∙12 H2O (s) + 3 H2 (g)
Calculate the percent yield by dividing the actual weight of the recovered alum by the theoretical yield and multiplying by 100 %.
PRE-LAB QUESTIONS
- List several common uses for alum.
2. Explain how this experiment demonstrates a “green” application of chemistry.
3. Identify the hazards associated with this experiment and explain why they are hazardous. List at least three.
4. Write the balanced chemical equation for the overall reaction in this experiment.
5. Define the terms "limiting reactant," "theoretical yield," and "percent yield."
Synthesis of Alum from Aluminum Foil Report Sheet
Name ________________________ Lab Partner(s) _____________________________
DATA AND OBSERVATIONS
Substance |
Mass (g) |
|---|---|
|
Aluminum foil |
|
|
Beaker + dry alum (final product) |
|
|
Empty beaker |
|
|
Recovered dry alum |
OBSERVATIONS
- Record observations during the reaction with KOH (e.g., gas evolution, color changes).
- Record observations during the addition of H2SO4 (e.g., precipitate formation and dissolution).
- Record observations during crystallization (e.g., crystal formation, time)
- Record any other relevant observations.
CALCULATIONS
Determining the Limiting Reactant: (Listed Reactants in the Order Used)
Substance |
Mass (g) or Volume (mL) |
Reactant Moles |
Moles of Alum that can be Produced
|
|---|---|---|---|
|
Al |
|||
|
1.4 M KOH |
|||
|
9.0 M H2SO4 |
Referring to the overall equation, the limiting reactant is _________________
Theoretical Yield Calculation (show your work):
Percent Yield Calculation (show your work):
POST-LAB QUESTIONS
- What factors could contribute to a percent yield lower than 100%?
- What factors could lead to a percent yield greater than 100%?
- Discuss the importance of using small pieces of aluminum foil in this experiment. How does it affect the reaction rate?
- Explain the purpose of washing the alum crystals with an ethanol and water mixture. Why is the solution chilled?
- Identify potential sources of error in this experiment and suggest improvements to these errors.