Question

2. Describe the temperature change that will occur in the reaction mixture when you add 3M KOH to the aluminum foil pieces in Step 3. Will this temperature change occur immediately? Briefly explain. 2ALL2keHCga) +6h0u) 2KMA9)+2AL COl)4t3l) 3 thl'e

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Answer 1

The description of a chemical reaction consists of a series of steps: 1) carrying out the reaction, 2) isolating the product(s), 3) purifying the product(s), 4) and characterizing the product(s) and determining its(their) purity. The isolation and purification of the products are based on physical properties such as the ability to form crystals, boiling point, melting point, solubility, etc. Characterization of the products may be either quantitative or qualitative. In a quantitative characterization, the chemical formula and the structure (i. e., how the atoms are connected) are determined. The former is usually accomplished using elemental analysis, mass spectroscopy, X-ray crystallography or some spectroscopic method. Sometimes it is sufficient to show only that certain ions or elements are present in a sample, and in this case a chemist will perform a qualitative test. Qualitative tests often use chemical reactions that result in a visible change (formation of an insoluble solid, a color change, or evolution of a gas) as a way to quickly show whether a particular chemical species is present or not.

Once the chemical reaction’s products are fully characterized, and the balanced chemical reaction is known, we can compute a theoretical and a percent yield. We do these final characterizations of the reaction because it is important to know how efficiently the reaction converts reactants to products. Chemists are always trying to strike a balance between the cost of the reactants, the value of the products, the time a reaction requires and the cost of any unwanted by-products that must be handled as hazardous waste. A reaction, even though it gives a valuable product, may be unusable because it has a low yield, takes too much time or generates too much waste.

In this experiment you will prepare and characterize alum (potassium aluminum sulfate dodecahydrate, KAl(SO4)2·12 H2O). The first step in this synthesis, which you will perform during Week 1, is to react metallic aluminum with a concentrated solution of potassium hydroxide (KOH) to form the potassium salt of the tetrahydroxoaluminate complex ion, [Al(OH)4]–. The balanced chemical equation for this oxidation-reduction reaction is

2 Al(s) + 2 KOH (aq) + 6 H2O () — + 2 KAI(OH)4 (aq) + 3 H2 (9)

The second step of the procedure is to convert the KAl(OH)4 to alum by addition of sulfuric acid (H2SO4) in an acid-base reaction. Under the experimental conditions, the alum has a limited solubility in water, and so it precipitates from the solution. The balanced chemical reaction that occurs in this step is

KAI(OH)4 (aq) + 2 H2SO4 (aq) + 8 H20 (1) KAl(SO4)2:12 H20 (5)

Alum is a hydrate, which means that it is a compound that has water molecules trapped within the solid. Hydrates will release some, or all, of their “waters of hydration” upon heating. If the chemical reaction between Al and KOH does produce alum as a product, we would expect that heating the product should result in a decrease in the sample’s weight corresponding to the loss of 12 water molecules per formula unit of alum. Thus, if one knows the starting mass of alum, and the amount (mass, and therefore number of moles) of anhydrous alum remaining after all of the water has been driven off, one can calculate the amount of water that was present in the alum (by the Law of Conservation of Mass). A comparison of the experimentally determined waters of hydration and the number expected from the chemical formula can then be used as evidence for the formation of the desired product. The process by which the waters of hydration are driven off is described by the chemical equation shown below, where the “Δ” written above the arrow indicates that heat was applied to the reactant(s).

KAI(SO2), 12H,0 (s) -- KAl(SO2), (s) + 12 H2O(g)

The reaction rate is strongly affected by temperature and the concentration of alkali, which acts as catalyst.