Stoichiometry

Abstract:

The objective of the experiment was to recover pure copper through a series of transformations. A series of chemical reactions are used to purify the copper. In the end 84 percent of the original copper was left, this means that when decanting and transferring some copper was lost.

Introduction:

Stoichiometry is known as the calculation of the quantities of reactants and products in a chemical reaction and in this lab one is trying to obtain the mass of copper after a series of transformations. In this experiment separation and purification are necessary to gain a product. Some ways this is done include precipitation (The process of separating a substance from a solution as a solid.), filtration (Passing a liquid through a filter to trap unwanted particles.), decantation (To pour off without disturbing the sediment.), extraction (To obtain from a substance by chemical or mechanical action, as by pressure, distillation, or evaporation.), and sublimation (To cause (a solid or gas) to change state without becoming a liquid.). Due to the law of conservation of mass one knows that the reactants are not destroyed and no new materials are created, therefore if any mass is lost it is due to human error.

Experimental:

A piece of copper wire with a mass of 0.098 grams was obtained. The sample was dissolved in 1 mL of concentrated nitric acid (HNO3). Next 100 mL of distilled water was added in the hood to dilute the solution along with 30 mL of sodium hydroxide. Following this 2 boiling chips were added and the mixture was heated over a meeker burner. The solution was stirred until almost boiling, at this point it was removed from the heat. Black precipitate was allowed to settle and the water was decanted into the sink and 100 mL of hot distilled water was added, and the mixture was stirred. After settling the supernatant liquid was decanted into the sink. In the hood 1 mL of 6.0 M sulfuric acid (H2SO4) was added and the mixture was stirred until the CuO was dissolved. An additional 1.0 mL of sulfuric acid (H2SO4) was added. 50-75 mL of distilled water was added with 2 drops of 0.1 M Sodium Fluoride (NaF) to break up the surface oxide layer. The boiling stones were removed. 0.3 grams of 30 mesh zinc (Zn) metal was added and the solution was stirred for 2-3 minutes or as long as it took for a rust red precipitate to form and for the water to clear from blue. When the gas evolution slowed the mixture was heated for a few minutes on a hot plate. 5 mL more of 6.0 mL sulfuric acid (H2SO4) was added and stirred for another 2-3 minutes and the supernatant liquid was decanted into the sink. The reddish brown precipitate was transferred to the pre-weighted porcelain dish and washed with 5 mL of distilled water and 3 mL of acetone. The supernatant liquid was decanted and the product was dried on a hot plate for 2-3 minutes and massed.

Data and Results:

Initial mass of copper wire 0.098 grams

Mass of zinc 0.306 grams

Mass of porcelain dish + mass of copper 38.588 grams

Mass of dish 38.506 grams

Mass of copper (Mass of combined ÐŽV mass of dish) 0.082 grams

Percent yield= (mass of recovered copper)/(initial mass of copper) ЎС 100% 84%

Q1: An equation for what is occurring can be written as: Cu (s)+ 4 HNO3 (aq) „Ñ- Cu(NO3)2(aq)+ 2NO2 (g) + 2 H2O (l))

Q2: This reaction can be written as Cu(NO3)2(aq)+ 2 NaOH (aq) „Ñ- Cu(OH) 2(s)+ 2 NaNO3 (aq).

Q3: When heating the blue precipitate black chunks begin to form then sample turns a dark teal, to a dark blue, to black, then as it settles it becomes black chunks sinking in clear liquid.

Q4: Chemical formula after the addition of sulfuric acid: CuO (s) + H2SO4 (aq) „Ñ- Cu SO4 (aq) + Cu (s).

Q5: After the addition of the zinc metal the formula is as follows: Cu SO4 (aq) + Zn (s) „Ñ- Zn SO4 (aq) + Cu (s).

Q6: Hydrogen gas is formed and one knows when all the copper in the copper (II) sulfate solution has reacted once the liquid turns from blue to clear.

Q7: The chemical formula has not changed since the addition of zinc metal.

Q8: Any trace of sulfuric acid left on the surface was removed by washing.

Q9/10/11/12: