Chemistry/Cu-Al Stoichiometry

The Stoichiometry of the Reaction Between Aluminum Metal and Copper (II) Sulphate

The object of this experiment is to find experimentally the mole ratio existing in the reaction between aluminum metal and copper (II) sulphate. The experiment has two distinct parts.

In the first part, you will react a piece of aluminum foil with an excess of copper (II) sulphate solution to produce copper metal. The unbalanced reaction equation is:

Al(s) + CuSO4(aq) ===> Al2(SO4)3(aq) + Cu(s) ............(1)

After filtering and washing the copper, you will react the copper metal with nitric acid, producing a solution containing copper ions.

3 Cu(s) + 2 HNO3(aq) + 6 H+(aq) ===> 3 Cu2+(aq) + 2 NO(g) + 4 H2O(l) ............(2)

The colourless NO(g) immediately reacts with the air to form red–brown NO2(g). After neutralizing the excess nitric acid, you will dilute the copper ion solution to exactly 250.0 mL.

In the second part, you will find the concentration of copper ions in the diluted volume using a titration procedure. The titration procedure is as follows. First, a large excess of iodide ion is added to a sample having a known volume of dissolved copper ions, producing the following reaction.

<center>2 Cu2+(aq) + 4 I(aq) ===> 2 CuI(s) + I2(s) .............. (3)

The I2 produced in this initial reaction is then titrated with a solution containing thiosulphate ions (S2O2-3) having a known concentration, producing the following reaction.

2S2O2-3(aq) + I2(s) ===> S4O2-6(aq) + 2 I(aq) ............. (4)

Once the volume of thiosulphate solution is known, the moles of thiosulphate can be calculated and eventually you can find the moles of copper that originally reacted with the aluminum.

CALCULATIONS AND QUESTIONS

1. (a) Average the two closest volumes of Na2S2O3 used in the titrations to calculate the average volume of Na2S2O3 used. (Show your work.)

(b) Use the molar concentration of the Na2S2O3 solution and the average volume of Na2S2O3 found in calculation 1(a) to calculate the moles of Na2S2O3 used. This result is the moles of S2O2-3 used in equation (4).
(c) Use equation (4) and the moles of Na2S2O3 found in calculation 1(b) to calculate the moles of I2 reacted by the Na2S2O3.
(d) The moles of I2 reacted in equation (4) equals the moles of I2 produced in equation (3). Use this fact to calculate the moles of Cu2+ reacted in equation (3).
(e) The moles of Cu2+ found in calculation 1(d) represents the moles of Cu2+ in each 25.00 mL portion of solution taken from the 250 mL volumetric flask. Use this information to calculate the total moles of Cu2+ contained in the 250 mL flask.
(f) The total moles of Cu2+ contained in the 250 mL volumetric flask equals the moles of Cu(s) reacted in equation (2) and also equals the moles of Cu(s) produced in equation (1) and the moles of CuSO4 reacted in equation (1). How many moles of CuSO4 reacted in your experiment? Note that you used an EXCESS of CuSO4 solution and that not all of the CuSO4 solution added in step 2 actually reacted; only the CuSO4 that reacted with the aluminum produced copper metal.

2. Use the mass of aluminum foil reacted to calculate the moles of aluminum used in the experiment.

3. Use the moles of CuSO4 that reacted and the moles of aluminum that reacted to calculate the value of the following ratio:

$mole ratio = \frac{moles of CuSO_4 reacted} {moles of Al reacted}$

4. Numbers such as 1, 2, 3, etc. are called “integers” and numbers such as 0.5, 1.5, 2.5, etc. are called “half–integers”. Round the value for the mole ratio found in calculation 3 to the nearest integer or half–integer.

5. Balance equation (1). What type of reaction is represented by this equation?

6. Does the mole ratio you found in calculation 4 agree with the mole ratio predicted from your balanced equation (1)? Explain your answer.