Determine the Reaction Stoichiometry and the Valency of Magnesium

Objective: To study the quantitative relationship between the amount of the reactants and products of a reaction. A known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry and the valency of magnesium.

Introduction:
Stoichiometry is the study of the quantitative relationship between amounts of reactants and products of a reaction. It can be used to calculate the amounts of products given the reactants and percent yield. In this experiment, a known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry.

Magnesium reacts with hydrochloric acid to produce hydrogen gas. The purpose of the experiment is to determine the value of X in the following equation: Mg + xHCl → MgClx + X/2 H2.

The mass of magnesium is measured by the analytical balance before use in the experiment. A known amount of magnesium is reacted with a large amount of excess hydrochloric acid, making magnesium the limiting agent in this reaction.

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The excess HCl ensures that all the magnesium will react completely. The formation of MgClx and H2 depends on the amount of magnesium used. Comparing the amount of hydrogen gas produced with the amount of magnesium consumed will enable the determination of the X value.

Materials and Apparatus:
Magnesium ribbon, 0.5M HCl, 50cm3 burette, 25 cm3 pipette, retort stand, electrical balance, watch glass, beaker, gauze, funnel, glass rod, thermometer.

Procedure:

1. The burette was used upside down to collect the hydrogen. The volume of the unmarked space in a clean, dry 50cm3 burette was determined by pipetting 25.
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   00 cm3 of water into the vertically clamped burette (right way up). The burette reading was recorded, drained, and repeated. The water in the burette was left for 10 min and checked for leaks.

2. A piece of magnesium was cleaned with steel wool. A piece was cut with scissors to the indicated length and curled up. A watch glass was tared on the four decimal balance, and the magnesium ribbon was accurately weighed between 0.0300g and 0.0360g, then placed inside a 600cm3 beaker.
3. A small filter funnel with a short stem (1.0-1.5cm long) was covered with gauze. The watch glass was inverted and placed over the magnesium.
4. The beaker was carefully filled with (tap) water until the level was approximately 0.5-1.0 cm above the end of the funnel stem. The burette was completely filled with 0.5M HCl. The beaker was inverted in the water, and the end of the burette was placed over the stem of the funnel, ensuring no air entered, and clamped into position.
5. The excess water was removed with a pipette until the level was just above the stem of the funnel.
6. More than 100cm3 of 0.5 M HCl was added to the beaker, and a glass stirring rod was used to ensure complete mixing so the HCl reached the magnesium. This was aided by gently tapping the watch glass with a glass rod.
7. The solution was stirred to initiate the reaction and then left to proceed unaided. At the completion of the reaction, the watch glass was tapped gently to dislodge any gas bubbles.

Data and Result:

Weight of magnesium(g): 0.0302
Volume of unknown marked space in 50cm3 burette(cm3): 5.5
Burette reading after completion of reaction(cm3): 22.6
Volume of hydrogen gas (cm3) = Burette marked region - Burette reading after completion of reaction + Volume of unknown marked space in 50cm3 burette = (50-22.6+5.5) cm3 = 32.9cm3

For n(H2): pV=nRT

n(H2)= pVRT = 1 atm×32.9× 10-3L / 0.08026 L atm K-1mol-1×298K = 1.345×10-3mol
For n(Mg): n(Mg) = mass / molar mass = 0.0302 / 24.305mol = 1.243×10-3mol
Mg + xHCl → MgClx + X/2 H2
X/2 = (1×1.345×10-3mol) / 1.243×10-3 mol
X= {2(1×1.345×10-3)} / 1.243×10-3 = 2.16 ≈ 2.0
∴ Mg + 2HCl → MgCl2 + H2

Discussion:

Magnesium, an alkaline metal, reacts with acid to produce hydrogen gas, and the reaction between magnesium and hydrochloric acid is exothermic. Since the HCl is added in excess, magnesium is the limiting reactant in this reaction. This experiment uses a known starting mass of magnesium and the amount of hydrogen gas collected to determine the x value and the valency of magnesium. The volume of unknown marked space in the 50cm3 burette was 5.5cm3. The volume of hydrogen gas was calculated as 32.9 cm3.

The x value in the equation was determined to be 2, indicating a complete equation for magnesium and hydrochloric acid of Mg + 2HCl → MgCl2 + H2. The molecular ratio between Mg and Cl is 1:2, corresponding with magnesium's position as a group 2 element with 2 valence electrons in its outer shell, forming a stable 2+ ion. Chlorine atoms gain an electron to form a stable 1- ion, and two chlorine atoms gain one electron from magnesium to form MgCl2, held together by an ionic bond.

The use of curved magnesium ribbon, due to its larger surface area, increases reaction speed compared to flat ribbon, facilitating faster reaction due to increased exposure to hydrochloric acid. Precautions include using gauze to prevent magnesium ribbon from floating on the hydrochloric acid surface and using a glass rod for complete mixing.

Question:

1. It is advisable that the temperature is not taken for at least 20 min after adding the HCl. This is because the reaction is exothermic, affecting the number of moles calculated. Temperature inversely affects the number of moles; an increase in temperature decreases the number of moles, and vice versa. The temperature must be stable to accurately determine the number of moles of H released in the reaction.
2. The moles of hydrogen gas present are calculated using the given method: n(H2) = pVRT = 1 atm×32.9×10^-3L / 0.08026 L atm K^-1mol^-1×298K = 1.345×10^-3mol.
3. The Ideal Gas equation is pV = nRT, where p is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature. At S.T.P., one mole of gas occupies 22.4 L.
4. If hydrogen gas leaks through the stopcock of the inverted burette, the calculation of the number of moles of H2 will be inaccurate, as the volume of gas is proportional to the number of moles.
5. Based on experimental results, the x value is 2, and the valency of magnesium is 2.

References:
1. http://1chemistry.blogspot.com/2011_06_01_archive.html
2. http://www.markedbyteachers.com/university-degree/physical-sciences/determination-of-the-valency-of-magnesium.html
3. http://www.science.uwaterloo.ca/~cchieh/cact/c120/idealgas.html