 # Determine the Reaction Stoichiometry and the Valency of Magnesium Essay

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:

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Stoichiometry is the study of the quantitative relationship between amounts of reactants and products of a reaction. Stoichiometry can be used to calculate the amounts of products with 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 is used to reacts with hydrochloric acid in order to produce hydrogen gases. 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 used in the experiment. A known amount of magnesium is reacted with a large amount of excess hydrochloric acid and hence the magnesium acts as a limiting agent in the particular reaction.

As the amount of HCl is used in excess, this can ensure that all the magnesium will be reacted completely in the reaction. The formation of MgClx and H2 are depends on the amount of magnesium used. The comparison of the amount of hydrogen gas produced with the amount of magnesium consumed will enable the X value to be determined.

Materials and Apparatus:

Magnesium ribbon,0.5M HCl,50cm3burette, 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 25.00 cm3 of the 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 scissor within the length shown. The ribbon was curled up. A watch glass was tare on the four decimal balance and the magnesium ribbon was ACCURATELY weighed between 0.0300g and 0.0360g on the watch glass which then was placed inside a 600cm3 beaker. 3. A small filter funnel with a short stem (1.0-1.5cm long) was taken and 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 in the beaker and the end of the burette was placed over the stem of the funnel, no air enters was ensured and clamped it into position. 5. The excess of the water was removed with a pipette until the level was just above the stem of the funnel. 6. Above 100cm3 of 0.5 M HCl was added to the beaker, a glass stirring rod was used to ensure complete mixing such that the HCl reached the magnesium. This was helped by the tapping of watch glass gently with a glass rod. 7. The solution was stirred to initiate the reaction and was not stir further to left the reaction proceeds unaided. At the completion of the reaction the watch glass was tapped gently to dislodge and gas bubbled. 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× 10-3L0.08026 L atm K-1mol-1×298K =1.345×10-3mol
For n(Mg): n(Mg)=massmolar mass
=0.030224.305mol
=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 is an alkaline metal which can react with acid will produce hydrogen gas. And the reaction between magnesium and hydrochloric acid is an exothermic reaction. Since the HCl is added in excess, hence the magnesium is the limiting reactant in this reaction. In this situation, the magnesium is used up completely to react with the excess hydrochloric acid.Based on the quantitative relationship between the amount of the reactants and products of the following equation: Mg + xHCl → MgClx + X/2 H2

A known starting mass of magnesium and the amount of hydrogen gas collected will be used to determine the x value and the valency of magnesium. In this experiment, the volume of unknown marked space in 50cm3 burette determined was5.5cm3. Volume of hydrogen gas was equal to burette marked region minus the burette reading after completion of reaction plus the volume of unknown marked space in 50cm3burette and it was 32.9cm3. From the calculation, the x value in the equation determined was 2.Therefore, the complete equation for magnesium and hydrochloric acid is Mg + 2HCl → MgCl2 + H2. The proportional molecular between Mg and Cl is 1:2.As magnesium is the categorized to the group 2 element in the periodic table which has 2 valence electrons presence in most outer shell. A magnesium atom will lose 2 electrons to form a stable 2+ ion. Chlorine is in group 7 of the periodic table which will gain an electron to form a stable 1- ion. The two chlorine atoms will respectively gain one e-from the magnesium to form MgCl2 which held together by ionic bond.

The magnesium ribbon used in the experiment was in the curved-shape. The curved shape magnesium ribbon was used instead of the flat magnesium ribbon because the former one can reacts faster than the latter one. The curved magnesium own larger surface area which increase the reaction proceed faster compared to the flat magnesium ribbon although they are in the same volume. As the surface area is increased, the magnesium atoms can have more surfaces to be exposed to the hydrochloric acid. Thus, the reactivity of the reaction increases. There was some precaution in this experiment. Gauze used to prevent the magnesium ribbon float on the hydrochloric acid surface, if using too much gauze will lead to hydrogen gas difficultly passed through the gauze. A glass rod was used to ensure complete mixing such that the HCl reached the magnesium. This was helped by the tapping of watch glass gently with a glass rod. To stop tapping the glass watch if magnesium reached the acid surface. Otherwise the magnesium attached to the burette wall and cannot react with acid.

Question:

1. It is advisable that the temperature is not taken for at least 20 min after adding the HCl.Why is this so? Ans: It is because the reaction between HCl and Mg is exothermic reaction. It will affect the no. of mole. Base on pV=nRT, temperature is inverse to the no. of mole. If the T is increase, the n will become smaller. The T is decrease, n become bigger. The temperature has to be stable before we determine the number of moles of H released in the reaction.

2. Calculate the moles of hydrogen gas present using the given calculation method. Ans: n(H2)= pVRT =1 atm×32.9× 10-3L0.08026 L atm K-1mol-1×298K =1.345×10-3mol

3. Give the Ideal Gas equation and specify what each variable is. Show one mole of the gas at S.T.P. occupies 22.4L. Ans: Ideal Gas equation pV = n R T, these equations are Boyles law and Charles law respectively. Similar expressions can be derived for V, n and T in terms of other variables. The standard temperature is 273 K and pressure is 1 atm or 101325 N/m2 and R is gas constant 0.08205 L atm / (mol·K).

For 1 mol gas ,V= nRTp
=1 mol×0.08026 L atm K-1mol-1×273K 1atm =22.4024L
≈22.4L

4. What will be the result if hydrogen gas, H2, leaks through the stopcock of the inverted burette? Ans: If hydrogen gas, H2 leaks through the stopcock of the inverted burette, the calculation of the numbers of moles of H2 will be interrupted. Based on the ideal gas equation, the volume of gas is proportional to the number of moles. The increase of volume of H2 will increase the number of moles while the decrease of the volume will decrease the number of moles H2 produced. 5. Given equation Mg + xHCl → MgClx + X/2 H2.Based on the experimental results, determine x and valency of magnesium. Ans: Based on the experimental results, the x value was 2 and the valency of the 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