Determination of the Valency of Magnesium

EXPERIMENT 6

TITLE:

INTRODUCTION:

Stoichiometry is the study of quantitative (measurable) relationships of the reactants and products in a balanced chemical reaction. It can be used to calculate quantities such as the amount of products that can be produced with the given reactants and percent yield. Stoichiometry rests upon the law of conservation of mass, the law of definite proportions (i.e., the law of constant composition) and the law of multiple proportions. In general, chemical reactions combine in definite ratios of chemicals. Since chemical reactions can neither create nor destroy matter, nor transmute one element into another, the amount of each element must be the same throughout the overall reaction. For example, the amount of elements x on the reactant side must equal the amount of element X on the product side. In this experiment, the known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry.

This experiment determines the stoichiometry of a reaction of magnesium and HCl. The relationship between moles of magnesium reacted and moles of hydrogen produced are plotted. Magnesium Ribbon is a strip of Magnesium that is solid at room temperature. When mixed with hydrochloric acid it produces Magnesium Chloride, which is a liquid and Hydrogen gas.

The below is the equation that occurs:

Mg + XHCl → MgClx + H2

The known amount of the magnesium is reacted with large excess of HCl, and the volume of H2 evolved is measured. As HCl is in excess, all the magnesium will be consumed, and the yield of both MgClX and H2 depend only on the amount of magnesium. A comparison of the amount of hydrogen produced with the amount of magnesium consumed will enable X value to be determined.

OBJECTIVE:

This experiment is:

To study the quantitative relations between amounts of reactants and products of a reaction.
To determine the reaction stoichiometry by using a known starting mass of magnesium and the measured collection of hydrogen gas.

APPARATUS AND MATERIALS:

Magnesium ribbon, HCl (0.5M), Burette (50 cm3), Pipette (25 cm3), Retort stand, Analytical balance, Glass watch, Beaker (1000 cm3), Gauze, Funnel, Glass rod, Thermometer

PROCEDURE:

The volume of the unmarked space in a clean, dry 50 cm3 burette was determined by pipetting 25.00 cm3 of water into the vertically clamper burette. The water in the burette was left for 10 minutes and any leakage was checked.
A piece of magnesium ribbon was cleaned with sand paper. The ribbon was then cut off with a pair of scissors and it was weighed between 0.0300 and 0.03600 grams on an Analytical balance with a watch glass. The ribbon was curled up and was then placed inside a 500 cm3 beaker.
A small filter funnel with a short stem was took and covered with gauze. It was then inverted and placed over the magnesium.
The beaker was carefully filled with (tap) water until the level is approximately 0.5 – 1.0 cm above the end of the funnel stem. The burette was completely filled with 0.5 M HCl, it was then inverted and placed in the water in the beaker. The end of the burette was placed over the stem of the funnel, ensuring that no air enters and it was clamp into position.
The excess of water was removed with a pipette until the level is just above the stem of the funnel.
About 100 cm3 of 0.5 M HCl was added to the beaker, a glass stirring rod was used to ensure complete mixing.
The solution was stirred to initiate the reaction. The filter funnel was tapped with the glass rod gently to dislodge any gas bubbles.

RESULTS:

The volume of the unmarked space:

From the marked (50 cm³) to the tap of the burette = 25.0 cm³ - (50.0 – 35.0) cm³

= 10.0 cm³

Table 1: Mass of Magnesium used

Weight of Magnesium used = 29.3596g – 29.3272g

= 0.0324 g

Initial volume of HCl used = (10.0 + 50.0) cm³

= 60.0 cm³

Final volume of hydrogen gas, H2 produced = 24.0 cm³

Volume of hydrogen gas, H2 produced ...

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RESULTS:

The volume of the unmarked space:

From the marked (50 cm³) to the tap of the burette = 25.0 cm³ - (50.0 – 35.0) cm³

= 10.0 cm³

Table 1: Mass of Magnesium used

Weight of Magnesium used = 29.3596g – 29.3272g

= 0.0324 g

Initial volume of HCl used = (10.0 + 50.0) cm³

= 60.0 cm³

Final volume of hydrogen gas, H2 produced = 24.0 cm³

Volume of hydrogen gas, H2 produced = (60.0 – 24.0) cm³

= 36.0 cm³

Temperature of the solution:

Initial temperature = 23.5°C

Temperature after 20 minutes = 24.0°C

CALCULATIONS:

The reaction between magnesium and hydrochloric acid in this experiment will have the equation as below:

Mg + XHCl → MgClx + H2

In order to determine the valency of magnesium and get the full chemical equation of this reaction, the X value needs to be found.

In this experiment, 0.0324 grams of magnesium and excess of HCl is used as the starting materials, hence the product is totally depends on the amount of magnesium.

In order to find out how much hydrogen gas can be produced by 0.0324 grams of magnesium, the number of mole of magnesium used must be determined first.

Number of mole of magnesium =

= 0.0013 mol

By using the ideal gas law, pV = nRT we are able to calculate the amount of hydrogen gas produced. Where p is the pressure, V is the volume of hydrogen gas produced in the experiment, n is the number of mole of hydrogen gas, R is gas constant and T is the temperature.

pV = nRT

(101.325 kPa)(36.0 cm³ 1000) = n 8.314472 JK−1mol−1 (24.0 +273.2) K

n =

n = 0.00148 mol

We can determined the value of X using the number of mole of hydrogen gas produced, as number of hydrogen gas produced is 0.00148 mol which is equal to in the chemical equation . Hence, the value of X is equal to:

X = 0.00148 mol x 2

= 0.00296 mol.

Hence by substituting all the value we found into the chemical equation below:

Mg + XHCl → MgClx + H2

It will become the equation below:

0.0013 Mg + 0.00296 HCl → 0.0013 MgCl0.00296 + 0.00148 H2

By dividing all the substituted value with 0.0013, we will get an equation as below:

Mg + 2.28 HCl → MgCl2.28 + 1.14H2

Hence based on the equation obtained from the calculation we can conclude that the value of X is approximately 2.

As the value of X represents the valency of magnesium, we can say that the valency of magnesium is 2.

DISCUSSION:

Stoichiometry is the study of the quantitative relations between amounts of reactants and products of reactions, which is, how many moles of A react with a given number of moles of B will produced how many moles of products. Different reaction will have different characteristic proportions. The method used to obtain these from chemical formulas, equations, atomic weights and molecular weights, and determination of what and how much is used and produced in chemical processes are all deals with stoichiometry. Same goes to this experiment, we uses stoichiometry to calculate the value of X and hence determined the valency of magnesium. The value of X calculated is 2.28, which is approximately 2. Thus, by knowing the value of X, we can conclude that the valency of magnesium is approximately 2. Although theoretically the valency of magnesium is 2, we are not able to obtain such accurate results due to some limitation.

A chemical reaction which involves breaking or formation of bonding among the molecules involved often correlated to exothermic or endothermic. As such reaction involves the absorption and release of energy which absorbs or produces heat. In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in form of, electricity or sound. While, endothermic describes a process or reaction that absorbs energy in the form of heat. In this experiment, the temperature of the solution is determined too. The initial temperature for the experiment is 23.5°C. The temperature was then measured again after 20 minutes and it is 24°C. The temperature of the solution increases, thus we can say that the experiment is undergoing exothermic reaction. The reason of measuring the temperature only after 20 minutes or above is because we can only determine one experiment whether it is exothermic or endothermic when the reaction is close to the peak of the experiment. In this experiment, when concentrated hydrochloric acid is added into the water, hydronium ion, H3O+ and chloride ion, Cl- are produced. In order to get an accurate temperature, we have to wait until these ions reacted with Magnesium ions, Mg2+. Besides, the temperature of the solution will only change when all the particles in the solution are exposed to the heat produced. Therefore, measuring the temperature too early in the experiment may not be able to see the rise in temperature of the solution.

With the value obtain from the calculation we are able to calculate the number of mole of hydrogen present in the experiment. By referring to the calculation, the general equation for this experiment is as below:

Mg + XHCl → MgClx + H2

We have calculated the value of X, which is 2.28. Therefore, the number of mole of hydrogen gas is around 1.14 moles. The number of mole of hydrogen present should be 2 x 1.14 moles as 1 hydrogen gas has 2 hydrogen atoms. Hence, the number of mole of hydrogen is 2.28 moles. However, this is not the number of mole of hydrogen present in the experiment. The actual number of mole of hydrogen present in the experiment should be calculated using the Ideal Gas law equation, pV = nRT. The calculation is as below:

pV = nRT

(101.325 kPa)(36.0 cm³ 1000) = n 8.314472 JK−1mol−1 (24.0 +273.2) K

n =

n = 0.00148 mol

From the calculation, we know that the number of mole of hydrogen gas is 0.00148 moles hence, by multiplying it with 2 we are able to get the number of mole of hydrogen atoms present in the experiment, which is 0.00296 moles.

We uses the Ideal Gas equation, so that we are able to determine the number of moles of hydrogen gas produced and hence, we are able to determine value of X and valency of magnesium. Ideal Gas equation is the equation from Ideal Gas law and the Ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. The state of an amount of gas is determined by its pressure, volume, and temperature. The equation is as below:

pV = nRT

Where p is the absolute pressure of the gas; V is the volume of the gas (usually in dm3); n is the amount of substance of the gas, measured in moles; R is the gas constant (which is 8.314472 JK−1mol−1 in SI units); and T is the absolute temperature, which is measure in Kelvin, K. In order to prove the accuracy of the equation, we can use it to calculate the volume of one mole of gas at S.T.P which occupies 22.4L. Pressure in S.T.P is 1 atm, the gas constant is 0.0821 L atm and the temperature is 0°C, which is also 273.2 K.

pV = nRT

(1 atm) V = 1 mole (0.0821 L atm) (273.2k)

V =

V = 22.4 L

Therefore, the volume of one mole of gas at S.T.P. calculated is 22.4L which is the same as the theoretical value.

Before or during the experiment, we must ensure that there is no hydrogen gas leaks out of the burette through the stopcock of the inverted burette. If the is a leak of hydrogen gas means the result obtained will be affected and hence the process of determining the valency of magnesium will be difficult and may be the valency of magnesium calculated will not be accurate anymore. Besides, the calculation using the ideal gas equation to determine the number of mole of hydrogen present will be inaccurate too. To avoid inaccuracy of result, we must be sure that no hydrogen gas is leak out of the burette.

There are several precautions need to pay attention to. For instance, hydrochloric acid is a strong acid; hence it is highly corrosive to our skin. So, one must be extremely careful when handling it. Safety gloves and goggles should be worn at all time to protect our eyes and hands from making direct contact with these acids. Other protection like lab coat should also be worn for self protection. Besides that, magnesium ribbon must be cleaned properly with sand paper before use in the experiment as magnesium will form magnesium oxide easily when it is exposed to air for a certain period of time. If it is not properly cleaned, this might affect the accuracy of the result. We should also beware of parallax error in order to increase the accuracy of the results. One should also handle all the glass ware with care. Lastly, wash your hands thoroughly with soap to prevent toxic or harmful substances to remain on the hands.

CONCLUSION:

The unmarked space of a burette from the 50 cm3 mark to the stopcock of a burette is calculated, which is 10.0 cm³. By using stoichiometry and with the help of ideal gas equation, we are able to determine the value of X, which is approximately 2. By using the value of X, we are able to determine the valency of magnesium, which is also 2, which are the same as the theoretical value. We also get to know that 2 mole of hydrochloric acid are require to react 1 mole of magnesium in order to produced 1 mole of magnesium chloride and 1 mole hydrogen gas. Besides that, we also get to know that the reaction between hydrochloric acid and magnesium is exothermic by measuring the temperature during the experiment. The ideal gas equation is studied and proven accurate by calculating the volume of one mole of gas at S.T.P. which occupies 22.4 L. Some precautions steps is learnt through this experiment.

REFERENCES:

Eldredge, A. (2007). Chemistry: Principles, Patterns and Applications. International Edition: Pearson.

Endothermic - Wikipedia, the free encyclopedia. (2009, July 19). Retrieved July 21, 2009, from Wikipedia, the free encyclopedia:

http://en.wikipedia.org/wiki/Endothermic

Exothermic - Wikipedia, the free encyclopedia. (2009, July 9). Retrieved July 21, 2009, from Wikipedia, the free encyclopedia:

http://en.wikipedia.org/wiki/Exothermic

Ideal gas law - Wikipedia, the free encyclopedia:. (2009, July 19). Retrieved July 21, 2009, from Wikipedia, the free encyclopedia:

http://en.wikipedia.org/wiki/Ideal_gas_law

Papers - Determination of the Valency of Magnesium. (2009, July). Retrieved July 21, 2009, from Papers - Determination of the Valency of Magnesium:

http://www.123helpme.com/view.asp?id=150375

Stoichiometry - Wikipedia, the free encyclopedia. (2009, July 11). Retrieved July 21, 2009, from Wikipedia, the free encyclopedia:

http://en.wikipedia.org/wiki/Stoichiometry

Determination of the Valency of Magnesium

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