Title: Experiment 2 Determination of the valency of the magnesium
Objective: -To study the quantitative relationship between the amount of 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.
- To identify the unknown X value in the chemical equation between magnesium and hydrochloric acid
- To determine the valency of magnesium
Introduction:
Stoichiometry is the study of the quantitative relationship between amounts of reactants and products of a reaction (that is, how many moles of A react with a given number of moles of B). In this section, 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 form hydrogen gases. The aim of this experiment is to determine the value of x in the following equation:
Mg + X HCI → MgCIX + H2
A known amount of magnesium is reacted with a 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 is dependent on the amount of magnesium used. A comparison of the amount of hydrogen produced with the amount of magnesium consumed will enable the X value to be determined.
Apparatus and Materials: Magnesium ribbon, 0.5M Hydrochloric acid, Burette (50 cm3), Pipette (25 cm3), Retort stand, Funnel, Glass rod, Thermometer, Watch glass, Beaker (500 cm3), Gauze, Electrical balance
Procedure:
1. The volume of unmarked space in a clean, dry 50 cm3 of burette was determined by pipetting 25.00 cm3 of water into the vertically clamped burette. The burette reading was noted. The burette was drained and repeated. The water was left in the burette for 10 min and checked whether leaks occur. 2. A piece of magnesium ribbon was cleaned with sand paper. A piece with scissors was cut off and the ribbon was curled up. A watch glass was tared on the four decimal balances and weighed ACCURATELY between 0.000 and 0.0150 g of the magnesium ribbon on the watch glass, which should then be placed inside a 500 cm3 beaker. 3. A small filter funnel with a short stem (1.0-1.5 cm long) was taken and covered with gauze. It was inverted and placed on the watch glass over the magnesium. 4. The beaker was filled carefully with (tap) water until the level is approximately 0.5-1.0 cm above the end of the funnel stem. The burette was filled completely with 0.5 M HCl, it was inverted (with supplied cork pressed flat over the open end) and placed it in the water in the beaker, the cork was removed and placed the end of the burette over the stem of the funnel, ensuring that no air enters, and clamped it into position. 5. The excess of water was removed with a pipette until the level is just above the stem of the funnel. 6. About 100 cm3 of 0.5 M HCl was added to the beaker, using a glass stirring rod to ensure complete mixing such that the HCl reaches the magnesium. This may be helped by tapping the watch glass gently with a glass rod. 7. The solution was stirred to initiate the reaction and then do not stir further so that the reaction proceeds unaided. At the completion of the reaction (say 30 minutes) tap the watch glass gently to dislodge any gas bubbles.
Note: Any magnesium that escapes through the fine gauze will still react with the 0.5 M HCl in the burette.
Results:
Volume of unmarked space of the burette: 8.1cm3
Mass of magnesium ribbon: 0.0108g
Initial temperature: 26°c
Final temperature: 26°c
Volume of hydrogen: 8.1cm3+(50.0-34.7)cm3=23.4cm3
Calculation:
Mg+ xHcl→ MgClx +x/2H2 n(Mg) = Weight of magnesium ribbon/ Relative Atomic mass for Mg
Mole of magnesium = 0.0108g/24 g mol-1= 4.5x10-4 mol
Actual mole of HCl used = 100cm3 x 0.5M / 1000= 0.05 mol
Number of mole of HCl / number of mole of Mg= 0.05 mol/ 4.5x10-4 mol = 111.11> 1 * Hence, Magnesium is a limiting agent.
Theoretical mole of hydrogen gas = 0.0234dm3 / 24dm3 mol-1= 9.75x10-4mol
Number of moles of magnesium: number of mole of hydrogen gas
1 (4.5 x 10-4mol) = X/2 (9.75 x 10-4mol)
Thus, X/2 = 4.5x 10-4mol /9.75 x 10-4mol
X=0.9231
X ≈ 1
Discussion:
Magnesium is a chemical element with the symbol Mg and atomic number 12. Its common oxidation number is +2 and it is an alkaline earth metal which is categorized to the Group 2 element in the periodic table. Magnesium is a rather strong, silvery-white, light-weight metal and tarnishes slightly when exposed to air, although, unlike the alkali metals, an oxygen-free environment is unnecessary for storage because magnesium is protected by a thin layer of oxide that is fairly impermeable and difficult to remove. Like its lower periodic table group neighbor calcium, magnesium reacts with water at room temperature, though it reacts much more slowly than calcium. When submerged in water, hydrogen bubbles almost unnoticeably begin to form on the surface of the metal—though, if powdered, it reacts much more rapidly. The reaction occurs faster with higher temperatures .Magnesium also reacts exothermically with most acids, such as hydrochloric acid (HCl). As with aluminium, zinc, and many other metals, the reaction with HCl produces the chloride of the metal and releases hydrogen gas. Since the Hydrochloric acid is added in excess, hence the magnesium is the limiting reactant in this reaction. Magnesium is a highly flammable metal, but, while it is easy to ignite when powdered or shaved into thin strips, it is difficult to ignite in mass or bulk. Once ignited, it is difficult to extinguish, being able to burn in nitrogen (forming magnesium nitride), carbon dioxide (forming magnesium oxide, and carbon) and water (forming magnesium oxide and hydrogen). Limiting reactant is the reactant which restricts the amount of products generated in the reaction. It is also the substance which is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent since the reaction cannot proceed further without it. If one or more other reagents are present in excess of the quantities required to react with the limiting reagent, they are described as excess reagents or excess reactant .In this situation, the magnesium is used up completely to react with the excess hydrochloric acid.
From the calculation, the value of X in the chemical equation between magnesium and hydrochloric acid was determined. The value of X is 1. So that, the complete chemical reaction for magnesium and acid is Mg (s) + HCl (aq) à MaCl2(aq) +1/2 H2(g). Based on the Las of Conservation of Matter, matter can be neither created or destroyed. An equation must have the same number of atoms of the same kind on both sides of the equation. The reaction between magnesium and hydrochloric acid is an exothermic reaction. This reaction releases heat energy from the reaction to its surrounding which caused the solution is the beaker become hotter. The temperature cannot be detected significantly because the rise in temperature was just slight different. The temperature is only increased by few degree Celsius due to the large specific heat capacity of the water. Furthermore, it is the opposite of an endothermic reaction. It also can expressed in a chemical equation: Reactants → Products + Energy
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.
Magnesium metal and its alloys are explosive hazards ,they are highly flammable in their pure form when molten or in powder or ribbon form. Burning or molten magnesium metal reacts violently with water. When working with powdered magnesium, safety glasses with welding eye protection are employed, because the bright-white light produced by burning magnesium contains ultraviolet light that can permanently damage the retinas of the eyes .
Furthermore, Results from a meta-analysis of randomized clinical trials demonstrated that a magnesium supplement can lower high blood pressure in a dose-dependent manner. Low serum magnesium levels are associated with metabolic syndrome, diabetes mellitus type 2 and hypertension. Low serum magnesium levels have been associated with a higher risk of developing metabolic syndrome. Magnesium therapy is recommended by the ACC/AHA/ESC 2006 Guidelines for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death for patients with ventricular arrhythmia associated with torsade de pointes who present with long QT syndrome as well as for the treatment of patients with digoxin intoxication-induced arrhythmias. Magnesium is also the drug of choice in the management of pre-eclampsia and eclampsia.
In addition to its therapeutic role, magnesium improves calcification. Patients with chronic kidney disease have a high prevalence of vascular calcification, and cardiovascular disease is the leading cause of death in this population. Several in vitro and animal studies point toward a protective role of magnesium through multiple molecular mechanisms. Magnesium is a natural calcium antagonist, and both human and animal studies have shown that low circulating magnesium levels are associated with vascular calcification. Results from an observational study conducted in the general Japanese population demonstrated that lower serum magnesium levels were significantly and independently associated with a greater average intima-media thickness and the risk of at least two carotid plaques. Magnesium supplementation might be useful in reducing the progression of atherosclerosis in chronic dialysis patients. Low serum magnesium may be an independent risk factor for death in patients with chronic kidney disease, and patients with mildly elevated serum magnesium levels could have a survival advantage over those with lower magnesium levels.
Since the kidneys are responsible for the excretion of magnesium, anyone with a heart or kidney disorder should not take any extra magnesium except under their doctor's supervision. It is very rare to overdose on magnesium from food, however, people that ingest large amounts of milk of magnesia (as a laxative or antacid), epsom salts (as a laxative or tonic), or magnesium supplements may overdose, especially if they suffer from kidney problems. Too much magnesium can cause several serious health problems, including nausea, vomiting, severely lowered blood pressure, confusion, slowed heart rate, respiratory paralysis, deficiencies of other minerals, coma, cardiac arrhythmia, cardiac arrest, and eventually death. The most common side effects of magnesium toxicity are stomach upset and diarrhea.
Questions:
1. Note that the temperature is not taken for at least 20 min after adding the HCl. Whyis this?
The temperature is taken after 20 minutes after adding the hydrochloric acid is because of the heat release by the formation of MgCl (heat of summation) will affect the calculation. Based on the equation pV= nRT, temperature is inverse to the number of moles. Increase in the temperature will decrease the number of moles while decrease in temperature will increase the number of moles. The temperature has to be stable before we determine the number of moles of H2released in the reaction.

2. Calculate the moles of hydrogen present using the given calculation method. pV=nRT p = 1 atm, R=0.08206, T=(26+273) Kelvin
V = 23.4cm3= 23.4ml/1000=0.0234L n=PV /RT= 1atm x 0.0234L/ 0.08206L atm K-1mol-1 x 299K= 9.54x 10-4mole

3. Give the Ideal Gas equation and specify what each variable is. Show one mole of gasat S.T.P. occupies 22.4 L.
-Ideal gas equation is the combination of Boyle’s law, Charles’ law and Avogadro’s law. Ideal gas equation: pV = nRT
Where p= pressure (must be in atm),
V= volume (measured in dm3or L),
T = temperature in Kelvin,
R = gas constant: 0.08206dm3atmK -1 n= number of moles.
V = nRT/p= (1mol) x (0.08206 Latm K-1mol-1) x (273K)/1atm = 22.4024 L ≈ 22.4 L

4. What will be the result if hydrogen gas, H2, leaks through the stopcock of the inverted burette?
-If hydrogen gas, H2 leaks through the stockpile 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 H2will increase the number of moles while the decrease of the volume will decrease the number of moles H2 .produced

Conclusion:
The unknown X value in the chemical equation between magnesium and hydrochloric acid is calculated as X=1.Magensium has a valency of 2.
References:
* Peter Hong Leong Cheah. (Aug 02, 2009). Experiment 6. Available: http://www.scribd.com/doc/18026423/Experiment-6 Last accessed 11/2/2014 * Magenesium From Wikipedia, the free encyclopedia. Available: http://en.wikipedia.org/wiki/Magnesium Last accessed 11/2/2014