Experiment - determination of the valency of magnesium

by vicchiam (student)

Title: Determination of the Valency of Magenesium

Objective:

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

Apparatus and Material:

Electrical balance, rubber band, watchglass, glass rod, burette, beaker, thermometer, pipette, gauze, barometer, retort stand, funnel

Procedure:

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 25.00cm3 of water into the vertically clamped burette (right way up).The burette reading was noted, drained and repeated. The water in the burette was left for 10 min. and checked for leaks.
A piece of magnesium was cleaned with steel wool. A piece was cut with scissors 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 between0.0300 and 0.0360g on the watch glass, which then was placed inside a 600cm3 beaker.
A small filter funnel with a short stem (1.0-1.5cm long) was taken and covered with gauze. The watch glass was invert and placed over the magnesium.
The beaker was carefully filled with (tap) water until the level was approximately 0.5-1.0cm above the end of the funnel stem. The burette was completely filled with 0.5MHCl. The beaker was inverted and placed 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 clamp it into position.
The excess of water was removed with a pipette until the level was just above the stem of the funnel.
About 100cm3 of 0.5M 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.
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 bubbles.

Result:

Mass of magnesium ribbon= 0.0339g
Initial reading of burette = 9cm3
Final reading of burette = 46cm3
Total volume of H2 collected = 37cm3

Calculation:
Mg (s) + X HCl (aq) MgCl2(aq) + X/2 H2(g)
Mole of magnesium = 0.0339g/24 g mol-1
= 1.413x10-3 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/ 1.413x10-3 mol
= 35.39 > 1
Magnesium is a limiting agent.
Theoretical mole of hydrogen gas = 0.037dm3 / 24dm3 mol-1
= 1.541x10-3 mol
Number of moles of magnesium: number of mole of hydrogen gas
1.413x10-3 mol : 1.541x10-3 ...

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Result:

Mass of magnesium ribbon= 0.0339g
Initial reading of burette = 9cm3
Final reading of burette = 46cm3
Total volume of H2 collected = 37cm3

Discussion:

Stoichiometry is the branch of chemistry and chemical engineering that deals with the quantities of substances that enter into, and are produced by, chemical reactions. Stoichiometry provides the quantitative relationship between reactants and products in a chemical reaction. In this experiment, magnesium is used as the medium and the hydrogen gas release will be measured as the data for calculation of stoichiometry reaction.

Magnesium reacts with hydrochloric acid to release hydrogen. The experiment conducted is to determine the value of X in the following equation.

Mg + XHCl → MgClx + X/2 H2

Magnesium which will be reacted with excess hydrochloric acid has a known amount, so when the displacement reaction when the two is reacted will release hydrogen gas and the amount of gas collected can be used as a data to calculate the value of x as the magnesium amount is limited, so the hydrogen gas evolved also are limited.

Magnesium which is in the group 2 in periodic table is an alkali metal, which evolve hydrogen gases when reacted with acid. From the calculation, the value of X in the chemical equation between magnesium and hydrochloric acid was determined and the value of X is 2, the complete chemical reaction for magnesium and acid is Mg (s) + 2 HCl (aq) → MaCl2(aq) + H2(g). An equation need to have the same number of atoms of the same kind on both sides of the equation to form a balanced reaction. 1.413x10-3 mol of magnesium was used up completely to react with 2.966x10-3 mol of acid to produce 1.541x10-3 mol of magnesium chloride and hydrogen gas.

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 rise in temperature cannot be detected significantly because the temperature has only slight different as the temperature is only increased a little due to the large specific heat capacity of the water.

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.

Conclusion:

The experiment was carried out to determine the valency of magnesium and it is found out that it is with 6 electron valency as 2 more electron is needed to have an inert octet electron. 2 mole of hydrochloric acid is needed to react with 1 mole of magnesium so that it can have a complete electron configuration.

Question:

1. Note that the temperature is not taken for at least 20 min. after adding the HCl. Why is this?

The temperature is taken after 20 minutes after adding the HCl 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=(27+273) Kelvin

V = 16.7cm3= 16.7ml/1000

=0.0167L

n=PV /RT

= 1 x 0.0167 / 0.08206 x 300

= 6.784 x 10-4 mole

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’slaw. 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 -1and

n= number of moles.

V = nRTp= (1mol) x (0.08206 L atm 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, H2leaks 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 H2 will increase the number of moles while the decrease of the volume will decrease the number of moles H2 produced.

Experiment - determination of the valency of magnesium

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