Mg + X HCl → MgClx + X/2 H2
We know that magnesium will react with hydrochloric acid to produce hydrogen gas and magnesium chloride (MgCI) salt. MgCI is soluble but not hydrogen gas, so the gas can be collected in the burette using a filtering funnel.
In this experiment, a limiting factor is needed to control the reaction. Magnesium is the best choice since it is in solid form and can be easily fixed in a space compared to hydrochloric acid in aqueous form, so excess hydrochloric acid will ensure that all the known amount of magnesium have reacted.
When the volume of the hydrogen gas is obtained, the mole of the gas can be calculated, then substituted into the formula and to get the amount of mole of each molecule, and the difference between how many magnesium we had put in and how many actually reacted can be compared.
Apparatus :
- Burette(50cm³)
- Pipette (25 cm³)
- Funnel
- Thermometer
- Barometer
- Glass rod
- Retort Stand
- Electrical Balance
- Watch Glass
- Beaker (1000 cm³)
Materials :
- Magnesium ribbon
- HCI (0.5M)
- Gauze
- Glove
- Rubber band
Procedures :
- 25 cm³ of water was pipetted in to the 50 cm³ burette. The burette was clamped to the retort stand and the reading was taken. The burette was drained and the step was repeated. The water was left in the burette for 10 minutes and any leaks were observed. The volume of the unmarked space between the 50 cm³ and the tap of unknown volume was determined.
- A piece of magnesium ribbon was cleaned with steel wool. A piece was cut off and curled up. The magnesium was weighed on an analytical balance.
- The magnesium curl was placed on a watch glass and placed in a 600 cm³ beaker.
- A small filter funnel with short stem was covered with gauze. It was inverted and placed on the watch glass over the magnesium.
- The beaker was carefully filled with tap water till the level is approximately 0.5-1.0cm above the end of the funnel stem.
- The burette was filled full with hydrochloric acid, with a cork pressed flat over the open end, and was inverted and placed in the water in the beaker.
- The cork was removed from the burette and the burette’s end was placed over the stem of the funnel, ensuring that no air enters and was clamped into position.
- The excess water was removed with a pipette until the level is just above the stem of the funnel.
- 100 cm³ of hydrochloric acid was added into the beaker using a glass rod to ensure complete mixing such that the hydrochloric acid reaches the magnesium.
- The solution was stirred to initiate the reaction. Then, the stirring was stopped to ensure the process proceeded unaided. At the end of the completion of the reaction, the watch glass was tapped gently to dislodge any gas bubbles.
Results :
Weight of Magnesium ribbon used in the reaction : 0.0123g
Blank volume of the burette that used to collect the hydrogen gas :
1st reading = 5.2 cm³
2nd reading = 4.4 cm³
3rd reading = 4.3 cm³
Average volume = 4.6 cm³
Reading of hydrogen gas in the burette : 40.9 cm³
Analysis & Calculation
Reading for the Hydrogen gas volume at the burette: 40.9 cm³
Volume of the hydrogen gas = Blank volume of burette + (50 – reading at the burette)
=4.6 + (50 - 40.9)
=13.7 cm³
= 13.7 ml
Assuming the pressure is 1atm, the following formula were used,
PV = nRT
Volume of Hydrogen gas : 13.7ml=0.0137L
Temperature: 22°C = 295K
P= 1atm
The mole of hydrogen gas is calculated,
PV=nRT
n=0.0137/(0.0821 x 295)
n=0.000565 mol
Mole of hydrogen gas : 0.000565 mol
Based on the following equation:
Mg + X HCl → MgClx +X/2 H2
The ratio of H atom in HCl and H2 is (1:2),
So the mole of HCl is actually double of the H2 ,which is 0.000565mol x 2 = 0.00113mol
From the data, we had only used 0.0123g of magnesium ribbon as the reactant, using 0.0123g to calculate the mole of magnesium ring we used.
Mole of magnesium used = (0.0123g) ÷ (24g/mol)
=0.000512mol
So the valency of electron of Magnesium is = 0.00113mol ÷ 0.000512mol
=2.2 ≈2.0
SO, this means 1 mole of Magnesium can react with 2 mol of Chlorine.
Mg + 2 HCl → MgCl2 + H2
Now, we are going to find the theoretical result of the reaction. The ratio of mole of hydrogen gas and magnesium is the same which is (1:1), so it means the mole of the hydrogen gas had produce is = 0.000512 mol.
Theoretical yield = 0.000512 mol
The mole of magnesium we used is 0.000512mol. The amount is different from the calculation by using hydrogen gas which is 0.000565mol.
Actual yield = 0.000565mol
To calculate the percentage yield:
Percentage yield= (Actual yield - theoretical yield) ÷ theoretical yield x 100%
= (0.000565x0.000512) ÷ 0.000512 x 100%
=10.35%
From the calculation, we get the percentage yield is 10.35%.
Discussion
From the experiment, we successfully obtained hydrogen gas in the burette through the filtering funnel during the effervescence, by reacting magnesium with hydrochloric acid. The weight of magnesium is 0.0123g and the amount of hydrochloric acid is100 cm³. At the beginning of the process, the reaction is considered slow compared to after stirring the solution outside the filtering funnel. This might be because the solution is not homogenous enough at the beginning. The reaction became the fastest after 1 or 2 minutes when the solution was stirred, it is because the solution became more homogenous since the HCl outside filtering funnel diffused in.
About 15 minutes later, some solution in the beaker were removed and additional HCl solution was added into the beaker in order to speed up the reaction. This is because after 15 minutes, the reactants in the beaker would have decreased in concentration and the product concentration becomes higher compared to the initial. The products may resist the reaction to continue according to the law of chemical equilibrium. So, removing some of the solution in the beaker can reduce the amount of product and additional HCl can increase the amount of reactant.
In the lab manual, it is requested to measure the temperature of the solution in the beaker after 20 minutes. During the experiment, we actually measured the temperature of the solution but it is not include in the raw data, so we decided not to show it in the data & result section but show it inside the discussion. The temperature of the initial is 22°C and it is 22°C in the 10 minutes, and we did not measure the temperature during 20 minutes, this is a major error we found after the experiment.
For the temperature, we believe this reaction is an exothermic reaction, but there is no increase in temperature in the first 10 minutes. We believe this is because of the environment is cool and the additional of HCl during the 15 minutes, additional solution may change the temperature, and we estimated that the temperature is equal or lesser than 22°C.
In the calculation, when the theoretical yield and actual yield were compared, the actual yield is more than theoretical yield, which is 0.000053mol in access compared to the actual yield. This is due to the dissolved gases in the solution might have followed the gases formed in the effervescence, and they became bubbled together and got collected in the burette.
Magnesium easily looses it’s electrons, it is then more likely to react with elements that would accept them, it can be oxygen, which has 6 electrons and need 2 to be more stable (octet rule), that’ll give magnesium oxide (MgO). Or it can react with 2 halides which have 7 electrons and need one each to give MgX2. All these bonds formed are covalent bonds.
Conclusion:
The valency of magnesium is 2.2, which is close to the theoretical valency of magnesium (which is 2). The theoretical yield of the reaction is 0.000512mol and the actual yield is 0.000565mol, and the percentage error yield is 10.3%
References
- http://en.wikipedia.org/wiki/Stoichiometry [Last modified on 2/3/2008][view on 4/3/2008]
- Tan Yin Toon (2003), Physical Chemistry for STPM, Penerbit Fajar Bakti SDN.BHD [Page 50-52]
Question
- Note that the temperature is not taken for at least 20 minutes, after adding the HCl. Why is this?
In this experiment, we assumed that the temperature of hydrogen gas is in equilibrium with the temperature of water. Therefore, the temperature taken within 20 minutes will directly affect the accuracy of x value (the valency of the magnesium) during the calculation.
- Calculate the moles of hydrogen present using the given calculation method.
Volume of Hydrogen gas: 13.7ml = 0.0137L
Temperature: 22°C = 295K
P=1atm
R=0.0821L·atm÷(mol·K)
PV=nRT
n=0.0137/(0.0821 x 295)
n=0.000565 mol
Mole of hydrogen gas = 0.000565 mol
- Give the Ideal Gas equation and specify what each variable is. Show one mole of gas at S.T.P. occupies 22.4L.
By using the ideal gas equation, we must set a few things to constant because that is the ideal condition.
Pressure = 1atm
Temperature = 273K
n = 1 mol
This is the way of calculating the volume of 1mol of gas in S.T.P.
PV=nRT
(1)(V) = (1)(0.0821)(273)
V = 22.4133L ≈ 22.4L
4. What will be the result if Hydrogen gas leaks through the stopcock of the inverted burette?
If this happened, the volume of the hydrogen gas will be surely less than what we get in the experiment, this will cause the error in the result which made the valency of magnesium very little and high percentage error yield. Hydrogen gas is the product of the reaction, it must be quantitatively collected in order to determine x.
