From this equation I can see that 1 mole of Magnesium reacts with 2 moles of hydrochloric acid to give 1 mole of Magnesium chloride and 1 mole of Hydrogen.
1cm of Magnesium = 0.012g, which means that 3cm = 0.036g
Therefore using the equation: moles= mass/RAM
Moles of Mg= 0.036/24=0.0015
The ratio between Magnesium and Hydrogen is 1 : 1
So: 0.0015 : 0.00.15
Which means that therefore the volume of Hydrogen released is
0.0015 x 24dm3 = 0.036 dm3 or 36 cm3
From this I can see that the volume released is a reasonable amount since the maximum capacity of the burette is 50 cm3.
This leaves me left with working out the volume of Hydrochloric acid that I should use. Using the equation from before I can state the following:
The ratio between Magnesium and Hydrochloric acid is 1 : 2
So: 0.0015 : 0.0030
Using the equation n=cv/1000
0.0030= 0.5xV/1000
3=0.5xV
V=6 cm3
So therefore 10 cm3 in excess
Safety
There are a lot of safety measures that should be taken care of, as a lot of hazardous substances are being used and made. These are the following substances:
-
Ethanoic acid- CH3COOH
- Hydrochloric acid- HCl
-
Magnesium Chloride- MgCl2
-
Hydrogen- H2
- Ethanoic acid and Hydrochloric are strong irritants and are highly corrosive.
- Magnesium Chloride and Hydrogen gas are highly flammable.
Preventions
There is a lot which can be done to prevent the hazards from taking place. As both acids used are both quite corrosive the person carrying out the experiment should always wear goggles and optionally gloves. This is to prevent the acid from coming in contact with skin or eyes. In the case of the acid coming in contact with the skin than the person should wash it off immediately. If the acid comes in contact with eyes than the person should immediately go to an eye-wash station and rinse his eyes.
A lot of care should also be taken when the acid is heated. It shouldn’t be heated directly since this might cause the acid to spit which can be extremely dangerous. The acid should be therefore heated in a water bath.
Percentage error for the planned method
-
estimate for oxygen given off will be = 36 cm3
- graduated tube measured to nearest mm
- therefore error at top = 0.5mm
- error at bottom = 0mm
- total error = 0.5 mm
-
therefore percentage error= (0.5÷36)x100=1.39%
As you can see the percentage is very low which means that the accuracy is quite high. This is the percentage of error for every reading taken in the experiment.
Percentage error for the planned procedure 2
- Estimate for oxygen given off will be = 380 sec
- Therefore error of getting it earlier = 0.5 sec
- Error of getting it later = 0.5 sec
- Total error = 1 sec
-
Therefore percentage error= (1÷380)x100=0.26%
- As you can see the percentage is very extrememly low which means that the procedure is very reliable.
Percentage error for the temperature reading (Procedure 2)
- Estimate for temperature rise will be = 50 ˚C
- Therefore error of getting it earlier = 0.1 ˚C
- Error of getting it later = 0.1 ˚C
- Total error = 0.2 ˚C
-
Therefore percentage error= (0.2÷50)x100=0.4%
- As you can see the percentage is very extrememly low which means that the procedure is very reliable.
Procedure 1:
The aim of the first procedure is to find the order of reaction with respect to Magnesium. I am going to work out the order by using a continuous method where I will collect the hydrogen released in the reaction.
Mg(s) + 2HCl (aq) MgCl2 (aq) + H2 (g)
I will collect the gas using the following apparatus:
- Conical Flask
- Rubber bung with delivery tube
- Graduated tube
- Water bath
Once the results have been obtained I will plot the graph of time against release of Hydrogen from where I will be able to work out the order of the reaction.
Procedure 2:
The aim of the second procedure is to work out the activation energy. To do this I will have to do the experiment which measures the effect of temperature on the rate of reaction between hydrochloric acid and magnesium. The acid which is in a boiling tube should be heated to temperatures between 30˚Cto 70˚C in a water bath. I would than measure the time it takes for a magnesium ribbon to dissolve completely in the Hydrochloric acid.
I this experiment I would again use 0.5 M of Hydrochloric acid with 1.5cm of Magnesium. Due to safety precautions I would cover the test tube with cotton-wool to prevent the escape of Hydrogen gas.
After I obtained the results I will work out the activation energy using the Arrhenius equation. This is the equation:
ln k = constant – EA/R (1/T)
Where k is the rate constant of the reaction, R is the gas constant,
8.31 J K-1mol-1. EA is the activation energy of the reaction in J mol-1
and T is the temperature in kelvins.
We can work out the activation energy using the graph I would have drawn which 1/T against ln rate
Set of results for Procedure 1
These are the set of results for the collection of hydrogen, between Magnesium and different concentrations of Hydrochloric acid.
This below is the table which would show what order of reaction it is with respect to Hydrochloric acid.
The following results are obtained by reacting Magnesium with a weak acid which in this case is ethanoic acid.
Procedure 2
This is the reaction between Magnesium and Hydrochloric acid at different temperatures so that I can work out the activation energy for the reaction.
Therefore using the average:
Evaluation
Anomalous results:
On the graphs I have marked any anomalous results. These results do not go along with the main trend making them stand out as irregular results.
These anomalous results can be explained by human error, which I believe is one of he major factors; however I believe that the key factor is the limitations of the agreed method.
Accuracy & reliability
My results are very accurate. My average volume of gas evolved curve lies and follows the same pattern as the total average volume of gas evolved; my rate of reaction curve follows the same pattern and lies close to the total average rate. However the reliability cannot be proven because no second tests have been carried out.
The accuracy cannot be justified because as the concentration of the enzyme concentration increases so does the standard deviation. The inaccuracies can still be accepted because as the rate increases, increasing the volume of gas evolved per second giving rise to human error. And therefore the increasing volume of gas evolved would increase the level of human error. This is where delay time plays a big role because it would be very difficult to get an exact reading corresponding to the exact minute. This is because oxygen is constantly being released so if you miss the minute board line maybe by just a few seconds than it would count as not accurate because there would have been some oxygen released in those few seconds. This error would have showed most effect on the 100% solution because the reaction is very fast in this solution.
To improve the reliability of my results I could have used a gas syringe which would have been much easier to use and slightly more accurate as well because than I could read the actual volume of oxygen released instead of having to subtract the value from the total capacity of the burette. Setting the gas syringe would be much easier as well because I would than not have to waste valuable practical time with filling the burette up with water. To improve the accuracy I would not only take the class averages but the entire year groups, a larger sample giving a larger perspective and a more accurate average. The nature of the reaction cannot be controlled, but the apparatus can be manipulated.
Human errors
Human error played a small role in the course of the experiment. There was a difficulty in obtaining a reading because of the small multiple lines indicating the decimals. So basically there is a chance of getting the reading wrong by maybe +0.1cm3 or -0.1cm3. Inserting the different amounts of solutions could have also caused an error because some of the solutions could have been spilled while trying to put them in the conical flask.
Limitations of the method and apparatus
The method and apparatus was set to give the best method for collecting the gas, however the problem was in the addition of the magnesium into the conical flask. The problems that occurred were that there was a sudden burst of gas that surged through the system and into the gas burette when the Magnesium was being added.
A practical error which should have been taken care of was not to agitate the mixture during the course of the reaction because this would have resulted in a very bad outcome of results. Agitating the mixture would have given the particles extra kinetic energy which means that more oxygen would have been produced at the time when the solution gets agitated.
