G.C.S.E Science coursework October 03
Rate Of Reaction.
I am going to investigate how Hydrochloric acid (HCL) when added to magnesium (Mg) at different concentrations, will affect its rate of reaction. I will measure all sulphuric acid concentrations in moles (M).
In the reaction between hydrochloric acid and magnesium ribbon, the hydrochloric acid will dissolve the magnesium and produce hydrogen All chemical reactions involve reactants which when mixed may cause a chemical reaction which will make products.
In my case the reactants are hydrochloric acid and magnesium ribbon. The chemical reaction takes place when the magnesium ribbon is dropped into the hydrochloric acid. The products that are formed during this reaction are hydrogen gas and magnesium chloride. The formula equation for this experiment is:
Mg + 2HCl (r) MgCl2 + H2
Magnesium + Hydrochloric acid (r) Magnesium Chloride + Hydrogen
The following information was drawn from Microsoft Encarta 97 and encylopedia.com
Magnesium, symbol Mg, silvery white metallic element that is relatively un-reactive. In group 2 (or IIa) of the periodic table, magnesium is one of the alkaline earth metals. The atomic number of magnesium is 12.
Properties and Occurrence
The metal, first isolated by the British chemist Sir Humphry Davy in 1808, is obtained today chiefly by electrolysis of fused magnesium chloride. Magnesium is malleable and ductile when heated. With the exception of beryllium, it is the lightest metal that remains stable under ordinary conditions. The metal is not attacked by oxygen, water, or alkalis at room temperature; it reacts with acids. When heated to about 800° C (about 1472° F), it reacts with oxygen and emits a brilliant white light. Magnesium melts at about 649° C (1200° F), boils at about 1107° C (about 2025° F), and has a specific gravity of 1.74; the atomic weight of magnesium is 24.305.
Magnesium ranks sixth in natural abundance among elements in crystal rocks. It occurs in nature only in chemical combination with other elements, particularly as the minerals carnallite, dolomite, and magnesite; in many rock-forming silicates; and as salts, such as magnesium chloride, in ocean and saline-lake waters. It is an essential constituent of animal and plant tissue.
Magnesium forms divalent compounds, chief among which are magnesium carbonate (MgCO3), which is formed by the reaction of a magnesium salt and sodium carbonate and is used as a refractory and insulating material; magnesium chloride (MgCl2·6H2O), which is formed by reacting magnesium carbonate or oxide with hydrochloric acid and is used as dressing and filler for cotton and woollen fabrics, in paper manufacture, and in cements and ceramics; magnesium citrate (Mg3(C6H5O7) 2·4H2O), which is formed by the reaction of magnesium carbonate with citric acid and is used in medicine and effervescent beverages; magnesium hydroxide (Mg(OH)22), formed by the reacting of magnesium salt and sodium hydroxide and used in medicine as the laxative "milk of magnesia," and in sugar refining; magnesium sulphate (MgSO4·7H2O), well known as Epsom salt; and magnesium oxide (MgO), called burnt magnesia, or magnesia, prepared by burning magnesium in oxygen or by heating magnesium carbonate and used as a heat-refractory and insulating material, in cosmetics, as a filler in paper manufacture, and as a mild, antacid laxative. Alloyed forms of magnesium have considerable tensile strength. The metal is used when lightness is an essential factor: alloyed with aluminium or copper, it is used extensively in making castings for airplane parts; in artificial limbs, vacuum cleaners, and optical instruments; and in such products as skis, wheelbarrows, lawn mowers, and outdoor furniture. The unalloyed metal is used in photographic flash powders, incendiary bombs, and signal flares; as a deoxidiser in the casting of metals; and as a getter, a substance that achieves final evacuation in vacuum tubes.
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The estimated world production of magnesium in 1989 was 350,000 metric tons. The estimated United States production in the same year was 148,000 metric tons.
Chemical compound, HCl, a colourless, poisonous gas with an unpleasant, acrid odour. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity. Hydrogen chloride is prepared commercially by the reaction of sulphuric acid with sodium chloride (common salt); niter cake, a mixture of sodium bisulfite and sulphuric acid that is a by-product of nitric acid manufacture, is sometimes used in place of sulphuric acid. Hydrogen chloride is also produced as a by-product of the manufacture of chlorinated organic chemicals. It can be prepared directly by reaction of hydrogen and chlorine gases; the reaction is very exothermic and takes place readily in sunlight or at elevated temperatures. Although anhydrous (water-free) hydrogen chloride is commercially available as a high-pressure compressed gas in steel cylinders, most of the gas produced is dissolved in water to form hydrochloric acid a commercially important chemical. Pure grades of hydrochloric acid are colourless, but technical grades, commonly called muriatic acid, are often yellow-coloured because of impurities such as dissolved metals. Most hydrochloric acid produced has a concentration of 30% to 35% hydrogen chloride by weight. The major use of hydrochloric acid is in the manufacture of other chemicals. It is also used in large amounts in pickling (cleaning) metal surfaces, e.g., iron before galvanizing. It reacts with most common metals, releasing hydrogen and forming the metal chloride; with most metal oxides and hydroxides it reacts to form water and the metal chloride. Hydrochloric acid is also used in small amounts in processing glucose and other foods and for various other uses. Concentrated solutions are strong acids and highly corrosive. Hydrochloric acid is not an oxidizing agent but can be oxidized by very strong oxidizing agents, liberating chlorine gas. In dilute solutions of the acid the hydrogen chloride is almost completely dissociated into hydrogen and chloride ions. A solution containing 20.24% hydrogen chloride by weight is azeotropic, boiling at a constant temperature of 110 at atmospheric pressure. Hydrogen chloride also forms monohydrates, dihydrates, and trihydrates that are liquids at room temperature.
To carry out this investigation I will use the following equipment:
- Safety Goggles
- HCL (0.25, 0.50, 1, 1.5, 2m)
- Magnesium strips
- Delivery tube
- Gas syringe
- Clamp stand
- Pen/pencil and paper to record results
We must also make sure that before we carry out the experiment that it is safe, safety rules are as follows:
- Wear safety goggles at all times
- Put bags, chairs/stools out of the way or under the table
- Clear tables of all items which are not required for the experiment
- Always walk carefully around the classroom#
- Handle the glass carefully
- Act in an appropriate manner, and if there is a problem ASK THE TEACHER
I must make sure that this experiment is carried out fairly because I want to get the most accurate results as possible. This is what I will do to carry out a fair test:
- Use the same volume of hydrochloric acid
- Use the same length pieces of magnesium ribbon
- Make readings the same time
The factors that could affect the rate of reaction of my experiment are as follows:
· Concentration of acid
This could affect the rate of reaction because the higher the concentration of the acid then the more acid particles per 100cm3 so more collisions per second and then there will be more successful collisions per second.
· Temperature of the acid
If the starting temperature of the acid is different each time the speed at which the acid particles collide with the magnesium ribbon will increase more the higher the temperature goes. This means the acid particles move with more energy, which means they will collide with the magnesium with more energy, which will give more successful collisions per second.
· Surface area of the magnesium
If the magnesium had a bigger surface area each time the experiment was done, then the acid particles will have a bigger area to collide with, so more collisions will occur every second and the more collisions per second than the more successful collisions per second.
· Type of acid used
If you changed the type of acid then the rate of reaction would change. Hydrochloric, Sulphuric and Nitric acid all would produce a different rate of reaction, so if I do change the type of acid then all three kinds would produce a different set of results.
I predict that as I increase the concentration of the acid I am using, the rate of reaction will also increase.
What makes me say this is that, if I increase the concentration of the acid there will be more acid particles present in the same, fixed volume. If there is more acid particles present in the same fixed volume, then there will be more collisions per second, which will also mean that there are more successful collisions per second, resulting with a faster rate of reaction. This is proven to me through the collision theory, which states that, the more collisions between particles in a given time the faster the rate of reaction.
However, I also predict that as I double the concentration of the acid I am using, the rate of reaction will uniformly double as well.
I consider this the case because if you double the concentration of the acid, then I'd suspect the number of acid particles within a fixed volume of the solution doubles. If I rely my opinion on this, then I would expect that the number of successful collisions will also double, resulting with a chemical reaction double as fast.
Firstly I will carry out a preliminary test using the weakest and the strongest molars of acid. The preliminary work that I will be conducting is to find out the optimum length of magnesium ribbon and the optimum volume of hydrochloric acid. I also want to find out how long I should record my results for.
To do this I will be measuring out a volume of hydrochloric acid and a length of magnesium ribbon and reacting them together. If there is still some magnesium left over when it has stopped effervescing then I will have to increase the volume of hydrochloric acid.
If the reaction takes too long to finish then I will have to shorten the length of magnesium that I use, however if the reaction is too short then I will have to do the opposite and increase the length of magnesium that I use. The optimum rate that I am trying to find is a reaction that isn't too short but isn't too long, so I can get enough results to plot a good graph. I need to find the optimum volume of hydrochloric acid so that it is in excess after the reaction is over.
To further my experiment more I could of also found out how to keep the temperature change down. This is necessary because as the reaction is taking place the temperature will rise because the reaction is exothermic, and this could cause my results to be inaccurate as the temperature change will heat up the acid and give the acid particles more energy so they will move faster and collide with the magnesium with greater force causing more successful collisions per second.
From my preliminary results I have found that I will be able to carry out a good experiment if I use 5cm of magnesium ribbon and time it every 10seconds for one minute.
These are the steps I shall take to carry out my experiment:
- Set up the practical (as shown in diagram)
- Cut the magnesium into strips (exact same lengths) using a pair of scissors and ruler
- Measure the desired amount of acid in a beaker and then pour into the conical flask.
- Make sure that the stopwatch is set to zero
- Place a magnesium strip into the conical flask and place the delivery tube back in the top, as soon as this is done start the stop watch
- Observe the magnesium strip and the volume of gas in the gas syringe
- Record results at the desired times (times, volumes, cm of magnesium will be decided in the preliminary results)
- Repeat these steps until you experiment is finished.
We have decided to carry out this experiment 4 times for each of the different molars of acid. So this method will be carried out 20 times. The reason for repeating this experiment 4 times is so we can take an average, lets say we carried this experiment out 3 times, and one of the experiments gave us anomalous results then we would only have 2 sets of results left to find an average. This is why we have chosen to carry out this experiment 4 times.
In my first preliminary test we used only 2cm of magnesium ribbon and decided to record the results every 20secs, for 2minutes. I did this twice once for the 0.25 m and once for the 2m. My results are as follows:
(The magnesium ribbon had dissolved in 20secs giving us only one reading)
These results clearly show that the strip of magnesium ribbon was too small. We repeated a few more tests until we came to the conclusion that we would use 5cm of magnesium ribbon and we would record our results every 10 seconds for only 1minute.
The tables below show my results:
(The arrows indicate that if the experiment were left to continue over a longer period of time then the graph would go on)
The averages will be used to plot graphs.
(The tests, which are filled in with yellow are anomalous results therefore cannot be used for the experiment)
When carrying out the 1.5 molar test for the first time we found out that the magnesium ribbon dissolved too quickly so we decided to change this part of the experiment by recording the volume of gas every 5 seconds instead of every ten. This change will not affect our experiment in any way; it just means we will be able to plot point on the graph more accurately.
Although the magnesium had all finished before the end of the experiment that didn’t matter too much, as it was only important to have at least 3 reading to plot our graphs. If you study the graphs carefully you can see that the syringe didn’t always end up with the same volume of gas in it at the end, which it should have done. This may have been because of the room temperature or the size of the strip of magnesium. They may have not been the exact same size.
Again the magnesium ribbon had dissolved before the minute was up, but as previously said is not important, what I have noticed though is that there was a higher volume of gas in the syringe it reached above 100+ were as in the previous experiment it had dissolved at around 95 – 100. Eventually if all of the experiments would have been left until the magnesium had totally finished we should have ended up with the same volume of gas each time. But the size of the magnesium may have been slightly out, or the temperature may have changed, also the acid may have been contaminated.
(Graph is included on separate page)
I have taken the results from my experiments, found the average, plotted a graph and found the reaction rates for each concentration of hydrochloric acid. I can see from my results that the reaction rate increases as the concentration of acid increases. This is because there are higher amounts of hydrogen in the acid, so therefore more ions will collide with the magnesium. This also makes the reaction time faster overall, as the magnesium is used up faster. I can also see from my graphs that the reaction rate is faster at the beginning of the reaction, and as the magnesium is used up as the reaction progresses the rate decreases. In my hypothesis I said that the higher the concentration of the hydrochloric acid is, the faster the reaction will be, which is because of the collision theory. As I increased the concentration of the hydrochloric acid there were more particles in a certain volume of it, and therefore there was a greater chance of the H+ ions colliding with the Mg particles than in a lower concentration of acid. Therefore my hypothesis was correct.
The equipment I used to carry out this experiment was quite accurate for the investigation. The procedures used were, in my opinion the best for which we could possibly do with the equipment we had available to us. The evidence obtained was quite accurate.
The results were quite accurate for the equipment. To make it more accurate we could have taken smaller measurements and even used other methods, for example, the effects temperature and pH have on the sulphuric acid. Better equipment would give us better results e.g. it would be possible to use a highly-sensitive pressure pad connected to a computer with the pad inside a glass jar. As the gas moves into the jar it will compress against the pad and give off a more accurate reading. We would also need a one-way tube so that the gas cannot go back into the experiment.
My evidence appears to be quite reliable. The slight "fluctuations" in my graph may have been caused by reading off the measurements inaccurately, or from not keeping the temperature of the chemicals the same. However, I think that my results are sufficient to support my conclusion.
I think that to improve my results I should have ensured that the temperature of the hydrochloric acid is the same and that the strips of magnesium are exactly the same length. I could also test the effects that temperature has on the solution; the effects that increasing/decreasing the amounts of hydrochloric acid or the magnesium has.
I used information from school textbooks, and information from my teacher and also used the information available on the Internet to complete this investigation.