Chemistry investigation Essay
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Aim: I am going to carry out an investigation measuring the rate of reaction between hydrochloric acid and magnesium. The independent variable will be the molar of the hydrochloric acid; I will be diluting it with water to change the concentration. The dependent variable could be one of four things; Temperature change, Mass loss, Gas produced using a burette or gas produced using a gas syringe. The dependent variable of the primary experiment will be decided once I have performed four preliminary experiments each with the different dependent variables.
The results of these preliminaries will help me determine which factor is the most accurate to measure.
Background Information: As I am planning to investigate the rate of reaction by changing the concentration of the acid I should take into consideration how the reaction is affected by concentration.
If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed.
As you increase the concentration of the acid, there are more acid particles in the same volume. Therefore there is a greater chance of acid particles colliding, and reacting, with particles on the surface area of the magnesium ribbon.
Here is an example of how the rate of reaction will increase if the concentration gets higher:image00.png
With this in mind it is clear to see that as I decrease the concentrate of the acid by adding water to it I will be decreasing the rate of reaction. There is a positive correlation between the two variables.
The experiment equation that will effectively happen is:
MAGNESIUM + HYDROCHLORIC ACID — MAGNESIUM CHLORIDE + HYDROGEN
Mg + 2HCl —- MgCl2 + H2
Magnesium; Magnesium is a light, silvery-white, and fairly tough metal. It tarnishes slightly in air, and finely divided magnesium readily ignites upon heating in air and burns with a dazzling white flame. Because serious fires can occur, great care should be taken in handling magnesium metal, especially in the finely divided state. Water should not be used on burning magnesium or on magnesium fires.
Hydrochloric acid; Hydrochloric acid is the aqueous solution of hydrogen chloride gas (HCl). It is a strong acid, the major component of gastric acid, and of wide industrial use. Hydrochloric acid must be handled with appropriate safety precautions because it is a highly corrosive liquid.
Hydrogen; when mixed with oxygen across a wide range of proportions, hydrogen explodes upon ignition. Hydrogen burns violently in air. It ignites automatically at a temperature of 560. Another characteristic of hydrogen fires is that the flames tend to ascend rapidly with the gas in air.
Magnesium chloride; this is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2 (H2O) x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water. Anhydrous magnesium chloride is the principal precursor to magnesium metal, which is produced on a large scale.
Method: In this preliminary experiment I choose not to alter anything but to observe how the temperature increased within the time of the reaction. I used 10ml of hydrochloric acid and 3cm of magnesium ribbon. I carried out three repeat measurements to gain an idea of what the results would look like if compared.
Evaluation of Preliminary Methods
Each of the experiments above has limitations that strongly influenced the results. This has given me an idea of what degree of accuracy each method can be measured to and if they are suitable to collect data for the primary experiment.
Temperature: Measuring the temperature is a simple way of seeing where the rate of reaction begins, peaks and ends. It is a clear indication to how the magnesium reacts and at what temperature. There are, however, some inaccuracies to the formation of the set up. Some of the thermal energy detected by the thermometer is sometimes transferred into the glass of the conical flask which led to an inaccurate reading. Furthermore the thermometer is slow to respond to the rise in temperature and also only gives reading in whole numbers so slight elevations in heat are difficult to read and thus inaccurate.
Mass Loss: Measuring the mass at different points in the experiment provides markers to see how much magnesium and hydrochloric acid has been reacted. The electric scale I used was too sensitive to get a general reading; this affected the results severely. The mass would sometimes increase during the reaction due to a small piece of debris getting onto the scale or because a gust or air; consequentially the results were askew.
Gas Produced using a burette: This is a very good way of measuring the rate of reaction. The measurements are clear and can be recorded to a decimal place which makes the results very accurate. There are some limitations, however, to using a burette. When the gas is produced it takes time for the hydrogen to travel along through the tube and up into the burette. This extends the time of the reaction slightly because not all the gas has been collected which the magnesium has been dissolved. Furthermore some amount of hydrogen have escaped the burette and simply floated to the surface of the water not being accounted for. This is a small inaccuracy but can still alter the results.
Gas Produced using a gas syringe: This is a very accurate piece of equipment. It does not allow for gas to escape and records the data consistently and with speed. The only limitations in using a gas syringe are that the reading can only be reading in whole numbers and that the syringe sometimes collects water vapour as well. The water vapour, however, can be eliminating by adding an extra conical flask to the tube which collects the vapour as it is slightly cooler.
The gas syringe is the most advanced piece of equipment in each of the preliminary methods and also is the most accurate in recording the data. Therefore to calculate the rate of reaction between magnesium and hydrochloric acid my dependent variable will be the gas produced using a gas syringe.
Hypothesis: Now that I have determined what I am going to measure and how I can now make my prediction over what I think will be the final outcome of the experiment.
Rate of reactions are all based on the collision theory. This states that the more collisions in a system, the more likely combinations of molecules will occur. As a result of this the reaction will accelerate, and the rate of that reaction will increase. The concentration of a substance will raise the number of collisions and thusly speed up the rate of reaction.
On this basis I believe that if the molar of the hydrochloric acid is decreased there will be less gas produced and so the rate of reaction will slow. Here is a diagram:
We can see in the figure above that if the molar is halved then there are fewer particles in the water so fewer collisions can happen. The gas produced will therefore be reduced and the overall time of the reaction will decrease.
How it is controlled
Why it is controlled
Concentration of Magnesium
By measuring out a 3cm piece of magnesium each time I take a repeat measurement.
I am measuring the concentration of the hydrochloric acid not the magnesium.
Concentration of Hydrochloric acid
I will start with 10ml of hydrochloric acid and replace 2ml of it with water every time I want to reduce the concentration.
I want to observe what effect it will have on the rate of reaction.
Adding an extra, cooler conical flask to collect the water vapour.
I am measuring the gas produced between hydrochloric acid and magnesium; water vapour is not a product.
By putting the flasks in a thermostatically controlled water bath for 5 minutes before each experiment.
I am measuring the concentration and do not want the higher temperature to give the particle more energy to collide.
500ml conical flasks- To hold the magnesium and hydrochloric acid reaction and to collect the water vapour.
Conical flask bung (with hole for tubing) – To allow for gas only to travel through the tubing.
Glass tubing- To direct the flow of gas.
100 ml² gas syringes- To measure the gas produced.
Retort stand- To hold the gas syringe.
Clamp- This is attached to the retort stand.
50 ml measuring cylinder- To measuring the hydrochloric acid accurately
stop-clock- To measure the time of the reaction
10ml of Hydrochloric Acid plus varying amounts of water- To measure the concentration of the acid
Magnesium ribbon (3cm per experiment) – To react with the acid.
Ruler and scissors – To measure and cut the magnesium ribbon.
Here is a diagram:
Other apparatus: image10.pngimage09.pngimage11.png
Obtaining precise and reliable results
My experimental design permits me to take very accurate measurement; however, the only fault in its design is that the syringe only measures in whole numbers. This should not pose as a problem because I will take lots of reading to compensate. I will then find the mean of easy set of results so that I can compare them accurately in addition to this I will also take the range of the results which will allow me to observe whether they overlap.
The limitations of my other apparatus are common. Although my stop can measure to a 10th of a second my reaction time will be a lot slower then this. The ruler and scissors should also be taken into consideration, measuring and cutting the magnesium could cause some variation. The measuring cylinder also has to be estimated to a rough degree when pouring the hydrochloric acid. All of these small inaccuracies can add up to a large error so I need to be very consistent so each result is rounded to the same degree of accuracy.
Risk assessment: during the practical various measures must be taken to ensure the experiment is carried out safely. These measures are;
• Always wear safety goggles (at all times) to ensure no chemicals make contact with eyes.
• Wear laboratory aprons to ensure there are no spillages onto clothing.
• Use a bench mat to stop spillages onto bench
• Avoid contact of acid on skin – it is corrosive. If acid does touch skin it must be immediately washed off.
• Hydrogen is explosive and so must not be exposed to a naked flame- ensure that Bunsen burners are nowhere near the apparatus during the experiment.
• Basic Laboratory Rules i.e. do not run in labs, No food or drink allowed etc.
• No concentration of hydrochloric acid greater than 2 molar, anything larger is dangerous.