Preliminary experiment
In the preliminary experiment we started by timing how long the 2 cm of magnesium took to react with the hydrochloric acid. But although we found that the reaction occurred quicker with the higher concentration of acid we realised that this method was very unreliable because we had to rely on our eyesight and reflexes to find out the speed of the reaction. We needed to trust our reflexes but as the speed was too fast we weren’t sure at which point the reaction was over. When we saw that there was no more action (no more bubbles of hydrogen released) we stopped timing, but the reaction may have finished before our timing.
In stead we collected the gas in a measuring cylinder filled with water. We timed how long the reaction took to create 10cm3 of hydrogen in the tube. We found that this method was a lot more reliable than by doing the experiment before as we could easily see when the required amount of gas was produced. So after doing the preliminary experiment, I am still none the wiser about how the results will turn out. All I found out was the best method of collecting the data which is by timing how long it takes for 10cm3 to accumulate.
Aim To study the affect of changing the concentration of hydrochloric acid on the rate of reaction between hydrochloric acid and magnesium.
Fair Testing
Make sure that the concentration of acid is the required amount and adds up to 100cm3
Make sure the magnesium is cut to 2cm each time so that the variable remains constant
Method
- Cut a piece of magnesium 2cm long and fill a conical flask with the correct concentration of water / hydrochloric acid (see in table below). Measure the amounts of each substance by using a measuring cylinder.
The amounts must add up to 100 cm3 in each solution to ensure a fair test. Connect a tube to the conical flask, which is turned upside down a bowl of water ( the measuring cylinder must be filled with water and have no air bubbles, as this is the cylinder that we use to time how long it takes for 10cm3 of hydrogen). The tube must enter at the bottom of the measuring tube. Make sure that there is no air inside the tube.
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Add the magnesium into the mixture of water and hydrochloric acid and time how long it takes for the gas to fill 10cm3 in the measuring tube. Do the experiment three times to make sure your answers are reliable. More results can help to show a pattern emerging and can also show up any anomalous results.
- Make sure that all of the different concentrations of hydrochloric acid have been done but you will need to replace the substances each time so that the raw materials are the same so that there is no margin of error.
To make sure that this test is fair, we need to do many things. We need to check the measuring on water and hydrochloric acid carefully and at eye level due to the fact that liquid looks like it is a different amount, depending where you look at it.
Fair Testing
Make sure that the concentration of acid is the required amount and adds up to 100cm3
Make sure the magnesium is cut to 2cm each time so that the variable remains constant
Make sure that you place the bung over the conical flask as soon as possible to stop any gas escaping.
We also need to check that we have cut the magnesium accurately to 2cm. We also have to time the reaction as soon as the magnesium touches the water until 10cm3 of gas has filled up the measuring tube. Before the actual experiment we have to make sure that there is no air in the measuring tube, as this will make the reading of the results harder as you won’t know when 10cm3 of hydrogen has been collected.
Results
Because the results are so spread out, I will have to put the time taken into a rate of reaction. This is done by 1/average time. This will enable the user to see the rate of reaction and how it increases with the concentration of acid.
Conclusion
As you can see from the graphs, there is a link between the concentration of hydrochloric acid and the rate of reaction. The higher the concentration of hydrochloric acid the faster the rate of reaction. Graphs 1 shows all three sets of results without 0.2 mols/dm3 of acid mainly to show the deviation in results. I chose to do this as when I showed the complete set of results you couldn’t see the deviation in results, as the times were so close. Graph 2 shows the average time on the same graph, excluding the result for 0.2mols/dm3. Graph 3 shows the average time of all results including the results for 0.2 mols/dm3. This too shows the trend that the lower the concentration of acid the faster the reaction goes. Graph 4 shows the rate of reaction compared to the concentration of acid. This shows the trend that the higher the concentration of acid, the higher the rate of reaction due to more collisions because of the increased number of acid particles.
My hypothesis was proved to be correct. The higher the concentration of hydrochloric acid, the quicker 2cm of magnesium will react. The results show that with more particles of hydrochloric acid per cm3, the faster the magnesium will react due to the increased probability of collisions. This is proved by the results, which shows how long it took for the react to produce 10cm3 of hydrogen gas.
The formula for the reaction is as follows
Hydrochloric acid + Magnesium Hydrogen + Magnesium chloride
2HCL+ 2Mg H2 +2MgCl
Hydrogen and magnesium chloride were the products in this reaction.
Evaluation
You can see in this diagram that because there are more hydrochloric particles and because there are more particles there are going to be more collisions with the magnesium ribbon. Because there are more collisions the rate of reaction speeds up.
The lower concentration of hydrochloric acid means that there are fewer collisions with the magnesium ribbon. This means that the rate of reaction is decreased because the collisions are less frequent.
I noticed that the results show a pattern where when the concentration of acid reaches the lower values, such as0.2 and 0.4 mols/dm3 the time taken for 10cm3 to accumulate increases. The difference between the results is at first quite small but later on the gap increases. Such as 0.4 to 0.2 mols/dm3 has a difference of 112 seconds whereas 0.6-0.4 has a difference of 48seconds. This may be because the amount of hydrochloric acid is so small that when the particles collide with the magnesium, then it may be another 10 seconds before they collide again. Also with less particles, more energy will be used up to lesser effect. What I mean by this is that if you have a small amount of a reactant such as hydrochloric acid, and you have the other reactant magnesium as a constant ( the mass stays the same when the concentration changes ) the small amount of hydrochloric acid will have to react with more particles than itself compared to the higher concentrations of hydrochloric acid. (eg proportionately 1 hydrochloric acid particle to every 50 magnesium particles where as higher concentrations may be more like 1:2 . This is because as they collide the particles change direction it will take longer to produce 10cm3 of hydrogen than if you were to have more particles.
Our test was fair. We measured the amount of hydrochloric acid, water and magnesium as accurately possible, when it came to the liquids we measured at eye level. Also when we were timing how long it took for 10cm3 of hydrogen to be produced, we measured this at eye level too so that we could get an accurate measure of time. There was a slight deviation in timing, as we had to take into account the human reaction time being slightly slower than the actual experiment time.
When we added the magnesium we had to take the lid off of the beaker, which may have lost some gas. We also had to time as soon as the magnesium left our hand to fall into the hydrochloric acid. This may not be accurate but we had to make a rough guess to when to time.
To improve the experiment I would test the concentrations of hydrochloric acid against other reactants such as copper, iron and lead and see if the relationship is the same as it was the reaction with magnesium. I would also compare the results to see if the hydrochloric acid reacts slower or faster at the same concentration but reacting against other elements. This will help to determine the nature of reaction. Such as the how the number of electrons in the outer shell has an affect on the speed of the reaction.
There were three results in the middle of the experiment (1.6,1.4 and 1.2), which are off the predicted pattern. This may be because we used a different roll of magnesium. This roll was (unbeknown to us) cleaned and rubbed down. This exposed the magnesium surface to the acid, there fore increasing the rate of reaction due to the impurities being rubbed off of the magnesium so the magnesium could react with the acid straight away. With an extra layer of impurities, the reaction will take longer as there is more for the acid to do to get to the magnesium and for the reaction to start. The rest of the experiment we used uncleaned magnesium as we thought that if we rubbed the magnesium down to get rid of the “dirt” on the surface then we may decrease the amount of magnesium and make it thinner.
This happens due to the fact that there are more frequent collisions between the reactants, hydrochloric acid and magnesium.
This is illustrated in the diagram below.