Rate of Reaction Between, Hydrochloric Acid and Magnesium

Aim

I aim the find out whether the concentration of acid effects the amount of gas given off in a set amount of time between hydrochloric acid magnesium ribbon.

Prediction

I predict that the higher the concentration, the quicker the gas will be given off. The atomic collision theory describes how the reaction quickens. I looked up the collision theory in a textbook "Science in Action", and found the following information.

The collision theory explains how a reaction can only take place when reactive particles meet, under certain conditions. It stands to reason that if you have more reactive acid particles (a higher concentration of particles) in the same place, they are more likely to collide with another particle, and react.

However, other aspects also affect the reaction, but are all made more likely if there are more molecules. For the molecules to react they have to have

Enough energy to achieve an activated complex. This energy that they need is activation energy.
The right orientation for the reaction to take place. ]

Also, when these reactions take place quickly, the produce a lot of heat, so if the reactions do happen more quickly with the stronger acid, they will be quickened still be the increase in heat. This increase in energy makes the particles vibrate quicker, and therefore collide with other particles, again increasing there chance of reaction.

I also think that when I double the strength of the acid, the rate of reaction will double because the amount of chorine atoms available for the magnesium to react with will have doubled, therefore the amount of chorine atoms reacting with the magnesium will double, and because there are twice as many particles reacting, the rate of reaction will be twice as much.

Apparatus

I will need

A test tube
A stop clock with an accuracy of 1/10s
Magnesium ribbon
Hydrochloric acid (0.5 – 2.5 mole)
Gas Syringe

Method

From looking at textbooks and asking my teacher, I found that there are two ways measure how quickly the reaction was taking place. I could either measure the amount of hydrogen given off, or I could observe the change in weight, as the hydrogen was given off. I decided to measure the ammount of gas given off, because I knew that a change in wright would require equipment that was acurate to many decimal places, and the school did not have this equipment.

I will set up the apparatus as follows

I have used this method as some research I have found that this is the most widely used method.

I will drop the 2cm magnesium into the hydrochloric acid and start the timer. I will then shout out the amount of gas (hydrogen) in ml at certain intervals, determined by the strength of the acid. I have changed the time intervals because I would have to mark the results every 2 seconds for the 2.5 mole acid, to get accurate results, but it would be pointless recording this accurately from the 0.5 more acid.

I am using 2cm of magnesium as my preliminary tests show that it keeps the reaction quick and is easier to work with.

This is a preview of the whole essay

I am using 2cm of magnesium as my preliminary tests show that it keeps the reaction quick and is easier to work with.

For my preliminary test I tested different lengths of magnesium ribbon to see how long it would take them to dissolve completely in acid. I did this so I would know how much magnesium ribbon to use in the final experiment. I used all the lengths with 0.5 and 2.5 mole acid. I did this because I wanted a sensible length for both, with the 0.5 mole time being short enough to be sensible, and the 2.5 mole time being long enough to make the recording accurate. I tested length of magnesium from 0.5 to 4, in 0.5 cm intervals. I did this because a 1cm difference would be too large to choose between and any smaller than 0.5 would make the measuring of the pieces a difficult task, and 1mm would not really make a difference in the amount of time taken. I would not repeat the experiment, because the results do not have to be very accurate, but they will only need to give me a rough outline of quickly magnesium dissolves.

These were my results.

As I didn’t repeat the experiment, I will put these results in a graph. The results should lie on straight lines. I will know from looking at the graphs, if I have any outliners.

As you can tell from the straight lines. There are not any outliers and my results are correct.

From this test I have decided that 2cm would be the best length to work with, as it provides a long enough time to record accurately the 2mole acid test, and it is also a reasonably short time for the 0.5 mole acid.

The reaction is as follows:

Magnesium + Hydrochloric acid → magnesium chloride + hydrogen

Mg + 2HCL → Mgcl2 + H2

Metal + acid → salt + Hydrogen

The magnesium displaces the hydrogen atoms from the chorine to give magnesium chloride salt solution and hydrogen gas. This happens because magnesium is more reactive than hydrogen.

I am using a gas syringe to measure the amount of hydrogen produced as I can then work out the rate of reaction, and give more accurate results.

I will then repeat the experiment using 0.5, 1, 1.5, 2 and 2.5 mole acid.

I will make this a fair test by

Ensuring that only the acid strength changes if

The size of the magnesium changes
The area of the magnesium changes.
The size of the test tube changes.
The ease of movement by the gas syringe changes

It will affect the experiment.

As always when working with high strength acid you should always wear goggles.

I will repeat each acid strength 3 times and average the results to hinder an anomalous result.

Results

It is difficult to see any trend using only a table so I have produced a graph that held all the data:

As you can see from the graph, the time at which the reaction stopped is more or less the same ml3. This is because I used the same amount of magnesium, and the same amount of acid, so I will get the same amount of hydrogen, no matter how strong the acid is.

You can also see from the graph the speed at which the reactions took place. 0.5 moles being the slowest and 2.5 being the quickest. This proves my prediction that the stronger the acid, the quicker the reaction

However, other aspects also affect the reaction, but are all made more likely if there are more molecules. For the molecules to react they have to have

Enough energy to achieve an activated complex. This energy that they need is activation energy.
The right orientation for the reaction to take place.

Also, when these reactions take place quickly, they produce a lot of heat, the where quickened in the stronger acids, because of the increase in heat. This increase in energy makes the particles vibrate quicker, and therefore collide with other particles, again increasing there chance of reaction.

This is why 2.5 mole acid had the quickest rate of reaction.

I then need to prove my prediction that the rate of reaction would double when the strength double, so I calculated how much gas per second was given off by calculating, a rate of reaction, using the formula.

Rate of reaction = Time taken stop reacting

Gas given off

Again to illustrate my results I put these results into a graph. I would hope for a completely straight line because when you double the acid strength, the rate of reaction should double.

As you can see from the graph and results table my prediction was correct until it came to the 0.5 mole acid.

These are the results rounded to show my prediction. Note that the 0.5 mole acid is missed out

I have several theories of why the 0.5 mole acid did not react as expected.

The temperature raised considerably on the other 4 due to the quickness of the reaction. This heat would quicken the reaction due to the fact that the molecules would be vibrating and therefore colliding more often (as explained before).
Sometimes the magnesium floated on the top of the acid. This was not a problem in most cases because the bubbles over lapped the magnesium so it also reacted from the top. However, due to the 0.5 mole slow reaction, this did not happen.

Because of all these inconsistencies, it is difficult to say how reliable my results are. I think that because the results are what I expected, and because I carried out the experiment with care, also the fact that I repeated the experiment many times makes it highly unlikely that the results are inaccurate enough to not be able to draw a valid conclusion from.

Evaluation

This was a good experiment because it clearly showed my prediction, and where it didn't I was able to spot the errors and am now able to make the experiment better. I worked as I kept a fairly high degree of accuracy, and the experiment had a high margin of error, due to the length of time some of the results could to take.

My results were fairly accurate but my error in the rate of reaction of the 0.5 mole acid could have been down to accuracy, but I seriously doubt it, as I asked around to see if other people had encountered the same problem. Everybody had.

I have several theories of why the 0.5 mole acid did not react as expected.

The temperature raised considerably on the other 4 due to the quickness of the reaction. This heat would quicken the reaction due to the fact that the molecules would be vibrating and therefore colliding more often.

Sometimes the magnesium floated on the top of the acid. This was not a problem in most cases because the bubbles over lapped the magnesium so it also reacted from the top. However, due to the 0.5 mole slow reaction, this did not happen.

To make my experiment more accurate I could have

Weighing the magnesium instead of just measuring the length of it. This was an obvious problem as I think my spread of results for the end amount of hydrogen given off was too high. I would have preferred if it were only 1 or 2 ml. But it was 4.33ml
Setting up another system for getting the magnesium into the acid. When I did the experiment I just dropped the acid in and attached the gas syringe as quickly as possible. The disadvantages with this were:

It was inaccurate
The start of the reaction would be when most gas was given off.
The time of attaching the gas syringe was always different.
The gas syringe often jumped forward slightly when I put it on.

Repeated the experiment more times.
Used more acid. This would shop the temperature problem as the temperature would be less likely to change, due to the increase in energy it would take to heat the water.

Because of all these inconsistencies, including the 0.5 mole acid result, it is difficult to say how reliable my results are. They are not accurate enough to study the experiment in-depth, however for a general hypothesis such as “Aiming to find out whether the concentration of acid effects the speed at which gas is given off, between hydrochloric acid magnesium ribbon” and because the results are what I expected, and I carried out the experiment with care, also the fact that I repeated the experiment many times, it is reasonable to presume that I can draw a simple conclusion like, “the higher the concentration, the quicker the gas will be given off “

If I were to do the experiment again I would change the way I inserted the magnesium into the flask. I think I would have a double chambered flask that would be able to have the wall removed. See diagram.

I could combine this idea with the alternative way I could do the experiment, as described in my planning. The method would be to:

Place magnesium and the acid in a flask, which is then plugged with cotton wool, to prevent any liquid splashing out, during the reaction. Next, the flask is weighed, then tipped up to let the reactants mix and a clock is started. The mass is noted at regular intervals, until the reaction is complete.

I would use the same volumes for all the chemicals in the new experiment, as I see no good reason changing them. I would expect the graph for the result to be much the same, but obviously with different axis labels and values. For example

In conclusion, the experiment did prove my prediction that the rate of reaction doubles with when the acid strength doubles.

Daniel Hill 10S 25308.doc Page of