Factors affecting the rate of a chemical reaction.

Introduction

A chemical reaction is a process by which one or more substances change into one or more other substances. Reactants are the substance(s) you start with and products are what you take out. A chemical reaction between two substances takes place when particles of these substances collide with each other. This either uses or releases energy. A reaction that releases energy is known as exothermic, where as a reaction that uses it up is called endothermic. This investigation looks at what affects the rate of a chemical reaction.

Factors affecting the rate of a chemical reaction

Temperature: Kinetic theory says that the more energy a particle has, the more it will vibrate. In a solid, particles vibrate but do not move. They are fixed in position and cannot move past each other. They are close together and the forces between them are strong. But, if you heat them then they become a liquid. In a liquid, particles can move past each other, and are not held in fixed positions. They are instead joined in small groups and the forces between particles are not so strong. Heat up these particles some more and they become a gas. In a gas, the particles have hardly any forces between them, and are a very long way apart. They are moving quickly and so spread out.

By heating substances, you are giving the particles more energy and move around more at a faster speed. If you heat the reactants in a chemical reaction then the particles will move around more and more collisions will occur. This means that the reaction will happen faster.

Concentration: If one of the reactants in a reaction were dilute, for instance hydrochloric acid in water, then the concentration of the acid would have a bearing on the rate of the reaction. If the acid were at a higher concentration, there would therefore be more particles per cubic centimetre of water. This would increase the probability of the acid colliding with the other reactant, thus speeding up the reaction.

Pressure: The greater the pressure of something, the closer the particles in that substance are. If you put two reactants together at a very low pressure, then the particles will be far apart and the collision of these particles will be less likely. In the same way, when reactants are put together under greater pressure then the likelihood of collisions occurring is increased. This speeds up the rate of the reaction.

Surface Area: If you are reacting a solid with a liquid, then the surface area of the solid will affect the rate of reaction. This is because the liquid can only react with the particles of the solid that it comes into contact with. This is easiest to understand if we look at it in 2 dimensions:

If we use the example of Calcium Carbonate in hydrochloric acid, we can see that whilst both of these pictures contain the same mass of CaCO3, picture1’s block has a perimeter of 80 units, where as picture2’s 4 blocks have a total perimeter of 160 units. This means that more of the substance is exposed and the reaction happens faster. So, the small the particles are, the greater the rate of reaction.

Catalysis: A catalyst is a substance which you can add to a chemical reaction that speeds it up without being used up itself. An example of this is the decomposition of Hydrogen Peroxide. On it’s own this reaction takes hours, but if you add some Manganese Oxide it will speed up rapidly. The catalyst may change during the reaction, but for it to be a catalyst, it will change back before the reaction is over. Catalysts speed up reactions, but are not used up themselves. They do not make up part of the final products of that reaction.

Plan

I will investigate how the concentration of an acid affects the rate of a reaction. This is the easiest factor to change and still be able to draw an accurate graph. Temperature is hard to quantify because all of the reactions that are possible to do in class are exothermic and this will affect our results. The reactions available to us are: Magnesium + Hydrochloric Acid, and Calcium Carbonate + Hydrochloric Acid. Surface area is also hard to quantify, as the surface area of magnesium ribbon will not change much when broken into smaller pieces, and the surface area of Calcium Carbonate chips is hard to measure. Pressure is impossible to measure with the equipment provided.

Concentration is easy to measure as it is just a case of diluting already prepared acid to make the concentration you want. This is also very accurate if done well. I will measure as many different concentrations of acid as possible to make a good graph. These concentrations will be: 0.25molar, 0.5molar, 0.75molar, 1molar, 1.25molar, 1.5molar, 1.75molar and 2molar. Since the surface area of Magnesium changes very little compared to it’s volume, we will use this in our experiment instead of the marble chips.

To work out how much acid and magnesium to use, we must know a ...

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To work out how much acid and magnesium to use, we must know a little bit about the mole. Quite simply, a mole is just an expression of a specific number the same as a dozen or a gross is. Where as a dozen is 12 somethings, a mole is 6 x 10²³ somethings. So, 1mole acid means that for every 1 dm³ of water there are 600,000,000,000,000,000,000,000 Hydrochloric Acid molecules. Avogadro discovered that 1 mole of something weighed the same number of grams as it’s atomic mass. So, since Magnesium’s atomic mass is 24, 1mole of pure magnesium would weigh 24 grams. The formula for Hydrochloric Acid is HCl, so one mole of Hydrochloric acid would weigh the atomic number of hydrogen (1) + the atomic number of chlorine (35). One mole of hydrochloric acid would weigh 36 grams, not including the substance it was dissolved into.

We can use this method and the formula of reaction between hydrochloric acid and magnesium to calculate how much of each substance reacts with each other exactly, and how much of each product there is. The chemical formula for magnesium + hydrochloric acid is:

Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen

Mg + 2HCl → MgCl2 + H2

From this, we can tell that 24g of Mg reacts with 72g of HCl to make 94g of MgCl2 and 2g of H2. We also know that 1mole of gas at room temperature occupies 24dm³, or 24 litres. We will be using the preliminary test to find out how to measure this reaction, and one of the methods we will be testing involves collecting gas, we can see that 24 litres is much too much to produce and measure in the lab. For the sake of accuracy, we will use 10 cm³ (10ml) of acid because it is easy to convert from 1 dm³ into this measurement. We will use 0.25g of Magnesium because as well as it being a nice number to work with, I have calculated that with 0.75g of HCl it will produce 0.25dm³ (250ml) of hydrogen. 10mls of 0.25molar HCl will produce less than this, but I don’t know how much less. This is something else that I will use my pre-test for, and the amount of gas produced will help me decide on my method of collecting data. I will not change the temperature of any of my experiments; they will all be done at room temperature. I will not use any catalysts, nor will I change the pressure at which the experiments are conducted. The surface area of the Magnesium will change only marginally between experiments anyway, as we will be using filings, not ribbon, because they are easier to weigh out.

Preliminary Testing

The two methods of collecting information that I need to choose between are: temperature and gas collection. This is how I will set up the temperature experiment:

Every 15 seconds for 5 minutes we will read the temperature on the thermometer. I expect to see a temperature change since this is an exothermic reaction. My only worry with this experiment is that when the 2molar reaction is taking place, the heat given off will affect the other particles and speed up the reaction, distorting our results. We are testing with 0.25, 1 and 2 molar acids because these are the top, middle, and bottom values that we will be testing.

This is how we will set up the gas collecting experiment: (See next page)

Every 15 seconds for 5 minutes we will read off each measuring cylinder how much gas has been produced. We are using 250ml measuring cylinders, and as I explained in my earlier calculations, this will easily be enough to contain all the Hydrogen produced. My only concern is that the 0.25 molar experiment will not release enough hydrogen to make a good graph.

These are my pre-test results:

From these results, we see that the gas collecting is the best way of collecting results in this experiment, as it will only go one way, where as the temperature drops at the end of the reaction. When I am drawing graphs of the results, collecting gas will give me a graph like this graph A and temperature will give me a graph like graph B.

Graph A is easier to read from, as the rate of reaction at any given point can be found just by measuring the gradient. I would not know how to measure the rate on Graph B, and after the peak, the heat would take a long time to fall anyway. Also, it is much easier to read off the measuring cylinder than the thermometer, which is another reason for choosing the gas collecting method.

Range

The greatest concentration of acid available to me is 2 molar, and from this I can make any lower concentration simply by diluting the acid with water. I will measure as many different concentrations as possible within the time given to give me a good graph. These concentrations will be from 0.25 molar acid all the way up to 2 molar acid with intervals of 0.25 moles.

Fairness

To get meaningful results, the other factors that could affect the rate of this reaction must be controlled. This should not be too difficult.

The experiments will all be conducted at room temperature, so the only influence on temperature will be that produced by the reaction itself.

The pressure will remain the same, as there is no real way that I can change it anyway.

The surface area of the magnesium filing may change marginally with each experiment, but the change will be so minimal that it will not affect our results.

The affect of catalysis is easy to control, since I will just not put a catalyst into the boiling tube!

Reliability

Reliability can be achieved in this experiment quite easily just by being careful when taking results and by checking all of my equipment and materials before use. I will:

Make sure that my boiling tube is dry so as not to dilute the acid.

Be careful in weighing out the magnesium and make sure that it all goes in the boiling tube.

Start my timer as soon as the acid goes in the boiling tube with the magnesium, not when the bung goes in.

Check my results against those of someone else doing a similar experiment so as to check that the pattern is the same.

Take repeat results to make an average. This should eliminate any irregular results and make my graph more accurate.

Keep a sketch graph as I go along. This should identify any irregular results early so as I can re-take them to improve the accuracy of my graph.

Prediction

Using the theory I already know and my pre-test results, a prediction should be quite easy to make. The simple prediction is that the higher the concentration of acid, the higher the rate of reaction will be. My graph will look something like this:

The numbers to the right of each line represent the concentration of acid used in that experiment.

Results

For full table of results, see Appendix 1. This is a summarized table where all the values are averages. All of the values are correct to the nearest whole number.

Observations

None of my sketch graphs seemed to differ too much from the lines I had predicted, but when I put together the 3 averages from each strength of acid, it is clear that some of my experiments were slightly off. For instance, I have not counted test 2 from both the 1 molar test and the 1.75 molar experiments because the results from those differ greatly from the other two tests and affect the average result. The above table is the averages table without these ‘incorrect’ results. The only other observation I made is that as the concentration of acid increased, so did the heat produced by the experiment. This could not be changed as it was an exothermic reaction, but I don’t think that the heat produced affected our experiment.

Graphs

Graph 1 is the graph of all of my results, given to the nearest cm³. Graph 2 is the only first 1:30 minutes, and is a magnified version of the first part of Graph 1. The tangents on Graph 2 are explained in the conclusion. The circled points are points which I think are incorrect. There are no error bars on the graphs because it is hard to quantify the amount of possible error when diluting the acids, and the possible error in collecting gas is ½ cm³ either way which is too small to fit on the graph.

Conclusions

All the lines on my graph show the pattern I predicted. The steeper that gradient is, the faster the reaction is happening. This suggests that the reactions were happening faster with the greater concentrations. It also shows that the reactions happened early on in the 5 minutes that we were timing the experiment for. To compare the rates, I will draw a tangent on the line at 30 seconds and measure the gradient of that to get the gradient at that point.

The gradient of any straight line can be calculated as: Rise_

Tread

In each case here, the tread is 15, as I have drawn the tangents over the 15 seconds around the 30-second point. The rises are different for each one, and are shown here:

0.25 molar: Gradient = Rise = 3 = 0.2

Tread 15

0.50 molar: Gradient = Rise =10 = 2/3 or 0.333…

Tread 15

0.75 molar: Gradient = Rise =13 =13/15 or 0.8666…

Tread 15

1.00 molar: Gradient = Rise =20 = 1 1/3 or 1.333…

Tread 15

1.25 molar: Gradient = Rise =23 =1 8/15 or 1.5333…

Tread 15

1.50 molar: Gradient = Rise =31 =2 1/15 or 2.0666…

Tread 15

1.75 molar: Gradient = Rise =26 =1 11/15 or 1.7333…

Tread 15

2.00 molar: Gradient = Rise =24 =1.6

Tread 15

The fact that the gradients for 1.75 molar and 2 molar acid are smaller that 1.5 molar acid suggests that these reactions had already begun to slow down. For greater accuracy I will draw some more tangents over the 15-second mark and attempt to measure the gradients there. In all cases I expect them to be steeper.

0.25 molar: Gradient = Rise = 5 = 1/3 or 0.333…

Tread 15

0.50 molar: Gradient = Rise =15 = 1

Tread 15

0.75 molar: Gradient = Rise =20 =1 1/3 or 1.333…

Tread 15

1.00 molar: Gradient = Rise =27 = 1.8

Tread 15

1.25 molar: Gradient = Rise =32 =2 2/15 2.1333…

Tread 15

1.50 molar: Gradient = Rise =40 =2 2/3 or 2.666…

Tread 15

1.75 molar: Gradient = Rise =44 =2 14/15 or 2.9333…

Tread 15

2.00 molar: Gradient = Rise =48 =3.2

Tread 15

The gradient in this case is the same as the speed of the reaction. Obviously the speed of this reaction is positively affected by the strength of the acid, as I predicted. The speed of this reaction can be measured in cm³ of H2 produced / second. I will draw a third graph of rate of reaction against the strength of acid to see if the line I draw has an obvious pattern.

I have drawn a straight line of best fit to this graph up to the point of 1 molar acid. The graph then begins to steady out, as I believe that beyond that point, the speed of the reaction makes much of it happen before the 15-scond mark. This flattens out the gradient. To be more certain of this I would have to take more measurements, both above and below 2 mol/dm³ of acid. The intervals of these measurements would have to be closer together. From the line that I have drawn though, I can assume that below 1 molar acid, the amount of hydrogen produced in the first 15 seconds of the reaction relates directly to the strength of the acid. Using the gradient of the straight line, I can tell that after 15 seconds of a reaction, the amount of hydrogen produced, in cm³ is 1 5/6 x the concentration of acid in mol/dm³.

In conclusion, the concentration of the acid does have an effect on the rate of the reaction. The more concentrated the acid is, the faster the reaction goes.

Evaluation

My first two graphs look reliable, and I have a fair idea about why the third one doesn’t look so reliable. I have plotted enough co-ordinated close enough to the lines to be confident that my conclusions are correct. There is very little spread in my results, which again makes me confident that my conclusions are correct.

There are two big possible sources of error. One is human error in reading the level of gas in the measuring cylinder. This is a minimal source of error, as we made sure that we were very careful in reading the cylinder. The possible error is about 1cm³ either way, which would not affect my results or conclusions at all. The other possible source is in the actual concentrations of the acids. They may not have been exactly diluted as we expected, which could have produced major in accuracies. This is particularly evident in Graph 1. The amounts of gas produced after 5 minutes should be evenly spaced between concentrations. There is a big gap between 1.25 molar and 1.5 molar acid, and a very small gap between 1.5 and 1.75 molar acid, suggesting that the 1.5 molar acid was actually at a higher concentrate than we first thought.

One possible reason that several of the points for the 15-second mark are below the lines drawn for them is that we lost some of the gas as we put the bungs in. This resulted in the difference between the x-axis and the first point being smaller than it should have been. The differences between the first and second points are correct because the bung then stayed in for the rest of the experiment.

Cambridge Co-ordinated Chemistry by Jones, Jones and Acaster agrees that increasing the concentration of an acid will indeed speed up a reaction. It does not give a formula or equation for this, so I cannot check my graph 3 against anything else to see if it is correct.

On the whole I think that this has been a successful experiment. I have collected the results I predicted, and proved that increasing the concentration of a reactant will increase the rate of the reaction. If I had more time and resources, I would take more measurements of this reaction from different concentrations, both above and below 2 molar acid. This would allow me to see if my prediction for the line on Graph 3 is correct or not.

-Mike Stead