Chemistry Coursework Plan: The aim of this experiment is to analyse and understand rates of reaction.

Chemistry Coursework- Plan

Aim: The aim of this experiment is to analyse and understand rates of reaction. In order to understand this, I have experimented using different concentrations of hydrochloric acid and magnesium strips. Timing how long it takes for the magnesium to dissolve. This experiment and following information will give me a clear understanding of this

Preliminary work: Before this experiment, we carried out a number of other experiments in order to investigate the affect of things like concentration and temperature had on a reaction between an acid and usually marble. We used the laptops using a programme called “Science investigation”. We investigated the time it took different concentrations of acid to dissolve pieces of marble.. We also investigated different ways of looking at the results for example, recording the time increase during the actual experiment converting this time into the rate of reaction. This helped give me an idea of the type of experiments I was doing and helped me decide what I wanted to do and how I wanted to do it.

My pilot study was extremely important as well because it allowed me to check how well my experiment would be done and if any changes needed to be made. I changed my experiment. I used magnesium instead of marble, because it is more reactive. During the pilot study I was also able to see which concentrations of acid would be ideal in order to collect a more accurate set of results.

These previous experiments and plans were extremely helpful and ensured that I would be able to carry out a smooth and efficient experiment. Understanding this I was able to make a hopefully right hypothesis.

Reaction between HCl and Mg

In the reaction between hydrochloric acid and magnesium ribbon, the hydrochloric acid will dissolve the magnesium and produce hydrogen gas. All chemical reactions involve reactants which when mixed may cause a chemical reaction which will make products. In my experiment 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 rate of reaction between the product and the reactant will increase or decrease depending on certain factors. The factors that may affect the rate of reaction are:- temperature of the Hydrochloric Acid, mass of the magnesium ribbon used, concentration of the Hydrochloric acid, surface area of the magnesium ribbon used
All of these factors will change the rate of reaction because of the Collision Theory. This is a theory that is used to predict the rate of a reaction. The Collision Theory is based on the idea that for a chemical reaction to take place, it is necessary for the reacting particles to collide with each other with enough energy to break or form new bonds between the other particles, which is called a successful collision. If when they collide and they do not have enough energy to break or form new bonds then they will simple bounce of each other, causing an unsuccessful collision.

Collision Theory

The theory that we use to explain how different variables change the rate of reaction is called the collision theory.

For a reaction to take place, the particles of the substances that are reacting have to collide. If they collide, with enough energy then they will react. The minimum amount of kinetic (movement) energy that two particles need if they are going to react when they collide is called the activation energy

There are therefore two main ways of increasing the rate of a reaction:

1) Increase the number of collisions
2) Increase the amount of movement (kinetic) energy so that more collisions lead to a reaction

The effect of temperature

When we increase the temperature at which a reaction is taking place, the particles move more quickly. This has two effects:

1) More collisions take place

2) When a collision occurs, there is more chance that the collision will lead to a reaction, because the amount of energy is more ...

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There are therefore two main ways of increasing the rate of a reaction:

1) Increase the number of collisions
2) Increase the amount of movement (kinetic) energy so that more collisions lead to a reaction

The effect of temperature

When we increase the temperature at which a reaction is taking place, the particles move more quickly. This has two effects:

1) More collisions take place

2) When a collision occurs, there is more chance that the collision will lead to a reaction, because the amount of energy is more likely to be greater than the minimum amount of energy needed.

At a lower temperature, the number of collisions is lower because the particles are moving more slowly. Also when a collision occurs, there is less chance of a reaction taking place because the movement energy in the particles is less

The effect of concentration

The concentration of a solution is how strong the solution is. For example, if we consider the reaction between marble chips (calcium carbonate) and hydrochloric acid:

Calcium carbonate + hydrochloric acid —> calcium chloride + water + carbon dioxide

Then a stronger acid contains more acid particles and less water particles than a weaker acid.

Increasing the concentration of a solution leads to more collisions (greater frequency of collisions) so the rate of the reaction goes up.

In a less concentrated acid, the number of collisions is low, so the rate of the reaction is slower.

Hypothesis: I predict that as the concentration decreases the time for the magnesium to dissolve will gradually increase. I think this because magnesium will obviously react faster and better with a higher concentration. As we add the distilled water the time will increase, this is because we are diluting the acid and thus it becomes less efficient to dissolve the magnesium.

Predicted graph:

In order to carry out this experiment successfully, I will have to choose the most accurate equipment.

Equipment:

12 test tubes (including repeat)
Hydrochloric acid concentrations: 1.0/2.0/2.5/3.0/3.5/4.0 (molar)
12 magnesium strips (2.2 cm each)
Measuring cylinder (20 cm3)
Pipette (to extract excess diluted water or hydrochloric acid)
Stop watch
Goggles (safety precautions)
Distilled water
Test tube rack
Burettes

Method:

Get together all the equipment you need.
Measure out all the different concentrations of hydrochloric acid. (using burette for accuracy).
Place these in test tubes.
Cut out equal strips of magnesium ribbon. (2.2cm)
Place a strip in your first concentration.
Time it until the fizzing stops. (When the reaction has ended).
Continue doing this for the rest of the concentrations.
After you have your results, record them in a table.
Repeat the experiment in order to have accurate results.

Results:

My table to record the results will look like this.

In order to make these concentrations, I will have to dilute them. This is the amount of both hydrochloric acid and distilled water I will use.

Variables:

Temperature:

The temperature depends on how fast the particles move around. When the particles vibrate faster or slower, there will be more or less of a chance that two particles from the separate reactants will collide. This means that there will also be a faster or slower over-all reaction time. It is essential that I keep this variable the same; I will use all my hydrochloric acid at room temperature.

Ribbon length: (2.2 cm)

The surface area of reactants also has a noticeable affect on the results. The length of the magnesium ribbon must be the same. If a piece had a higher surface area, it would react faster because there would be more of its particles on the surface in contact with the hydrochloric acid

Purity of water used:

I haven’t used tap water in this experiment because there might be impurities in it, which affect the results for example, it might be slightly acidic I have therefore chosen to use distilled water in order to be absolutely sure that it won’t affect the results.

In this experiment my input variable is the concentration of hydrochloric, which will be changed to different concentrations in order to see the different effects it has.

My output variable is the time taken for the magnesium to dissolve or the rate of reaction. This is what I will be measuring to see what affect changing the concentration has had on the experiment.

I think overall my plan is a good one as it basically allows for a quick, efficient and fair investigation into how different concentration of hydrochloric acid can affect the rate of reaction. I have tried to be a precise as possible with any mistakes or miscalculations, which might occur so that my results are accurate.

Results

These were the results after carrying out the experiment.

Both the graph and the results above supports my original prediction of: the more concentrated the acid the faster the rate of reaction because it shows the time difference between the different strengths of acids. In a higher concentration there are more acid particles to react with the magnesium ribbon and therefore it is eaten away faster.
I conclude that changing one factor does have a significant effect on the rate of reaction as we have seen.

Looking at the set of results obtained, you can clearly see that they all follow the expected pattern. This is pattern suggests that the reaction rate increase when the concentration of the acid increases because if you increase the concentration of the acid you are introducing more particles into the reaction which will in turn produce a faster reaction because there will be more collisions between the particles which is what increases the reaction rate.

Analysis of results

From the results in the table and the graph we can see a steady increase in the rate of reaction as the concentration of the acid decreases. This complies with my prediction. The graph shows that there is an increase in the rate of reaction as the concentration increases because the graph has it's largest gradient or it is steepest at this point.). Also we can see that as the reaction continues the concentration of the reactants decrease and so does the rate of the reaction as we can see the decreasing gradient on the graph steadily falling and coming to a stop when the reaction is complete and the magnesium has completely disappeared.

Conclusion

I can conclude that if you double the concentration of the acid the reaction rate would also double, this is because the ions are closer together in a concentrated solution. The closer together they are, the more often the ions collide. The more often they collide, the higher the chance of a reaction between the magnesium and the hydrochloric acid. Also because there are more particles in the solution which would increase the likelihood that they would hit the magnesium so the reaction rate would increase. The graph gives us a good device to prove that if you double the concentration the rate of reaction doubles. If you increase the number of particles in the solution it is more likely that they will collide more often. The activation energy of a particle gets higher with heat, the particles which have to have the activation energy are those particles which are moving, in the case of magnesium and hydrochloric acid, it is the hydrochloric acid particles which have to have the activation energy because they are the ones that are moving and bombarding the magnesium particles to produce magnesium chloride.

Evaluation

There are many reasons why our results could have been slightly wrong.

When the reaction takes place bubbles of H2 are given off, which might stay around the magnesium, which therefore reduces the surface area of the magnesium and so the acid cannot react properly so this affects the results.
We could have controlled factors in the investigation better (e.g. the stirring of the solution because if this is not done properly it can lead to incorrect results). 3. Using larger concentrations of acid would give a bigger more accurate conclusion instead of just using 10ml test tubes use 1litre test tubes, this way graphs would be more spaced out and give an accurate form or curve.

Before looking at the factors that can alter the rate of reaction, we must consider what happens when a reaction take place.
First of all, the particles of the reacting substances must collide with each other and, secondly, they need a certain amount of energy to break down the bonds of the particles and form new ones. This energy is called the activation energy or Ea. If a collision between particles can produce sufficient energy (i.e. if they collide fast enough and in the right direction) a reaction will take place. Not all collisions will result in a reaction.

The investigation could be done using one variable and therefore have a set of results which were related in some way. The variables that could be used are:

1. Concentration
2. Particle size/surface area
3. Temperature

These variables can be used because:

- The more concentrated the reactants, the greater the rate of reaction will be. This is because increasing the concentration of the reactants increases the number of collisions between particles and, therefore, increases the rate of reaction.

- When one of the reactants is a solid, the reaction must take place on the surface area of the solid. By breaking up the solid into smaller pieces, the surface area is increased, giving a greater area of collisions to take place and so causing an increase in the rate of reaction.

- An increase in temperature produces an increase in the rate of reaction. A rise of 10º C approximately doubles the rate of reaction. When a mixture of substances is heated, the particles move faster. This has two effects. Since the particles are moving faster they will travel greater distance in a given time and so will be involved in more collisions. Also, because the particles are moving faster a larger proportion of the collisions will exceed the activation energy and so the rate of reaction increases.

Before doing the actual experiment I decided to do some preliminary work. These were to tell me the details that I would need to know for my investigation to be successful. I saw from these preliminary investigations that the magnesium ribbon started to react with the hydrochloric acid the moment that I dropped it in. I decided that it would be a good idea to start timing the second that I dropped it in. When the ribbon had been eaten away by the acid, it stopped fizzing. I decided that I would stop timing the second that the fizzing stopped. I discovered from my preliminary experiments that when I used a low concentration of hydrochloric acid, for instance, 1.0 molar, it took a long time for the magnesium ribbon to be dissolved. I decided that it would be impractical to spend time acids that take too long for the magnesium to dissolve. As well as high concentrated ones that dissolve it to quickly.

When the magnesium ribbon reacts with the hydrochloric acid, magnesium chloride is formed. I wrote down the equation to show this:

Magnesium + Hydrochloric acid = Magnesium Chloride + Hydrogen
Mg + 2HCl = MgCl + H

The graph above supports my original prediction of: the more concentrated the acid the faster the rate of reaction because it shows the time difference between the different strengths of acids. In a higher concentration there are more acid particles to react with the magnesium ribbon and therefore it is eaten away faster.
I conclude that changing one factor does have a significant effect on the rate of reaction as we have seen.

The evidence I have been able to gather from this investigation seems to lead to a quite firm conclusion. I might not have been able to find the exact speed of the reactions but the pattern seems to be correct as I have repeated readings three times and as it agrees with the information I have researched.
I used the variable of concentration, which seemed to be of a good choice as it would show the results of how more acid molecules reacting with magnesium, would result in a faster reaction.

There will always be ways in which you can improve your investigations and the same thing goes to my investigation. Every time I washed a test tube or a measuring cylinder, I did not dry it before using it. This may have affected the rate of reaction, as water would dilute the acid. To improve my results, I could dry the test tubes and the measuring cylinder after they are washed to prevent diluted acids. The size and weight of the magnesium would have affected the rate of reaction. The experiment could be improved by measuring, adjusting and weighing the magnesium ribbons so they all are the same size and weight.