How does changing the concentration of hydrochloric acid affect its rate of reaction with magnesium?

Shaheena Baig

Aim:

The aim of this experiment is to find out how changing the concentration of hydrochloric acid will affect its rate of reaction with magnesium. The rate of a chemical reaction is the time it takes for the reactants to react. I.e. in this experiment I will be monitoring how long it will take for magnesium to react in different concentrations of hydrochloric acid.

Explaining chemical reactions

In order for a chemical reaction to occur, the following needs to take place:

The particles involved need to collide with each other.
The collision needs to have sufficient energy called activation energy.
Particles have to be in the correct orientation (right way round).

There are four factors which affect the rate of reaction between two chemical reactants.

Concentration: Increasing the concentration of a reactant, increases the rate of reaction. This is because, in a higher concentration, there are more particles per unit volume, which means that there is a greater chance of a successful collision. Therefore the rate of the reaction between the two reactants will increase.

Temperature: Increasing the temperature of a reactant, increases the rate of reaction. This is because a higher temperature means a higher energy level. This makes particles move faster, which means that there is a greater chance of a successful collision. Therefore the rate of the reaction between the two reactants will increase.

Surface area: Decreasing the size of solid particles increases the rate of reaction. This is because decreasing the size of solid particles increases their surface area, which means that there is a larger surface area for collisions to occur. Therefore increasing the rate of reaction.

Catalysts: A catalyst is a substance which alters the rate of a chemical reaction but is chemically unchanged at the end of the reaction. The reaction has a lower temperature, which means that there is lower activation energy. There are an increased number of successful collisions, therefore an increase in the rate of reaction.

In this experiment, I am going to investigate the effect of concentration on the rate of reaction between two chemical reactants.

Preliminary experiment

Before doing the actual experiment, I did a preliminary study to help me find a suitable range for the concentrations and how to control the temperature for the main experiment. The temperature was controlled at room temperature, which can be seen from the results we obtained from this preliminary experiment. These indicate that the temperature was controlled as the temperature recorded was 27ºC at 0 seconds for each of the solutions. The preliminary study has also helped me familiarize myself with the method that I could be using for the main experiment.

I did the experiment using five different concentrations of hydrochloric acid, which were as follows:

I made the concentrations using hydrochloric acid and dilute water. I measured the liquids using measuring cylinders to make the required concentrations. I then poured the solutions into five beakers and measured the amount of magnesium needed. We used 5cm of magnesium for the preliminary experiment. We set up the equipment needed and did the experiment with the five different solutions. While one person put the magnesium into the beaker, the other started timing using the stop watch. We also kept record of the temperature using a thermometer. In this way, the experiment went smoothly and in an organized way. We recorded the temperature for one minute at 10 second intervals. We had 7 temperature recordings including the one from the start of the experiment. In this preliminary experiment, the independent variable was the concentration of hydrochloric acid and the dependant variable was the temperature. The controlled factors were the surface area of the magnesium, and the temperature, which was kept constant at room temperature.

Results:

These are the results we obtained from the preliminary study:

When the solution had 50 cm³ of hydrochloric acid and no distilled water, the reaction took place very rapidly. This can be seen by looking at the temperature difference at the start of the experiment and at 10 seconds. It increased by 6.5ºC, which is quite a big change. This reaction was exothermic because heat was released and we have evidence to support this, because the temperature starts to decrease in the 50cm³ and 40 cm³ of hydrochloric acid. In the main experiment, I will lower the range of concentrations to a maximum of 25cm³ of hydrochloric acid and minimum 5cm³. This is because the reaction was too fast with 50cm³ and it was difficult to record our results as the change was too quick. A lower range should minimize this effect. I will also use a bigger strip of magnesium to make the reaction time longer in order for us to get the best possible results. Also to make sure that surface area does not limit the rate of reaction, I will use a 15 cm strip of magnesium, which is in excess and enough for all the acid to react. Overall; this preliminary study has helped me to get familiar with the method and the apparatus I will have to use in the main experiment. I have a better idea of the rate of reaction between hydrochloric acid and magnesium at different concentrations. Therefore, I will be able to make a prediction based on these results.

Main experiment

Hypothesis

I predict that as the concentration of hydrochloric acid increases, so will the rate of reaction with magnesium. So at a higher concentration of hydrochloric acid, the rate of reaction will be much faster compared to the rate with a small concentration of hydrochloric acid. I predict that the rate at which hydrogen is being produced will be much quicker in the highly concentrated solutions, which means that there is faster rate of reaction between the reactants. I predict my graph to look like this:

The highest concentration graph would be the steepest because the reaction would take place at the fastest rate. The graph would also eventually level out, but with the higher concentrations, it would level out first. The lowest concentration, which is of 5cm³ of hydrochloric acid, would be the less steep graph compared to the rest of the concentrations. This is because it will take the longest for the reaction to take place and the graph would also take long to level out.

Scientific explanation

In order for magnesium to react with hydrochloric acid, the particles need to collide successfully with each other and the collisions need to be with sufficient energy. The rate of reaction depends on the amount of successful collisions in a specific amount of time.

Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen gas

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

This is a preview of the whole essay

Scientific explanation

Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen gas

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

Increasing the concentration of hydrochloric acid will increase the rate of reaction with magnesium. In a concentrated solution, there are more particles to react with, which means that there are more Hydrochloric acid particles for the magnesium to react with. This would mean that there is greater chance for particles to successfully collide with each other and react. When the magnesium strip is put into the beaker with the hydrochloric acid, the particles will rapidly start to react with the magnesium. However if there are too little particles for the reaction, then the reaction will take place much slowly, whereas if there were more particles of the reactant, then there would be more frequent successful collisions, increasing the rate of reaction.

In the less concentrated solution, there are fewer acid particles, which means that there is a smaller chance of them colliding with the magnesium atom. The particles will eventually react, but at a slower rate. In the highly concentrated solution, there are more acid particles for the reaction to take place much quickly. There is a greater chance for the acid particles to react with the magnesium particles at a faster rate. In the reactions, the particle will react and be used up. This is why the graph will start to level out since fewer and fewer particles are reacting. The particles react fastest at the beginning of the experiment, which is why the graph is the steepest at the start. From this explanation, I can make a final prediction which is: ‘’the higher the concentration, the faster the rate of reaction’’.

Controlling factors:

The controlling factors are the things I will keep constant during the experiment. They are as follows:

Temperature- this will be kept constant at room temperature. (as shown from the preliminary experiment)
Total volume of solution- this will always be 50cm³. The liquids will be measured using a measuring cylinder.
Amount of magnesium- this will be in excess so that the surface area doesn’t limit the rate of reaction and in effect, the amount remains constant.
Surface area- this will be kept constant by using magnesium as the second reactant as it is uniform (the same size) unlike calcium carbonate, which is not. A 15cm strip of magnesium should be enough for all the acid to react with.

Independent variable: This will be the thing that I am going to change in the experiment and it will be the concentration of hydrochloric acid. I am going to find out how it affects the dependant variable.

Dependent variable: This will be the thing I am going to measure in the experiment and it will be the volume of hydrogen gas produced with time, which determines the rate of reaction. I am going to see how it will be affected by the independent variable.

Equipment:

Hydrochloric acid (enough to make five solutions of different concentrations).
Distilled water.
Magnesium strip (15 cm × 5).
Five conical flasks to put the HCl solution in.
Biurette to measure the amount of hydrogen produced.
Glass bowl to put the water in.
Clamp stand to hold the biurette in place.
Rubber bung to cover the conical flasks.
Measuring cylinders (25ml and 10ml) to measure the liquids.
Pipette to make a precise measurement.
Ruler to measure the magnesium.
Scissors to cut the magnesium.
Pencil to coil the magnesium around.
Stop clock to time the experiment at 10 second intervals.
Goggles for safety.
Pen and paper with results table to record our results.

Method:

There are many ways in which this experiment can be conducted, and the rate calculated, but the three main methods of monitoring the reaction are:

The first method is to observe the magnesium dissolving into the hydrochloric acid. We have to put a cross under the conical flask with the acid. When we have put the magnesium into the conical flask, we have to time how long it takes for the cross to disappear. The quicker it disappears, the faster the rate of reaction. This is probably the simplest method as it doesn’t require a lot of equipment and it is easy to follow. However, this method is not very accurate because the time it takes for the cross to disappear is not the precise time it takes for the reaction to take place. There is no definite time at which the reaction could have taken place, which is why it is difficult to record the time. Therefore, it would be difficult to measure the dependant variable exactly and the results will not be accurate.

The second method is to monitor the mass loss of hydrogen. This can be done using a balance. We have to record the mass of the reactants before and after the experiment to calculate the mass lost. When the reactants react, the gas is formed and escapes from the conical flask or the beaker. This method is suitable for reactions that produce carbon dioxide or oxygen. However, it is not very accurate for this experiment because hydrogen produces a very low mass loss and it would be difficult to monitor the reaction.

The third method to monitor the rate of reaction is by monitoring the time it takes for hydrogen to be produced in suitable intervals. This will be done using a 50cm³ burette. This is the most accurate method to monitor the rate of reaction between magnesium and hydrochloric acid. I can work out the amount of hydrogen produced, which determines the rate by subtracting the amount of water produced by 50. This is a complicated method compared to the other two methods, but it will be the most accurate.

Justification of method:

I have chosen to conduct the experiment using a burette so that I can record the amount of H2O that is being produced. From these results I can work out how much hydrogen is produced by subtracting it from 50. This will then determine the rate of reaction.

Before the experiment, I must make sure that I am wearing goggles and the area is clean and tidy to ensure that the experiment goes smoothly. Firstly, I will be making all the solutions so that they are ready for the main part of the experiment. As I have found out from the preliminary study that there was a big temperature range between the different concentrated solutions, I will be making the solutions more dilute in the main experiment to minimize this temperature range. I will be making five solutions with different hydrochloric acid concentrations. They will be as follows:

I will measure the appropriate amount of hydrochloric acid and distilled water using measuring cylinders and a pipette for a more precise measurement if necessary. For example, for the first solution, I will measure out 25 cm³ of HCl and 25³ of distilled water. This will give me one of the solutions for the experiment. I will pour the liquids directly into the conical flasks from the measuring cylinders. I will do this to all of the other solutions as well. When I have made all of the hydrochloric acid solutions, I will label them so I know which is which for the main experiment. I will also have to cut up 15cm strips of magnesium for the experiment. I will do this using a ruler and scissors. I should have five strips in total, one for each of the five solutions.

After I have made all of the solutions and cut up the magnesium strips, I will be ready to move on to do the actual experiment. I first need to get the all the equipment in place and ready to use. I need to get the clam stand and the conical flask with the 25cm³ of HCl in the right place. I will then half fill the glass bowl with water and place it on top of the clamp stand. I will then get the biurette and fill it up with exactly 50cm³ of water. Then I will hold it upside down and place it in the water bowl. I will then attach the burette and the conical flask using a pipe. The last thing I will have to do to set up the experiment is to coil up the 15cm of magnesium using a pencil and put it into the conical flask and start timing. I will immediately cover the conical flask using the rubber bung after I have put the magnesium in to minimize any heat loss from the reaction. I will record my results every 10 seconds for as long as needed for the reaction to take place completely. I will repeat this whole procedure for the other four concentrations until I have a complete table of results. At the start of each experiment, the amount of water in the burette will be 50cm³ and as the reaction progresses, this will decrease at different rates according to the concentration of the hydrochloric acid. After I have recorded all of the readings from the burette, I will convert these results into the volume of hydrogen gas produced by subtracting it from 50. I will then be able to work the rate of reaction between magnesium and HCl in the five different concentrations. After I have recorded all of my results, they will be ready to produce a graph for analysis.

Fair test:

To ensure a fair test in this experiment, I need to make sure that the following factors are controlled and kept constant throughout the experiment: The temperature needs to be kept constant and I can do this by doing the experiment at room temperature in one session for all of the concentrations. If I do the experiment in different parts, I could be getting inaccurate results because the room temperature may change. I also need to make sure that the amount and surface area of the magnesium is the same for all of the experiments. I will be using a 15 cm magnesium strip, which is uniform (the same size) to ensure a fair test. I will also have to keep the amount of hydrochloric acid solution the same to keep it a fair test. I will have 50cm³ of solution in each conical flask. I will also have to make sure that the conical flasks are covered immediately after the magnesium is put in, to ensure that no hydrogen gas escapes. I will be using the same equipment for all of the experiments incase some differ is size or shape.

Accuracy:

In order to improve the reliability of this experiment, I will repeat it at least twice. I can then find an average of the two results, which will make the overall results more accurate. When measuring the acid and the distilled water I will use the smallest measuring cylinders because they give us a more accurate reading. I will also use pipettes to get the precise measurement of the liquids. When starting the stop clock, I will make sure that the reading is 0 seconds at the start of each experiment. I have to fill up the burette before each experiment and to improve the accuracy, I must make sure that it is exactly 50cm³; otherwise we could be getting a higher or lower reading from the burette.

Safety:

The safety of others and myself in this experiment is essential because it involves using hydrochloric acid and a highly reactive metal; magnesium. I must firstly make sure that I am wearing safety goggles throughout the experiment especially when handling the acid since it can irritate the skin and the eyes if it comes in contact. I must also make sure that the area in which I am conducting the experiment is neat and tidy to ensure that all of the equipment is laid out in an organized way. This will ensure that the experiment goes smoothly without any problems. However, if any such problems do arise such as glass braking, I will handle it with care and dispose it appropriately. If any acid spills, I will clean it up immediately using paper towels. After the experiment, I must make sure that I safely dispose the chemicals into the sink and put the equipment back in their allocated places. Other than these safety precautions, I must make sure that the experiment is carried out in an organized manner to limit the potential dangers.

Results:

These are the results I obtained from the main experiment. I repeated the experiment two times, so I have two sets of results for each of the concentrations. I have found an average of the results and I have also calculated the hydrogen produced by subtracting the average volume of water in the burette by 50. This is because the burette had 50cm³ of water at the start of the experiment. A graph can now be plotted using the results for the average volume of hydrogen produced and the time taken.

Volume of hydrochloric acid: 25cm³

Volume of hydrochloric acid: 20cm³

Volume of hydrochloric acid: 15cm³

Volume of hydrochloric acid: 10cm³

Volume of hydrochloric acid: 5cm³

The following table is the summary of my results for the first two minutes. I will use only this information to produce the graphs.

Conclusion:

In conclusion, I have found out that as the concentration of hydrochloric acid increases, so does the initial rate of reaction. I can support this conclusion by looking at my results table and graph, which show that at a higher concentration, the rate at which the water in the burette decreases is high; therefore the rate at which hydrogen is produced is much faster. This can be seen by looking at the five different concentration graphs. The graph of the highest concentration is the steepest and the lowest concentration graph is the least steep.

Analysis:

In my Hypothesis, I stated that as the concentration of hydrochloric acid increases, so would its rate of reaction with magnesium. The results I obtained form this experiment support my hypothesis. I also stated that the rate would change during the experiment, and my graph supports this, since they are not straight lines and the gradient of the graphs are not same.

The solution which had 25cm³ of hydrochloric acid had the fastest reaction when magnesium was put into it. The water in the biurette decreased within ten seconds, so hydrogen was produced in the quickest amount of time. This therefore suggests that the rate of reaction was very fast. The solution which had 20cm³ of hydrochloric acid had the second fastest reaction when magnesium was put into it. The third solution, which had 15cm ³ of hydrochloric acid, had the third fastest reaction and this pattern continues till the fifth and final solution, which was the least concentrated of only 5cm³ of hydrochloric acid.

In this experiment, the magnesium needed to react with the hydrochloric acid at different concentrations. In order for the reaction to take place, there needed to be successful collisions between the magnesium and acid particles. These collisions also needed to have sufficient energy to react fully. An equation for this reaction is as follows:

Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen gas

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

The reason why the solution with a higher concentration of hydrochloric acid had the fastest reaction with magnesium is because there were more particles for the reaction to take place. In a highly concentrated solution, there are an increased number of particles per unit volume, which means that there is a greater chance of a successful collision occurring with the other reactant. This therefore increases the rate of reaction. In the less concentrated solution of hydrochloric acid, there are fewer particles to react with the magnesium. These particles will eventually react, but at a much slower rate. They also have a less likely chance to collide successfully, so the rate of reaction will be very slow. This can be seen from the graph I have produced using my results. Within two minutes, the graphs for the 25cm³, 20cm³ and 15cm³ of hydrochloric acid started to level out, which suggests that the magnesium and the hydrochloric acid have reacted completely. However, the graphs for the 10cm³ and 5cm³ of hydrochloric acid didn’t level out in two minutes, which suggest that there weren’t enough hydrochloric acid particles to react with the magnesium in two minutes. This means that there was a slow rate of reaction.

In my hypothesis, I also stated that the rate of reaction would change during the experiment. My graph proves this correct, since it is not a straight line, but smooth curve. The reason why the rate of reaction between the hydrochloric acid and the magnesium particles decreases is because, as the experiment progresses there are fewer and fewer acid particles that remain for the magnesium to react with. When a hydrochloric acid particle reacts with a magnesium particle, the same particle cannot react again. Therefore there are fewer particles that remain compared to the start of the experiment. At the end of the reaction, there are no particles to react with, which is why the graph eventually starts to level out. The horizontal line in the graph suggests that the reaction has completely finished and that there would b not more particles reacting.

I also worked out the gradient for each of the graphs, so that I can calculate the rate of reaction in the different concentrations of hydrochloric acid. I divided the ‘x’ axis, which was the average volume of hydrogen produced by the ‘y’ axis, which was the time taken for the reaction. The graph is not a straight line, so the gradient will differ throughout the graph, but I took the gradient at the start of the graph, at 10 seconds, as this is where the graph is straight and at this point, the rate of reaction should be at its fastest. The table below shows the gradient for the different graphs:

The gradient of the 25cm³ hydrochloric acid solution graph is the highest of 2.35cm³/s. This therefore means that the rate of reaction is 2.35cm³/s. The gradient for the 20cm³ hydrochloric acid solution graph is 1.05cm³/s, which therefore means that the rate of reaction is 1.05cm³/s. The gradient of the 15cm³ hydrochloric acid solution graph is 0.35cm³/s, so the rate of reaction in this concentration is 0.35cm³/s. The fourth concentration graph of 10cm³ of hydrochloric acid had a gradient of 0.12cm³/s, so the rate of reaction in this solution was 0.12cm³/s. The graph of the lowest concentration had no gradient at all in the first ten seconds. This could have been due to the slow reaction between the acid and magnesium particles. However, when the reaction starts to take place after 20 seconds, the gradient of the graph at this point is 0.025cm³/s. Therefore, the rate of reaction is 0.025cm³/s, which is very slow compared to all of the other graphs. Overall, I have found out that an increase in the concentration, increases the rate of reaction.

Evaluation:

Overall, I think this experiment was done well because we obtained accurate and reliable results that were free from any major anomalies. I think our method was suitable for obtaining basic results, which were needed for the analysis and to support my prediction. The experiment was carried out in an organized way, which is why I think it went smoothly, without any problems. The preliminary experiment also helped us plan our time and to help decide the correct range for the hydrochloric acid concentrations for the main experiment.

The lines of best fit on the graphs are quite close to the points for most of the graphs. However, for the 10cm³ hydrochloric acid concentration graph, the points are not as close to the line of best fit as the other four graphs. If the points on that graph were to be joined up, there would have more fluctuations in the graph. The points of the graph may be like this, because there may be more fluctuations in the rate of reaction for that concentration or we may have made an error in our reading. This could be from the trapped bubble in the burette, which is the result of an unsteady flow. The reasoning behind some of the anomalous that we got in this experiment could be because of the collision theory. In order for particles to collide successfully and increase the rate of reaction, there needs to be sufficient energy. This energy may not be available to all particles and therefore not every collision will be successful. This may be the cause of the fluctuations in the 10cm³ HCl concentration graph in which not all particles are reacting. Other than this, the results I obtained from this experiment are free from any anomalies, which suggest that these results are quite accurate and reliable.

The method I chose was the hardest method out of the three I had choice from, but it was also the most accurate. I think I measured the liquids accurately using the measuring cylinders, however, I still think I can improve the accuracy of measurement and perhaps find a new way to measure the liquids more accurately. A pipette could be used instead of a measuring cylinder to improve the accuracy. I think it would have been better if we already had the different concentrations of hydrochloric acid made, because this would save us time and we would be content that these solutions are accurate. However, we can still try to make these solutions in a more accurate way by using smaller measuring cylinders and other measuring equipment. I had also kept a good record of time in the stop clock and I never delayed the reading, which is why I think I didn’t have any major anomalous results.

To improve the procedure of this experiment, I think there could be a better way to put the magnesium into the conical flask before we start timing. To ensure a fair test, we need to make sure that we start timing exactly when the magnesium is put into the conical flask for every experiment. To stop bubbles from forming in the burette, we could use a gas syringe instead as this will limit the number of trapped bubbles being produced and give us a better reading of the volume of water present. In this experiment, I think the range for the concentrations was fine, but the intervals could have been smaller. We could experiment with 21cm³, 22cm³, 23cm³ and 24cm³ of hydrochloric acid as well to improve the accuracy of our results. We can also decrease the time intervals to every five seconds instead of 10 seconds. This will ensure that we have more readings and the graph we get will be much more detailed.

Bibliography

These are the resources I used to help obtain relevant information for this investigation: