The Rate of Reaction Between HCl and Mg

Chemistry Sc1- The Rate of Reaction Between HCl and Mg

Aim

To investigate the affect on the rate of reaction by changing a factor of the experiment.

Introduction

When hydrochloric acid and magnesium react, the hydrochloric acid will dissolve the magnesium and produce hydrogen gas. All chemical reactions involve reactants which when mixed together may react to leave a product. In this case, the reactants are magnesium and hydrochloric acid. The reaction occurs when the magnesium is dropped into the hydrochloric acid. The equation for this experiment is:

Magnesium (s) + Hydrochloric acid (aq) → Magnesium Chloride (aq) + Hydrogen (g)

Mg + 2HCl → MgCl2 + H2

The magnesium will react with hydrochloric acid, because it is higher in the reactivity series that hydrogen. When the two chemicals react, a displacement reaction will take place and the magnesium will displace the hydrogen in the hydrochloric acid forming magnesium chloride and hydrogen gas. Therefore, the products of the reaction are magnesium chloride and hydrogen gas.

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 the idea that reactions occur when the particles in the reactants collide with each other with enough energy to break or form new bonds between the other particles. This 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 off each other, causing an unsuccessful collision.

Hypothetically, there are also several ways to record the results (although we were only given the option of the last way):

Volume of gas evolved- I could use a gas syringe to collect and measure the gas that is evolved from each test of the experiment.
The weight before, during and after the experiment- I could put a chosen volume of hydrochloric acid in a beaker and put that on a set of very accurate scales and record the weight of it. Then I could put the magnesium into the beaker with the hydrochloric acid and measure the decrease in weight at chosen intervals. The weight will decrease because the hydrogen will be displaced from the hydrochloric acid.
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Volume of gas evolved- I could use a gas syringe to collect and measure the gas that is evolved from each test of the experiment.
The weight before, during and after the experiment- I could put a chosen volume of hydrochloric acid in a beaker and put that on a set of very accurate scales and record the weight of it. Then I could put the magnesium into the beaker with the hydrochloric acid and measure the decrease in weight at chosen intervals. The weight will decrease because the hydrogen will be displaced from the hydrochloric acid.
How long the magnesium takes to dissolve- I could use a stop watch to measure the length of time the magnesium takes to dissolve.

Factors

The factors that could affect the rate of reaction of my experiment are:

Concentration of acid- This could affect the rate of reaction because the higher the concentration of the acid then the more acid particles per volume of acid, therefore more collisions per second and then there will be more successful collisions per second.

Temperature of the acid- Rates of reaction increase with a higher temperature because the particles involved in the reaction have more energy to move from the heat so they collide more rapidly. This will give more successful collisions per second.
Surface area of the magnesium- If the magnesium had a bigger surface area each time the experiment was repeated, then the acid particles will have a bigger area to collide with, so more collisions will occur every second, which would mean there are more successful collisions per second.
Type of acid used- If you changed the type of acid then the rate of reaction would change. Hydrochloric, nitric and sulphuric acid would all produce different rates of reaction, because they have different ions, so if I were to change the type of acid then all three kinds would produce a different set of results.

Plan

I decided to change the concentration of the acid to measure the average rate of reaction. The results will be measured with a stopwatch and the length of time the magnesium takes to dissolve will be measured. The results will be taken down in a table, and then a graph will be drawn from the information. The concentration of hydrochloric acid is calculated by moles and I will vary the concentration from 0.0M- 4.0M. Each time, a 1cm strip of standard width magnesium ribbon will be used. Each time the experiment is repeated, I will use 10cm3 of HCl and each mole of acid will be tested 5 times with an average to try to keep the results reliable. 1.0M is double the concentration of 0.5M and 2.0M is double the concentration of 1.0M. We will only be using a measuring cylinder to measure out the volume of acid to use and we could improve the accuracy of the experiment by using a syringe. The heat given off during the reaction may be a problem too, because the higher the concentration of acid, the more heat is given off which will mean the reaction occurs even quicker. However, the heat given off during the reaction in relation to the concentration of acid will be the same for each experiment so the results should not be affected.

Prediction

I predict that the higher the concentration of the acid, the higher the rate of the reaction will be. This is based on the Collision Theory. The higher the concentration of acid, then the higher the concentration of acid particles there will be in a certain volume. This will mean that the higher the concentration of acid, the more the successful collisions there will be in 1 second. Therefore, the rate of reaction increases.

Results

I observed small bubbles emerging on the surface of the magnesium, which rose towards the surface of the acid. The gas evolved was collected and a lit splint was held in the gas. There was a popping sound and the splint went out. I also felt that as the reaction took place, the aid warmed.

Analysis

The graph shows that the higher the concentration of acid, the quicker the magnesium is dissolved. The curve of the graph decreases rapidly at first as the time taken to dissolve the magnesium decreases rapidly, the more concentrated the acid become, but begins to level out towards the end. The table and graph also shows that double the concentration of acid used, halves the time taken for the acid to dissolve the magnesium. In the 1/Time graph, the trend was positive and the y-axis was proportional to the x-axis, which is what should happen because when the concentration of the acid doubles, the time taken for the magnesium to dissolve halves. The explanation for this is that when the acid concentration doubles there will be double the amount of hydrogen ions to react with the constant reactant. The fact that the concentration is the only factor optionally changed means that the time taken for the hydrogen ions to be displaced by the magnesium and the chloride ions to bond with the magnesium is halved.

Conclusion
I can conclude that if you double the concentration of the acid the rate of reaction would
also double. This is because the ions are closer together in a more concentrated solution. The closer together they are, the more often they collide. The more often they collide, the
faster the reaction because there are more particles in the solution, which would increase the likelihood that there would be successful collisions and that they would hit the magnesium so the reaction rate would increase. The graphs prove that if you double the concentration the rate of reaction doubles. The activation energy (energy needed to start off a chemical reaction) of a particle gets higher with heat; the particles that need the activation energy are those particles that are moving. In the case of magnesium and hydrochloric acid, it is the hydrochloric acid particles that need the activation energy because they are the ones that are moving and bombarding the magnesium particles to produce magnesium chloride. The hydrochloric acid particles needed a high activation energy, or they would have bounced harmlessly off the magnesium. I can also conclude that the gas that was evolved was hydrogen gas as stated in the introduction.

My conclusion agrees with my prediction in that I predicted: …that the higher the concentration of the acid, the higher the rate of the reaction will be. However my conclusion was more detailed than my prediction in stating by how much the rate of reaction increased with a certain concentration of acid.

Evaluation

Our method to obtain the results went well because it was accurate and the consequence was that we could draw reliable graphs. The results were reliable and quite accurate and there were no anomalous results. The only slightly off point on the graphs was for 4.0M. This may have been because the heat produced from the reaction sped it up more in relation to the other concentrations. The procedure was suitable for the results we wanted to obtain because it was quick, fair and quite accurate. Although, the results were excellent, they were not perfect. Some of the reasons for this may be:

When the reaction takes place, bubbles of H2 are given off, which might stay on the magnesium, therefore reducing the surface area of the magnesium so the acid cannot react properly so this affects the results.
There may have been some slight human error when stopping the stopwatch.

The results helped to give a firm and reliable conclusion and it would have been hard to improve them. However, to improve the experiment, we could use a computer to time the reaction. We could also carry out the experiment with HCl of higher concentrations than 4.0M and repeat the various concentrations still further, although I feel that for this experiment, five times was sufficient.