To investigate the individual factors which affect the rate of reaction between Magnesium and Hydrochloric acid.

Aim

Background Information

For a reaction to take place between two or more substances, in this case magnesium and hydrochloric acid, the reactants must first collide with each other. However, they also have to collide with a minimum amount of energy for a reaction to occur. This is called activation energy.

The word equation for, when magnesium reacts with hydrochloric acid is:

Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen

By looking at this equation I can tell that this will be an exothermic reaction, as hydrogen will be released in the form of energy. To explain this exothermic reaction, there is a simple energy profile below which helps to describe how the activation energy works.

The graph above shows how much activation energy is needed for the particles to react for an exothermic reaction. If the particles collide with less than the required amount of activation energy then a reaction cannot take place.

The graph above shows that the vast majority of the particles do not have enough energy for them to react. To allow these particles to react we can either change the shape of the curve or move the activation energy further to the left.

To do this I can change one of the factors/variables that affect the rate of this reaction. These factors are, surface area, concentration, temperature and a presence of a catalyst in the reaction. I will explain these briefly below.

SURFACE AREA

The greater the surface area, the faster the reaction: a powdered solid will produce a faster reaction than if the solid with the same mass was present as a single lump. This is because increasing the surface area of the solid increases more chance of collision between particles resulting in a faster rate for the reaction. The diagram below will make it easier to understand the theory. The diagram below shows a reaction between magnesium and hydrochloric acid.

CONCENTRATION

Increasing the concentration of the reactants increases the rate of reaction.

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

For this reaction the more diluted the hydrochloric acid the longer it will take for the reaction to take place via the more concentrated the acid the faster the reaction. This is because for any reaction to take place the particles must collide first. And when the concentration is higher, this means that there are more particles in a given space therefore the chances of collision are greater.

The rate of the reaction depends on the concentration of the reactants for a general reaction in ...

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

The rate of the reaction depends on the concentration of the reactants for a general reaction in which A and B are the reactants

A + B products

The general rate equation is

Rate = k[A]m[B]n

TEMPERATURE

As the temperature is increased the rate of reaction also increases. Approximately, for many reactions happening at around room temperatures: the rate of reaction doubles for every 10˚C rise in temperature. The explanation is that particles can only react if they collide and if you heat a substance the particles will move faster and therefore collide more often. Linking this back to activation energy, increasing the temperature would increase the number of particles with enough activation energy for the reaction to take place.

CATALYSTS

A catalyst is a substance, which speeds up a reaction but is chemically unchanged at the end of the reaction. Using a catalyst provides an alternative route for the reaction has a lower activation energy, this allows for more particles to react; as the number of successful collisions increases.

The graph below shows the effect of adding a catalyst:

From all these variables the one I have chosen for my investigation is concentration.

Prediction

I predict that the more diluted the hydrochloric acid the slower the reaction will take place and the more concentrated the acid the quicker the reaction will take place. I know this because of my own knowledge and research about the collision theory of particles. In a low concentration of acid the particles are spread far apart and therefore they will only collide with the magnesium particles occasionally, whereas in a higher concentration of acid, the particles collide with the magnesium particles more often. This increases the rate of reaction.

Apparatus

The apparatus I will need for this experiment are:
2 measuring cylinders
Conical flask
Delivery tube with a bung
3cm magnesium ribbon strips
Troth
Stopwatch
2-mol dm3 hydrochloric acid
Pipette
Bowl

Diagram

A diagram of how the apparatus should be set out:

Method

Set up the equipment as shown in the diagram above and wear safety goggles.
Fill the bowl with tap water.
Using a measuring cylinder measure out 10cm3 of 2mol dm3 of hydrochloric acid and mix it with 15cm3 of water (a concentration of 4:6) so the total volume of the solution is 25cm3.
Pour the solution into the flask
Add the magnesium ribbon to the flask and replace the bung as quickly as possible. Get your partner to start the stopwatch at the same time.
Another person should be holding the measuring cylinder in the bowl as shown in the diagram during the reaction is taking place.
During the reaction is taking place hydrogen gas should travel through the delivery tube into the measuring cylinder. Read off the volume of hydrogen produced in the measuring cylinder from the meniscus every 10 seconds.
Record the results in a table until the reaction hasn’t stopped.
Repeat all the steps above to do this experiment 2 more times, so that an accurate average can be obtained.
Repeat the experiment with different concentrations (from steps 2-9) for the following concentrations:

12.5cm3 of 2 mol dm3 hydrochloric acid mixed with 12.5cm3 water (a concentration of 5:5)
15cm3 of 2 mol dm3 hydrochloric acid mixed with 10cm3 water (a concentration of 6:4)
17.5cm3 of 2 mol dm3 hydrochloric acid mixed with 7.5cm3 water (a concentration of 7:3)
20cm3 of 2 mol dm3 hydrochloric acid mixed with 5cm3 water (a concentration of 8:2)
22.5cm3 of 2 mol dm3 hydrochloric acid mixed with 2.5cm3 water (a concentration of 9:1)

After I have recorded the results I must find the rate at which each reaction took place and then find an overall average rate for each concentration. To find the rate I will use the formula rate = volume/time.

Fair test

I will keep this experiment a fair test by only changing the concentration of the acid and keep every thing else the same. This is because the independent variable in this experiment is concentration. I must make sure that the size/mass of magnesium is the same also that the temperature of the water is kept the same. I will also repeat this experiment 2 more times to make sure the results I have are reliable. I must also make sure that when taking the hydrogen level readings that I am reading from the meniscus, therefore it will be more reliable. The apparatus I use in each experiment are the same size when conducting the experiment.

Errors

The possible errors that can occur in this experiment are:

The length of magnesium ribbon provided could be different sizes, which can cause changes in the result, as the mass will be different.
When closing the bung it is possible that some gas could escape. This means that our reading would not be reliable.

Results

A table on the next page to show the amount of hydrogen produced at different time intervals.

Analysis and conclusion

The graphs that I have drawn from the results I have obtained seem very accurate to me, as I can see a pattern in the results and the curve is very consistent; all the points seem to fit well to make an accurate curve. This is because I see a pattern in all my readings from the graph as most of my repeated experiments have very similar results for example when the concentration of hydrochloric acid was 20 mol dm-3 the volume of hydrogen produced after 10 seconds was 6 cm3, etc. From the graph I am also able to back up my theory. I predicted that as the concentration of the acid increases so will the rates of the reaction. This is because if there are more acid molecules in a given space then there will be more frequent collisions and therefore a higher rate of reaction. If the concentration of the acid is lower then collisions between the acid and magnesium will be less frequent and therefore will slow down the reaction.

The graph shows the gradient of the line getting steeper, the higher the temperature. From looking at my results and further research I have found out that as the concentration doubles so will the rate of reaction. As you can see from the table that when the concentration was 20-mol dm-3 the average rate was approximately 0.5(1 d.p.) cm3s-1 and when the concentration was 40 mol dm-3 the average rate was approx. 1.1 (1d.p.) cm3s-1.

From my background knowledge I had said earlier that this reaction will be an exothermic reaction and from this graph I can tell that I was right. This is because when the reaction was in progress, heat was given off therefore the graph is in a curve. I should’ve had a straight line on the graph but due to the fact that heat was given off we had no control over it therefore I know that this reaction was a second order reaction.

Evaluation

As my method and plan was to make the experiment a fair test and if I had made sure that the results were exactly accurate, the results should have been near perfect. But because I could not ensure exact measurement was used, this could be a possible reason for the slight inaccuracy in the results. There is an inaccuracy that cannot be ruled out of my experiment, unless there is another way of performing the experiment, and that is human error. For example one of the major problems was that when some help from a friend was needed to start the experiment and this lead to a small delay in starting the stopwatch. Another problem was that some of our measurements were in decimals; we had to use a measuring cylinder, which was not very accurate either. Next time I could use a more accurate measuring device or keep it simple and just use whole numbers. To control the excess heat I could use cooling bars, which takes away any excess heat so it as at room temperature.

Overall besides these possible errors I think my results are reliable because they show a pattern they back up my prediction and support the

Generally the results I have obtained are reliable enough to support the conclusions I have come to about the values for the rates of reaction, the collision theory and the order of reactions because I did my experiment as accurately as possible. Disregarding the small sources of errors, which do not affect my results vastly.