Investigate, analyse and evaluate the effect on the rate of reaction of varying the concentration of hydrochloric acid in the reaction with magnesium.

Sam Daniels – Set 2 07/05/2007

GCSE Chemistry Coursework 2003/4

Sam Daniels

Chemistry Coursework

AN INVESTIGATION INTO THE REACTION BETWEEN HYDROCHLORIC ACID AND MAGNESIUM

My aim is to plan, investigate, analyse and evaluate the effect on the rate of reaction of varying the concentration of hydrochloric acid in the reaction with magnesium.

The chemical equation of the reaction I am investigating is:

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (G)

Rate of Reaction

Rate of Reaction = Gas Produced

Time taken

To measure the rate of reaction I can use three ways: Precipitation - this is when the reaction’s product is a precipitate which clouds the solution. Change in mass – a reaction which produces a gas will lose mass and this can be shown by carrying out the reaction on a balance. Volume of gas given off – A gas cylinder gas be used to collect the gas given off.

Rate of reaction depends on four separate things: Temperature, concentration (or pressure for gases), size of particles (or surface area for liquids) and the presence of a catalyst. Rate of reaction is a change in mass or volume or concentration per unit of time. An increased rate could be the product of any of these factors: an increase in temperature, an increase in concentration, an increase in surface area or the introduction of a catalyst.

Variables

The variables available to investigate are: Mass of Magnesium Used

Temperature of surroundings

Time of reaction taking place

Pressure (atmospheric)

Volume of HCl used

Volume of H2 produced

Concentration of HCl

Shaking/Stirring

Surface Area of Magnesium

Turnings or Ribbon Magnesium

A dependent variable is a factor which changes due to another variable changing. This variable is an independent variable and it is the variable that is changing. For my dependent variable I will use Volume of H2 produced and collected, and also I will measure the temperature of the reaction at the start and at the end of my reaction and for my independent variable I will change the concentration of hydrochloric acid used in terms of mol/dm³.

Fair Test

In my investigation I need carry out a fair test so my results will be accurate and reliable. To do this the only change I will make is the concentration of the hydrogen hydrochloric acid because if I change any other variables I will not know which of them affected the results of the dependant variable (the volume of Hydrogen Gas produced). I will use the same amount of magnesium turnings for each experiment so the surface area of the reagent does not change and alter the rate of reaction, I will use the same volume of HCl in each experiment although the concentration of it will change, I will use the same set up of apparatus for each experiment and the same air temperature and air pressure and I will also time the reaction for the same amount of time on each experiment so all the results have the same variables changing and remaining constant, this way I should not get any anomalous results. I will also use pure, distilled water so there are no impurities to affect the experiment and I will take the funnel out of the burette while filling the conical flask so no drips will make my measuring inaccurate.

Preliminary Experiments

I plan to use a range of five concentrations which will give me good readings but to do this I will need to do some preliminary experiments to identify the amounts of each reagent I will use. I am aim to produce a maximum of 100cm³ of hydrogen gas and to do this I will need to use a certain amount of reagents. I can estimate the amounts to use of reactants by performing a chemical calculation:

Calculating the mass of magnesium turnings:

1 mole of Magnesium: 6 x 1023 atoms of Mg ...

This is a preview of the whole essay

Preliminary Experiments

Calculating the mass of magnesium turnings:

1 mole of Magnesium: 6 x 1023 atoms of Mg weigh 24g.

Relative Atomic Mass of Mg = 24

1 mole of Hydrochloric Acid: 6 x 1023 atoms of Hydrogen = 1g

6 x 1023 atoms of Chlorine = 35.5g

35.5 + 1 = 36.5g

Relative Atomic Mass of HCl = 36.5

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

24g 73g 95g 2g

1 mole of Mg reacts with 2 moles of HCl

24g of Mg reacts with 73g of HCl

From these calculations I can also see how much HCl I will need to use so that it is in excess and so all the Mg will be used up.

I will vary the concentration five times during the experiment. There needs to be a range of concentration so I will obtain accurate results. To obtain an idea of the exact concentrations to use in my main experiment I conducted some preliminary experiments using the method I will use in my main experiment. These are the results:

I have decided to use a range of concentration 0.2 mol/dm³ apart between 0.5mol/dm³ and 1.5 mol/dm³. These concentrations will be: 0.6 mol/dm³, 0.8 mol/dm³, 1.0 mol/dm³, 1.2 mol/dm³, 1.4 mol/dm³.

Plan

I plan to conduct an accurate experiment and I will make sure I keep to this. I will measure the volume of gas produced to one decimal place and the mass of magnesium used to two decimal places. I will use a burette to accurately measure the volume of HCl I will use, I will make sure there is no bubble at the bottom of the burette which will make my readings void.

I propose to repeat experiments because consistent results mean reliable, accurate results. I will average my results so I get an idea of the mean view of the amount of gas produced in the reaction.

I also intend to identify anomalous results, examine how they could have come about and what I could have done to improve the accuracy of the experiment.

I will use these pieces of apparatus: Burette, conical flask, measuring cylinder, beakers, rubber bung, rubber delivery tube, timer, retort stand, water trough, weighing boat, balance, and thermometer. I will set up the apparatus as shown below:

I will firstly measure out a mass of magnesium turnings using a balance set to two decimal places. I will place a weighing boat on the balance, zero it, and then carefully add a few turnings at a time to get a mass of 0.10g of magnesium. I will then measure out 25.0cm3 of diluted hydrochloric acid and I will obtain the certain concentrations from an original 2.0 mol/dm³ as follows.

If 2 mol/dm3 = 25.0cm3

1 mol/dm3 = 12.5cm3

I will use concentrations which increase by 0.2mol/dm3 each time.

2mol/dm3 ÷ 0.2mol/dm3 = 10

25.0cm3 ÷ 10 = 2.5cm3

Therefore I will increase the volume of HCl by 2.5cm3 for each concentration increase of 0.2mol/dm3.

I will then carefully pour this concentration of HCl into a conical flask. I will fill the measuring tube with water and clamp it upside down in the water trough so the gas will displace it. I will place the end of the rubber delivery tube under the measuring cylinder and then add the magnesium to the conical flask. I will quickly place the rubber bung on the flask and start the timer. I will then take the bung off the conical flask when the timer reaches sixty seconds and record the volume of gas collected in the measuring cylinder. I will then repeat the reset the apparatus and repeat the experiment more than once so that I can judge the reliability and the accuracy of the results.

Safety Precautions

Whilst performing the experiment, I will take into account some personal and general safety concerns. I will make sure that I wear safety goggles and wear a protective lab coat at all times so if, accidentally some chemicals gets spilt or I will not get it in my eyes, on my clothes or on my books because my books will be off the table and out of the way. I will be standing up so I can move out of the way quickly if necessary and the stools will be pushed underneath the desks and out of the way. I will tuck in any loose clothes such as my tie, and I will take off my blazer. If there are any spills or breakages, I will inform my teacher immediately and get it cleaned up as quickly and as efficiently as possible. There will be no naked flames or sources of heat in the laboratory due to hydrogen gas being very flammable.

Prediction

I predict that the more concentrated the hydrochloric acid is, the faster the rate of reaction will be, and the faster the hydrogen will be given off. I also think that the amount of hydrogen gas given off will be proportional to the concentration of the hydrochloric acid. These predictions can be justified in terms of the collision theory: for two molecules to react, they have to collide. The rate of a chemical reaction depends on how often the molecules collide (the frequency of collision) and with how much energy they collide (the energy of collision). Sufficient energy is important so there is enough energy to break the bonds in the reacting molecules. Energy will then be released when new bonds and new molecules form. Molecules also have to collide so that the reactive parts of them come together (with the correct orientation).

More collisions increase the rate of reaction and all methods of increasing the rate of reaction can be explained in terms of the increase of the number collisions between the particles of the reagents. Temperature, concentration (or pressure), size of solids particles (or surface area) or the presence of a catalyst increases the number of collisions.

I also predict that the graph of volume of gas produced against concentration and rate of reaction against concentration will be a straight line because the gas produced is proportional to the concentration.

Results

Experiment 1

Experiment 2

Experiment 3

Analysis

I will calculate the rate of each experiment to two decimal places:

Experiment 1

Experiment 2

Experiment 3

Average

From the table of results and the graphs I have drawn I can see a stable increase in the rate of reaction as the concentration of the acid increases. This conforms to my prediction that “the more concentrated the hydrochloric acid is, the faster the rate of reaction will be”. The graph of average rate of reaction for each concentration shows that there is an increase in the rate of reaction as the concentration increases because the line of best fit becomes steeper and steeper.

The results of the experiment can be explained by the Collision Theory. The more hydrochloric acid ions (the concentration), and magnesium atoms there are the more they collide with each other and therefore the more chance they have of reacting with each other to produce hydrogen gas and magnesium chloride.

The graph of the results of experiment 1 shows that the amount of gas produced increases with the concentration of the HCl. It increases steadily, but there is one anomalous result which I have noticed for a concentration of 1.4mol/dm³ which I have circled. The graph of the results of experiment 2 shows also shows the same information and still there is one result which does not fit in and I have identified this on the graph. The graph of the results of experiment 3 however is slightly different. It does not curve steadily up like the others. It forms a shallow ‘S’ shape by the volume of gas starting to increase, but then not increasing as much from a concentration of 1.2 – 1.4 mol/dm³.

The graph of the rate of reaction of experiment 1 shows that the rate of reaction increased with the concentration steadily up to 1.2mol/dm³ but at 1.4mol/dm³ there is an anomalous result the same as the one in the results graph and I have circled it. The rate of reaction against concentration graph for experiment 2 shows a very similar pattern to experiment 1: the rate increases steadily in the curve shown. The graph of experiment 3 shows that the rate increases the same as the others but the last two results do not fit the pattern of the rate increasing by more and more for each concentration.

The graph showing the average amount of gas produced for each concentration shows the broad scale of my experiments. The data collected most fulfils my prediction that “the amount of hydrogen gas given off will be proportional to the concentration of the hydrochloric acid”. The graph for the average rate of reaction shows this as well but in both, the anomalous results from the three experiments have affected the average results and I have identified these points on the graph.

The majority of my results were in accordance with my predictions, but the graphs were not a straight line as I had predicted. This was because of the exothermic nature of the reaction. It lets out heat into the surroundings and this heat produces heats up the reagents and therefore increases the rate of reaction and the speed at which the hydrogen is produced. When the hydrochloric ions and the magnesium atoms are heated up, they gain energy and speed up. This makes their collisions more frequent and more violent, and this will therefore increase the possibility of there being a successful collision where the ions and the atoms react. This effect is only noticeable during the reaction with the greater concentrations.

As we can see on the diagram opposite, the 45 degree straight line is what I predicted the line would be like without considering the heating effect and the collision theory, and the curved line is the actual outcome, and the difference is shaded in.

Evaluation

Firstly I need to determine and evaluate how accurate my experiment was. Mostly my experiment was very accurate; I measured the magnesium to two decimal places, and the volume in a burette to one decimal place. The conical flask used made the reading of the gas produced more accurate because it funneled all the gas through the delivery tube, if I had used a beaker the gas would have just stayed in the beaker. I filled the burettes full of pure water and HCl using a funnel and I then turned on the tap to get rid of any bubbles in the bottom of the burette and to zero the measurements. After filling the burettes up, I removed the funnel so no drips from it affect the measurements. The procedure was overall quite accurate, I asked someone else to start the timer while I combined the Mg and the HCl and then replaced the bung, but there could still have been a small amount of time for gas to escape.

My results are mainly consistent, but at 1.4mol/dm³ concentration there is consistently an anomalous result and I have identified this result on my graphs. This result only occurred at the highest concentrations, where I observed that the production of the gas was fastest. This anomaly is most likely to be caused by the procedure of replacing the bung after combining the Mg and the HCl. When the reagents are mixed, they start producing hydrogen immediately and if I did not get the bung on quick enough some gas will have escaped and therefore made the reading lower. The heating effect due to the exothermic nature of the reaction will also have increased the rate of the reaction and I have explained this in my analysis.

There are some improvements I could make to my experiment to make the results more reliable and accurate. These include using a measuring gas syringe and using another person as well as some one to start the timer to replace the bung quicker so no gas is lost. Using a gas syringe will increase the accuracy of the results because it is graduated more than a measuring cylinder and the graduations can be seen clearer than on a measuring cylinder because there is no meniscus of the water to affect the level.

The experiment was quite reliable due to the method I used and the apparatus I used. My experiment was repeatable, this means that if I repeated my experiment over and over again, the results would turn out roughly the same. I would like them to always be within 5% of each other and at most there should be a 10% difference between the results.

I could perform further experiments to support my investigation. This would give me an idea about what other factors affect the rate of reaction and in what way. I could extend my experiment by for example: reading the amount of gas produced at intervals during the reaction is occurring and hence see how the rate of reaction changes from the start to the finish of the reaction. I could use a different metal to see if the reactivity of the metal affects the reaction, I could use a different acid, a smaller or larger mass, a smaller or larger volume of reactants to increase or decrease the rate of reaction. I could also add the element of temperature into the investigation: I could submerge the apparatus and the reactants in a water bath set at a certain temperature, leave them for a while to become the desired temperature and then perform the experiment. This method would take longer to prepare but I believe it would be worth it.