Investigating varying concentrations, affecting the Rate of Reaction between hydrochloric acid (HCL) and magnesium (Mg)

GCSE Chemistry Coursework

Candidate name: Alexander Willink

Aim:

The aim of this experiment is to find out the effect of concentration of acid, in the reaction between dilute hydrochloric acid and magnesium ribbon.

Prediction:

I predict that with an increase in concentration the rate of reaction will there for also increase. This will (I predict) be proven by the amount of hydrogen gas produced. If we increase the concentration, we are in essence, increasing the total number of collisions. This, in turn, will increase the number of effective collisions, and the rate will increase.

Introduction: magnesium + hydrochloric acid magnesium chloride + hydrogen

Mg (s) + 2Hcl (aq) MgCl2 (aq) + H2 (aq)

For a successful collision reaction to take place the reacting particles must collide with enough energy, this energy will need to be sufficient to break the magnesium bonds. Room temperature should be adequate. The rate of reaction can be measured by how many successful collisions occur in units of time, (the amount of hydrogen gas produced). When the concentration increases, so will the reaction. This is because there should be more particles to react with the magnesium. Whilst magnesium and hydrochloric acid particles are making successful collisions their particles are forming magnesium chloride and hydrogen. So eventually the reaction will slow down and come to a halt because there are no more magnesium particles left to react.

There are many other methods of increasing the reaction, (1) By varying the surface area to volume ratio i.e. cutting the magnesium into smaller pieces, there can be more particles surrounding it, and therefore an increase in reaction. (2) An increase in the temperature will increase the average amount of kinetic energy; this would increase the amount of successful collisions. (3) A catalyst will give the specimen a ‘sticky’ surface allowing for more collision.

Risk assessment:

Be aware that magnesium is highly flammable and MUST BE KEPT FROM A NAKED FLAME. Encase of it burning, blue glass or partially open fingers must be used to view the flame. Eye protection must be worn. Wear a lab coat and avoid eating or drinking in the lab. The hydrochloric acid should not come in direct contact with the skin, because it is corrosive and may cause burns. Standard lab procedures must be taken seriously.

Fair test, accuracy:

To ensure an accurate test is carried out, the following variables need ...

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Risk assessment:

Fair test, accuracy:

To ensure an accurate test is carried out, the following variables need to be kept under the same conditions; the temperature, pressure and Mg surface area. In this experiment I will not be using a catalyst.

All instruments should be placed in the middle of the working area, to minimize the possibility of damage. I.e. falling on the floor.

The utmost care and speed must be applied to qualify good results when adding the magnesium to the acid solution to minimise the amount of gas escaping. The reaction will start as soon as contact is made between Mg and HCl.

The experiment will be done three times to allow for a possible freak result.

Apparatus:

100cm³ conical flask
Measuring cylinder
50cm³ hydrochloric acid
0.06g of magnesium*
Stop watch
Thermometer
Flexible, rubber tube
Gas syringe
Specimen tube
Glass tube
Single whole bung

*We use 0.06g, of magnesium because we know from a chemistry textbook that it will produce a measurable amount of hydrogen.

The range of HCL concentrations I am going to use are; (de-ionized water)

0.25m
0.50m
0.75m
1.00m
1.25m
1.50m
1.75m
2.00m

Method:

(1) The gas syringe must be connected to the flexible tube. This tube must then be connected with the single whole bung.

Separately the conical flask must be filled with 50cm³ of hydrochloric acid (at the concentration specified for that experiment). A specimen of 0.06g of magnesium must be weighed and placed in a specimen tube. And using tweezers, must be placed in the conical flask containing the Hydrochloric acid solution, it is important that specimen tube is placed upright in the flask. The bung, must the finally be connected to the conical flask. When all this is ready, the flask must be tipped to allow the specimen to come in contact with the HCL. The timer must be activated immediately, and the results observed and recorded with accuracy. We repeated the experiment three times to ensure a fair test.

Theory: (collision theory)

The collision theory describes how the rate of reaction increase (the time taken for the magnesium ribbon to disappear when it is reacted with hydrochloric acid) when the Concentration of HCL increases. The theory states that if, the more concentrated the Reactants, the greater the number of collisions between particles increases. This also explains why the greatest rate of reaction is usually as soon as the reactants have been mixed, i.e. they are both at their highest concentrations. As the reaction continues, the Concentration of the reacting substance decreases and so does the rate of reaction. We must consider what happens when a reaction takes place. First of all the particles of the Reacting substances must collide with each other, and secondly a fixed amount of energy Called activation energy* (Ea) must be reached for the double bonds to be broken into single ones. If the Particles can produce the right amount of energy (i.e. if they collide fast enough and in the Right direction) a reaction will take place. The reaction is speeded up if the numbers of Collisions are increased. In this investigation we must consider the topic of variables. Clearly, the time taken for the magnesium to disappear when it is placed in different concentrations of HCL is related in some way. The higher the concentration of HCL you use, the less time it takes for the magnesium to disappear and so the rate reaction increases. The concentration of HCL you use is the independent variable because it will Vary, and the time taken for the rate of reaction to take place (i.e. the magnesium to Disappear) is the dependent variable because it depends upon the concentration of Hydrochloric acid. Other variables throughout the investigation, which will vary are the Volume of water used, and the volume of hydrochloric acid. The variables, which will remain unchanged, are the temperature (room temperature) which will stay the same in order for it to be a fair test. Because if the temperature changes it will affect the rate of reaction between the reactants, either by speeding it up if the temperature rises because the Particles move faster and travel a greater distance in a given time and so will be involved in more collisions. Or the temperature may slow the reaction down due to particles moving slower. The amount of magnesium used will stay the same 0.06g, so that it is a fair test? The time it takes for the magnesium to disappear will be measured Accurately using a stop clock as soon as the HCL is poured into beaker with the magnesium in it. This way most results will be accurate.

*Activation Energy

Even if the species are orientated properly, you still won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction. Activation energy is the minimum energy required before a reaction can occur. You can show this on an energy profile for the reaction. For a simple over-all exothermic reaction, the energy profile looks like this:

Conclusion:

I think in al the experiment went well. The evidence shows that the reaction rise is due to the increase in concentration of hydrochloric acid. A clear example of this is the graphs produced from the results. Although there seems to be no correlation between the peak results of each individual test, it is clear that with an increase in concentration, the magnesium reacts quicker, which is due to the amount of successful collisions. The concentration increases the number of collisions, and therefor increasing the probability of a successful collision. This can be seen by the line of best fit drawn from the points. Because of this increase in speed, it became necessary to record readings at different times. For example the 2.00moles of HCL reacted so fast that every 5-second readings were not accurate enough. I decided to record every 2 seconds.

EVALUATION:

There are many reasons why our results for the time graph did not prove the point that Concentration/time, such as?
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 isn’t done properly it can lead to incorrect results).
Using larger amounts of magnesium would give a bigger more accurate conclusion. More points graph
When the bung was placed on the conical flask, a small pressure was created. The measuring cylinder consequently read this. It would usually amount to no more than 2cm³. I did solve this problem in as accurate a way as possible i.e. I submitted all the results to a computer program called excel, where, by using a formula, I was easily able to subtract each starting value from each set of results. By doing this, it would create the illusion that all experiments started at 0cm³.
I should have been more accurate in cutting the magnesium; my graphs prove that the magnesium was cut inaccurately. I would expect all the results to end at the same level. This, however, is not the case. One other explanation of the inaccuracy (you must understand I am trying to shift the blame a little) is the constant minor change in room temperature, emphasized by the fact that the tests were not executed all in the same setting.
I could also have gained more results by simultaneously recording the temperature. I could then ensure that the reaction was taken whilst in similar temperature conditions….different temperatures. How is done.
Another anomaly that is clearly spotted in the graph is the different concluding results for each experiment. I expected all the results to level out at roughly the same level. However there is a clear difference between
The line of best fit drawn from the graph can see the accuracy of the results. If it is clear that a point is not following the line, then it can be accounted for as an anomaly. There may be many reasons for such an occurrence.