Looking at the activation energy results I think it will be possible to lower the temperature more to gain extra results, which will result in a better indication of what the activation energy is. I also think that 60oC is a suitable starting temperature as it is not close to the boiling point of either acids.
Apparatus
- Hydrochloric Acid - 0.5M, 1.0M, 1.5M, 2.0M, 2.5M & 3M.
- Ethanoic Acid - 0.5M, 1.0M, 1.5M, 2.0M, 2.5M & 3M.
- Magnesium Ribbon – 1 metre.
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Gas syringe – 50cm3.
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Conical Flask – 100cm3.
- Stop Clock – accurate to 1/100 of a second.
- Test tubes
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Measuring Cylinder – 10cm3
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Thermometer- 100oC
- String.
- Beaker.
- Clamp, boss & tripod.
Procedure
To find obtain results for the reaction order experiment, the apparatus should be set up in the following manner.
Firstly, measure out 20cm3 of 0.5M-3M of hydrochloric acid into separate beakers. Pour the 20cm3 of 0.5M acid into a dry conical flask, which should be washed and dried after each reaction to prevent excess acid reacting with the magnesium. Cut 2.5cm of magnesium ribbon and place in onto a piece of string and conical flask as shown by the diagram. Then connect the gas syringe to the conical flask using a connecting tube, as shown by the diagram again. Make sure the gas syring reads 0cm3 before starting the reaction. Reset the stop clock to zero and then pull the string from the conical flask to release the magnesium strip. As soon as the magnesium lands in the acid, start timing the reaction. Once the volume of hydrogen collected has reached 10cm3, stop timing. Record the time and release the connecting tube. Dispose the magnesium and acid, and then prepare the conical flask, after washing and drying, with 1M hydrochloric acid. Repeat the entire experiment with 1.5M, 2M, 2.5M and 3M hydrochloric acid. To create accurate results, the all the concentrations should be carried out two more times. Once the times have been recorded for hydrochloric acid, repeat the experiment with ethanoic acid using the same concentrations.
Firstly heat the water to 60oC, using the thermometer in the water to measure the temperature. This will also be considered as the temperature of the acid in the test tube. The water will not be heated to above 60oC as it is nearing the boiling point of 85oC boiling point of hydrochloric acid. However, for this experiment a test tube will be used for the reaction rather than a conical flask. In this experiment the concentration of the acid will stay constant, and the temperature will vary. The concentration of hydrochloric acid and ethanoic acid that will be used is 0.5M. Once the test tube is connected to the gas syringe, record the temperature of the acid and pull the string to release the magnesium strip. As the strip lands in the water start timing till 10cm3 of hydrogen is collected and then record the time. Let the temperature decrease by a specific amount, approximately 4-5oC and then repeat the experiment. Carry on doing this till the temperature reaches 20oC. Redo the entire experiment again, but this time using ethanoic acid. The repeats for activation energy cannot be carried out until all the original results are collected.
Risk Assessment & Safety
There are a number of dangerous substances I will be handling, therefore I will take a number of safety precautios, to avoid any injury to myself, or others. Firstly there is hydrochloric acid. This is a corrosive substance, and therefore may cause burns, as well as it being an irritant to the respiratory system. However, as I am using less than 6.5 M, it should be labelled as an irritant. Therefore as a safety precaution I will use safety goggles, and wear safety gloves.
Next there is Ethanoic acid. This, like HCl, is corrosive, but again, as I am using low concentrations, it can be labelled irritant. Therefore I ill use the same safety measures as I used for HCl, which is to wear a pair o safety goggles, and a pair of safety gloves.
Finally, I am using magnesium. This is a highly flammable metal. Therefore one safety measure I will use, is to keep it away from all oxidising agents. As I am using sharp strips, I will wear safety goggles, in order to prevent damage to the eye.
Justification of Apparatus
Acids – I have chosen to use hydrochloric and ethanoic acid, as they are both available in a variety of concentrations and they are both monobasic. This means that the only difference between the acids is their strengths. This will therefore make the experiment fair.
Concentrations – I have decided to use the concentrations from 0.5M to 3M, as they will provide enough results to draw a suitable conclusion. Using concentration greater that 3M will cause a reaction to occur too quickly. This will make timing inaccurate and provide unreliable results.
Gas & Acid Volumes – I am using 2.5cm strips of magnesium for each reaction, which weighs 0.025g. The reaction equations show that one mole of magnesium reacts to produce one mole of hydrogen gas. This means that 0.025g of magnesium will produce 25cm3 of gas. This theorectical volume is more that the amount that I am going to collect, therefore this will allow me to collect initial rate times. The concentration of the acids needs to be similar in all the reactions so that the experiment remains fair. Therefore the acid needs to be in excess to show how the rate of reaction varies with concentration. For the acid to be in excess, it has to be at least five times the number of moles of magnesium. The equations also state that one mole of magnesium reacts with two moles of acid. There are 0.001 moles of magnesium in 0.025g and therefore, at the lowest concentration, there should be 0.01 moles of acid. The lowest concentration that I am using is 0.5M and the volume is 20cm3, which gives the number of moles of acid as 0.01. This is ten times the number of moles of magnesium; hence the acid is in excess.
Magnesium Ribbon – I have decided to magnesium ribbon over zinc powder. This is because cutting the magnesium ribbon strips will be easier than weighing the powder. Using the magnesium strips will be less time consuming and provide more accurate results, as the reacting container will be closed when using the magnesium strips. If I were to use the zinc powder, the conical flask would be open when the reaction started, making the results less accurate.
Gas Syringe – This willl be used to collect and measure the volume of hydrogen gas given off in the reaction. The maximum volume is more than that required for the experiment. Therefore, there will not be any risk of not being able to collect the required volume.
Concial Flask – This will be used to contain the reacting substances and will have a lower chance of the magnesium strip sticking to the side of the container. If I were to use a test tube, the width is equal all the way down the tube. However, a conical flask’s width increases, therefore reducing the chance of magnesium sticking to the sides.
Stop Clock – This will be used to measure how long it takes for 10cm3 of gas to be collected in the experiments.
Test tubes – This will be used in the activation energy experiment as the conical flask will not fit into the glass beaker.
Measuring cylinders – These will be used to measure 10cm3 of acids and to create the mixtures of acids for a specific concentration such as 1.5M.
Thermometer – This will be used to measure the temperature of the acid in the activation energy experiment.
Beaker – This will be used to contain water in the activation energy experiment. Together they will act as a water bath to maintain and lower the temperature of the acids in the activation energy experiment.
Clamp, boss, tripod – This will be used to hold the gas syringe steady in both experiments.
Justification of Method
Dry conical flask – The flask needs to be dry with no water. This is because the water will dilute the acid and therefore, alter the concentration of acid. Also if the conical flask contains other substances, it might result in unwanted effects when the reaction occurs.
Reset gas syringe & stop clock – Making sure both of these read zero will reduce the errors that occur when the experiment is carried out. If the gas syringe or stop clock doesn’t read zero, the time or volume of gas recorded will be incorrect.
Time as soon as Mg lands in acid – This will measure the time exactly from the start of the reaction till the finish.
Dispose of products – This is to make sure that products from the previous reaction do not react with the reactants of repeat, or new concentration experiments. If the products weren’t diposed of, there could more than the required volume of acid reacting with magnesium.
Two repeats – A repeat is necessary to make sure that the original result is valid. Repeating the experiment two more times will make sure that there is at the least one correct result. Repeating makes the data more reliable and accurate, and also giving the opportunity to average results.
Lowering the temperature by 4-5oC – Lowering the temeperature from 60oC to 20oC in 4-5oC steps, will allow me to collect as many results as possible. The greater number of results will allow the conclusion of the investigation to be more reliable and accurate. I have decided to lower the temperataure rather than raising it, as it will be more difficult to increase the temperature to a specfic point. Lowering will allow me to maintain a specific temperature more easily. I have also decided to lower the temperature between two points. This is because it will be impossible to decrease the temperature by an exact amount, for example exactly 5oC.
Six Results (at least) – I will aim to collect six results for the order experiments and at least six in the activation energy experiment. I have chosen to collect six results, as this will be enough to draw a reliable straight-line graph showing the order of reaction.
Fair Experiment
To enable the results of the experiment to be as precise as possible, the experiment needs to be carried out under fair conditions. Firstly, all the acid volumes and the length of magnesium used for each experiment needs to be the same. If these varied, it would be impossible to analyse and interpret the results as there is more than one variable. In the activation energy experiment, the temperature of the water must be below 100oC, as you do not want the water to evaporate. This is because then the volume of the water will change and therefore alter the heat capacity of the water, resulting in unreliable results.
IMPLEMENT
Order Experiment Results
Hydrochloric Acid (3sf)
Average Time
Ethanoic Acid (3sf)
Average Time
Activation Energy Experiment Results
Ethanoic Acid (3sf)
Hydrochloric Acid (3sf)
- Activation Energy = Gradient x Gas Constant
= -3780 x 8.31
= -31400 J
Therefore, the approximate value for activation energy = 31.4 kJ mol-1
- Activation Energy = Gradient x Gas Constant
= -3110 x 8.31
= -25800 J
Therefore, the approximate value for activation energy = 25.8 kJ mol-1
ANALYSIS
From the results collected, it can be seen that the order of reaction for ethanoic acid was one, whereas the order for hydrochloric acid was two. Using the line of best fit on the concentration-rate graph for ethanoic acid it can be seen that there is a directly proportional relationship between rate and concentration. The reason for this relationship is due to the mechanism of the reaction. The line of best fit on the concentration-rate graph for hydrochloric acid shows a direct relationship between rate and concentration. Therefore, the reaction mechanisms must be different for the acids to have different orders. There are three steps in the mechanism for this acid metal reaction. Firslty the H+ ions must form and to react with the magnesium metal. Secondly, the H+ ions must form hydrogen gas by accepting an electron. The last step is for metal ions to react chlorine or ethanoate ions and make the metal “disappear”. Clearly the second and last step would occur at the same speed for both acids. This is because, irrespective of the acid, hydrogen ions will form hydrogen the same way and metal ions will react to form a salt in the same way. Therefore, the rate-determining step must be the first step. This is because hydrochloric acid will produce a greater number of H+ as it is a stronger acid, i.e dissociates completely. However ethanoic acid is a “weak acid” as it does not dissociate completely, meaning it produces less H+ ions compared to hydrochloric acid. Therefore step one is quicker with hydrochloric acid since there are more hydrogen ions reacting with the magnesium. Hence increasing the concentration of acid will dramatically increase the rate of reaction; producing a second order reaction. Increasing the concentration of ethanoic acid doesn’t have such a great effect; producing only a first order reaction.
The activation energies calculated show that ethanoic acid has an activation energy of 25.8 kJ mol-1 and hydrochloric acid 30.1 kJ mol-1. The reason for the difference is activation energy can be understood by considering the different bond energies. When HCl reacts with magnesium, the hydrogen-chlorine intermolecular bonds are being broken. When ethanoic acid reacts with magnesium, the oxygen-hydrogen bonds are being broken. As activation energy is the minimum energy required to start a reaction, the energy required to break the hydrogen-chlorine bond must be less than the energy required to break the oxygen-hydrogen bond. This is the reason why hydrochloric acid has a lower activation energy than ethanoic acid. Covelant bonding is the type of bonding that is present in the hydrogen-oxygen/chlorine bonds. Therefore, the reason for ethanoic acid to have a larger activation energy is not due to the type of bonding. The reason is due to the fact that the hydrogen bonds to the electron in the 3p sub-shell in HCl, but bonds to the electron in the 2p subshell in ethanoic acid. Therefore, as the electrons in the 3p sub-shell are further away from the nucleus than the 2p sub-shell, they will be subject to less forces of attraction from the nucleus. As the force of attraction is less, it will be easier for the bond to break, resulting in HCl having a lower activation energy. The electrons in ethanoic acid experience stronger forces of attraction from the nucleus, resulting in the acid having a greater activation energy. The ease at which the bonds break can be confirmed due to the classification of the two acids. Hydrochloric acid is considered to be a strong acid, whereas ethanoic acid is a weak acid. A strong acid discociates into H+ ions easily compared to a weak acid, which only partially dissociates into H+ ions. This means that the bonding between oxygen and hydrogen in ethanoic acid must be strong than chlorine’s bond to hydrogen, as it only partially dissociates. This is comfirmed by looking at the average bond enthalpies. The oxygen and hydrogen bond has a bond enthalpy of 463kJ mol-1 whereas the hydrogen and chlorine bond only has a cond enthalpy of 431kJ mol-1. For this reason ethanoic acid has a higher activation energy than hydrochloric acid.
EVALUATION
I think that the results obtained for this investigation was accurate enough to provide a suitable conclusion. However from the table of results and graphs it can be seen that there are a few anamolous results. These were, the first time recorded for the 1.5M hydrochloric acid order experiment, and also the times recorded for 2.5M and 3M. The times recorded for the last two concentrations were almost identical meaning, there must have been an error with the method. The various errors that might have occurred during the order experiment are shown below.
The first source of error was the time delay after putting the magnesium into the acid, and eventually starting the stop clock. As there was only one person carrying out this experiment, it is impossible to start timing as soon as the magnesium lands in the acid. There is a delay of between one-second to two seconds between the magnesium landing and the stop clock being started. Obivoiusly this affects the results by providing times that are too long. However, it is not possible to alter the results since I do not know by what margin of time to amend the results by. Therefore, the improvement that could be made is to have two people carrying out this experiment. One person puts the magnesium in, while the other person is ready to start timing as soon as the magnesium lands in the acid. This will consequently improve the accuracy and reliability of the times recorded.
The measuring cylinders were only accurate to 0.5cm3, which might also result in there being errors. This will affect the results, as there will be slight variations in the volume of acid used, also meaning slight variations in concentration. Again this will affect the reliability of the results, as times will be faster or slower. The improvement that could be made is to use pipettes and pipette fillers to accurately measure the required volume of acid. Therefore, this will improve the accuracy and reliability of the results as the method for measuring will be more precise.
The thermometers were only accurate to 0.5oC, meaning this might lead to errors in the experiment. This will affect the results, as the actual precise temperature is not known. In the experiment I am allowing a range between 4-5oC temperature drop. However, if the thermometer is only accurate to 0.5oC, the actual temperature drop might be greater. The only way to improve the accuracy of temperature recordings is to use a digital thermometer. This will provide temperatures readings with greater accuracy and therefore provide trends on the graphs that are more obvious.
Another source of error is the fact that the reaction was exothermic, meaning heat was given out during the reaction. Therefore, in the activation energy experiment the acid temperatures could be incorrect due to the fact that the water temperature was being measured. The water temperature was assumed to be the acid temperature, however since the reaction was exothermic, the temperature of the acid could have been greater. Therefore using the actual acid temperature will give points on the graph that are slightly different to the present results. The improvement that could be made is to actually have the theremometer in the test tube containing acid and magnesium. Having a bung that has a hole big enough for a thermometer (ordinary or digital) will be able to measure the temperature of the acid accurately. This will provide temperatures readings with greater accuracy and therefore provide trends on the graphs that are more obvious.
The measuring cylinders and thermometer caused the greatest errors that can be be improved upon. When I measured 10cm3 of acid in the measuring cylinder, I can be sure that I had more than 9.5cm3 but less than 10.5cm3. The error would be be: 0.5/10 x 100 which equals 5%. Using the thermometer would provide a percentage error of 2.5% for a reading of 20oC. Therefore, if I am measuring a temperature change, and I measure one temperature too high and the other too low, the total error would be twice the error for each reading.