In contrast to nitric acid, sulphuric acid reacts too quickly with the magnesium ribbon, which means it cannot be measured accurately. It can be finally observed that hydrochloric acid is the most suitable for the experiment as it completely reacts with the magnesium in the most suitable time. Ethanoic acid is the only weak acid available; therefore it will be used to draw comparisons to investigate the difference between strong and weak acids.
The experiment was initially conducted with a 2. 00M solution of hydrochloric acid at room temperature and at 60. 0oC. At 60.0 oC the reading was 18. 22 seconds. The intense speed of the reaction proved a disadvantage as it made the experiment hard to control. It was then decided against using this concentration. This could have been overcome by using a higher volume of acid e. g. 20. 00cm3. This is an awkward measurement to use with a standard sized 10. 00 cm3 pipette, as it would have to be measured twice and placed into the test tube. This will lead to more inaccuracies in the experiment. It was finally decided to use a concentration of 1. 00 molar. Choosing the Length of Magnesium Ribbon:
To choose the appropriate length of magnesium ribbon several different lengths were to be tested in a reaction with 10. 00cm3 of 1. 00M acids at room temperature (21. 0 oC). The following results were obtained: Length of magnesium (cm) Time (seconds) 0. 50 42. 23 1. 00 42. 28 1. 50 79. 42 2. 00 60. 53 The results above show some anomalies, for example the 1. 50 cm reacted in a longer time than with the 2. 0cm. This is inconsistent with the background research that was conducted because it suggested that a longer length of magnesium ribbon would result in a slower rate of reaction.
The table below shows the time it took to displace the various lengths of magnesium at 1. 00M solution of hydrochloric acid at 60 oC. Length of magnesium (cm) Time (seconds) 0. 50 42. 35 1. 00 42. 15 1. 50 63. 28 2. 50 79. 33 3. 00 92. 21 It is clear from the table above that the timings produced with the 1. 00M of solution are manageable to use. A 2. 00cm strip of magnesium ribbon will be used in the final experiment, as it is easier to measure out and to cut practically. The experiment shall have set temperatures that will range from 20. 0 oC to 60. 0 oC in ten-degree intervals.
The highest temperature will be 60. 0 oC because as observed in the background research, any temperature above this would lead to the acid evaporating. Each temperature should be repeated twice in order to rule out any anomalous results. This will then improve the reliability of the experiment. Quantities Used in the Final Experiment: The following quantities will be used in the final experiment: 10cm of magnesium ribbon weighed = 0. 07g Therefore 2. 0 cm weighs 0. 014/5 = 0. 014g Number of moles of magnesium strip = mass/ moles
It is important to note that the volume of acid used in the experiment is an important factor, which must be kept constant. This is because in theory if more magnesium ribbon was added to excess in the system, eventually there will be a blockage point in which the magnesium 2+ ions will find it difficult to get away from the active site. These will in turn slower the rate of the reaction. This is called crowding effects. Using 10 cm3 of 2 molar acids Number of moles = (volume/ 1000) x concentration = 10/1000x 2 = 0. 02 moles of acid Ratio of acid to.
It is also clear that the number of moles of acid is greater than the number of moles of magnesium. This indicates that the volume of acid used is in excess because it is five times larger. This will add an advantage to the experiment, as it will ensure that the reaction on hand is completed. Also the excess acid will absorb any heat evolved, as it is an exothermic reaction because bonds are being broken. The volume of hydrogen produced = number of moles x 24000cm3 = 8. 3 X 10-4 moles x 24000 = 17 cm3 of hydrogen gas evolved. The Variables: The dependent variable in the reaction is the temperature varying from the first experiment.
This will determine the activation energy of the reaction. Another factor to consider is that the magnesium ribbon has already reacted with the air, forming a layer of magnesium oxide. Before each experiment the magnesium ribbon was rubbed ten times in order to remove the magnesium oxide layer and leave the pure metal. The number of strokes will be kept constant as too many can decrease the mass of the magnesium, therefore decreasing the number of moles which may decrease the rate of the reaction. Also the same concentration of acid is needed for the first experiment.
The water, which will start at room temperature, will absorb some of the excess heat of the exothermic reaction to keep the temperature the same. Apparatus: The following diagram shows the apparatus, which was used for all the experiments: Final Practical Procedure: When conducting the final experiment it was important to note that any change to the compiled method (compiled from the preliminary experiment) would result in inaccurate results. The final experiment was conducted using the following method. 1. Arrange the apparatus shown in the diagram. 2. Cut a 10. 0 cm strip of magnesium ribbon.
3. Stroke x10 with sandpaper over the magnesium ribbon 4. Measure out 2. 0cm and mark with a pencil. Then cut the magnesium ribbon 5. Fill the beaker 200. 0 cm3 with tap water halfway 6. Place 10. 0 cm3 using a pipette of the required acid i. e. HCl or ethanoic acid in a boiling tube 7. Place the boiling tube containing the acid in the beaker of water. 8. Place a thermometer in the acid 9. Heat the system using the Bunsen burner until the required temperature and stop when the temperature is accessed 10. Remove the Bunsen burner to avoid over temperature rising above the required temperature.
11. Place the magnesium strip immediately after removing the heat 12. Immediately start the stop clock. And stop when the magnesium disappears via effervescence. 13. Repeat the process three times for every temperature required 14. Place results in a table. The second experiment on hand focuses on the order of the reaction as the concentration of the acid is varied. The order of the reaction with respect to the reactant can be found and analysed using an initial rate method. This will be done by analysing the effect of concentration of the reactant on the rate of reaction.
In this experiment the reactant will be is hydrochloric acid. The method used will be similar to the first experiment however the temperature will be kept constant. Rate = k [X]a If Ln is removed from each side of the equation, Ln Rate = a Ln X + Ln constant. A graph with Ln Rate on the vertical axis and Ln [H+] on the horizontal axis will be produced, where the gradient will be the order of the reaction. The second experiment will make use of the same acids as the previous experiment, where ethanoic acid is used for the weak acid and hydrochloric for the strong acid respectively.
In this experiment with a constant temperature and a variability of the concentration of hydrochloric acid, it is likely that the hydrochloric acid will produce faster timings because it is a stronger acid. The rate equation and procedure will be kept the same as the previous experiment. Also the errors are the same in the experiment. The temperature of the experiment must be kept constant throughout. Placing the system into a water bath to maintain room temperature will do this. Also using excess in terms of volume of acid will help maintain a constant temperature and concentration, for the same reasons as the fist experiment.
In conducting the second experiment, firstly the rate of the reaction will be calculated using the above method. Secondly, the experiment will be repeated for each concentration used in order to produce more results so an average can be taken. From the compiled results a graph will be constructed with a line of best fit. The graph will be used to determine the gradient of the line by Ln rate / Ln concentration. This will then in turn define the order of the experiment with respect to the acid used. From the order of the reaction it is possible to predict a likely mechanism for the reaction.
It is likely that order will be of second order and therefore the graph will be a curve. The curve will be relatively deep and the half-life wont be constant but will increase as the reaction proceeds. Rate=k [H+(aq)]a Ln rate=a Ln [H+ ] + Ln k Y= mx + c The indices a represent the order of the reaction, which is likely to be second. If this prediction is correct then it suggests that the reaction consists of two rate-determining steps, which are slow steps. Each rate-determining step has highest activation energy in the reaction. Therefore it determines the overall rate of the reaction. Prediction:
As the hydrochloric acid has pre-dissociated previously, increasing the concentration of solute will not have an overall effect on the actual order of the reaction. Section 2: Implementation The following shows the results obtained from both experiments compiled into a tabulated format so they can be analysed: Investigation 1 (10. 00cm 3 ) 1. 00M hydrochloric acid (strong acid) + magnesium ribbon: Investigation 1 (10. 00cm3) 1. 00M Ethanoic acid (weak acid) + magnesium ribbon: TEMP/o C TEMP/K (1/TEMP) in K-1 TIME /s Experiment 2 /s AVERAGE /s-1 RATE 1/av time /s-1 In (rate).