The general formula for working out the rate of a reaction is: -
rA = K[A]a [B]b [C]c
Where ra = Rate of concentration change with respect to A.
Where [A], [B] or [C] = concentration mol dm-3.
Where a, b, or c = the order of the reaction.
Where K = the rate constant.
The rate equation will take the form of:
RHCl = k[Mg]a[HCl]b
Since the concentration of magnesium is constant ([Mg]a) then it becomes part of k
RHCl = k[HCl]b
Before I can conduct sums to calculate the value ra, I need to find out what order a reaction is. This can be done using the order graphs on page 247 of the Nuffield Advanced Chemistry Students Book.
Once the graphs are complete, I will decide whether the graph is zero order, first order or second order using the Nuffield Student Book.
When this has been determined, I will apply this to the above equation and work out the rate of change with respect to concentrations of hydrochloric acid or sulphuric acid. This will be done by completing a graph with Log10 Rate on the vertical axis, and Log [acid] on the horizontal axis. The order of the experiment will be gradient of the line.
This sum relates to the practical because, I will be altering the concentration of known substances and deducing ra.
In the experiments, I will be looking at the following null hypotheses to prove if they are correct or incorrect.
Experiment one.
- The effect of basicity of an acid has no relevance to the activation energy.
My method for testing my null hypothesis will be by means of timing how long it takes for a specific size/quantity of either magnesium or zinc to dissolve in a specific amount of an acid at a noted temperature.
Experiment two.
- Different basicity have no effect on the order of a reaction.
For this experiment, the variable I will be changing is the concentration of two acids. This means that all other variables will be kept the same, including heat, size of sample, and volume of liquid.
I will experiment on two acids to try to prove that my null hypothesis is incorrect, I will do this by making several different concentrations of the two acids using a gas syringe. I will use a gas syringe because I know that a metal and an acid produces a metal-salt and hydrogen gas, it is also the best way I know to record the amount of gas given off. Then record how long it takes for a certain amount of gas to be produced.
Experiment one.
Calculation of reagents
For this experiment, the variables that will be changed is the basicity level of acid at given temperatures. This means that all other variables are kept the same, including size of sample, concentration and so on.
Different acids with different basicity levels will be tried first. Different bases include Mono, di, and tri basic acids. Some of the possible acids that could be used are listed below.
Before starting the preliminary experimental work, Firstly, I had to find what amount of reactants kept the acids in excess. This mean that the metal had run out and not the H+ ions donated by the acid. In addition, another reason of keeping the acid in excess is to reduce the temperature rise. This would be a contributing factor for the change in activation energy.
Equations for determining quantities of reactants and temperature change.
Mg(s) + 2H+(aq) → Mg2+(aq) + H2 (g)
Mg(s), H2 (g)
∆Hөf = [Products] – [Reactants]
∆Hөf = [-466.9] – [0]
∆Hөf = -466.9
KJ to J = -466.9 ÷ 1000
J = -0.4669
If use of 30mm of magnesium ribbon, it would weigh 0.04 grams.
Energy exchange in joules = Specific heat capacity × Mass × Temperature change.
The formula can be re-aranged to give the temperature change in joules.
Temperature change = Energy change × Specific heat capacity × Mass.
∆T = Q × c × m
∆T = -0.4669 × 4.18 × 50
∆T = -97.582˚C with 1 mole of Magnesium ribbon.
∆T = -3.903˚C with 0.04 moles of magnesium ribbon.
From this equation, It can be deduced that 50cm3 of acid and 0.04 grams of magnesium, which equates to 30mm length of magnesium ribbon should be used. If scaled up the amount of acid in the reaction, there would be in greater proportion of the reactants; this will decrease the temperature change and make the practical more controlled. In practice this is impossible, because too much acid will be difficult to mix, also the cost will be greater, unnecessarily.
Other points about this reaction is that the need to investigate whether to continue to use magnesium, or use another metal instead such as zinc. I know from the reactivity series of metals that magnesium is more reactive then zinc, so it will react faster with the and weaker acids.
The first preliminary experiment involved a 30mm strip of magnesium ribbon, and a lump of zinc being placed into 2 molar hydrochloric acid and seeing which one of them reacted. From the reaction, the magnesium reacted well with what looked like a constant rate of reaction. On the other hand, zinc reacted much more slowly; this would have been expected due to its relatively low place in the reactivity series.
Preliminary experiment one.
Aim
To find the best concentration of acid and the acids that should be used for the experiment.
Prediction
The initial rate of reaction
The rate of reaction is dependent on the concentration of the chemicals, it is the initial rate of reaction that I will experiment. This is because in this early phase of the reaction the rate of reaction is a true reflection of the rate given by that particular concentration.
Method
Apparatus: - 1 Water Bath
2 50 cm3 Conical flasks
2 thermometers with 0.5°C increments
1 Stopwatch
1 bottle of distilled water
1 Measuring cylinder with 1 ml increments
12 pieces of 30mm magnesium ribbon
200cm3 of 2 molar HCl
200cm3 of 2 molar H3PO4
200cm3 of 4 molar HCl
- Take the water bath and set it to 30˚C for the first experiment and 60˚C for the second experiment, then place a thermometer in it.
-
Take the measuring cylinder and measure out 50cm3 of 2 mole HCl.
-
With the 50 cm3, place it into a conical flask and put into the water bath.
- Place the other thermometer into the conical flask.
- Once both thermometers are reading 30˚C, remove the thermometers and the conical flask from the water bath.
- Drop in 1 piece of magnesium and start the stopwatch.
- Once all the magnesium has stopped fizzing, stop the stopwatch and record the time on a spreadsheet.
- Then repeat the experiment again so it is possible to take an average of the 2 results.
The end point will be the time it takes to dissolve 30 millimetres of magnesium ribbon.
Repeat this method using the phosphoric acid and the 4 molar hydrochloric acid
Results
2 Molar Hydrochloric acid
2 Molar Phosphoric acid
4 Molar Hydrochloric acid
Evaluation
These experiments show it would be better to use a 2 molar acid, the times indicate a long enough period to time. Also choosing a longer period will help keep the errors to a minimum.
Four molar acids would be too fast a reaction especially for sulphuric acid, because it has a high disassociation value it will have a faster time than hydrochloric acid. This would greatly increase my error so the use of the 2 molar acids for experiment will be better.
After this a second experiment was conducted. The collection of the gas given off, it was tested it with a lighted splint. The result was a squeaky pop from the test-tube, this shows that the gas produced was hydrogen. The acids used in the experiment will be hydrochloric acid, sulphuric acid and phosphoric acid.
Experiment two.
Calculation of reagents
The following equation is how to work out the amount of gas that would be produced.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2 (g)
Or
Mg(s) + H2SO4(aq) → MgSO4(aq) +H2 (g)
Gas produced = Volume of solution cm3 / 1000cm3 × Concentration × 24000
Gas produced = 50cm3 / 1000cm3 × 0.04dm-3 × 24000
Gas produced = 48 cm3
For this reaction, 50 cm3 of acid and 0.04 grams of magnesium ribbon will be tested. The time will be stopped when my gas syringe reaches 40cm3.
Preliminary experiment two.
Aim
To see if the correct amounts of reactant have been used and to see what results they produce for analysis. At this stage, I will be evaluating my method of instrumentation. I know from the equation at the top of this page that the gas given off from the reactants is hydrogen. The most accurate way of measuring a gas given off is by using a gas syringe.
Method
Apparatus: - 1 50 cm3 gas syringe with 1 cm3 increments.
1 Clamp & stand
4 50 cm3 conical flask
1 50cm3 measuring cylinder
1 stop watch
100 cm3 of 1 molar HCl
100 cm3 of 4 molar HCl
100 cm3 of 1 molar H2SO4
100 cm3 of 4 molar H2SO4
8 30mm strips of magnesium ribbon
-
Take the measuring cylinder, measure out in turn 50cm3 of all the acids, and place them in to a separate conical flask.
- Take the clamp & stand and set up the gas syringe with the appropriate tubing to fit all conical flasks.
- Take a magnesium strip and place in a conical flask quickly inserting the tube from the syringe and press start on the stopwatch.
-
Once 40cm3 has been reached, press stop on the stopwatch and record the time.
- Repeat the experiment with the other acids
- Repeat whole experiment for two sets of results for each acid.
The end point will be the time it takes to produce 30cm3 of gas
Results
Hydrochloric acid
Sulphuric acid
Evaluation
Unfortunately, due to the nature of equipment pressure could only build to produce 35cm3 of gas. Another reason for the lack of gas in the gas syringe, is the amount of tubing that was used to connect the conical flask to the gas syringe. The tubing was 3 millimetres diameter, and 200 millimetres long. It could also be due to the lack of an airtight seal, somewhere on the apparatus. This will not change my overall result though. This is because the reduced volume will not change throughout the experiment. The method of investigation and it will use 30cm3 as the measured volume to time.
As you can see from the results, even at this point the sulphuric acid has a faster reaction time than hydrochloric acid. This reaction went reasonable well and with the changes I have made it will be suitable for investigation.
Experiment one.
Hypothesis
- The effect of a basic acid has no relevance to the activation energy.
Method
Apparatus: - 1 Water Bath
3 50 cm3 Conical flasks
2 Thermometers with 0.5°C increments
3 Stopwatches
1 Bottle of distilled water
3 50 cm3 Measuring cylinders
63 Pieces of 30mm magnesium ribbon
1050cm3 of 2 mole H2SO4
1050cm3 of 2 mole HCl
1050cm3 of 2 mole H3PO4
Cotton wool
- Take the water bath and set it to 30˚C.
-
Measure out 3 times 50cm3 of sulphuric acid
- Place all 3 samples in to separate conical flasks.
- Place the conical flasks into the water bath and wait until temperature has reached 30˚C. Using 1 thermometer for the conical flask and 1 thermometer for the water bath.
- Place the magnesium strip in one of the conical flasks, then start the stopwatch and add the cotton wool to the neck of the conical flasks.
- Once the magnesium has stopped fizzing, stop the stopwatch and record the time.
- Repeat stages 5 and 6 with the remaining conical flasks and record the results.
- Repeat with Hydrochloric acid and phosphoric acid using separate measuring cylinders.
- Remove the conical flasks from the water bath and wash with distilled water.
- Repeat stages 2 to 8 twice more and record the results. For triplicate results.
- Once 3 results have been obtained for 30˚C for all acids change the water bath temperature to 35˚C
- Repeat stages 2 to 10 but instead of 35˚C change the water bath to 40˚C.
- Repeat stages 2 to 9 using 45, 50, 55, 60˚C
Conical flasks and cotton wool used to stop fizzing from reducing the conc. of the acid.
Experiment two.
Hypothesis
- Different concentrations have no effect on the order of a reaction.
Method
Apparatus: - 14 50cm3 Beakers
1 gas syringe with 1cm3 increments
1 Clamp & stand
3 50cm3 conical flasks
3 50cm3 measuring cylinders
1 stop watch
500cm3 of 2 molar HCl
650cm3 of 4 molar HCl
500cm3 of 2 molar H2SO4
650cm3 of 4 molar H2SO4
42 30mm strips of magnesium ribbon
- Make up the following dilution series. Using 2 molar acids hydrochloric acid, sulphuric acid and distilled water. Measure out the amounts of water twice for the different acids, and place them in separate beakers. Then take the hydrochloric acid and add the correct amounts for each concentration. Then do the same with the beakers that are left with sulphuric acid. Use separate measuring cylinders for each reagent.
- Then label them, so that the different concentrations and different acids are not mixed up.
- Then do another dilution series to get other concentrations, this time using a 4 molar acid and eight beakers for the two types of acid.
- Set up the gas syringe and tubes with a conical flask attached.
-
Take 50cm3 of 1 molar hydrochloric acid and pour it in to the conical flask.
- Then add the piece of magnesium, replace the lid and set the stopwatch going.
-
Once 30cm3 has been reached, stop the stopwatch and recorded the results.
- Then repeat the same experiment twice.
- Then repeat stages 5 to 8 with 1.5 molar hydrochloric acid.
- Then repeat stages 5 to 8 with concentrations 2, 2.5, 3, 3.5 and 4.
- Finally, repeat stages 5 to 10 with sulphuric acid.
Experiment one.
Results
Manipulated Data
Experiment two.
Results
Manipulated Data
Calculations
Working out the activation energy
From the graph measure the x and y value.
And complete the following sum :-
EA = Y
X
EA Χ 10-3
EA Χ R
R= 8.31
Phosphoric Acid
EA= Y= 1.00
X= 0.3375
EA= 2.96 Χ 103 Χ 8.31
EA = 24622.22 J mol-1
1000
EA = 24.6 KJ mol-1
Hydrochloric Acid
EA= Y= 0.825
X= 0.35
EA= 2.36 Χ 103 Χ 8.31
EA = 19587.86 J mol-1
1000
EA = 19.6 KJ mol-1
Sulphuric Acid
EA= Y= 0.75
X= 0.45
EA= 1.67 Χ 103 Χ 8.31
EA = 13850.00 J mol-1
1000
EA = 13.85 KJ mol-1
Working out the order of reaction
Order of reaction of the line of best fit = Y
X
Hydrochloric Acid Sulphuric acid
Order = 1.1 Order = 0.5
0.55 0.35
Order = 2 Order = 1.4
Experiment one.
Conclusion
The null hypothesis in this case was correct. I didn’t find a direct correlation between the basicity of the acid and the activation energy. As you can see from the activation energy calculations, they show that the activation energy does not increase as the number of H+ atoms in that acid increase. Therefore, it tells me from the calculations that there is no realstionship between the activation energy and the amount of hydrogen atom being produced.
The results that I obtained are in the right pattern from my basic understanding of chemistry. Phosphoric acid reacts the slowest, therefore I needed to increase the activation energy for it to react with the magnesium. Sulphuric acid is the fastest to react, as it has the lowest activation energy there needs not much energy for it to react with the magnesium molecules, and hydrochloric acid that reacts somewhere in-between.
A more advanced look at the chemistry of the molecules gives me a better look at why the activation energies are different, and why they don’t conform t the patterns I suggested in my null hypothesis. Seen below are the three molecular models of acid.
The reason why Sulphuric acid has such a low activation is due to the pie bonds. These pie bonds attract the electrons toward them from the O – H
Sigma bond to form a cloud nearer the sulphur molecule. As the pie bonds are shorter and closer to the sulphur, it makes them stronger and so attracts these electrons. The movement away of the electrons from the O – H bond makes the hydrogen molecule fall off comparatively more easier than the other acids.
Hydrochloric acid is the next easiest molecule to break down, and has the next lowest activation energy. It is more difficult to pull this molecule apart because it is very simple, it also has a full outer shell of ionic bonding. This full outer shell means that the molecule is “happy” speaking metaphorically. I.e. the molecule doesn’t want to loose an electron. Due to the intermolecular forces it keeps the electron and hydrogen molecule very close forming the larger activation energy for the removal of the hydrogen.
Phosphoric acid has the highest activation energy, this is because the phosphate molecule pulling equally on the oxygen molecules that are intern pulling on the hydrogen atoms, this molecule is intermoleculally very stable. There is only one double bond pulling the hydrogen’s electron away and if this value is shard between all the hydrogen molecules the force is minimal on all sharing electrons.
Evaluation
There are only some minor points about this experiment that could have affected the results. In the method (Experiment 1 Method 5), I dropped the piece of magnesium from the neck of the conical flask down into the acid. One problem that arose was that the piece of magnesium would not fully submerge in the acid. This means that only a small proportion of the surface area of the magnesium was in contact with the acid. This would have the effect on the results, as to lengthen the time. This is why I chose to triplicate my results. Although to improve this experiment I could also repeat the experiment many more times, this would improve the accuracy in the results.
Another point was that the time from dropping the magnesium, to pressing the stopwatch, there was a small delay. Due to the continued delay throughout all of the readings, all readings are still valid, this is because the continual delay would have been apparent in all of my readings.
One point in method 9 was I washed the conical flasks and the measuring cylinders. I washed them with distilled water. If these were not completely dry, then the addition of water would decrease the concentration of the acid, although the amount of water left would be so insignificant it would not really affect the results, also the water addition would have been constant throughout all of my results, so would not affect the overall results.
Experiment two.
Conclusion
I concluded from this experiment that the order of sulphuric acid was two, and for hydrochloric acid, an order of 1.4 obtained.
I believe that the figure obtained with hydrochloric acid is incorrect, due to inaccuracies of equipment; I think this value should be an order of one. Therefore, I can explain, that as the basicity of acid increases so does the order, i.e. the basicity of sulphuric acid is two, also the order is two. I would also go as far as saying, if I experimented on the order of phosphoric acid, it would be an order of three, this is because its basisity is three. This is
Evaluation
In experiment, two I used a gas syringe. The amount of hydrogen gas was measured and the time taken when it reached 30cm3. An issue of using a gas syringe is that there is connecting rubber tubing. This tubing also contains hydrogen. The true value of gas was there for greater than 30cm3. This factor was taken in to consideration; to stop the variation in length of rubber tubing I used the same gas syringe and same tubing for all of experiment 2.
The time between, I replaced the lid and started recording is also another factor that could have altered my results slightly. Again if I replaced the lid at different timing to starting the stopwatch, my results would be slightly altered, I reality though it was a good point of chemistry. Due to the reactive-ness of magnesium, the point at which dropped the magnesium in the acid would have got rid of all the magnesium oxide build up on the surface of the magnesium.
There are always ways in which I could have improved my results, one way I could have used is by using higher-grade equipment. Alternatively, for this experiment I could have immersed all the equipment in water. This would show me any leaks in the apparatus. The experiment was done on two consecutive mornings; therefore, the temperature would have been different. I could have done the experiment in a controlled closed room that had a constant temperature, therefore the mixture of acid and magnesium and the gas syringe would have all been at the same temperature.
Further points of Error Analysis for experiment 1 and 2
There are many possible sources of error in this series of experiments. With most of them careful working can minimise the impact of them on the final results, but with some others the errors have to be quantified as it is almost impossible to eliminate. The results and graphs already take into account the possible errors.
Absolute Temperature Errors
The thermometers used in this particular experiment were standard “Philip Harris” mercury laboratory thermometers. When checking their readings against each other it was found that they were reasonably accurate, giving temperature readings all within 0.5oC of each other. However, it was found that the water bath which were available allowed the temperature of the water to deviate up to ±2.5oC (0.8% on the kelvin temperature scale), and that the water bath did not have a very accurate thermostatic dial. (It gave a temperature of between 44oC and 39oC when the dial was set to 35oC.) By having beakers of the chemical within the water bath helped to absorb this deviation, and the deviation of temperature within the beaker was around ±1.0oC, therefore the temperature is considered to be accurate to within ±1.0oC. In the temperature range 20 ± 1.0oC to 80 ± 1.0oC the errors in 1/Absolute Temperature is between ±0.000008 and ±0.000012 (0.25% typically). Given the scale of the graph, this error can be considered negligible, and its effect is likely to be swamped by errors produced in other measurements.
Errors in the concentration of the standard solution.
A 3-figure balance was used to measure the amount of magnesium to be used in a standard flask, but as there were draughts in the lab it was usually only possible to measure the mass to within ±0.01g. The 50cm3 standard flask that was used had a specified error of ±1 ml at 20oC. Other possible errors may occur which affect the concentration, for instance, the beaker in which the solution was made up may contain impurities despite repeated rinsing with distilled water, and the magnesium may not have been adequately rinsed out of the beaker before the standard flask is filled with distilled water. The total quantifiable error in the concentration is the 1% resulting from the errors in mass (assuming a typical mass of 1.0g) and the 0.4% resulting from uncertainties in the volume, and therefore has a total possible error of 1.4%. However, at times new batches of standard solution had to be brought in during an experiment and this could explain why some of the points deviate from the line of best fit by a fair amount.
Errors in starting and stopping the timer.
As mentioned above, the human reaction time is about 0.5s. The average time for a run of catalysed experiment is around 20s, therefore the percentage error is 2.5%, and all the times should be treated as ±0.25s.
Other sources of random errors.
The errors listed above all have a random nature, and it is therefore
impossible to remove the error by adding a ‘correction factor’. Even after a correction factor is added, there would still be other sources of random errors such as operator errors and differences of room temperature across the 4-week period.
It can be clearly seen from the acid concentration graph that there is an indirect proportionality between acid concentration and rate, so therefore hydrochloric acid is almost a first order reagent. The same graph shows a negative curved relationship and therefore is second order. When a log-log graph is plotted a straight-line relationship is obtained, confirming this is second order
Bibliography
.
Relating to the rates of reaction.
Frank Brescia et al., Experiment 41: Determination of the Rate Law for a Reaction; Catalysis, Fundamentals of Chemistry - Laboratory Studies,
Fundamentals of Chemistry - Laboratory Studies, Academic Press (1966, New York and London), p.149.
Beverley A. Ardron et al., Potentiometry and kinetics, Education in Chemistry September 1986, Education in Chemistry (1986, London), pp.151-152.
Alex Lu, Scientific Investigation Report: The factors which affect the rate of reaction between acids and limestone, The Edinburgh Academy (1994, Edinburgh).
General References
Hazcards, BDH Chemicals Handbook, BDH Chemicals (1994, Reading).
Nuffield, Nuffield Advanced Science: Book of Data, Nuffield Science Foundation (1984, Birmingham).
