The temperature may be difficult to measure, as the temperature of the acid if heated may be different from the temperature of the agar piece, and at high temperature the neutralisation reaction may be too quick to record.
Prediction
I predict that as the concentration of the acid is doubled, the rate of diffusion will double. I believe this because if the concentration is doubled, there are twice as many HCl molecules per unit volume, so the concentration gradient is doubled (twice as steep). The “driving force” is then twice as strong so diffusion should occur twice as quickly. The reaction itself should also take half the time, also because there are twice as many HCl molecules per unit volume, so it is almost certain that twice as many (successful) collisions will occur per unit volume, so the reaction will end in half the time.
The preliminary experiment
Equipment/Reactants
- 1 boiling tube
- 1 test tube
- 1 Petri dish containing NaOH enriched agar jelly, and incorporated phenolphthalein indicator
- 1 large cork borer
- 1 “large” straw (smaller than large cork borer)
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2 x 5cm3 0.2moldm-3 HCl(aq)
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2 x 5cm3 0.4moldm-3 HCl(aq)
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2 x 5cm3 0.6moldm-3 HCl(aq)
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2 x 5cm3 0.8moldm-3 HCl(aq)
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2 x 5cm3 1.0moldm-3 HCl(aq)
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10cm3 measuring cylinder
- Test tube rack
Method
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Measure out the 5cm3 of acid using a measuring cylinder, and then pour into the boiling or test tube.
- Collect a piece of agar using either the straw or the cork borer.
- Drop the agar sample into the acid, and simultaneously start the stopwatch.
- Holding a piece of white paper behind the boiling or test tube, keep checking to see how much of the agar has gone colourless.
- Stop the stopwatch when the whole section of agar is colourless.
Results
I originally thought that I would only need 5cm3 of acid in each experiment, and I was right.
I also chose to use the “larger” straw instead of the cork borer to get our agar sections, as the larger samples took too long to be neutralised.
Dilution Table
I used this table to make the other concentrations of HCl (other than 1.0moldm-3), using the 1.0moldm-3 acid.
To do this I would need a 10cm3 measuring cylinder, to measure out both ingredients, and the boiling tube I would be using in the experiment.
Method to get 0.2moldm-3 acid
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Using a pipette, I would collect some water, and squirt it into the measuring cylinder until the bottom of the meniscus reached the 4cm3 mark.
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I would collect some 1.0M hydrochloric acid, and squirt it into the measuring cylinder until the bottom of the meniscus reached the 5cm3 mark.
Safety
In order to make sure the experiment runs smoothly, I need to keep several safety precautions in force, the main one being to wear goggles during the experiment, to keep our eyes safe if there is a problem.
In high concentrations, hydrochloric acid is corrosive, but in the concentrations I am using it can be irritant, so I should take care when handling the acid, or preferably wear gloves and protective clothing.
Hydrochloric acid should not be inhaled as it damages the respiratory system.
Sodium hydroxide is actually corrosive, so I should probably wear gloves, so that it does not come into contact with my skin.
Reliability
To make sure my results are reliable, I will carry out an experiment with each concentration of acid 3 times, and then take an average of the results, after dispelling any anomalies. They are only anomalies if they are more than 10% outside of the other results.
To make the experiment a fair test, I will not change any other variables-:
- I will check the temperature of the acid before and during each experiment, to make sure the temperature stays the same during the reaction (unless it is endothermic or exothermic). If the temperature is not correct, I can hold the test tube until the contents have warmed up slightly.
- I will keep the size (and hence surface area) of the agar section constant by using the same Petri dish (my designated Petri dish), and using the same straw to collect it each time.
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I will use the same amount of acid (5cm3), and it must be an excess of acid, so the reaction will definitely be completed. I will make sure I use exactly 5cm3 by using a 10cm3 measuring cylinder to measure out the amount of acid I need.
Method (Actual Practical)
The method for this experiment will be the same as the method used in the preliminary experiment. I will also use the same volume of acid. This time I will use a boiling tube if that is what is available- the tube used is unimportant, and I will use a straw, to save time during the practical.
Following the diffusion/reaction
I will time how long it takes for the agar section to turn colourless, and then using the equation (1/s), turn the time into a rate.
Sketched graph
The graph showing rate of diffusion over concentration should result in a straight line:
Observation
The results table
NB: Results in red are anomalies, and were not used when the average time was calculated- the repeated results were used instead.
Accuracy
It would have been better if I had timed the time the diffusion took to 1 decimal place, as it would have made my rate of reaction more accurate, but by the time I had realised this, I had recorded around half of my results, so I would either have to repeat them all to get times to 1 decimal place, which would take a very long time, or fabricate my results, which would not be scientific at all.
Detecting the end point
To detect the end point of the diffusion, when it turned colourless, I held the test tube up to a piece of white paper in relatively bright light to see when all the phenolphthalein had turned colourless.
Analysis
Statement about the graph
The graph shows that the rate of diffusion has increased with the concentration of the hydrochloric acid, so a good summary statement for this would be:
“As the concentration is increased, the rate of reaction increases”
- OR -
“The higher the concentration, the faster the diffusion/the higher the rate of diffusion.”
Gradient of graph=
My Conclusion
The graph has shown that my initial hypothesis is correct. My prediction was “as the concentration of the acid is doubled, the rate of diffusion will double.”
When the concentration of the acid was doubled from 0.2moldm-3 to 0.4moldm-3, the rate of diffusion doubled exactly from 0.0013s-1 to 0.0026s-1. Also, when the concentration of the acid was doubled again from 0.4moldm-3 to 0.8moldm-3, the rate of diffusion doubled from 0.0026s-1 to 0.0054s-1. With this evidence I can keep my prediction, and use it as my conclusion.
The rate of diffusion is directly proportional to the concentration of the acid because when the concentration is doubled there are twice as many HCl molecules per unit volume, so the concentration gradient is twice as steep (agar piece contains negligible acid), so the driving force making the acid molecules move will be twice as great, and the acid particles will move twice as quickly. This would mean that the agar would be supplied with twice as much acid in the time given, so the reaction would take half the time (the rate of reaction would also double.)
Validity
The data reinforces my prediction, as the points on the graph show exactly what my prediction states (see above).
Evaluation
The procedure
There were several mistakes during the procedure, which led to numerous anomalies, which had to be repeated and consumed time. Sometimes when I was collecting an agar piece it was not taken up with the straw, so I had to try again. As I was trying to save my portion of agar, I tried to take up the same piece, but I could not do it properly, so some of it was cut off, resulting in a smaller piece than normal (as well as slightly decreasing the surface area. A smaller piece meant there was less distance for the HCl molecules to travel in order to neutralise the whole section, and certain sides of the “centre” would be neutralised before others. If the piece is smaller it does not affect the rate of diffusion, as the concentration gradient is not changed (the agar contains negligible if any acid), but it will affect the rate of reaction, because it will take less time for the entire section to be neutralised, When it is inverted, it will be abnormally high, which will appear as a anomaly on a graph. If the piece is not too different in size from the normal sections, the results for the time and rate of reaction will not be too far from a normal result, and so would appear as a consistent anomaly on a graph.
I think this is why we got so many anomalies- results more than 10% outside of the others for their data point (concentration).
Occasionally I started the stopwatch too late, or I accidentally paused the timing for about 2 seconds during the reaction, as the buttons on the stopwatch were very sensitive. This decreased the precision of my results, but they did not really affect the rate or diffusion as I was working with such small numbers (the rates).
The evidence
The data was very useful to me in calculating and graphically representing how the rate of diffusion is affected by the concentration of the acid. It was recorded to a reasonable degree of accuracy (nearest second, no decimal places) for a school laboratory experiment.
For all the different concentrations the times recorded are all quite precise (close to each other), no more than 16 seconds between all three of them. This made my average accurate, so my best fit line would be more accurate.
For several reasons I did obtain some blatant anomalies- and they are all highlighted in red on the results table (page 6). Using these to calculate the average time for each data point would have been silly, as it would change the average, by lowering it, so the line would not be anywhere near straight.
Sources of error and areas of improvement
The main reason that I obtained anomalies in my results was the strange shapes of the agar sections, if not collected properly. Using the straw, it was difficult as usually a vacuum is required to suck up the agar piece. The only simple way to do this was to cover the top of the straw with my finger, so very little or no air can go in or out. The straw, being made of plastic, is also bendable, so it could bend in the agar, possibly resulting in squashed agar. One way to resolve this problem would be to have a small metal tube, very similar to a small cork borer, which could still be covered to create a vacuum. This would mean that the agar pieces will be basically the same every time!
Also, the agar could be fed through something similar to a meat grinder, but much, much smaller, to cut up the agar into pieces of the right size.
It was quite difficult to control the temperature during the experiments, and I believe that it may have increased as the test went on. This could be controlled by using a thermostatically controlled water bath, which held the test tube during the experiment. I could set the temperature it was supposed to stay at, and the water bath would make sure it stayed at that temperature.
When I was coming towards the end of my testing, someone sabotaged my agar portion by squirting acid on it, so that the acid was neutralised. Due to the phenomenon of diffusion, the sodium hydroxide present in the untouched areas of my agar moved towards the neutralised parts that had a low concentration of sodium hydroxide, so equilibrium was reached. This would mean that the overall NaOH concentration in the agar would be significantly lower compared to what it was before the sabotage. This should not affect the rate of diffusion, as the concentration gradient (of the acid) will remain the same. However the rate of reaction may decrease.
The timing may have started a bit late- I was slow to react when I dropped the agar section into the acid. This is a classic example of human error, and can really only be resolved by being quicker to react.
Detecting the end point when the neutralisation has been completed and the agar section has turned colourless can be quite difficult- as it can be hard to actually see the section in front of the paper. A different indicator could be used, for example universal indicator, so I can track the progress of the neutralisation, by seeing what colour it is. When it has turned green (neutral, pH 7), I will know that the reaction is over. The table below shows the colours of the Universal Indicator, and their respective pHs. (From )
A colorimeter could be used instead, to announce or record when the agar has turned completely colourless.
It would be advisable to use fresh equipment, because when I carried out my experiment, there was some residue in my boiling tube and measuring cylinder. They could have been reactive, and they could have reacted with the acid. This cross-contamination could adversely affect my results.
Proximity of the graph points to the best fit line
The points on my graph are somewhat far away from best fit line- they are a maximum of 2.2cm away. This gives me a slightly weak conclusion.
The points above the line may be excessively far away from the line because I did not dilute the acid correctly, so the rates of reaction are abnormally high- I may have used too much acid or too little water, so the concentration of the 0.2moldm-3 acid may actually have been around 0.33moldm-3 as the best fit line on the graph suggests. The opposite applies to the 2 points below the line, as the concentrations may have been lowered than they were supposed, because I may have used too much water (or too little acid).
I think this kind of thing would only have happened on a small scale. I measured out the acid and water using a measuring cylinder that measures to the nearest cm3, so it is unlikely that I will be more than 1cm3 out. It is possible that I may have misread the side, thinking that the bottom of the meniscus was exactly on the mark, but it was actually slightly above or below it. Small inaccuracies like this would not affect the concentration of the acid too much, but they could slightly alter the rate of reaction, and also the point plotted on the graph. The more inaccurate I am in measuring out the acid, the further away the point will move form the best fit line.
The points may be far away from the line because the temperature changed during the experiment, when I was not checking the temperature. If the temperature increased (and not due to heat given off by reaction) as the room warmed up, the reaction would occur faster, so the time taken would decrease, so the rate of diffusion would be higher.
I may have detected the end point too late- that is I did not notice that the whole agar section was colourless until a long time after the neutralisation had been completed. This would increase the time, and decrease the rate of diffusion.
The anomalies were excluded when the averages were calculated, so they would be correct.
The rate of diffusion for the 0.8moldm-3 hydrochloric acid was abnormally low at 0.0052s-1, and it was very close to the point for the 0.6moldm-3 acid, so I am calling it a consistently anomalous result. I think it was caused by a combination of the sources of error in each repeat.
Other experiments
Another experiment, similar to this one, could be conducted to see if my conclusion (and also prediction) is applicable to other substances, or over a wider range of data points (concentrations), but in the same type of experiment.
In this second experiment I could have 5 aerosol cans, each containing a gas of different concentration- ranging from 0.2moldm-3 to 1.0moldm-3, increasing in 0.2moldm-3 increments (like in the last experiment). In each experiment, a person would be standing 2 metres away from the spray can, when the gas inside it is sprayed, for 1 second exactly. At that moment the stopwatch will be started, and someone will time how long it takes for the “sniffer” to smell the gas that has been sprayed. When the sniffer smells the gas, they will give a quick signal, and the stopwatch will be stopped accordingly. The same will be done for each concentration of gas, at least 3 times for each one, to maintain reliability. The conclusion should be the same, or very, very similar to the one for the previous experiment.