- Safety goggles must be worn at all times.
- Good ventilation is required – some chemicals, eg. Ammonia solutions have very potent smells.
- Sodium hydroxide is corrosive. Wash hands carefully and thoroughly.
- Hydrochloric acid and ethanoic acid are irritants.
- Handle the thermometer with care – it contains mercury, which is poisonous.
- Handle all glassware with care – it may be hot after the reaction has taken place.
FAIR TEST
These points should be followed to ensure the experiment is fair.
- Make sure only one input variable is changed at a time.
- Make sure the temperature has reached its maximum before you record it – wait for it to drop.
- Make sure exactly 20ml of each chemical is used in each experiment.
- Make sure the pH is chosen precisely when recording it. In case of doubt, use a second indicator, eg. Phenolthaline.
- When swirling the acid and alkali together in a conical flask, do not put your hand around it. Hold it at the top. Otherwise, the contents will be warmed up from your body heat.
MEASUREMENTS
I have decided that I shall repeat each experiment I do three times. In doing this, I hope that any anomalous results will have a chance to show up. It is important to repeat any experiments more than once. I will record my time to the nearest second and I will record my temperatures to the nearest half ºC. When I’m finding the pH of the solution, I shall use Universal Indicator. To find the nearest matching pH, I shall use a pH colour card to determine the colour/ pH, of the acid/ alkali.
APPARATUS
- Four 100ml beakers + Four 25ml-measuring cylinders.
- A timer.
- 2M hydrochloric acid.
- 2M ammonia solution.
- 2M sodium hydroxide.
- 2M ethanoic acid
- One 100ml conical flask – bung with hole for thermometer.
- Thermometer.
- Universal indicator / phenolthaline
METHOD
- Firstly, an order of experiments to do must be decided. Here is a suggested order: Strong acid and weak alkali, strong acid and strong alkali, weak acid and weak alkali, then finally weak acid and weak alkali.
2. Get four 100ml beakers to hand. For whichever experiment you are doing first, measure out the required chemicals in an amount of 60ml.
3. Measure out 20ml of each chemical into a 25ml measuring cylinder.
4. Take the temperature of the two chemicals. Record these temperatures.
5. Add the two chemicals together into a conical flask and put a bung with a hole for a thermometer in it. Put the thermometer in it.
6. Start the timer.
7. Carefully swirl the chemicals together in the conical flask. Watch the thermometer carefully.
8. Note the maximum temperature it reaches and the time it took.
9. Use Universal Indicator to test the pH. Using a pH colour card, record the pH.
10. Wash the apparatus carefully.
11. Repeat this same experiment twice more so you have three sets of results.
12. Conduct the same experiment, three times each, for your other necessary tests.
13. Wash hands.
PREDICTIONS
I predict that the DH will be negative, because neutralisation is an exothermic reaction. I also predict that:
- Hydrochloric acid + ammonia solution = ammonia chloride and water.
HCl + NH4OH = NH4Cl + H2O
- Hydrochloric acid + sodium hydroxide = sodium chloride and water.
HCl + NaOH = NaCl + H2O
I can predict this because I know that an acid + an alkali = a salt and water.
I predict that the stronger the acid and the stronger the alkali, the bigger the DH will be. This means that a strong acid and a strong alkali should have the biggest DH, then a strong acid and a weak alkali should have the next biggest DH, equal to that of the weak acid and strong alkali. The smallest DH should theoretically be from the weak acid and the weak alkali. I think that this is true because the stronger the acid/alkali, the more particles there are – so therefore there are more collisions. This means that the reaction is faster, but should also give off a lot of heat.
Obtaining Evidence
Whilst I worked on this experiment, I used extreme care and followed all the points covered in the risk assessment.
When I conducted this experiment, I had to re-do it. This is because the method I primarily used wasn’t fair. My teacher pointed this out to me. To start with, I was going to do the experiments in the same way, but put a one-minute time limit on it. I thought that that would be an efficient way to conduct it, because then each experiment would be the same length. I believed that this would be fair because I didn’t think very carefully about it. This became obvious to me, after I had conducted several experiments, that this wasn’t fair. This was because I was trying to find the temperature, the maximum. If I put a one-minute time limit on this, then I’m not giving the temperature a fair chance. The temperature may have reached its maximum, then dropped, or it could still be on its way up. My teacher suggested another way I could have done this. He said that I could take the temperature at 10-second intervals. However I chose to do it differently. I didn’t put a time limit on it, but I still timed the reaction, so that I had more numbers to play with. This experiment was now fair.
RESULTS
To work out DT, you take initial temperature from final temperature:
DH = m x s x DT
Where: m= mass of chemicals being used (20ml)
: s= specific heat capacity of water (4.2J/k/g)
: DT= change in temperature, initial – final.
Analysing and Considering Evidence
Looking at my tables, I can see that what I have predicted is true. I predicted that because neutralisation is an exothermic reaction, the DH would be negative. Shown above is the table for DH. All of the DH are negative.
I also predicted that the biggest change in temperature would be from the strong acid and the strong alkali. Looking back at my tables I can see that the biggest change in temperature is indeed -11ºC, the strong acid and the strong alkali. To back up the other predictions, I find that the weak acid and strong alkali, and the strong acid and weak alkali had a very similar result, -9ºC and –9.5ºC. The weak acid and weak alkali had, as I predicted, the smallest change in temperature, -7.5ºC.
I also predicted that the biggest DH would be from the strong acid and strong alkali. Looking at my table, I see that this is correct. As with the change in temperature, the strong acid and weak alkali had practically the same result as the weak acid and strong alkali. This is what I predicted. The weak alkali and weak acid had the smallest DH, as I predicted.
I have drawn some graphs to show/present my data in a different way. They show what I have explained above a little more simply. They show the relationship between the strong acid and strong alkali, the strong acid and weak alkali, weak acid and strong alkali and the weak acid and weak alkali. They show the change in temperature as well as DH.
Evaluating
I think that this investigation was successful. I achieved the results that I predicted I would gain. My prediction was correct and I had scientific knowledge to back it up. All of my results seem to support the predictions I made. I do not seem to have any anomalous results. I think that my results were very accurate. Although I achieved what I predicted, I believe that I could have made improvements to my method to gain even truer results. I conducted the experiment in an inappropriate way to begin with. I had to alter the method then, so that my test was fair. If I hadn’t spent time doing this inappropriately, then I could have spent more time on my experiment and gained even more results. Ideally, I would have liked to repeat each experiment one more time. Although doing the first experiment incorrectly did mean that I learnt not to put time limits on experiments. It wasn’t something I had thought of before. As soon as there was an indication that something was wrong, I worked out what it was in my head. This meant that I worked it out myself and I have learnt from previous experience. This means that in future I wont make the same mistakes.
If I were to do this experiment again, I would change some of the equipment. I would use a pipette to gain a more precise measurement of acid and alkali, and in a larger scale-measuring cylinder. This would make my experiment better because I actually would be gaining a neutral solution. I took the pH only for interest and to make sure I was gaining the right sort of results. However if I were to do this experiment again, I wouldn’t use a timer. I thought that it may prove useful to have more numbers to play with but it was only a hassle that I didn’t need to have, as I didn’t use these figures at all.
I believe that my experiment was fair. I followed all of the guidelines that I set to make it a fair test. The only real difficulty I met during this investigation was when I realised I was making the test unfair by putting a time limit on it. I over came this quickly, by altering my method and taking the time limit off.
To gain extra information for this experiment, it is possible to refer to textbooks from school or the library. The Internet has some amount of useful information. Informative CD ROMs have some information, such as Encarta. More theory work could be gained from this extra research.
