This is a titration so the standard equipment is needed:
- Burette (50 cm3)
- Pipette (25 cm3)
- Pipette filler
- Burette stand
- Beaker 100ml
- Beaker 50ml
But to Measure the temperature accurately I also need:
- Polystyrene cup (Polypot)
- Thermometer
Fair Test
- Using titration equipment, much more accurate
- Polypot keeps heat in
- Use the same bottle of acid or alkali
- Using 2 mole solutions rather than 1 molar to get a bigger difference
Prediction
I predict that because the Sulphuric acid molocule has twice as many hydrogen ions, you will need half as many molecules as hydrochloric acid needs to neutralise the Sodium Hydroxide. This is because one sulphuric acid molocule can neutralise sodium hydroxide at a ratio of 2 sodium hydroxide molecules to one sulphuric acid. Whereas it is a 1 to 1 ratio with the hydrochloric acid.
Other Predictions
On a general basis, I predict that the amount of heat energy will rise, slow down and on the graph this will probably be represented by a very gentle curve, more a line, plateauing with a flat(ter) line at the end. I believe this because to begin with there will be a high concentration of reactant ions (higher concentration meaning more ions in a given volume) so reactants are more likely to come together and react, the concentration will drop and it will take longer for the ions to meet, I expect this to make only a small difference.At the end, the room temperature acid added at the end, which doesn’t react, reduces the average heat energy (temperature), but not the amount of heat energy, in fact it adds to it, I am not exactly sure whether that will make a downward slope because of heat loss or a flat line on the graph, I guess a slope. As soon as the solution is neutralised, the amount of heat energy will start going down because it’s being lost and not added to, I am not sure by how much though.
I predict that the temperature will have an effect o the rate of reaction but this will probably not be apparent.
Method
- You must wash the burette you’re using with water and again with the acid to be used, be it HCl or H2SO4 so that you get a constant concentration (not diluted by the water left in there when it is rinsed
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Measure with a pipette exactly 25cm3 of 2M NaOH into a polystyrene cup (Polypot) as this insulates, meaning less heat is lost, and measuring from the bottom of the meniscus for accuracy.
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Checking the burette tap is closed, funnel in the acid. For HCl add about 30cm3 because the theory tells us that the ratio of OH- ions to H+ ions is a 1:1 ratio, (because of this ratio no preliminary test to establish a range was necessary) which means approximately equal amounts of each will create a neutral solution
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Add the HCL to the NaOH, 3cm3 at a time.
- After you have added some acid, stir the Polypot with the thermometer and quickly record the temperature and move on
- Carry on until the change in temperature is small and several negative temperature change results are recorded, indicating the alkali has been neutralised
Safety:
- The acids that will be used are 2 molar, twice as strong as the usual strength used, this makes it far more important to use goggles and not to spill on clothes or you.
- Check carefully to see if the burette is secure, if it isn’t then it may fall and break, this would result in acid and glass shards on the desk and floor, not safe for reasons too obvious to mention.
- Make sure the burette doesn’t leak or drip.
- You should always be standing during an experiment, but in this case, with stronger corrosive solutions, this becomes even more important.
- This may not have much to do with the experiment, but while carrying the acids and alkalis around, remember to watch out for bags and stools or any obstacles which may cause you to trip up.
- The thermometers we use contain mercury which is poisonous, but also, so is the vapour, so any fabric or other thing which soaks it up must be washed because it could give you poisoning, especially clothes.
Obtaining Evidence
The results I used weren’t mine, so I can’t use this section, so I haven’t tried
Tables Of Results
Analysis
Change in heat
Sulphuric: 37 – 25 = 12 ml added
Hydrochloric: 53 – 25 = 28 ml added
Sodium Hydroxide: 25 ml added
Sulphuric: 12 ml X 2 H+ ions = 24 \
Hydrochloric 28 ml X 1 H+ ions = 28 ¦- They all are about the same
Sodium Hydroxide: 25 ml X 1 OH- ions = 25 /
The graph clearly shows that far less Sulphuric acid was required to neutralise the sodium hydroxide than the hydrochloric acid, as predicted. Also the ratios where correctly predicted. On the analysis above of the proportion of ions, you can see that these are all approximately the same, considering that the millilitre was the smallest unit used, except that the hydrochloric acid seemed a little out, but that’s for the evaluation.
Looking at the “temperature rise” in the tables (see obtaining evidence) the results done twice matched each other very well proving that they were both accurate, the chance of making the same mistake to the same level is very unlikely
The lines on the graph tells us that the temperature rose at a high, nearly constant rate as the acid was added to the alkali (no sign of the possible curve I predicted coming from temperature increasing the rate of reaction in one way or reactant concentration falling in the other way, it is possible that they cancelled each other out though).
The temperature then declined after it reached a peak, I think this is due to loss of heat to the surroundings.
Energy Change = Temp Rise X mass of liquid X 4.2
(Joules) degrees C 1 ml = 1 g specific heat capacity
4.2 is the Specific Heat Capacity of water, which is the number of joules required to raise 1 ml 1cm3 1g (all are equal) of water 10C. The solutions used are 2M, which is a fairly low concentration, and so, being nowhere near the saturation point, it’s safe to use the value for water is used. This, of course is a generalization, and did cause some of the errors. Both the acids released the same amount of heat, the reason that the HCl peak is below the H2SO4 one is due to the heat being lost over a longer period of time because in theory they should be at exactly the same point, because the same amount of energy is released in the heat of neutralisation.
Conclusion
I conclude that that measuring heat energy released during an exothermic reaction, is an accurate enough method.
I drew on the graph a line from the points of neutralisation and stopped above the starting ml (assuming that the energy level dropped at a fairly constant rate) that the amount of energy really released is around 3,200 joules, no matter what is used to neutralise the alkali.
Evaluation
The main part of the prediction was correct, that the amount of energy released by neutralisation is always the same. Some others weren’t, the idea of a curved line was wrong (the two factors may cancel each other out).
The process was quite inaccurate looking at the two peaks shows you how much heat was lost in how relatively. But it was all surprisingly accurate, the doubled results were all close together so that the fact they were not greatly different shows that they were either both wrong (unlikely) or most likely, both about right. Not being more precise than a millilitre when your dealing with three millilitres is bound to cause problems, same with a small range of temperatures. Altogether, taking into account:
- Measuring to millilitres
- Measuring to degrees C (not decimal places)
- Dilution due to pipette / burette
- Heat Loss
- Human error on the burette tap (time and amount)
- The specific heat capacity being a bit different
- Splashing caused by stirring, reduces amount in pot
Altogether it is about 10% accurate, considering some inaccuracies work against each other, like 4 and 6, using a lower specific heat capacity than you really have creates heat whereas 4 is about actually losing it. The graph was mostly accurate, and it showed up only one anomaly, though I have no explanation for it.
Ideally, I would have a thicker poly pot with a lid. A magnetic stirrer would mix it, an electronic probe measuring to 2 decimal places stuck in the lid (same for the end of the burette) and a stop-clock to measure the interval of 20-30 seconds between adding the acid.
Obvious further work would be trying out different combinations of acids and alkalis, but, you can also measure heat produced in other exothermic reactions, or try measuring the heat used by endothermic reactions.
