When the solutions of acid and alkali are mixed, hydrogen ions, H+ (aq), and hydroxide ions, OH- (aq) combine to form water molecules (H+ + OH- H2O)
Sodium ions, Na+, and chloride ions, Cl- remain in the solution, which becomes a solution of sodium chloride, a salt. If you evaporate the solution you obtain solid sodium chloride.
Neutralisation is the combination of hydrogen ions from an acid and hydroxide ions from a base to form water molecules. In the process, a salt is formed.
I predict that as the concentration of the acid is doubled, the temperature rise is also doubled because there are double the number of H+ ions per unit volume, which can collide and react with the alkali.
When an acid neutralises an alkali, heat is given out. When acids react with alkalis in a neutralisation reaction to form water there is energy given out in an exothermic reaction. This produces more collisions between the particles of the acid and alkali, which gives out heat and therefore produces a higher temperature rise. When there is a higher concentration of acid, there will be more acid particles to react with the alkali, therefore producing more collisions and a higher temperature rise.
I predict that the average graph will look like this:
This shows that at 2M the acid and alkali are neutralised. When more acid is added, the temperature rise may be slightly lower. This is because there is an excess of acid.
Apparatus
- Two 10cm³ measuring cylinders-one for the acid and one for the alkali
- Two teat pipettes-one for the acid and one for the alkali
- A 500 ml glass beaker
- Safety glasses
- A bottle of hydrochloric acid and a bottle of sodium hydroxide
- Thermometer
- Heat-proof mat
- The required amount of water
Method
- Collect together the apparatus in the list above and carry out the safety procedures necessary.
- Choose a sensible range of values for the concentration of acid.
- Measure out the volume of acid and water needed in a 10cm³ measuring cylinder to prevent large percentage errors. To get the exact volume, use a teat pipette to add the last few drops for perfect accuracy.
- Measure out 10cm³ of alkali in a 10cm³ measuring cylinder, again using a teat pipette to get the meniscus exactly on the line, for accuracy.
- Take the temperature of both the acid solution and alkali. Record them. Work out the average of theses two temperatures. When adding the water to the acid, mix well so they don’t separate.
- Take a 500ml glass beaker and tip the acid solution and alkali into it.
- Stir, and record the highest temperature reached. Work out the temperature rise.
- Repeat the experiments using different molarities of acid and record the starting temperature, finishing temperature and temperature rise.
- Repeat the whole procedure three times to record more accurate, reliable results.
Preliminary Work
I have carried out the experiment mentioned above, in the method, to decide which acid concentration values I will use in my final experiments. First, I carried out the experiment talked of in the ‘information’ at the beginning of my coursework. Here are the results:
This showed that, due to the in-accurate conditions in which the experiments were carried out through, there was a difference of 1.6ºc compared to the experiment in ideal conditions, mentioned in the ‘information’ section above.
Then, I carried out an experiment to find out the range I will use. I did this by seeing the results of the highest, lowest and a middle value of concentrations. The results are below:
From these results I have decided to use the range:
2.0M, 1.6M, 1.0M, 0.6M and 0.4M. This is because they are quite evenly spread out. I have also chosen these because I can process them and try to prove my prediction.
I have decided not to use the acid concentration value 0.2M because this will give a high percentage error if the temperature rise is different on the separate tests e.g. say on one test the temperature rise is 1ºc and on another the temperature rise is 1.5ºc, the percentage error is 50 %, and this is very in-accurate. Also, we can only get to 0.5ºc accuracy, so lower values of the temperature rise are more difficult to measure accurately.
Results
These are the results I have gained from my experiments: -
Test 1
Test 2
Test 3
Average Results Table
In the average results table I have rounded up all my results to the nearest 0.5 ºc. as the apparatus I have used was unable to read my accurately, to say 0.1ºc.
Analysis
From the experiment and my graph, I have found out that if you double the concentration of the acid, the temperature rise also doubles. This is shown on my graph: at 1M the temperature rise is 6ºc and at 2M the temperature rise is 12ºc. My graph looks like that graph I predicted.
My graph is drawn to a line of best fit. This line is a straight line through the origin. This shows that the temperature rise is directly proportional to (∞) the concentration of the acid. As the concentration increases, the temperature rise increases. This is because there are more hydrogen ions in higher molarities of acid, which therefore causes more collisions with other particles, producing more heat.
The following reaction has taken place:
ACID +ALKALI WATER + BASE
Alkalis dissolve in water and the resulting solution contains hydroxide ions, OH: -
Hydrochloric + Sodium Sodium + Water
Acid Hydroxide Chloride
H+Cl- (aq) + Na+ OH-(aq) Na+ Cl- + H2O
Erase spectator ions
H+ (aq) + OH- (aq) H2O (l) Exothermic reaction
Water has been formed in the reaction between the acid and the alkali, and heat has been given off in the process thus causing a temperature rise. This is the heat of the neutralisation reaction and when an acid and an alkali are neutralised, water is formed.
My results support my prediction. I thought that as the concentration of acid was doubled, the temperature rise would be doubled. I found out that this prediction was correct. My graph also supports my prediction in the sense that the line of best fit goes through the origin, proving that the temperature rise is directly proportional to the concentration of the acid.
Looking at my average graph, there aren’t many anomalous results. The line of best fit is a good one and there are only three points that are off the line. But, these are all close to the line so aren’t that anomalous. The point that is the furthest away from the line is only 0.6ºc out. But, the slight un-accuracy may have occurred due to the heat that was able to be lost due to our conditions.
Looking at my results tables, I think that the whole of test one’s results are anomalous. They do not come very close to the other two sets of results, which are very close and reliable. All in all, my results are fairly accurate in relation with each other.
Evaluation
Even though the experiment I carried out was as accurate as my equipment and the surroundings could allow, there are some improvements that could be made; Instead of using a glass beaker, where there is much heat loss, I could use a polystyrene or plastic beaker to try and prevent this. I think that this would be an improvement, because these materials are good insulators.
During the experiment the bottles of acid and alkali frequently ran out, so more had to be made by our lab technician. This meant that there could have been a slight fluctuation in concentration from bottle to bottle. To be more accurate within the experiment I could use one large bottle and carry out the experiments on the same day, to reduce temperature effect.
The equipment I used was the not the most accurate. The thermometer was just a normal mercury thermometer, which you can only read accurately to 0.5 of a degree. To be more precise and gain exact results I could use a digital thermometer that reads to 0.1 of a degree. I also had to use a measuring cylinder, and even though a teat pipette was used, there is more accurate equipment that I can use to get the exact quantities needed. The equipment that could be used are a pipette and a burette. These are very accurate.
Another way to improve the experiment is to use larger quantities of acid and alkali, to give more percentage accuracy, as there is less of a percentage error in this case.
I think that this experiment could be furthered to give us more information and evidence. Ways in which I could do this is by using a more thorough range, say to 0.1 molarity. This would give more results to work on and use to process evidence.
I could also extend the procedure using higher concentrations of acid to see if the pattern is the same at the top end of the scale.
The glass beaker could be put in a tin can and have insulating material stuffed into it. This would try to prevent heat loss to the surroundings, meaning that the heat couldn’t escape and only heat the temperature, producing more accurate readings. Yet another extension I could make to the enquiry.
My results are fairly reliable. I have concluded this because they are all reasonably close in comparison with each other and each other table. For example, for the 2.0M, tests 1 and 3 have the same value as that in the results table, and the result for test 2 is only1.25ºc out. This is not that much, so we could say that the results are reliable. Also, the results for 0.6M are all fairly close: 3ºc, 4.25ºc and 4ºc. These are reliable and show that the whole experiment was reliable, in the conditions that we had to deal with.
