If the strength of the acid is reduced, the temperature of the product will decrease. This is because the bonds in the products will not be as strong due to a weakened acid reactant. Because the bonds will be weaker this means that they will require more energy to join together. As a result there will not be as much unused energy and therefore there will be less thermal energy dispersed.
Here is a diagram showing why there will be excess energy during the neutralisation reaction:
1. The acid and the alkali are selected.
*(n.b this is only a rough diagram - the molecular structures are not correct)
base (alkali) = sodium
NaOH (sodium hydroxide) = hydrogen + oxygen
. (hydroxide)
acid = hydrogen
HCl (hydrochloric acid) = chlorine
2. The acid and the alkali molecules react together – the stored energy is used to form new products.
3. The new products have formed stronger bonds than when they were reactants (when they had weak bonds) – therefore they have not required as much energy to form as they already had stored in them.
= NaCl (sodium chloride)
= H2O (water (hydrogen x 2 + oxygen))
This means there is some excess energy that is diffused into the surroundings as heat.
I will use equipment that is suitable for my experiments and that I can use to give me the most accurate measurements.
As my variable will be the strength of my acid, the amount of my alkali will stay the same. I have decided to use 40ml of alkali throughout my investigation. I have chosen 40ml because I will change the strength of my acid 8 times and 8 divides into 40 well. The amount of my acid will vary depending on how strong it is.
The eight different strengths of my acid will be measured in molar and they will be: 2.00m, 1.75m, 1.50m, 1.25m, 1.00m, 0.75m, 0.50m, and 0.25m. The acid I will be using is hydrochloric acid.
To dilute my acid I will use water. For example if the strength of the acid is to be 1.00m then I will use 20ml of acid and 20ml of water for my solution.
The alkali I will be using is sodium hydroxide.
I will have a 500ml beaker of 2m acid, a 500ml beaker of 2m alkali and three other beakers. One of them will have the correct amount of alkali to be used in it and the other will have the mixture of acid and water in it. The third beaker will be the reaction vessel. I will place a thermometer in each of the three beakers. This is so I can take an average temperature of the reactants before the reaction and a temperature of the final product. This will enable me to work out the temperature difference from before neutralisation to after.
To accurately measure the reactants I will use two 100ml measuring cylinders; one for the acid and water and one for the alkali. I will also have two 40ml measuring cylinders to use when I am dealing with smaller measurements in order to make them more precise.
By doing some preliminary work I have found that a 100ml beaker would be better than a 500ml beaker to use as the reaction vessel and to hold the reactants in. This is because the reactants have more space to move around and bond together in. Also the thermometer has a bigger surface area in the solution meaning there is a better chance of the temperature being more accurate.
I will use a stop clock to time the reaction and make sure I do not go over the intended reaction time.
In this investigation I will be measuring the temperature change from before the neutralisation reaction to after. I will take an average temperature of the reactants previous to the reaction and the temperature of the products after the reaction. I will then calculate the difference.
I have decided to take the temperature of the products after 1 minute. I conducted a preliminary experiment for 1 minute and 2 minutes and I found that there was no difference in temperature between them. Therefore I have concluded there is no need to carry on the experiment for longer than is needed.
Preliminary Work
After I had determined which measurements to use I carried out one complete experiment (but using only the first three strengths of acid) to see if it would produce the accurate and predicted results required.
From this set of preliminary results I can see that the measurements I have chosen are correct and will work. The temperature difference is decreasing as the strength of the acid is reduced, as predicted.
I encountered no problems as I was carrying out the experiment and I was confident that I was measuring the liquids accurately.
In the final experiment I will do three tests per strength of acid, this is so they will be more accurate and there will be less chance of getting anomalous results and also I can calculate an overall average. In total I will be doing 24 tests; 3 per strength of acid.
At the end of an experiment by taking the average start temperature of the reactants away from the temperature of the products I will be able to calculate the temperature difference. I will do this for all of the eight strengths of acid. This will tell me how the temperature changes during a neutralisation reaction
In my investigation the variable will be the strength of the acid used. The amount of alkali will always stay the same; 40ml. I will be changing the strength of the acid by diluting it with water. Therefore the amount of water is also a variable but the two are connected and I will say that the acid strength is the variable in my investigation.
As I dilute the acid further with more water I predict that the temperature of the acid and water will decrease as the ratio between them gets smaller. This because the water will be coming out of a tap so it will be colder than the acid and the alkali which will have been standing at room temperature for a number of hours. I will not be able to control the temperature of the water. Consequently the average start temperature of the reactants will not stay the same and will decrease. This means the finish temperatures will not necessarily follow-on from each other and there may not be an obvious pattern. This is until I have calculated an average start temperature for the reactants, then they should match up with their finish temperature.
I will have to hope the room temperature will stay the same as I cannot control it but if there is any change it will be very minimal. The temperature of the surfaces the reactants will be on will stay the same because I will not change them during the experiment.
Firstly I will fill my two 500ml beakers, one with hydrochloric acid and one with sodium hydroxide. I will then get my three 100ml beakers, my two 100ml measuring cylinders and
my two 40ml measuring cylinders. I will put one 100ml beaker and one of each of the two measuring cylinders with the 500ml of acid and one beaker and one of each of the two measuring cylinders with the 500ml of alkali. Now I have two groups; the acid group and alkali group. They will be separated from each other. This way there is little chance of using the wrong chemical or getting them mixed together by accident. The final 100ml beaker will be used as the reaction vessel and will be placed between the two groups in the centre.
I will then get my three thermometers and place one in each 100ml beaker. I will get my stop clock and position it next to the reaction vessel ready to time the reaction. I am now ready to carry out my experiment.
Each individual experiment, e.g. at acid strength 2m, will be done three times.
To start with I will measure 40ml of alkali in the 100ml measuring cylinder and pour it into my 100ml alkali beaker. I will then measure out the required amount of acid and the required amount of water in the measuring cylinders and mix them together in the 100ml acid and water beaker. Now I take the temperatures of the two reactants and note them down. I will work out the average later.
Now I am ready to put the two reactants together. I will pour them into the reaction vessel and start the stop clock. I will leave them for 1 minute and then take the temperature.
I have now completed the first experiment. Now I have to thoroughly wash out the three beakers, the measuring cylinders and the thermometers I used to make sure no acid/alkali/water is left. I will then dry them as well to make sure there is nothing left in them.
Now I will repeat the experiment two more times. Once I have done this I will have completed 2m. I will now do the same thing for all the other strengths of acid, just changing the amount of acid and water each time.
If I run out of acid or alkali I will place some more into the 500ml beaker.
When I have completed the whole of the experiment I will then work out the averages of the start temperatures. Then I will work out the difference between them and the finish temperature. Now I will have the temperature difference.
While I am conducting my experiments I will be wearing safety goggles at all times. This is to prevent acid or alkali getting into my eyes. After I have completed each strength of acid I will wash my hands to make sure there is no acid or alkali on them. All the chemicals will be placed in beakers to help prevent spilling and also so I am sure which substance is which.
Here are the results for my investigation. Because I did three experiments per strength of acid I could not fit all the results into one table. Instead I have done three; each one has only one of the experiments in it for each of the strengths.
- Table
Experiment 1
Experiment 2
Experiment 3
I now have all of my results. To make it easier to comprehend these results I have calculated an average of all the temperature differences at each strength of acid:
Here is a graph showing my results:
- Graph
I do not feel I will need to repeat my recordings as they are as I predicted and they follow a trend; they are not random. All of my three experiments, for each strength of acid, produced roughly the same readings as each other. Therefore there is no need to repeat them as they are obviously correct and I would not be able to improve them in anyway, keeping the method the same.
I can see that energy is released in a neutralisation reaction. It is released as thermal energy.
From my results I can see that as the strength of the acid decreases so does the temperature difference from before the neutralisation reaction to after. There is a steady decrease from a 9.8oC increase in temperature to a 3oC increase as the strength of the acid goes from 2m to 0.25m.
The reason the temperature increase is not as great when using a weakened acid is because not as strong bonds are formed in the product so there is less thermal energy given out, as I
explained in my scientific reasoning earlier
There was a range of 6.8oC in temperature from the strongest acid to the weakest acid. This proves my prediction that in a neutralisation reaction there will be a temperature change but when the products form stronger bonds then there will be a higher one. When the acid was at its strongest, strong bonds were formed and plenty of thermal energy was dispersed, but when the acid became less strong, weaker bonds were formed and not as much of a neutralisation reaction took place. For that reason less thermal energy was dispersed and so I did not detect as high a temperature increase. See back to my Scientific Reason for more information.
In my overall average results there are no anomalous results. This means that everything went to plan and all the recordings followed a trend. None of the results were random and they progressed as I predicted. The graph has an obvious line of best fit and none of the figures are placed far away from it.
Overall I think I think I did perform a safe, well-planned and precise investigation into the temperature changes during neutralisation and I have come to a conclusion.
From my results I can see that there is a change in temperature during a neutralisation reaction. The temperature rises depending on how strong the acid is but in my investigation it was between 9.8oC and 3oC after 1 minute. The stronger the acid the more the temperature increases.
I think my results would be able to be used to predict further results when changing the strength of the acid.
Overall I think my results are reliable. None of them are anomalous and they all follow a trend. They follow my prediction and scientific reasoning. If there were a number of anomalous results then would question their reliability but as there are not I know that they are reliable. They follow the line of best fit incredibly well, better than I had predicted.
Because all of my three experiments, for each strength of acid, got about the same recordings this suggests to me that they are accurate and reliable and there will be no need to repeat any of them.
I think the only way I could have improved my investigation would be to have used a more accurate measuring device. This would mean that the recordings would be more accurate. This option was not available to me and at the time I could only use thermometers that only measure in 1oC scales.
As a related investigation I could investigate how the strength of an alkali effects neutralisation. I could see if the results of it are similar to the results I have for the strength of acid. I could also see how the quantity of the reactants affects the temperature during neutralisation, whether more of them would mean a higher temperature, or the opposite. Another option would be to investigate other chemicals and see if they have the same properties as hydrochloric acid and sodium hydroxide, i.e. if they gave off the same amount of thermal energy during a neutralisation reaction. All of these would certainly give me more information about the energy changes associated with neutralisation.
