Variables
Independent Variable - This is the thing that is changed to do the experiment, in this case it is the concentration of the acid.
Dependent Variable - This is the reading that is read off to collect the results, in this experiment it being the change in temperature of the acid/alkali.
Control Variables - These are the things that are kept constant so they do not alter or change the results of the experiment or make it an unfair test.
- Repeat the experiment three times so the results can be averaged.
- Use the same volumes all the way through.
- Use the same equipment to minimise the experimental error.
Range
I plan to have a range of 5 results each with two repeats so I can produce an average from these results. The range of hydrochloric acid will be 0.25, 0.5, 1, 1.5 and 2 Molar concentrations for every 1 Molar of sodium hydroxide.
Reagents
2M Hydrochloric acid, 1M Sodium Hydroxide
HCl + NaOH H2O + NaCl
Hydrochloric acid + Sodium Hydroxide Water + Sodium Chloride
Diagram
Apparatus
100cm3 Conical Flask
100cm3 measuring cylinder
Thermometer
Goggles
Clamp
Clamp Stand
Boss
Burette
25cm3 Pipette (Grade ‘B’)
Phenolphthalein (indicator)
Method
The apparatus was set up as above. Measure out 25cm3 of the Sodium Hydroxide using the pipette into the conical flask. Put 5 drops of the indicator into the conical flask as well. Measure the temperature of the solution in the conical flask and record. Dilute the Sulphuric acid with water to the concentration needed (0.25, 0.5, 1, 1.5 and 2M) and fill the burette. Titrate the sodium hydroxide into the conical flask until the solution decolourises. Measure the temperature and record. Repeat this method three times for each different concentration.
I will record the data in a table like this:
Results
Conclusion
From this experiment I conclude that as the concentration of the acid increased so did the temperature. An increase in the concentration of the hydrochloric acid produced a subsequent increase to the end temperature after the neutralisation had occurred. At the end of the graph at 2M of hydrochloric acid the temperature had only slightly raised which presumably means that soon it will stop and then have the same end result.
Analysis
It can be shown from the results obtained that in a neutralisation reaction there is always heat given out. This is because the reaction between an acid and an alkali is exothermic. This means it gives out heat. Every time a molecule of acid reacts with a molecule of alkali, it gives out a similar small amount of heat. Collisions are more likely to happen as the concentration of the acid has increased so as to make the neutralisation time faster. One influence of the reaction time is the number and rate of collisions of the molecules involved. “If you are neutralising the same amount of alkali each time, you will always use the same number of acid molecules to do this” (GCSE Bitesize Revision – www.bbc.co.uk/education/revision). As the concentration of hydrochloric acid increased, the expected number of collisions also increased. My prediction was correct because as the concentration rose there was an increase in the change in temperature.
Evaluation
The experiment went pretty well to plan. Unfortunately quite a few of the results turned out to be anomalous, from my results I can see that I have one obvious anomaly but two slight anomalies, although the two slight anomalies are still pretty close to the line of best fit.
I feel that to increase the chance of improving the results in future experiments, then I should take more readings so I can leave the anomalous results out, to increase the chance of gaining a more successful set of results and averages.
There are a few reasons why the results didn’t go to plan:
- Lack of care when measuring, maybe a smaller measuring cylinder would reduce the possible chance of a human error.
- The alkali and water may not have been mixed properly.
If the experiment were to be repeated then there would be a few changes to be made in order for the results to be more accurate:
- Using more concentrations to give a better range.
- Take more care when cutting the potato pieces.
- To use a more precise pipette (as the one used was only a grade ‘B’ standard) to measure correct amount of acid, alkali and water.
To extend the experiment to see if different variables work you could try using other acids or alkalis, also you could try using different volumes instead of concentrations. This would provide additional evidence for the conclusion and also show how you could extend the experiment.