An Experiment to determine What Factors Affect Neutralisation of 25cm Sodium Hydroxide
An Experiment to determine What Factors Affect Neutralisation of 25cm³ Sodium Hydroxide
Introduction
Neutralisation is the reaction between an acid and an alkali that produces a neutral solution - a solution neither acidic nor alkaline. During the reaction, the acid dissociates and produces hydrogen ions, H+, and the alkali dissociates and produces hydroxide ions, OH-. If the number of H+ ions are equal to the number of OH- ions, then neutralisation occurs because there are no extra ions from either the acid or alkali to make it an acidic or alkaline solution -
This neutral solution is water: H + OH = H2O. A salt is also produced, depending on what type of acid and alkali you use.
The factors that affect neutralisation:
- The concentration of the acid
- The strength of the acid
- The strength of the alkali
- The temperature
Preliminary Investigation
We carried out a preliminary experiment to find the best conditions to carry out an investigation to determine how concentration affects the volume of acid needed to neutralise an alkali.
We took three different acids and three different alkalis and tested the volume each solution needed to neutralise either Hydrochloric acid or Sodium Hydroxide. We kept all the concentration of the reactants the same, so the tests would be fair, and the three acids were tested with 20ml of 1M Sodium Hydroxide, while the three alkalis were tested with 20ml of 1M Hydrochloric acid. We used a 10ml measuring cylinder to measure all of our reactants, and a Universal Indicator was used to show the exact moment of neutrality between the acids and alkalis.
Results:
Name of acid
Volume needed to neutralise 20ml of 1M Sodium Hydroxide (ml)
Hydrochloric acid
20.3
Ethanoic acid
25.4
Sulphuric acid
9.7
Name of alkali
Volume needed to neutralise 20ml of 1M Hydrochloric acid (ml)
Sodium Hydroxide
20.3
Calcium Hydroxide
9.6
Potassium Hydroxide
20.4
Conclusion:
From these results we chose not to use a measuring cylinder because it isn't accurate enough, and for the main experiment we will use a burette instead. We also decided that for the main experiment we would use Hydrochloric acid and Sodium Hydroxide because they dissociate fully, and this would allow us to be confident that our experiment is reliable and fair as there will be the same number of ions present in the reaction from the each reactant.
During the preliminary tests, we experienced some problems with the Universal Indicator, as it was difficult to know exactly when the solution became neutral. Due to this, our results may not be as reliable as possible, and so for the main experiment we need an indicator that has obvious colour changes. For this we ran some tests to find an indictor that showed clear colour changes:
Indicator
Colour of Indicator in Acid
Colour of Indicator in Alkali
Methyl orange
Red
Yellow
Bromothymol blue
Yellow
Blue
Phenol red
...
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During the preliminary tests, we experienced some problems with the Universal Indicator, as it was difficult to know exactly when the solution became neutral. Due to this, our results may not be as reliable as possible, and so for the main experiment we need an indicator that has obvious colour changes. For this we ran some tests to find an indictor that showed clear colour changes:
Indicator
Colour of Indicator in Acid
Colour of Indicator in Alkali
Methyl orange
Red
Yellow
Bromothymol blue
Yellow
Blue
Phenol red
Yellow
Red
Phenolphthalein
Clear
Pink
Litmus
Red
Blue
The clearest colour changes that we found were from the indicators Litmus, Phenolphthalein and Bromothymol blue. We conducted another test to see at which pH level the colour changed:
Indicator
pH at which the colour changes
Litmus
6-7
Phenolphthalein
7-8
Bromothymol blue
4-8
As the Bromothymol blue changed colour over a range of pHs, it would be difficult to know exactly when the solution in the experiment turned neutral. Due to this we decided not to use it, and also decided not to use Litmus because it suddenly changes at pH 7 with no immediate colour. Phenolphthalein had a distinct colour and changes colour at pH 8, allowing us to be more accurate with our readings, and so we will use this for our main experiment.
Main Investigation
For our main investigation we will be investigating the factor affecting neutralisation: the concentration of acid.
Prediction:
I think that as the concentration of acid increases, the volume needed to neutralise 20ml of 1M Sodium Hydroxide will decrease. By increasing the concentration of a solution means increasing the number of particles (in this case ions) present within that solution, without increasing the volume.
Ions have kinetic energy, and they move and collide with other ions around it. If there are more ions within a reaction, then more collisions take place, and so reactions occur at a faster rate. This means that for a low concentrated acid, more acid is needed to neutralise alkali, whereas if the acid is highly concentrated less acid is needed:
So, if we halved the volume of acid in the highly concentrated reaction, the number of ions present will also be halved, and the reaction will occur at the same speed as the low concentrated reaction, but less acid will be needed:
This means that the volume of acid needed to neutralise 20ml of 1M Sodium Hydroxide is inversely proportional to the concentration of the acid.
Method:
25ml of 1M Sodium Hydroxide will be safely pipetted into a conical flask. This will then be placed onto a white tile underneath a burette filled with Hydrochloric acid. Phenolphthalein indictor will then be added to the alkali. The reading on the burette will be noted down, and then the acid will be run into the alkali and indicator solution. The conical flask will be agitated constantly so the acid and alkali are mixed together thoroughly, and when the indicator changes colour, the acid is stopped running and the volume of acid will be noted down. Each test will be repeated three times for each different concentration of acid, and we will use five different concentrations of acid.
Diagram:
Apparatus:
Hydrochloric Acid of 1.0M, 1.25M, 1.5M, 1.75M and 2.0M
Sodium Hydroxide
Phenolphthalein
Burette
Burette filler
Conical Flask
Pipette
Safety bulb
White Tile
Results
These results show the readings on the burette at the start and at the end of the main experiment:
Concentration (M)
Volume of Acid (ml)
Trial 1
Trial 2
Trial 3
Start
End
Start
End
Start
End
.0
0.2
25.2
0.5
25.6
0.6
22.0
.25
0.5
22.3
22.3
44.2
.2
24.0
.5
0.2
8.9
8.9
37.7
0.4
9.1
.75
.3
6.9
6.9
32.4
32.4
48.1
2.0
.2
3.7
3.7
26.3
26.3
38.4
Trial 3 for the1.0M acid looked incorrect compared to trials 1 and 2, so we decided to carry it out again:
Concentration (M)
Volume of Acid (ml)
Trial 1
Trial 2
Trial 4
Start
End
Start
End
Start
End
.0
0.2
25.2
0.5
25.6
0.4
25.5
The results of trial 4 satisfied our expectations, so we chose to use the results of trial 4 instead of trial 3.
We then had to find out the volume of acid used in the experiment for each trial. To do this, we subtracted the end results from the start results.
We also found out the average volume of acid used:
Concentration (M)
End - Start (volume of acid used, ml)
Trial 1
Trial 2
Trial 3
Average
.0
25
25.1
25.1
25.07
.25
21.8
21.9
22.8
22.17
.5
8.7
8.8
8.7
8.73
.75
5.6
5.5
5.7
5.6
2.0
2.5
2.6
2.1
2.4
Analysis
The results show that as the concentration of the acid increased, the volume of acid, needed to neutralise the alkali, decreased.
I will draw graphs to show this relationship (next page).
My first graph shows an inverse proportional relationship between the concentration and volume of acid, so I decided to draw another graph to show volume against 1/concentration. This shows a directly proportional relationship.
From both these graphs I am able to conclude that the relationship between the volume and concentration of acid is inversely proportional (or the volume is directly proportional to 1/concentration). The formula for the second graph is volume = 25 x 1/concentration, and the gradient of the line of best fit, is 25. This shows that as 1/concentration of acid increased by 0.1M, the average volume of acid used to neutralise the Sodium Hydroxide increased by 2.5ml.
My results match and prove my prediction correct, since I predicted that as the concentration of the acid increases, the volume would decrease. This occurs because when an alkali is mixed with an acid, the hydroxide (OH-) ions in the alkali and the hydrogen (H+) ions in the acid vibrate and collide with each other, thus constructing H2O (water) in the process of neutralisation.
In the experiment, as the concentration of Hydrochloric acid was increased, the number of hydrogen ions, which is produced by dissociation, was also increased. When the higher concentrated Hydrochloric acid was mixed with the Sodium Hydroxide, less acid was needed for neutralisation because there are more H+ ions present in a smaller volume:
25cm³ of 1.0M 25cm³ of 2.0M
Hydrochloric acid Hydrochloric acid
As you can see, with the 25cm³ of 1.0M Hydrochloric acid all of the solution was used up, but in the 25cm³ of 2.0M Hydrochloric acid, only half was used because half of the solution contained the same number of molecules as the full solution of 1.0M Hydrochloric acid.
Evaluation
I feel that I obtained a good set of results because our procedure was fair and very reliable. The apparatus that we used allowed us to measure accurately, and as we carried out each experiment three times, I believe that our final set of results were precise. Our results were also very similar to other groups who conducted this experiment in our class and so I believe that our results are valid and repeatable.
We didn't have many problems during the experiments, but we acquired an anomalous result for our third trial of the 1.0M experiment. This could have occurred because the conical flask wasn't washed properly after our second trial, so there was still some acid contained inside. This would have affected our final results, but we noticed the error and carried out another trial to replace the anomalous one, and this provided sufficient evidence to support my conclusion.
Even though this investigation did produce a reliable set of results, we could improve this experiment by using a pH meter and an electronic burette. A pH meter is a device that measures the exact moment a solution becomes neutralised. An electronic burette pours 1ml of the solution inside it every second.
agitating the solution could obviously not move at the exact same speed for every separate test carried out, our results could be slightly unfair and inaccurate. By using a machine the experiment would not be affected by human error thus making the experiment more accurate and even fairer.
We could also investigate the factor, strength. Strength is the amount of ions free in a solution. If a solution is strong then more ions are free to react.
In a weak solution there are some ions that are not free to react. For such an experiment, we could use different strengths of acid to find out how the volume of acid needed for neutralisation is affected by strength. The experiment would be very similar to the investigation we have just conducted, but it will allow us to find out the relationship between volume of acid and strength, and see if it is the same as the relationship between the volume of acid and concentration.
