Investigate the effect of changing the concentration of sodium hydroxide (alkali) on the volume of hydrochloric acid needed to neutralize a fixed volume of alkali by measuring the temperature and noting colour changes of the solution mixture.

                

        

Graph showing exothermic reaction

Predictions:

        Based on my scientific knowledge I can predict that in this experiment, as the NaOH is being added there will be a temperature increase of the reaction mixture. After neutralization has occurred the temperature will decrease. Also there will be slight colour changes as the volume of NaOH in the reaction mixture is increased. At the point of neutralization the colour of the mixture should change completely. Neutralization will occur when there is an even distribution of H and OHֿ ions.

        It is also expected that the volume of acid needed to neutralize the alkali will depend on the concentration of the alkali. The higher the concentration of the alkali, the higher the volume of acid needed to neutralize it. As the reaction is to be repeated using different concentrations of alkali, it is expected that the rate of reaction will be affected. When the concentration of the alkali is increased, so is the rate of the reaction. This will be proved on the graphs showing the results. This prediction is made because in the dilute alkali there are not many OHֿ ions. In concentrated alkali there are a lot more. Because the number of alkali particles in a higher concentrated alkali solution is more than that in a concentrated one, more successful collisions will occur causing the reaction to speed up.

         

TABLE SHOWING EXPLANATION AND JUSTIFICATION OF APPARATUS AND MAERIALS

Variables:

        Independent variable→Volume of acid

        Dependent variable→temperature

        Controlled variable→Volume and concentration of alkali

Method:

1. Draw a table headed up as follows:

2. Using the measuring cylinder, 20cm³ of sodium hydroxide was added to the polystyrene cup. A few drops of phenolphthalein indicator were then added to the acid. The colour of the changed from colourless to pink solution changed to pink.
3. The temperature and colour of the mixture in the polystyrene cup were recorded.
4.  Using the burette, 2cm³ of dilute hydrochloric acid was gradually added to the cup. The mixture was stirred briefly using the thermometer, and the temperature and colour recorded.
5. Step 4 was repeated until a total of 40cm³ of dilute hydrochloric acid was added to the cup.
6. The procedure from steps 1-5 were repeated three times using three different concentrations of alkali;0.5,1 and2 moles, and 1molar concentration of the acid.
7. In the case of wide variations in the replicates, the experiments were repeated.
8. The results were then be plotted on a graph of neutralization as shown bellow:

Safety:

        In this experiment the chemicals to be used are corrosive. Safety gears such as goggles and lab coats should therefore be used to protect the eyes and skin. Also the area where the experiment is to be carried out should be cleared so that unnecessary accidents will be prevented.

Fair test:

        To ensure that my results are accurate and reliable, I shall do the following:

A reliable three sets of readings will be taken. This will ensure that accurate temperature readings are obtained. The readings will also ensure that there are no anomalous readings.

 Ensure valid-control of all factors which can affect my results.

TABLE SHOWING FACTORS THAT CAN AFFECT THE EXPERIMENT AND THE WAYS IN WHICH THEY WILL BE CONTROLLED.

        

Preliminary experiment: A trial experiment was carried out. In this experiment the main error was with incorrect temperature readings. The readings remained constant and this was due to the use of an inappropriate thermometer.

        To rectify the problem, the correct thermometer was obtained and used to carry out the main experiment.

TABLE SHOWING THE TEMPERATURE OF THE REACTION MIXTURE IN (˚c) AGAINST THE VOLUME OF 1 MOLAR HYDROCHLORIC ACID IN (cm³) ADDED TO 0.5 MOLAR SODUM HYDROXIDE.

TABLE SHOWING THE TEMPERATURE OF THE REACTION MIXTURE IN (˚C) AGAINST THE VOLUME OF 1 MOLAR HYDROCHLORIC ACID IN (CM³) TO 1 MOLAR SODIUM HYDROXIDE.

TABLE SHOWING THE AVERAGE TEMPERATURE OF THE REACTION MIXTURE IN (ºC) AGAINST THE VOLUME OF 1 MOLAR HYDROCHOLRIC ACID ADDED TO O.5 AND 1 MOLAR SODIUM HYDROXIDE.

Analysis: The collected data shows that different concentrations of sodium hydroxide are neutralised by different volumes of hydrochloric acid. In the experiment carried out, 20cm³ of 0.5 molar sodium hydroxide was neutralised by 8cm³ of 1molar hydrochloric acid, and 20cm³ of 1molar sodium hydroxide was neutralised by 22cm³ of 1molar hydrochloric acid. The volume of the alkali remained the same however; the concentration was changed from 0.5 to 1 and caused the alterations in the rate of reaction. It was also seen that before neutralisation had occurred there was a raise in temperature as opposed to the temperature fall after the reaction was completed. Before neutralisation had occurred the indicator remained pink. When neutralisation had occurred the indicator changed from pink to colourless to show that neutralisation was completed.

        

Conclusion: The results came out exactly as I had predicted. The volume of acid needed to neutralise 0.5 molar sodium hydroxide was less than that needed to neutralise the 1 molar sodium hydroxide. 8cm³ of the acid was needed to neutralise the 0.5 molar alkali, whereas 22cm³ of acid neutralised the 1 molar alkali. According to previous scientific knowledge, alkalis consist of OHֿ ions and acids consist of H ions.

When the 0.5 molar was made to react with the 1 molar acid the concentrations of the ions were different. There were fewer OHֿ ions than hydrogen ions present in the reaction mixture. In order for the hydrogen and hydroxide particles to have reacted together, they had to collide with each other and the collisions also needed enough energy. In a dilute acid, there are not many hydrogen particles. This means that there is not much chance of a hydrogen ion hitting a hydroxide ion. In this reaction the concentration of the alkali was less than that of the acid which made it possible for more successful collisions to have occurred and increase the rate of reaction than when the 1 molar alkali was used. Because the rate of the reaction with 0.5 molar alkali and 1 molar acid was faster than that of the 1 molar alkali and 1 molar acid, less acid was needed to neutralise the 0.5 molar alkali.

There was also a temperature change in the reaction. When reacting, substances are heated. They move very fast, which means they collide more often and with more energy. This accounts for the raise in the temperature of the reaction mixture before neutralisation was completed. After the reaction, there is no need for particles to collide. Hence, no collisions take place and instead of a heat gain there is heat loss. This is why temperature fell after neutralisation had occurred.

        

Evaluation: Titration is a method of analysis that allows one to determine the precise endpoint of a reaction and therefore the precise quantity of reactant in the container holding the reaction mixture. A burette was used to deliver the second reactant to the flask and universal indicator was used to detect the endpoint of the reaction. This method, used to investigate the ‘effect of changing the concentration of an alkali on the volume of acid needed to neutralise a fixed volume of alkali’ is very effective method as the results obtained were approximately accurate.

        The results were not precise but approximate results. To improve the accuracy of the results obtained I would have made some changes to the apparatus used. A measuring cylinder was used to measure the volume of alkali added to the mixture. Measuring cylinders are used for mixing, transporting, and reacting, but not for accurate measurements. The volumes stamped on the sides are approximate and accurate to within about 5%. In the place of the measuring cylinder it would have been more appropriate to use a pipette. Pipettes are used to measure small amounts of solution very accurately. Pipettes come in a variety of sizes and are made to rigorous specifications. They are designed to deliver the stated volume, at the temperature specified, when used in the correct manner.

The temperature of the room in which the experiment was carried out was not at normal room temperature. The room utilised was air-conditioned therefore temperature was lower than usual.

The results obtained were reliable. However, if the experiment was to be repeated using the pipette and at normal room temperature, I believe that the results would not be the same. They would be more accurate than those previously obtained.

There were no anomalous results. The temperature readings were in close proximity of each other therefore there was no problem in drawing lines of best fit on the graphs. Three sets of readings were taken for each part of the experiment. The results obtained all corresponded with the general pattern in the experiment. Therefore, they were good enough to draw firm conclusions.