To determine the concentration Essay Example for Free

To determine the concentration Essay To determine the concentration of unknown standard sodium hydroxide solution and the enthalpy change of neutralization between different pairs of acid-base used (Thermometric titration) Principle of method: Neutralization is the reaction between an acid and a base, which is an exothermic reaction. In this experiment, two methods are used to determine the concentration of sodium hydroxide solution and the enthalpy change of neutralization. H+(aq) + OH-(aq) H2O(l) Method 1 is to measure the temperature change of the reaction mixture when different definite ratio of acid and base are reacted. There are totally 7 ratios. For each ratio, the initial temperature of acid and base is recorded. When they are mixed, the final temperature of the mixture is recorded so that: Let Va, Vb, Ta, Tb, T be the volume of acid, volume of base, initial temperature of acid, initial temperature of base and final temperature of mixture if they are not reacted (which is actually the initial temperature of mixture before reaction) respectively. Thus, Final temperature of the mixture after reacted T = Temperature change due to reaction. Method 2 is to measure the temperature of the reaction mixture when each small portion of acid are added to a fixed amount of base solution with using titration of acid against base. For both of the method, different pairs of acid and base are used and compared. To find the concentration of sodium hydroxide solution, the volume ratio of acid to base which makes the maximum temperature change(which means the reaction is most complete) should be found out and so: (Since the basicity of both acid and base is 1) By conservation of energy, the enthalpy change of neutralization = heat change of solution The density and specific heat capacity of solution are assumed to be equal to those of water since the differences between them are negligible. Results: Method 1 Using nitric acid: (A graph is attached at page 6) Volume of HNO3(aq) (cThis result was acceptable since it was consistent and close to the actual value. The enthalpy change of neutralization using nitric acid was found to be -47. 65kJ and -51. 48kJ respectively by method 12. The enthalpy change of neutralization using ethanoic acid was found to be -48. 24kJ and -61. 11kJ respectively by method 12. The enthalpy of neutralization found out in this experiment was not reliable because the result was not consistent for method 12 and there were great errors in the experiment. The most significant evidence that shows the result was not reliable was that theoretically, the enthalpy change of neutralization between ethanoic acid and sodium hydroxide should never be higher than that between nitric acid and sodium hydroxide. This is because nitric acid is a strong acid while ethanoic acid is just a weak acid that it only slightly ionized in water. Therefore extra energy was needed to ionize the ethanoic acid molecules so that the enthalpy change of neutralization of ethanoic acid should be less negative than that of nitric acid. However the result of this experiment contradicted with the above statement, so that the results of this experiment were not reliable. The most significant error in this experiment was heat loss to surrounding. Although foam cup was already used in the experiment, the upper surface of solution also produced great heat loss to the atmosphere. Also, there may still be heat loss through the cup. This error was more significant in method 2 since a period of time was used to titrate the solution. During the time of titration, a lot of heat can be transferred to the air and thus the recorded temperature would be decreased. As a result, the calculated enthalpy change of neutralization would be lower than the actual one. To reduce this error, a Styrofoam cup instead of a polystyrene cup can be used since Styrofoam is a better insular of heat than polystyrene. Some cotton wool can be used to wrap the cup in order to further reduce the heat loss to surrounding. Also, a lid can be used to cover the upper mouth of the cup in order to reduce the heat loss to surrounding. However, the lid should allow the thermometer to stir the solution and also allow the burette to add acids into it in method 2. Obviously, the titration in method 2 must be carried out quickly in order to minimize the time for the heat to be lost from surrounding, and thus reduce the above error. The second error was the fluctuation of room temperature. In method 1, the initial temperature measured and the temperature of the solution just before reaction might be different. This was because of the change in room temperature since it would affect the temperature of solution by thermo equilibrium. In method 2, the room temperature before the titration and during the titration might be different. This would lead to an error on the measurement of temperature as the same case in method 1. To reduce this error, a more constant temperature in the laboratory can be constructed by an air-conditioner. This can reduce the error brought from the fluctuation of room temperature. In the experiment, method 1 is more suitable to determine the enthalpy change of neutralization since the temperature was measured just after the reaction in method 1 but the temperature was measured for several times in method 2 and this lead to error in the measure of temperature. However, method 2 is more suitable to determine the concentration of sodium hydroxide solution since the number of times of measuring the temperature of solution was much more than that in method 1. Then the peak of the graph can be found out more accurately. Thus the volume of acid used with the highest temperature change can be more accurate. Based on the results of this experiment, the enthalpy change of neutralization was more negative with weaker acids, having same base. However, this statement was obviously wrong since in weaker acids, some energy was used to ionize the acid molecules. Theoretically, the enthalpy change of neutralization should be more negative with stronger acids. This was not shown on the results in this experiment since the error was too large. Conclusion: The concentration of the sodium hydroxide solution was successively found out and it was acceptable, but the enthalpy of neutralization found out was not reliable since it was not consistent and it contradicted to the theoretical values. On the whole, the results was not acceptable and so the objective of this experiment was not fulfilled. Reference: http://en. wikipedia. org/wiki/Styrofoam http://en. wikipedia. org/wiki/polystyrene.