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Lab Report. Objective: 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)

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Introduction

Laboratory Report: Experiment 4 Determination of the enthalpy change of neutralization Name: Cheung Chun Hin, Harry Class: 6L (12) Date: 2-11-2009 Objective: 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 ...read more.

Middle

Initial Temperature = 23.1? Volume of acid added (cm3) 2 4 6 8 10 12 14 16 18 20 Temp. of mixture (?) 24.3 25.2 26.0 26.6 27.3 27.9 28.4 28.8 29.3 29.7 Volume of acid added (cm3) 22 24 26 28 30 32 34 36 38 40 Temp. of mixture (?) 30.1 30.1 29.8 29.6 29.4 29.2 29.0 28.8 28.7 28.5 The maximum temp. change in using ethanoic acid = 30.2-23.1 = 7.1? The corresponding volume ratio of acid to base = 23.6:25 = 118:125 Molarity of sodium hydroxide solution = 118�125�1.00 = 0.94M For enthalpy change, Discussion: In this experiment, the concentration of sodium hydroxide found in method 1 were 1.00M and 0.95 and that in method 2 were 0.98M and 0.94M which are both similar to 1M. This 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 1&2. The enthalpy change of neutralization using ethanoic acid was found to be -48.24kJ and -61.11kJ respectively by method 1&2. The enthalpy of neutralization found out in this experiment was not reliable because the result was not consistent for method 1&2 and there were great errors in the experiment. ...read more.

Conclusion

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 - End of Report - ?? ?? ?? ?? 1 ...read more.

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