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Explain how the enthalpy change of neutralisation can be used to determine the relative strength of an unknown acid.

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Introduction

Aim: To explain how the enthalpy change of neutralisation can be used to determine the relative strength of an unknown acid. Provided Information: Relative Formula Mass: 135 Appearance: White crystalline solid Properties: Very soluble in water Monoprotic (monobasic): An acid that has only one acidic hydrogen atom in its molecules; common examples are hydrochloric acid (HCl) and nitric acid (HNO3). Prediction and Scientific Background: Standard Molar Enthalpy of Neutralisation (/\Hn,) is the enthalpy change per mole of water formed in the neutralisation of an acid by an alkali, (298K and 1 atm). * Unknown Acid (aq) + NaOH (aq) --> Salt (aq) + Water (l) * In aqueous solution, strong acids and bases are completely dissociated and Hneut is approximately equal to -57.9 kJ mol-1. This neutralization process corresponds to the reaction: H+ (aq) + OH- (aq) --> H2O (l) * For weak acids, this enthalpy change is less exothermic because some input of energy is required to dissociate the acid. Therefore the Hneut of weak acids and bases is more positive than -57kJmol-1. Hydrochloric acid (strong acid): As HCl is a strong acid; it is a good proton donor, with near to complete dissociation in water. HCl (aq) --> H+ (aq) + Cl- (aq). Hence a strong acid completely dissociates into ions. Ethanoic acid (weak acid): As CH3COOH is a strong acid; it is a poor proton donor. The dissociation in the water is equilibrium, with the equilibrium position well to the left-hand side of the equation. CH3COOH (aq) H+ (aq) + CH3COO- (aq). ...read more.

Middle

Then 50cm3 of the sodium hydroxide will be run into a clean 100cm3 glass beaker. At this point, two comparable thermometers will be used to test the temperature of each solution for 3 minutes; a stopwatch will be used to record the temperature (used effectively to gain accurate recordings) at 0.5-minute intervals (every 30 seconds). When the temperature is efficiently recorded the unknown acid will be added to the sodium hydroxide. Afterwards a lid consisting of a small hole (in the centre) will immediately be placed securely on the cup to ensure a minimal amount of heat is lost (due to evaporation or convection), however in this gap of approximately 10-20 seconds a significant amount of heat could be lost or gained thus causing a change (major or minor) that may result in inaccurate recordings developing. When the initial drop of the acid will be applied to the sodium hydroxide the thermometer (that fits through the specific hole) will be used to stir the mixture and additionally record the temperature of the mixture, with the aid of the stopwatch at regular 0.5-minute intervals (every 30 seconds) for a period of five minutes. This experiment will be repeated a further two times and due to this a mean value can be obtained for the temperature change. Safety procedures: Safety glasses and gloves will be worn for health and safety reasons, and therefore as the chemicals are hazardous a risk assessment has been constructed in order to reduce the possibility of accidents or future problems. When chemicals have been used they must be sealed to confine any toxic gases being released. ...read more.

Conclusion

Requirements: To make sure measurements, observations and recordings are reliable the experiment will be repeated to ensure measurements were precise and as accurate as possible, and therefore due to this each experiment did not provide any anomalous results. To guarantee accuracy, the glassware was washed out with water after each time of use and then dried, to prevent any foreign ions entering the solution, as it may have altered the results. However a significant limiting factor to the experiment is apparatus percentage error, which consequently affects the reliability of results, as volumes were not prepared precisely: Percentage Error of Burette = ? 0.1 x 100 50.0 Therefore Overall Apparatus Percentage Error = 0.2 x 2 (as the burette was used twice) = 0.4% By adding the apparatus percentage error to the result gained from each acid, it may provide an enhanced understanding of supplementary inaccuracies and faults within the experiments. Due to these results, it is evident that further discrepancies occurred within the experiments in addition to the apparatus percentage error. A possible example was that solutions were made up inaccurately, or not made to the precise concentration of 1M. In addition, as the experiment was done in a class room there were constant alterations in the light intensity, temperature, pressure and chemicals (such as oxygen) in the atmosphere resulting in an increase or decrease in chemical reactions, therefore it was difficult to make this experiment completely accurate. Thus, in order to improve this factor the experiment could be performed in a vacuum, or under calculated standard conditions - a pressure of 100kPa, a temperature of 298K (25oC), and by using solutions of concentration 1 mol dm-3. ...read more.

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