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Investigation into the effect of acid/alkali strength on the heat change when acids and alkalis are mixed

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Introduction

Investigation into the effect of acid/alkali strength on the heat change when acids and alkalis are mixed Planning The main aim of this experiment is to investigate the heat change when acids and alkalis are mixed. The temperature of the acid and alkali that are going to be mixed will be taken prior to mixing, and after they have been mixed in a polystyrene cup. The maximum temperature rise will be noted as this will be the biggest heat change that has occurred. The highest temperature after mixing and the temperature prior to mixing will be subtracted to give the heat change. To ensure that this is a valid test the volume of the acid and alkali will be kept constant at 40cm3. The volume will be kept constant because if there were a differing volume of acid to alkali this would have an influence on the temperature rise as there is not the same amount of solution. The only variable in this experiment will be the strength of the acid and alkali. This will allow us to examine the manner in which the heat evolved differs for differing strengths. Comparisons can then be undertaken to see how concentration affects the heat change in set volumes of acid and alkalis. ...read more.

Middle

When the acid and alkali are mixed, the start temperature will be taken, as well as a finish temperature. Then two repeat temperatures will be taken of each start and finish temperature. This allow for an average for the rise in temperature to be taken. A preliminary experiment was carried out, and it was a thermometric titration of hydrochloric acid and sodium hydroxide. This allowed an insight into what to expect when two strong acid and alkalis are mixed. Obtaining Evidence Acid (20cm3, 2 molar) Alkali (20cm3, 2molar) Start temperature (oC) 2dp Finish temperature (oC) 2dp Average temperature rise (oC) 2dp ST1 ST2 ST FT1 FT2 FT Hydrochloric acid (HCL) Strong acid Sodium Hydroxide (NaOH) Strong alkali 18.00 18.00 18.00 34.00 34.50 34.25 16.25 Hydrochloric acid (HCL) Strong acid Ammonium Hydroxide (NH4OH) Weak alkali 16.50 16.50 16.50 30.00 30.00 30.00 13.50 Ethanoic acid (CH3OOH) Weak acid Sodium hydroxide (NaOH) Strong alkali 16.50 16.50 16.50 28.50 29.00 28.75 12.25 Ethanoic acid (CH3OOH) Weak acid Ammonium hydroxide (NH4OH) Weak alkali 16.25 16.50 16.38 28.00 28.00 28.00 11.62 As this experiment involved the usage of acids and alkalis, immense care had to be taken to prevent accidents from occurring. A number of safety precautions were taken. These were that eye protection had to be worn at all times. ...read more.

Conclusion

No anomalous results were obtained, but whilst undertaking the experiment some values that were obtained did not relate with the theory when compared with the previous results. It was later discovered that these incorrect results were due to small droplets of water being left in the bottom of the polystyrene cup and measuring cylinder, causing the concentration to lower of the acid and alkali. However these incorrect results were neglected, and a repetition was undertaken without the inaccuracy, and it was ensured that the cup and cylinder were thoroughly dried to obtain accurate results. Some improvements to the experiment would be: 1) A lid would be needed for the polystyrene cup so as to minimise the heat loss as much as possible. This would also be made of polystyrene. 2) The use of a more accurate thermometer. 3) The use of pipettes and burettes would be very helpful in carrying out the experiments to ensure that they were as accurate as possible. 4) Undertake titrations of the acids and alkalis in order to check their concentrations. To extend the investigation, some dibasic and tribasic acids and alkalis could be investigated. These are acids and alkalis that have more than one hydrogen or hydroxide ion. This can be seen below: H2SO4 HSO4- + H+ HSO4 SO4- + H+ This would give an insight into how the heat evolved is related to the strength of an acid or alkali. ...read more.

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This is a five star piece of work. The investigation is explained in detail and in a clear order so the reader can easily follow the plans. Excellent predictions made and justified and excellent results collected to support the prediction. More excellent calculations which are then evaluated in detail demonstrating a good scientific understanding. Great investigation with a good outcome. The layout could be better if titles were added for prediction and safety and improvements but other than that it is all well done.

Marked by teacher Patricia McHugh 08/04/2013

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