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‘The Relative Strength of an Unknown Acid’

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'The Relative Strength of an Unknown Acid' The aim of this experiment is to determine the relative strength of an unknown acid whose relative formula mass is 135. I am provided with the acid in as a white crystalline solid which is very soluble in water. The unknown acid monoprotic, which means for every one mole of acid, one mole of hydrogen is needed. The unknown acid can be completely neutralised by sodium hydroxide and the reaction is exothermic. The enthalpy change depends on the strength of the acid, so the stronger the acid, and the larger the enthalpy change. Therefore the weaker the acid, the smaller the enthalpy change. Below is a table with some typical values obtained by experiment. Acid Enthalpy change (kJ per mole of acid) HCl -57.9 HNO3 -57.6 CH2ClOOH -53.4 CH3COOH -50.1 HCN -38.2 To begin working out the unknown acids strength, I am going to use the enthalpy change equation (below) to work out the mass needed to make up a standard solution of the unknown acid. For this equation to work I am going to use 13?C as my hypothetical temperature rise and I am also going to choose an enthalpy change from the table above. ...read more.


Next using a dropping pipette (which will have also been cleaned with distilled water) I will add distilled water slowly to bring the meniscus up to the mark on the neck of the flask. I will be at eye level with the flask at this point to make sure it is as accurate as I can get it. I will then again place the stopper on the flask and turn it upside down and shake the contents, returning it upright, again repeating this ten times. Thermometric titration Before I start my titration I am going to leave my two solutions out in the room for a couple of hours, so they can adapt to the room temperature, and also so they are the same temperature. I must also make sure all my equipment is rinsed properly with distilled water. To clean the burette I will run distilled water through it, I will do this by setting the burette up with the use of a stand. I will close the tap at the bottom of the burette and fill the burette up to the top, I will then place a beaker under the burette and open the tap, letting the distilled water run out. ...read more.


The thermometer should always be read horizontal and at eye level, to make result as accurate as possible. Heat loss is inevitable but to minimise heat loss I used a polystyrene cup instead of a conical flask because polystyrene is a better insulator than glass. Hazards and safety precautions I must be careful throughout preparing the solution and the thermometric titration because the acid is unknown and could be very dangerous and the alkali is corrosive and toxic, and can be especially damaging to eyes. Any spillages must be cleaned up immediately; water can be used for this. To be careful, I must wear my lab coat and goggles at all times, gloves may also be helpful as most times. Below is a table of my hypothetical results: Volume added/cm3 0.0 0.5 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 Temperature/?C 19.0 20.6 21.6 23.5 25.1 26.7 28.9 30.8 31.8 31.5 30.9 On the next page is a graph I have drawn using these hypothetical results. It shows that the temperature rise is 13.0?C So from using the temperature rise of 13.0?C and the mass of 0.092g of the solid unknown acid, I worked out that the acid is a weak acid, and is CH3COOH. Skill 1 - Planning Charlotte Nellist Page 1 ...read more.

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