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The Disappearing Cross

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The Disappearing Cross In this experiment, we shall be adding sodium thiosulphate to hydrochloric acid together and placing a drawn cross underneath and seeing how long the rate of reaction lasts until you cannot see this cross. Before the reaction starts, the liquids are both clear. When added they turn cloudy and milky with a yellow tinge to it, due to the sulphur released. Na2S203 (aq) + 2HCL --> 2Nacl (aq) + H20 (l) + SO2 (g) + S(s) Sodium thiosulphate + hydrochloric acid --> sodium chloride + water + sulphur dioxide + Sulphur The aim of this experiment is to see how each factor affects the rate of reaction. The factors we are concentrating on are: * Concentration * Surface area/Particle size * Temperature * Catalyst Collision Theory: In order to understand rates of reactions, we use this model 1) All chemicals are made from particles (atoms/Molecules) 2) The particles have energy and can move 3) Chemical reactions only happen when particles collide 4) Some collisions are not successful. Thy must have enough energy to react Particle size. 6 surfaces therefore a slow reaction because of a smaller surface area Small chips therefore a faster reaction because of a ...read more.


This is because the collision theory deals with atoms vibrating as they receive more energy and they then hit more often. Apparatus o 1 thermometer o 1 beaker o 2 measuring cylinders o 1 conical flask o 1 tripod o 1 gauze o 1 heatproof mat o 1 stopwatch o 1 Bunsen burner o X board o 1 pair of tongs o 1 pair of goggles Method o Measure 10ml of hydrochloric acid into conical flask o Heat over Bunsen burner until desired heat o Remove immediately and place on top of hand-drawn flask o Add 25ml of Sodium thiosulphate o Time to see how long the cross takes to disappear completely o Repeat 3 times o Rinse and repeat to the heat 5 � over the last temperature. Safety A pair of goggles will be worn during the heating part of the experiment in order to protect the eyes. When handling hot beakers and measuring cylinders a pair of tongs will be used. A gauze and heatproof mat will be used while heating to avoid any damage to the equipment. ...read more.


The sketch graph I drew in my prediction matched the real graph showing that the science I used to explain my prediction was correct. Evaluation Looking at my results I can say that they were quite reliable and accurate. I had one anomalous result even after an average over three measurements. This was at 30� and I think may have been because the water we rinsed the beaker out in may have stayed in and caused the reaction to slow down. I can say that looking at my results when I repeated results they were quite close together. I think that I did the experiment quite well although I found it hard to spot where the exact moment when the cross disappeared. This is why we did an average over 3 measurements. To improve the experiment I would need to have a very accurate stopwatch to time exactly how long the cross took to dissappear so I could be really precise in my results. Ways in which I could extend this experiment are to use a different size of cross so that it doesn't disappear at such a low temperature this way I could carry on to see whether the collision theory is still right at higher temperatures. ?? ?? ?? ?? Chemistry Sharna Sutherland ...read more.

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    Apparatus List The apparatus in which I will need in order to undertake this experiment are: * Sodium Thiosulphate solution * Hydrochloric Acid * Measuring cylinder x2 (10ml for hydrochloric acid) (50ml for sodium thiosulphate - no 30ml cylinder). * Beaker * Water * Stop watch * Pen (to mark cross on paper)

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