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To measure the rate of reaction of thiosulphate ions with acid in solution, and find out how the rate depends on the concentration of both thiosulphate and acid.

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Chemistry - CH2 Practical Assessment Plan for Practical Assessment Aim: To measure the rate of reaction of thiosulphate ions with acid in solution, and find out how the rate depends on the concentration of both thiosulphate and acid. Background Information: Reactions can only happen when the reactant particles collide, but most collisions are not successful in forming product molecules. The reactant molecules must collide with enough energy to break the original bonds so those new bonds in the product molecules can be formed. All the rate-controlling factors are to do with the frequency of reactant particle collision. If the concentration of any reactant in a solution is increased, the rate of reaction is increased. Increasing the concentration, increases the probability of a collision between reactant particles because there are more of them in the same volume. The exact relationship between reaction rate and concentration depends on the reaction "mechanism". This is the process involving elementary reaction steps. The slowest step controls the rate. The nature of the slow step is not obvious from the balanced equation. Only experimental observation reveals the link between concentration and reaction rates. ...read more.


Wash any splashes or affected areas of skin with copious amounts of water. ? Sulphurous fumes produced in this reaction may be toxic by inhalation. Students who suffer from asthma need to take care. ? At the end of the experiment, small quantities of the chemicals can be diluted with running water and run to waste Apparatus and Chemicals We will need the following materials for the experiment: ? safety goggles ? three burettes for 1M sodium thiosulphate ? funnels ? 0.1M nitric acid ? Water ? 100 cm3 conical flask or beaker ? a black cross drawn on a white background ? test tubes ? stirring rod ? stopwatch Procedure The equation for the reaction whose rate we will be measuring is as follows: S2O32- (aq) + 2H+ (aq) S (s) + H2SO3 (aq) The reaction is easy to follow since one sulphur atom is formed for each thiosulphate ion reacting, and the sulphur makes the reacting solution more cloudy as its concentration increases. We will place the reaction vessel over a black cross, drawn on a white background. ...read more.


USE OF APPARATUS INITIAL RUN - PROCEDURE ? Using the 50 ml plastic measuring cylinder or a burette measure out into the conical flask: ? 6 cm3 of sodium thiosulphate. ? 4 cm3 of water. ? Using a burette or 10 ml glass measuring cylinder, measure out 10 cm3 of 0.1M nitric acid and put it into the test tube. ? Draw a small cross on the piece of paper and put the flask on top of it. ? Pour the nitric acid into the flask from the test tube and start the clock. ? Time (in seconds) how long it takes for the cross to disappear. ? Wash out the flask thoroughly . ? Repeat using the next volumes of sodium thiosulphate and acid as given in the results table (seven runs in all) After this we will carry out the calculations needed to complete the table: ? Calculate correct molarity, as solution dilutes the other ? 1/time versus concentration of thiosulphate at constant acid concentration ? 1/time versus acid concentration at constant thiosulphate concentration ? Using the figures obtained from our calculations as described above, we will plot two graphs, drawing the best line through the points on each graph. 1 ...read more.

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