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# The Determination of rate equation

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Introduction

The determination of rate equation Aim: The aim of this investigation is to determine the rate equation of the reaction between sodium thiosulphate Na2S2O3 (aq) and hydrochloric acid HCl (aq), using a graphical method. Background information The experiment is based on the effect of varying the concentration of the respective solutions and finding out the effect it has on the rate of the reaction; in this investigation i will change both sodium thiosulphate and hydrochloric acid. When hydrochloric acid is in excess to the sodium thiosulphate, the reaction may be different to when sodium thiosulphate is in excess. This will show the effect of change in concentration on the rate of reaction. Even though the reaction equation is known, it is not possible to work out the rate equation from the reaction equation and therefore the experiment has to be conducted. Theory: The rate of a reaction can be explained in terms of the rate of decrease in concentration of a reactant or the rate of increase in concentration of a product. The most general method used to determine the rate of reaction is to measure the change in concentration of the reactant(s) per second. The rate of a reaction may be represented by a mathematical equation related to the chemical equation for a reaction. Rate equation has the form rate = k [A]x [B]y which shows how the rate of a chemical reaction depends on the concentration of the reactants (A&B) and the rate constant k. A rate equation is used to describe how the concentration of a product increases or the concentration of the reactants decreases with time, the equation also indicates how the concentration of one or more reactants directly affects the rate. Occasionally it can even be the concentration of a product that affects the rate. In general the rate equation for the reaction: A + B C + D Is found by experiment to follow simple kinetics with the rate equation being written as: Rate = k [A]x[B]y k = rate constant, x = order with respect to [A], y = order with respect with [B]. ...read more.

Middle

3. Lay out all the equipment as shown bellow. 4. Label each test tube with the correct concentration level and organise them in ascending order, to prevent contamination. 5. Label the Graduated pipettes and the beakers for different chemicals to prevent contamination as HCl, Na2S2O3 and H2O are all clear liquid solution. 6. Pour HCl and Na2S2O3 into the correct labelled beaker from the main source. 7. Using the graduated pipette measure 4cm3of HCl and pour into the intended test tube in the test tube rack as shown below: 8. Then measure 41 cm3 of water into the HCl in the test tube and shake the solutions to dilute the HCl. 9. Measure 10cm3 of Na2S2O3 using the graduated pipette and pour it into the other labelled test tube. 10. Using a black marker pen draw a thick cross on the white tile as shown below; 11. Then place the white tile with the cross under the test tube of the diluted HCl as shown bellow. 12. Then add the 10cm3 of Na2S2O3 from the test tube and start the stop watch immediate. 13. Allow the same person to observe through out the whole experiment to make the practical more reliable. 14. Stop the stop watch when the cross is not visible through the solution from the birds eye view in other words when the solution becomes milky/cloudy. 15. Record the result in the result table shown below: Volume of HCl / cm3 Volume of H2O / cm3 Time taken for solution to become milky/ s Experiment 1 Experiment 2 Experiment 3 Average 4.00 41.00 8.00 37.00 12.00 33.00 16.00 29.00 20.00 25.00 16. Repeat the experiment with the same amount of HCl, Na2S2O3 and H2O another 2 times and record the result to make the experiment reliable and the data collected valid. 17. Now repeat the whole experiment using different ranges shown bellow: During these stages distilled water will be used to dilute the HCl and the volume of Na2S2O3 will be the same for the fist experiment. ...read more.

Conclusion

Identify those errors that are significant and justify your choice - justify improvements to the measurement A noticeable effect and the most significant measurement error on the rate of reaction which could have lead errors in measurement could have being; using naked eye to approach conclusion on the rate of reaction, the main problem with using a naked eye is that different people have different eyesight, even though the same person observed the rate of reaction through out the whole experiment, I believe this was not very accurate as even one second delay on the stopping the stop watch could cause an anomaly and also the fact that the person observing the reaction will not get the same clearness in each experiment will make difference in the result therefore instead of using naked eye I will next time use a colorimeter to measure the rate of reaction. Colorimeter is equipment used to measure the colour change in a reaction. By using this devise it will improve accuracy of the experiment as it will exactly tell me how long the rate of reaction has taken place for. Another factor which leaves a margin for error to certain extent was the equipment used to measure the reagents. The accuracies of each burette for example may differ slightly, which may affect the outcome of the experiment, as different burettes were used to measure different solutions. I noticed that it was very difficult to maintain the meniscus of the reagent on the mark as the burette was filled because some times air bubble were formed in the burette, This as a result leaves margin for error. When reading the burettes I was at the level of the graduation to avoid parallax error. The burette used to measure different reagents has an error of + 0.005 and hence may have affected the accuracy of the results obtained. To modify this error I would next time use a digital burette as it would measure the exact amount of reagent required in electric digits and therefore would improve the reliability and accuracy of the solution measured further. ...read more.

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