Determining the rate equation of a reaction - Sample Essay

The aim of this experiment is to determine the rate equation for the reaction between sodium thiosulphate and hydrochloric acid using the order of the reaction with respect to each of the reagents. HCl (aq) + Na2S2O3 (aq) –> 2NaCl (aq) + SO2 (g) + S (s) When Hydrochloric acid is added to sodium thiosulphate, a solid ppt of sulphur is formed. I will use this to judge when the reaction has ended. The rate equation for this reaction is R = k [HCl]x [Na2S2O3]y In this equation: R = the rate of reaction k = the rate constant x and y = order of reaction with respect to HCl and Na2S2O3 (e. g.0, 1, 2 etc… ).

[HCl] = the concentration of HCl [Na2S2O3] = the concentration of Na2S2O3 My experiment will enable me to calculate x and y through plotting the volume against the reciprocal of the time it takes for the ppt to form. Preliminary work: The demonstration done by my teacher show that 50 cm3 of thiosulphate and 20 cm3 of deionised water mixed with 5cm3 of hydrochloric acid took 14. 93 seconds to form the ppt. I will use this as a guide to decide which set of concentrations I will use to get a reasonable time which I can plot. When I implement my plan, I will plot my results.

Using the shape of the graph, I will then deduce the order of the reaction. The following graphs show the shapes I should get for each order of a reaction. I am not changing the volume of hydrochloric acid because I want to test the effect of changing the concentration of the sodium thiosulphate to be able to find out the order of the reaction with respect to the sodium thiosulphate. Apparatus:  White tile with “X” in the middle. I will place my reacting solution on this white tile and time until the cross can no longer be seen. Stop watch to time the experiment Stirring rod.

5 cm3 measuring cylinder to measure the hydrochloric acid  Two 50 cm3 burette to measure the deionised water and the sodium thiosulphate as it is more accurate than a measuring cylinder.  One 250 cm3 conical flask into which I will mix my reagents Two funnels to full my burettes, which I will remove before looking at the meniscus.  I will be supplied with deionised water, HCl and Na2S2O3 Concentrations: Run 1 Run 2 Run 3 Run 4 Run 5 Na2S2O3 (cm3) 50 45 40 35 30 Deionised Water (cm3) 20 35 30 35 40 Variables: Fixed Independent Dependent Temperature: the experiment will be done at room temperature.

I will control the temperature because changing the temperature changes the rate which will affect me results. Concentration of HCl: I will keep this constant because I can only change the concentration of one reagent to see the effect this has on the rate. If I change both reagents I will not be able to tell the order of the reaction is with respect to which reagent. The white tile and cross: changing this might affect my visibility of the cross and the time. Total volume: I will keep this the same to ensure a fair experiment and the same visibility due to smaller volumes being more shallow and I might see the ppt at different times.

My independent variable is the volume of the sodium thiosulphate. By altering the volume and adding deionised water, I am changing the concentration. This will enable me to observe how this will affect the rate of the reaction The dependent variable is the time taken for the cross to disappear. Method: 1. fill one of the 50 cm3 burette with sodium thiosulphate and the other with deionised water using the funnel and remove funnel before looking at meniscus. The bottom of the meniscus should be on the zero line.

2. measure into the conical flask the exact amount stated in the table above of the sodium thiosulphate and the water for run 1.3. measure using the 5 cm3 cylinder 5 cm3 of HCl. 4. place the conical flask on the white tile with the cross 5. add the hydrochloric acid and start the stop clock. 6. stir the mixture briefly. 7. look at the conical flask from the top and stop the stop clock as soon as the cross is no longer visible. 8. write the time taken in the table. 9. repeat steps 1-8 for the other runs. I will then convert the times into rates by doing 1/t. this will go along my y-axis. The volume of the sodium thiosulphate will go along the x-axis.

Based on the shape of my graph I will then decide what the order of the reaction is with respect to the sodium thiosulphate. Safety: Chemical Hazards HCl Solutions equal to or stronger than 2 mol dm-3 but weaker than 6. 5 mol dm-3 are IRRITANT. Na2S2O3 Sodium thiosulphate may be harmful if you swallow it. It may also irritate the eyes or lungs. Eye contact: Flush the eye with plenty of water. If irritation persists, call for medical help. Skin contact: Wash off with water. If swallowed: If the amount swallowed is not trivial, call for medical help. SO2 Major hazard : Inhalation Odour : Pungent and Irritating.

S Contact with the eyes can cause irritation. The solid may also irritate the lungs if breathed in. Sulphur burns to give toxic gases, so should not be burnt in the open laboratory. Wear safety glasses. Solid sulphur should pose only minor hazards unless heated to a temperature at which it will burn. Safety glasses required. NaCl May cause eye irritation.

Reference: http://www. cartage. org. lb/en/themes/sciences/Chemistry/Miscellenous/Helpfile/Kinetics/Rateorder. htm Chemistry 2 Cambridge Advanced Sciences Endorsed by OCR page 109 http://physchem. ox. ac. uk/~hmc/hsci/chemicals/sodium_thiosulphate. html.