Kinetics of the Acid Decomposition of Thiosulfate
KINETICS OF THE ACID DECOMPOSITION OF THIOSULFATE Aqueous solutions of thiosulfate, S2O32-(aq) are stable if neutral or basic, but decompose quickly when the thiosulfate is dissolved in acid according to the equation:
Sulfur dioxide is a gas at room temperature, but is very soluble in water. Sulfur, a water insoluble solid, forms a colloidal suspension. As a result, the solution first becomes cloudy and then opaque. We can take advantage of the developing opacity of the reaction system to do a kinetics study. A series of acidic thiosulfate solutions of the same volume, contained in identical Titertek wells, are prepared and placed on a piece of white paper clearly numbered. The decompositions are timed. While the reaction proceeds, the number can be clearly seen through the solution. Eventually, precipitated sulfur renders to solution opaque, the number become invisible, and the timing stops. After several runs, reaction times are correlated with the concentrations of the acid and thiosulfate ion so that the reaction mechanism and rate law can be determined. MATERIALS: 0.24 M sodium thiosulfate, Na2S2O3(aq), 6.0 M hydrochloric acid, HCl(aq), 2-1x8 (or 1x12) well Titertek strips, 4-micro-tip 1 mL Beral pipets, timer, cotton swabs, 24-well reaction plate, permanent marking pen PROCEDURE: 1. Practice adding water and starting the timer until you have mastered the technique. See the attached Titertek procedure sheet. Label three 1 mL Beral pipets for thiosulfate, S2O32-(aq), hydrochloric acid, HCl(aq), and water, H2O. Fill one well in a 24-well reaction plate with 0.24 M sodium thiosulfate, fill a second with 6.0 M hydrochloric acid, and the third well with deionized water. Fill the three labeled pipets from the reaction plate. Add 0.24 M sodium thiosulfate solution dropwise to one of the Titertek strips according to table #1. Repeat the procedure by adding the required amount of 6.0 M HCl(aq) to the second Titertek strip followed by deionized water. Place the two
Sulfur dioxide is a gas at room temperature, but is very soluble in water. Sulfur, a water insoluble solid, forms a colloidal suspension. As a result, the solution first becomes cloudy and then opaque. We can take advantage of the developing opacity of the reaction system to do a kinetics study. A series of acidic thiosulfate solutions of the same volume, contained in identical Titertek wells, are prepared and placed on a piece of white paper clearly numbered. The decompositions are timed. While the reaction proceeds, the number can be clearly seen through the solution. Eventually, precipitated sulfur renders to solution opaque, the number become invisible, and the timing stops. After several runs, reaction times are correlated with the concentrations of the acid and thiosulfate ion so that the reaction mechanism and rate law can be determined. MATERIALS: 0.24 M sodium thiosulfate, Na2S2O3(aq), 6.0 M hydrochloric acid, HCl(aq), 2-1x8 (or 1x12) well Titertek strips, 4-micro-tip 1 mL Beral pipets, timer, cotton swabs, 24-well reaction plate, permanent marking pen PROCEDURE: 1. Practice adding water and starting the timer until you have mastered the technique. See the attached Titertek procedure sheet. Label three 1 mL Beral pipets for thiosulfate, S2O32-(aq), hydrochloric acid, HCl(aq), and water, H2O. Fill one well in a 24-well reaction plate with 0.24 M sodium thiosulfate, fill a second with 6.0 M hydrochloric acid, and the third well with deionized water. Fill the three labeled pipets from the reaction plate. Add 0.24 M sodium thiosulfate solution dropwise to one of the Titertek strips according to table #1. Repeat the procedure by adding the required amount of 6.0 M HCl(aq) to the second Titertek strip followed by deionized water. Place the two