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Lab III - Exploring the Method of Initial Rates

Lab III - Exploring the Method of Initial Rates

Lab III - Exploring the Method of Initial

Exploring the Method of Initial RatesPurpose: To gain an understanding of the Method of Initial Rates and rate law determinationby studying how sodium thiosulfate decomposes in the presence of an acid.The Chemistry of Sodium ThiosulfateSodium thiosulfate, Na 2 S 2 O 3 , is a versatile ionic compound which we will use several timesthis year. In the photographic industry, it is known as “hypo” and is used in the developingprocess to remove unused photosensitive compounds. Later this year we will use thiosulfatein a titration to determine the concentration of sodium hypochlorite in bleach. The usefulnessof thiosulfate in oxidation-reduction reactions such as this is a consequence of its unusualLewis Structure which puts the two sulfur atoms in different oxidation states.When placed in an acid such as HCl, the following reaction occurs:S 2 O 3 2− (aq) + 2H + (aq) → S(s) + SO 2 (g) + H 2 O(l)We saw this exact reaction in our sunset demo last week, as the formation of sulfur in thesolution can effectively be used as a clock to follow the rate of the reaction. The clock willwork as long as the concentrations of the reactants do not change appreciably overthe clock period. If this is the case, then the average rate will be a good approximationto the instantaneous rate. In this experiment, the concentrations are set such that theseapproximations are valid.Preparing to ExperimentYou should find the following materials at your station• Two 8-well plates• Three micropipets containing 1 M HCl, 0.15 M Na 2 S 2 O 3 , and Distilled water• A stopwatch• A piece of white paper• A group of three people