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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 Rates

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<strong>Exploring</strong> <strong>the</strong> <strong>Method</strong> <strong>of</strong> <strong>Initial</strong> <strong>Rates</strong>Purpose: To gain an understanding <strong>of</strong> <strong>the</strong> <strong>Method</strong> <strong>of</strong> <strong>Initial</strong> <strong>Rates</strong> and rate law determinationby studying how sodium thiosulfate decomposes in <strong>the</strong> presence <strong>of</strong> an acid.The Chemistry <strong>of</strong> Sodium ThiosulfateSodium thiosulfate, Na 2 S 2 O 3 , is a versatile ionic compound which we will use several timesthis year. In <strong>the</strong> photographic industry, it is known as “hypo” and is used in <strong>the</strong> developingprocess to remove unused photosensitive compounds. Later this year we will use thiosulfatein a titration to determine <strong>the</strong> concentration <strong>of</strong> sodium hypochlorite in bleach. The usefulness<strong>of</strong> thiosulfate in oxidation-reduction reactions such as this is a consequence <strong>of</strong> its unusualLewis Structure which puts <strong>the</strong> two sulfur atoms in different oxidation states.When placed in an acid such as HCl, <strong>the</strong> 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 <strong>the</strong> formation <strong>of</strong> sulfur in <strong>the</strong>solution can effectively be used as a clock to follow <strong>the</strong> rate <strong>of</strong> <strong>the</strong> reaction. The clock willwork as long as <strong>the</strong> concentrations <strong>of</strong> <strong>the</strong> reactants do not change appreciably over<strong>the</strong> clock period. If this is <strong>the</strong> case, <strong>the</strong>n <strong>the</strong> average rate will be a good approximationto <strong>the</strong> instantaneous rate. In this experiment, <strong>the</strong> concentrations are set such that <strong>the</strong>seapproximations are valid.Preparing to ExperimentYou should find <strong>the</strong> 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 <strong>of</strong> white paper• A group <strong>of</strong> three people


1 2 3 4 5 6 7 8Figure 1: Tracing <strong>the</strong> wells on your paper• Examine your 8-well plates. The first thing you are going to want to do is trace <strong>the</strong>outlines <strong>of</strong> <strong>the</strong> wells onto your piece <strong>of</strong> white paper. Thus, after you are done youshould have 8 circles on your paper. Inside <strong>the</strong>se circles place <strong>the</strong> numbers 1-8 fromleft to right. When you are done, you should have <strong>the</strong> following diagram on your paper.• As shown in class, practice <strong>the</strong> “shakedown” technique for moving reagents from oneset <strong>of</strong> wells to ano<strong>the</strong>r.Using <strong>the</strong> micropipets, place <strong>the</strong> reagents into <strong>the</strong> wells as shown in <strong>the</strong> tables belowWell Number Drops <strong>of</strong> 0.15 M sodium thiosulfate1 102 93 84 75 66 57 48 3Table 1: Drops <strong>of</strong> sodium thiosulfate placed into each well <strong>of</strong> one <strong>of</strong> <strong>the</strong> 8 well platesWell Number Drops <strong>of</strong> 1M HCl Drops <strong>of</strong> Distilled Water1 2 02 2 13 2 24 2 35 2 46 2 57 2 68 2 7Table 2: Drops <strong>of</strong> HCl and Distilled Water Placed in each well <strong>of</strong> <strong>the</strong> second 8 well plate• Note that each well will have 12 drops <strong>of</strong> reagents in it when each is mixed. In addition,it is critical that you hold <strong>the</strong> pipets vertically when dropping reagents. Weneed each <strong>of</strong> <strong>the</strong> drops to be <strong>the</strong> same size. Holding <strong>the</strong> pipets at <strong>the</strong> same angle whiledropping helps to ensure this.


• Your group <strong>of</strong> three will divide labor in <strong>the</strong> following way– One shaker and observer– One timer– One recorderYou will do <strong>the</strong> following steps three times, rotating who does what each time (so thateach partner experiences all three jobs), recording your data in <strong>the</strong> table below.1. Add solutions to each <strong>of</strong> <strong>the</strong> 8 well plates as shown in Tables 1 and 2 above2. Use <strong>the</strong> shakedown technique to mix <strong>the</strong> solutions. The timer must start <strong>the</strong>stopwatch at <strong>the</strong> instant <strong>of</strong> shakedown3. The observer <strong>the</strong>n places <strong>the</strong> 8 well plate over <strong>the</strong> numbers on <strong>the</strong> paper, makingsure that well 1 contains <strong>the</strong> greatest number <strong>of</strong> drops <strong>of</strong> thiosulfate4. The observer watches <strong>the</strong> numbers under <strong>the</strong> wells, calling time when <strong>the</strong> numbercan no longer be seen in <strong>the</strong> well5. The timer tells <strong>the</strong> recorder <strong>the</strong> time at which each number disappears6. The recorder records this information for each well in <strong>the</strong> table below.7. Group members rotate positions and repeat a total <strong>of</strong> three times.8. Average <strong>the</strong> reaction times and place <strong>the</strong>m in <strong>the</strong> appropriate location in Table3 below.Well Number12345678ReactionTimeTrial 1(sec)ReactionTimeTrial 2(sec)ReactionTimeTrial 3(sec)AverageReactionTimeTable 3: Reaction Times for Each TrialAnalysisTo use <strong>the</strong> method <strong>of</strong> initial rates, we need three things:


• The general rate law for <strong>the</strong> reaction• The concentrations <strong>of</strong> each reactant for each experiment• The rate <strong>of</strong> each experiment1. To begin, write down <strong>the</strong> general rate law for <strong>the</strong> reaction, recalling that <strong>the</strong> balancedchemical equation is:Rate Law:S 2 O 3 2− (aq) + 2H + (aq) → S(s) + SO 2 (g) + H 2 O(l)Based on <strong>the</strong> experiment, explain why we can effectively ignore <strong>the</strong> effects <strong>of</strong> HCl on<strong>the</strong> rate in this experiment.2. Determine <strong>the</strong> initial concentrations <strong>of</strong> thiosulfate and HCl in each well, showing twosample calculations and <strong>the</strong>n completing <strong>the</strong> appropriate values in <strong>the</strong> table below:WellNumber12345678<strong>of</strong>Concentration<strong>of</strong> HCl (M)Concentration<strong>of</strong> Thiosulfate(M)DropsThiosulfateAvg. ReactionTime (sec)“Rate”(1/s)Table 4: Concentration and Rate Calculations


Sample Concentration Calculation for WellSample Concentration Calculation for Well3. Copy your average reaction times from Table 3 into <strong>the</strong> calculations table. Calculate<strong>the</strong> “rate” for each reaction by taking <strong>the</strong> reciprocal <strong>of</strong> each <strong>of</strong> <strong>the</strong>se times, againplacing your answers in <strong>the</strong> calculations table, and showing one sample below.Sample Rate Calculation for WellBased on our discussion in class, explain why using any data that is proportional to<strong>the</strong> rate is just as valuable as rate data in kinetics calculations.4. Create two graphs in Logger Pro• “Rate” vs. Concentration <strong>of</strong> Thiosulfate• “Rate” vs. drops <strong>of</strong> thiosulfate


Make sure each graph properly displays <strong>the</strong> data and is appropriately titled. You willadd <strong>the</strong>se graphs to <strong>the</strong> end <strong>of</strong> <strong>the</strong> report in your lab book.Select <strong>the</strong> appropriate curve fit for each <strong>of</strong> <strong>the</strong>se graphs. Based on <strong>the</strong>se graphs, whatis <strong>the</strong> order <strong>of</strong> <strong>the</strong> reaction with respect to thiosulfate? Explain your choice.Order <strong>of</strong> Reaction with respect to thiosulfate:Explanation:Discussion1. Is <strong>the</strong>re a relationship between your graph(s) and <strong>the</strong> value <strong>of</strong> <strong>the</strong> rate constant? Canwe actually determine <strong>the</strong> value <strong>of</strong> k in this lab? Explain.2. Did it matter in this lab if our independent variable was <strong>the</strong> concentration <strong>of</strong> thiosulfateor drops <strong>of</strong> thiosulfate? Explain


3. Explain why <strong>the</strong> assumption that each drop had <strong>the</strong> same volume was critical to thisexperiment.4. In <strong>the</strong> space below, clearly explain how you would change <strong>the</strong> experiment to determine<strong>the</strong> order with respect to HCl.Conclusion:

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