Chemical Kinetics: Reaction Rates Minneapolis Community and Technical College C1152 v.5.10

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1 Chemical Kinetics: Reaction Rates Minneapolis Community and Technical College C5 v.5.0 I. Introduction The Reactions Question #: Will a given chemical reaction occur? This question is very important especially if a reaction s products promis e new lifesaving drugs, stronger synthetic plastics or alternative fuels. The answer to this question is the focus of chemical thermodynamics and enthalpy and entropy considerations. However, in this eperiment, you will study a reaction that is known to occur in hopes of answering a second, important question: Question #: How fast does the reaction occur and in what way (if any) does it depend on the concentrations of the reactants? Such things are very important from a practical point of view since a reaction may be totally worthless if it requires many years to produce significant amounts of product. Fortunately, the reaction studied in this eperiment proceeds quickly enough for us to perform it many times within a reasonable amount of time. The reaction, shown below, involves hydrogen peroide and its reaction with iodide and hydrogen ions to form molecular iodine and water. Reaction # H O + I - + H + --> I + H O colorless blue in starch Aqueous starch, also present in the reaction vessel, turns the solution blue the moment molecular iodine (I ) concentrations increase. (Produced by the reaction: I + I - --> I 3 -, it is the I 3 - product and its reaction with starch that creates the blue color) Unfortunately, we can t get much useful information about the reaction s rate by simply miing the reactants. The reaction will produce enough I (I 3 - ), even in its early stages, to quickly turn the solution blue regardless of whether the reaction is fast or slow. A second reaction, occurring in the same container, is utilized in such a way that reaction rate information can be obtained. Reaction # S O I --> I - + S 4 O 6 - (fied amount thiosulfate) The reaction utilizes thiosulfate (S O 3 - ) ions to convert (or reduce) molecular iodine into the iodide ion (I - ). This process keeps the I concentration at zero and thus keeps the solution from turning blue. However, when all of the thiosulfate is used up, reaction # no longer occurs permitting I concentrations to quickly build and turn the solution blue. In this eperiment you will perform the above reaction sequence several times under different but carefully controlled conditions. However, in all cases, the amount of thiosulfate (S O 3 - ) is always the same and is the limiting reactant. How quickly the thiosulfate is consumed determines how quickly the solution turns blue. If a reaction miture takes more time to turn blue, it is because the reaction is relatively slow (low reaction rate) requiring more time to use up the available thiosulfate. A fast reaction uses up thiosulfate more quickly and turns blue in less time. Consider the analogous pie-eating contest (cartoon above). One way to organize such an event is to give each participant the same number of pies to eat (i.e. S O 3 - ). The fastest contenders finish with the pies in less time than those whose rate is slower. An individual whose rate is twice as fast finishes her pies in half the time as someone else. At the end of the race you might even see a few contestants turn blue (or green)!

2 Reaction Rates The rate of a chemical reaction often depends upon the concentrations of the reactants. In this eperiment, the reactants are hydrogen peroide, iodide and hydrogen ions (Reaction #). Mathematically, the rate of the reaction is related to these concentrations via the following rate equation: Rate k [H O [I - y [H + z In this equation, k is the reaction rate constant, [H O, [I -, [H + are the molar concentrations of the three reactants, and, y and z are eponents to be determined. When a reactant s concentration doesn t affect the reaction rate the reaction is referred to as zeroth order in that reactant (i.e. the eponent is a zero). Similarly, reactions found to depend directly or upon the square of a reactant s concentration are said to be first order or second order respectively (i.e. the eponent is a or ). Your goal in today s eperiment is to determine the reaction orders,, y and z, for all three reactants Consider a reaction known to be second order in some reactant (eponent is ). Tripling the concentration of the reactant will increase the rate of the reaction by a factor of 3 9. That is, the reaction proceeds at 9 the rate as before the concentration was changed. Naturally, 9 the rate means the reaction is finished in /9 th of the original time. Now consider two eperiments, identical in every way ecept that the first uses a different concentration of hydrogen peroide than the second. Their reaction rate equations can be written: Reaction #: Rate k [H O Reaction #: Rate k [H O [I - y [H + z Equation #A [I - y [H + z Equation #B Note that k, [I - y and [H + z are the same in both equations. Thus, if we divide these two equations by one another the identical terms cancel out: rate rate y + k [ HO [ I [ H y + k [ HO [ I [ H z z [ HO [ H O Equation # or rate rate [ H [ H O O [ H O [ H O Equation #3 As mentioned earlier, the rate of reaction is inversely proportional to the time it takes for the reaction to occur (i.e. twice as fast half the time). Therefore, the above equation can be rewritten as: t t [ H O [ H O Equation #4 where t and t are the times required for the reaction miture to turn blue.

3 Lastly, by taking the log of both sides of this equation we can solve for : t log t Equation #5 [ HO log [ HO Thus, by performing two trials in which only the initial hydrogen peroide concentrations are different, we can determine by measuring the respective times for each trial and substituting these values into the equation above. Other reaction orders (y and z) for the other reactants are calculated in the same fashion. II. MSDS: Chemical Information Potassium Iodide Solution Route Of Entry - Inhalation: NO Route Of Entry - Skin: YES Route Of Entry - Ingestion: YES Health Haz Acute And Chronic: ACUTE: MODERATELY TOXIC. MAY BE IRRITATING TO EYES. CHRONIC: SLIGHTLY TOXIC. Carcinogenicity - NTP: NO Carcinogenicity - IARC: NO Carcinogenicity - OSHA: NO Eplanation Carcinogenicity: NOT RELEVANT. Signs/Symptoms Of Overep: CHRONIC EXPOSURE MAY LEAD TO IODISM: RASHES, HEADACHE, FEVER. Med Cond Aggravated By Ep: MAY AGGRAVATE SKIN DISORDERS. Emergency/First Aid Proc: EYE: FLUSH WITH PLENTY OF WATER FOR AT LEAST 5 MINUTES. SKIN: FLUSH WITH PLENTY OF WATER. INGESTION: GIVE LARGE QUANTITIES OF WATER OR MILK. CALL PHYSICIAN IMMEDIATELY. INHALATION: REMOVE FROM EXPOSURE. SUPPORT BREATHING (GIVE ARTIFICIAL RESPIRATION/OXYGEN). CALL MD (FP N). Sodium Thiosulfate CAUTION! MAY BE HARMFUL IF SWALLOWED OR INHALED. MAY CAUSE IRRITATION TO SKIN, EYES, AND RESPIRATORY TRACT. Health Rating: 0 - None Flammability Rating: 0 - None Reactivity Rating: 0 - None Contact Rating: - Slight Lab Protective Equip: GOGGLES; LAB COAT Storage Color Code: Orange (General Storage) Potential Health Effects Inhalation: May cause mild irritation to the respiratory tract. Ingestion: May cause mild irritation to the gastrointestinal tract. Skin Contact: May cause mild irritation and redness. Eye Contact: May cause mild irritation, possible reddening. Chronic Eposure: Chronic eposure may cause skin effects. Aggravation of Pre-eisting Conditions: No information found. Molybdenum VI catalyst WARNING! MAY BE HARMFUL IF SWALLOWED OR INHALED. CAUSES IRRITATION TO SKIN, EYES AND RESPIRATORY TRACT. AFFECTS KIDNEYS AND BLOOD. Health Rating: - Moderate Flammability Rating: 0 - None Reactivity Rating: - Slight Contact Rating: - Moderate Lab Protective Equip: GOGGLES; LAB COAT; VENT HOOD; PROPER GLOVES Storage Color Code: Orange (General Storage) Potential Health Effects Inhalation: Irritant for t he upper respiratory system. Pungent taste in mouth and throat, coughing, labored breathing. Can be a route of absorption by the body with symptoms like ingestion. Ingestion: Irritant to the digestive system. Symptoms of sore throat, abdominal pain, nausea may occur. May cause anemia,gout, headaches, weight loss, joint pain, and liver or kidney damage. Skin Contact: Causes irritation to skin. Symptoms include redness, itching, and pain. Eye Contact: causes irritation, redness, and pain. Chronic Eposure: Prolonged or repeated eposure to this product may cause symptoms similar to ingestion. Aggravation of Pre-eisting Conditions: Persons with pre-eisting skin disorders, blood disorders or eye problems, or impaired liver, kidney or respiratory function may be more susceptible to the effects of the substance.

4 III. Procedure Recommendations: You will be working in pairs today. Assemble the apparatus shown at right. Use a clean, freshly rinsed 50 ml beaker for all trials. Obtain ~40 ml of Na S O 3 solution in a clean, dry 50 ml beaker. Measure solution volumes carefully. Warning: Keep the temperature control in the off position. Accidentally heating the reaction miture will dramatically affect your results. Solution #: A baseline for subsequent trial comparisons. Solution #: Half the H O concentration. (in comparison to Solution #) Refer to the table above for volume amounts. Follow instructions for Solution # Solution #3: Double the KI concentration. (in comparison to Solution #) Refer to the table above for volume amounts. Follow instructions for Solution # Solution #4: 0 the H + concentration (in comparison to Solution #) Refer to the table above for volume amounts. Follow instructions for Solution # Solution #5: The effect of a catalyst Only one trial Refer to the table above for volume amounts. Follow instructions for Solution #. Add all liquids ecept H O in order from left to right.. Reset the stopwatch. (HINT: Practice using the stopwatch before you need it.) 3. Steadily stir the solution using the magnetic stir plate/stir bar. Avoid splashing. 4. Measure the temperature of the miture using an alcohol thermometer. Record this temperature in your lab notebook. 5. Quickly pour the H O in the graduated into the reaction beaker while starting the stopwatch. 6. Stop timing when the solution abruptly turns blue. 7. Record the time in your lab notebook. 8. Discard the waste solutions as instructed. 9. Clean, rinse and shake off ecess water (Don t dry with paper towel) 0. Repeat a second time. Results should agree to within a few seconds.. Post your timing results on the blackboard.. Dispose of all waste solutions carefully in the lab sink. Solution #6: 0 o C warmer (in comparison to Solution #) Only One Trial. Refer to the table for volume amounts.. Warm the 50 ml beaker containing the miture on a hot plate (stir and use very low setting). 3. When the temperature is approimately 0 o C warmer than the original temperature remove the beaker from the hotplate and 4. Quickly add the H O and start the stopwatch. 5. Record the time for the reaction. 6. Don t forget to record the high and low temperatures for the trial. *Before leaving lab today, write down the concentrations as they appear on ALL bottle labels. Use these concentration values in your calculations.

5 Solution # Temp ( o C) A Water 00 ml grad B M Buffer 50 ml grad C M HC H buffer station D M KI 5 ml grad E % Starch 5 ml grad F M Na S O 3 5 ml volumetric pipette G M Mo(VI) catalyst station H M H O 0 ml grad R.T R.T R.T R.T R.T R.T Glassware Summary (see column labels in table above): Liquid A: 00 ml graduated provided for you. Solution B: 50 ml Graduated provided for you. Solution C: Graduated at acetic acid station (not to leave station) Solution D: 5 ml Graduated provided for you. Solution E: 5 ml Graduated provided for you. Solution:F i.) 50 ml beaker as reservoir. ii.) 5 ml volumetric pipette provided for you. Solution G: Graduated at catalyst station (not to leave station) Solution H: 0 ml graduated provided for you.

6 IV. Data Analysis The required initial concentrations of H O, I - and H + are not simply those that appear on their respective bottle labels. Rather it is necessary to calculate the new, diluted concentrations using 50 ml as the total volume (where did 50 come from?). Calculate the new initial concentrations and report these values in a data table (below). Several have been calculated for you and you should fill in the rest. Initial Concentrations (after dilution) Solution # [I- [H + [H O Report the times and temperatures for all trials in this table: Trial Trial Solution# Temperature ( o C) Time (seconds) Temperature ( o C) Time (seconds)

7 Determination of : [H O Substitute the concentration and time information for solutions # and # in Equation #5. Calculate the value of and round off to the nearest integer. Determination of y: [I - y Use the concentration and time information for solutions # and #3 to determine y. You will need to adapt Equation #5 appropriately to eliminate [H O and include [I -. Round your result to the nearest integer. Determination of z: [H + z Use the concentration and time information for solutions # and #4 to determine z. You will need to adapt Equation #5 appropriately to eliminate [H O and include [H +. Round your result to the nearest integer. Effect of a Catalyst: Calculate how many times faster the catalyzed reaction (Solution #5) proceeded in comparison to the cooler reaction (Solution #). (time uncatalyzed / time catalyzed ) Effect of Temperature: Calculate how many times faster the warmed reaction (Solution #6) proceeded in comparison to the cooler reaction (Solution #). (time cool /time warm )

8 V. Team Report Page : All must be word processed, clearly worded and on one page. Upper right corner: Student Names, lab section number and date of eperiment Answers to the following questions:. Why is it important that the total solution volume be kept constant from one trial to another? Eplain.. Based on your values of, y and z, how should the reaction rate be affected when a. the H O concentration is tripled (keeping all other factors constant) b. the I - concentration is doubled? c. the H + concentration is tripled? d. all concentrations are doubled? 3. How many times faster did the reaction occur with the addition of the catalyst (show work)? 4. Chemists often claim that reaction rates roughly double for every additional 0 o C. How many times faster did the reaction rate increase in your eperimental trial #5 (show work)? Does your result support the chemist s claim above? Page : Data tables: Obtain a copy of the two data tables from the lab-handout web site and fill in your values Calculations:. Show your calculations for, y and z (handwritten is okay).. Provide, y and z values with 4 decimal place accuracy. 3. Round off your, y and z values to the nearest integer. 4. Write the complete rate equation using your integer values of, y and z.