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1-Factors-Complete - Factors Affecting Reaction Rates...

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Unformatted text preview: Factors Affecting Reaction Rates Experiment Assignment & Mini-Report Guidelines READING Experiment — Lab Manual Pages / Handout Chemistry, S'h ed. by Silberberg — Ch.l6 Introduction and Sections 16.1-16.2 PRE-LAB Begin the prelab on a new page of your laboratory notebook. ALL elements of the pre-lab MUST be completed before an experiment is started. The COPY page from your notebook will be collected as you enter the lab. The original pages must stay in your notebook. Heading ' Title of experiment and number, your name, the dates of the experiment. Purpose ° Briefly, but specifically explain the purpose of each experiment individually (parts A-E). Procedure Summery ' Summarize the steps of each experiment individually (parts A-E). MINI LAB REPORT Begin the lab report section on a new page of the lab notebook. Heading ‘ Title of experiment and number, your name, the dates of the experiment. Data / Experimental Analysis ' Using the “Report Sheet” pages of the handout as a guide, record the necessary data for each experiment in your lab notebook. ° In your lab notebook, answer questions, provide calculations, analyses as directed by the PROCEDURE and REPORT SHEET for each part (A-E). Please recopy the report sheet questions in your notebook. ' Additionally, for part A, write a complete, balanced equation for each of the 4 reactions of magnesium metal with the 4 different acids. Include state symbols. Post-Laboratory Questions ° Answer the post-lab question directly on the handout (lab manual) pages and submit with the COPY pages of the mini-report. - To study the various factors that affect the rates of chemical reactions The following techniques are used in the Experimental Procedure Chemical kinetics is the study of chemical reaction rates, how reaction rates are con- trolled, and the pathway or mechanism by which a reaction proceeds from its reactants to its products. Reaction rates vary from the very fast. in which the reaction, such as the explosion of a hydrogen/oxygen mixture, is essentially complete in microseconds or even nanoseconds, to the very slow, in which the reaction, such as the setting of concrete, requires years to complete. The rate of a chemical reaction may be expressed as a change in the concentration of a reactant (or product) as a function of time (e.g., per second)-othe greater the change in the concentration per unit of time. the faster the rate of the reaction. Other parameters that can follow the change in concentration of a species as a function of time in a chem- ical reaction are color (expressed as percent transmittance, Figure 23.1). temperature, pH. odor, and conductivity. The parameter chosen for following the rate of a particular reaction depends on the nature of the reaction and the species of the reaction. We will investigate four of five factors that can be controlled to affect the rate of a chemical reaction. The first four factors listed below are systematically studied in this experiment: ' Concentration of the reactants . Surface area of the reactants - Nature of the reactants - Temperature of the chemical system - Presence of a catalyst Some substances are naturally more reactive than others and, therefore, undergo rapid chemical changes. For example, the reaction of sodium metal and water is a very rapid, exothermic reaction (see Experiment I 1. Part E), whereas the reaction between iron and water is very slow. Plastics. reinforced with fibers such as carbon or glass, are now being substituted for iron and steel in specialized applications where corrosion has historically been a problem. Experiment Factors Affecting Reaction Rates Iron reacts slowly in air to form ironllll) oxide, commonly called rust. When pure iron is heated and thrust into pure oxygen, the reaction is rapid. OBJECTIVE TECHNIQUES INTRODUCTION Figure 23.1 The higher concentration of light-absorbing species, the more intense is the color of the solution. Nonosecond: l X IO’gsecond Species: any atom, molecule, or ion that may be a reactant or product of a chemical reaction Nature of the Reactants Sodium metal and water: the reaction releases Hg{g) which ignites with the oxygen in the air to produce 0 yellow/ blue Home, the yellow resulting from the presence of Na‘ in the Home Experiment 23 27 5 Temperature of the Chemical System lnternal energ : the energy contained within the mo ecu/es/ions when they collide Presence of :1 Catalyst Concentration of the Reactants Surface Area of the Reactants EXPERIMENTAL PROCEDURE A. Vature of the Reactants As a rule oft/114mb. a 10°C rise in temperature doubles (increases by a factor of 2) the rate of a chemical reaction. The added heat not only increases the number of collisions1 between reactant molecules, but also, and more importantly, increases their kinetic en- ergy. On collision of the reactant molecules, this kinetic energy is converted into an in- ternal energy that is distributed throughout the collision system. This increased internal energy increases the probability for the weaker bonds to be broken and the new bonds to be formed. Activation energy for uncatatyzed reaction Activation energy for catalyzed reaction Potential Energy A catalyst increases the rate of a chemical re- action without undergoing any net chemical change. Some catalysts increase the rate of only one specific chemical reaction without af- fecting similar reactions. Other catalysts are more general and affect an entire set of similar reactions. Catalysts generally reroute the path- way of a chemical reaction so that this “alter— nate” path, although perhaps more circuitous, has a lower activation energy for reaction than the uncatalyzed reaction (Figure 23.2). Reaction Coordinate Figure 23.2 Reaction profiles of on uncotolyzed and 0 catalyzed reaction. An increase in the concentration of a reactant generally increases the reaction rate. See the opening photo. The larger number of reactant molecules increases the probability of an “effective” collision between reacting molecules for the formation of product. On occasion, such an increase may have no effect or may even decrease the reaction rate. A quantitative investigation on the affect of concentration changes on reaction rate is undertaken in Experiment 24. Generally speaking, the greater the exposed surface area of the reactant, the greater the reaction rate. For example, a large piece of coal burns very slowly, but coal dust burns rapidly, a consequence of which can be a disastrous coal mine explosion; solid potas- sium iodide reacts very slowly with solid lead nitrate, but when both are dissolved in solution, the formation of lead iodide is instantaneous. Procedure Overview: A series of qualitative experiments are conducted to deter— mine how various factors affect the rate of a chemical reaction. Caution: A number of strong acids are used in the experiment. Handle with (are; do not allow them to touch the skin or clothing. Ask your instructor which parts of the Experimental Procedure you are to com‘ plete. Prepare the hot water bath needed for Parts B and C. Use a 250‘mL beaker. I. Different Acids Affect Reaction Rates. Half—fill a set of four, labeled small test tubes (Figure 23.3) with 3 M H2804, 6 M HCI, 6 M CH3COOH, and 6 M H3PO4, respectively. (Caution: Avoid skin contact with the acids.) Submerge a l-cm strip of magnesium ribbon into each test tube. Compare the reaction rates and record your observations. 2. Different Metals Affect Reaction Rates. Half—till a set of three, labeled small test tubes (Figure 23.4) with 6 M HCI. Submerge l—cm strips of Zinc, magnesium, and copper separately into the test tubes. Compare the reaction rates of each metal in HCl and record your observations. Match the relative reactivity of the metals with the photos in Figure 23.5. 1A lO°C temperature rise only increases the collision frequency between reactant molecules by a factor of l.02—-nowhere near the factor of 2 that is normally experienced in reoction rote. 276 Factors Affecting Reaction Rates Disposal: Dispose of the reaction solutions in the "Waste Inorganic Test Solu- tions" container. 6 M s M a M H2304 HCt CH3COOH H3PO4 Figure 23.3 Setup for the effect of Figure 23.4 Setup for the effect acid type on reaction rate. of metal type on reaction rate. r.; Figure 23.5 Zinc, copper, and magnesium react at different rates with 6 M HCl. ‘ 3 Identify the metals in the photo according to their reactivity. Ask your instructor to determine if both Parts B and C are to be completed. You should perform the experiment with a partner; as one student combines the test solutions, the other notes the time. The oxidation~reduction reaction that occurs between hydrochloric acid and sodium thiosulfate, Na35203, produces insoluble sulfur as a product. 2 HCl(aq) + NaISZO3(aq) —~> 3(5) + 503(g) + 2 NaCl(aq) + H200) (23.1) The time required for the cloudiness of sulfur to appear is a measure of the reac- tion rate. Measure each volume of reactant with separate graduated pipets. I. Prepare the Solutions. Pipet 2 mL of 0.1 M Nazsgo. into each of a set of three ISO‘mm, clean test tubes. Into a second set of three 150~mm test tubes pipet 2 mL of 0.1 M HCl. Label the test tubes. 2. Record the Time for Reaction at the “Lower" Temperature. Place a NaZSQOK—HCI pair of test tubes in a salt/ice water bath until thermal equilibrium is established (approximately 5 minutes). Pour the HCl solution into the l‘la38303 so- lution, START TIME, agitate the mixture for several seconds. and retum the reac- tion mixture to the salt/ice bath. STOP TIME when the cloudiness of the sulfur appears. Record the time lapse for the reaction and the temperature of the bath. "using all certain digits (from the labeled calibration marks on the thermometer) plus one uncertain digit (the last digit which is the best estimate between the cali~ 3 bration markst." B. Temperature of the Reaction: Hydrochloric Acid-Sodium Thiosulfate Reaction System Experiment 23 27 7 3. Record the Time for Reaction at the "Higher” Temperature. Place a second NaZSZOK—HCI pair of test tubes in a warm water (<60°C) bath until thermal equi- librium is established (approximately 5 minutes). Pour the HCl solution into the Nazszo. solution. START TIME. agitate the mixture for several seconds, and re- turn the reaction mixture to the warm water bath. STOP TIME when the cloudi- ness of the sulfur appears. Record the temperature of the bath. 4. Record the Time for Reaction at “Room” Temperature. Combine the remaining set of NaZSZO3—HCI test solutions at room temperature and proceed as in Parts 8.2 and 8.3. Record the appropriate data. Repeat any of the above reactions as necessary. 5. Plot the Data. Plot temperature (y axis) versus time (x axis) on one—half of a sheet of linear graph paper for the three data points. Have the instructor approve your graph. Further interpret your data as suggested on the Report Sheet. Disposal: Dispose of the reaction solutions in the "Waste Inorganic Test Solu— tions" container. C. Temperature of the The reaction rate for the oxidation~reduction reaction between oxalic acid, HzCzOQ, Reaction: Oxalic and potassium permanganate, KMnO4, is measured by recording the time elapsed for Acid-Potassium the (purple) color of the permanganate ion, MnO4", to disappear in the reaction: Speggrgana‘e React'o" 5 H2C204(aq) + 2 KMnOJaq) + 3 HZSO4(aq) —> y 10 COZ(g) + 2 MnSO4(aq) + KZSO4(aq) + 8 H200) (23.2) Measure the volume of each solution with separate, clean graduated pipets. As one student pours the test solutions together, the other notes the time. 1. Prepare the Solution. Into a ISO-mm test tube pipet 1 mL of 0.01 M KMnO4 (in 3 M H2804) and 4 mL of 3 M H3504. (Caution: KMnO.1 is a strong oxidant and causes brown skin stains; H3504 is a severe skin irritant and is corrosive. Do not allow either Chemical to make skin contact.) Pipet 5 mL of 0.33 M H2C304 into a second, clean ISO-mm test tube. 2. Record the Time for Reaction at “Room” Temperature. Pour the HZCZO4 solu- tion into the KMnO4 solution. START TIME. Agitate the mixture. Record the time for the purple color of the permanganate ion to disappear. Record “room” temperature “using all certain digits (from the labeled calibration marks on the thermometer) plus one uncertain digit (the last digit which is the best estimate be— tween the calibration marks)” Record the Time for Reaction at the “Higher” Temperature. Place a second KMnOrI-IZCZO4 pair of test tubes in a warm water (~40°C) bath until thermal equilibrium is established (approximately 5 minutes). Pour the H3C304 solution into the KMnO4 solution, START TIME, agitate the mixture for several seconds, and return the reaction mixture to the warm water bath. Record the time for the disappearance of the purple color. Record the temperature of the bath. 4. Record the Time for Reaction at the “Highest” Temperature. Repeat Parts Cl and C3, but increase the temperature of the bath to about 60°C. Record the appro- priate data. Repeat any of the preceding reactions as necessary. La.) 5. Plot the Data. Plot temperature (v axis) versus time (x axis) on one-half of a sheet of Appendix C linear graph paper for the three data points. Have the instructor approve your graph. Disposal: Dispose oi the reaction solutions in the ”Waste Inorganic Test Solu- tions“ container. 278 Factors Aliecting Reaction Rates Hydrogen peroxide is relatively stable. but it readily decomposes in the presence of a I). Presence of a catalyst. Catalyst I. Add a Catalyst. Place approximately 2 mL of a 3% H303 solution in a small test tube. Add 1 0r 2 crystals of MnOz to the solution and observe. Note its instability. Ask your instructor for advice in completing both Parts E and F. E. Concentration of Reactants: Magnesium-Hydrochloric Acid System 1. Prepare the Reactants. Into a set of four, clean. labeled test tubes, pipet 1 mL of 6 M HCl, 4 M HCI, 3 M HCl, and l M HCl, respectively (Figure 23.6).2 Determine the mass (:0001 g)—separately (for identitieation)——of four l-cm strips of pol- ished (with steel wool) magnesium. Calculate the number of moles of magnesium in each strip. 6MHC| 4MHCt 3MHC| 1MHCl Figure 23.6 Setup for the effect of acid concentration on reaction rate. k.) . Record the Time for Completion of the Reaction. Add the first magnesium strip to the 6 M HCl solution, START TIME, and record the time for all traces of the magnesium strip to disappear. Repeat the experiment with the remaining three magnesium strips and the 4 M HCL 3 M HC1, and l M HCl, solutions. . Plot the Data. Plot figs/g (y axis) versus time in seconds (x axis) for the four tests ’9.) on one—half of a sheet of linear graph paper. Have the instructor approve your graph. Appendix C Disposal: Dispose of the reaction solutions in the test tubes in the ”Waste In- organic Test Solutions” container. CLEANUP: Rinse the test tubes twice with tap water and twice with deionized water. Discard each rinse in the sink; flush the sink with water. W090 emim Factors Affecting Reaction Rates Date Lab Sec. Nome Desk No. I. For a chemical reaction, such as A —> products, the concentration of a chemical reactant generally decreases with time. a. Sketch a plot of this relationship. b. Explain why the concentration of a chemical reactant generally decreases with time. 2. Chlorofiuorocarbons photodissociate in the stratosphere to produce chlorine atoms, Cl'. which may be responsible for accelerating a decreasing concentration of ozone. The decomposition of ozone follows a reaction sequence of 03 + Cl- —) CIO- + 02 ClO-+ O—>Cl. + 02 Describe the role of chlorine atoms in the decomposition of ozone. 3. A 20~mg strip of magnesium metal reacts in 2.0 mL of 3.0 M HCI over a given time period. Evaluate the % ratio for the reaction. Ll. Identify the major factor affecting reaction rate that accounts for the following observations: 21. Tadpoles grow more rapidly near the cooling water discharge from a power plant. b. More and more plastics are being used in construction, e.g., for the exterior of automobiles, but not merely because of their density being less than that of metal alloys. 0. Rubber tires deteriorate more rapidly in smog—laden areas than in the countryside. d. Unbumed hydrocarbons in the exhaust gases following the combustion of gasoline in the cylinder of an automobile engine are ideally converted to CO2 and H30 before being emitted from the tailpipe. ..—,_ Experiment 23 Report Sheet Factors Affecting Reaction Rates Date Lab Sec. Name Desk No. A. Nature of the Reactants 1. List the acids in order of decreasing reaction rate with magnesium: . 2. List the metals in order of decreasing reaction rate with 6 M HCl: , 8. Temperature of the Reaction: Hydrochloric Acid—Sodium Thiosulfate Reaction System 1. Time for Sulfur to Appear Temperature of the Reaction seconds _____°C seconds °C seconds °C 2. Plot temperature ()2 axis) versus time (x axis) for the three trials. Instructor’s approval of graph: 3. From the plotted data, interpret the effect of temperature on reaction rate. 4. From your graph, estimate the temperature at which the appearance of sulfur should occur in 15 seconds. Assume no changes in concentration. C. Temperature of the Reaction: Oxalic Acid—Potassium Permanganate Reaction System 1. Time for Permanganate [on to Disappear Temperature of the Reaction seconds °C seconds °C seconds °C 2. Plot temperature (y axis) versus time (x axis) for the three trials. Instructor's approval of graph: 3. From your plotted data, interpret the effect of temperature on reaction rate. 4. From your graph. estimate the time for the disappearance of the purple permanganate ion at 65°C. Assume no changes in concentration. Experiment 23 283 D. Presence of 3 Catalyst 1. What effect does the MnO2 catalyst have on the rate of evolution of 0: gas? 2. Write a balanced equation for the decomposition of H303. E. Concentration of Reactants: Magnesium—Hydrochloric Acid System Concentration mol H CI of HCl mol HCl Mass of Mg mol Mg —————- Time (sec) ”WWW—“MM l M molHCl 1 mol Mg 0 1. Plot axis) versus time (x axis). Instructor’s approval of graph: 2. How does a change in the concentration of HCl affect the time required for a known mass of magnesium to react? 3. From your graph, predict the time, in seconds. for 2 mg of Mg to react in 1 mL of 2.0 M HCI. ...
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