A STUDY OF REACTION RATES

Transcription

1 CH095 A Study of Reaction Rates Page 1 A STUDY OF REACTION RATES Chemical reaction rates are affected by changes in concentration of the reactants and temperature. How concentration and temperature affect chemical reaction rates are questions to be investigated in this experiment. In Part I, you will vary the concentration of a reacting species. In Part II, you will vary the temperature. In order to study the effect of one variable, you will keep other variables constant. The reaction you will study involves three different solutions as described below: Solution A: A mol/l solution of potassium iodate, KIO 3 (aq) Solution B: Contains some starch and the other reacting species HSO 3 - (aq) Solution C: A mol/l solution of potassium iodate, KlO 3 (aq) We are primarily interested in the reaction between iodate ions, IO 3 - (aq), and hydrogen sulfite ions, HSO 3 - (aq), as represented in equation (1). IO 3 - (aq) + 3 HSO 3 - (aq) --> I - (aq) + 3 SO 4 2- (aq) + 3 H + (aq) (slow) (1) In order to study the rate of a reaction, we need to be able to determine how fast one of the products is formed or how fast one of the reactants is consumed. In this reaction we can determine how long it takes to form a certain amount of I 2 (aq). In order to do this, we make use of the very fast reaction: 5 I - (aq) + 6 H + (aq) + IO 3 - (aq) --> 3 I 2 (s) + 3 H 2 O(l) (fast) (2) You will notice that in reaction (2) iodine is formed. The iodine produced in reaction (2) remains as I 2 (aq), and as such reacts with starch (present in Solution B) to form a blue-coloured substance. Only a very small amount of iodine is required to form the blue-coloured substance with starch. In this first set of experiments, the iodate ion concentration will be varied for five mixtures. By noting the time required for the blue colour to appear, we can determine the effect of iodate concentration on reaction rate.

2 CH095 A Study of Reaction Rates Page 2 PROCEDURE Part I: The Effect of Concentration There are five different combinations of Solutions A and B to be investigated in this part of the experiment. They are listed in Table 1. You should investigate the reaction using Mixtures 1 thru 5. The solutions for this part of the experiment should all be at room temperature. a. Obtain 70 ml of solution A and 140 ml of Solution B in clean dry beakers. b. Use clean graduated cylinders marked A and B. Carefully measure the volumes of the two solutions for each mixture as indicated in Table 1. c. Pour ml of Solution B into a large test tube. Pour solution A into B quickly and start timing. Mix the solutions by inverting the test tube three times. Be sure to put your thumb on the end of the test tube. Watch for the first sign of blue to appear and record the time from the point of mixing to the appearance of the blue colour. d. Use all other mixtures outlined in Table 1. e. Repeat the above mixtures and record the time for the reaction to the nearest second. f. Average the time of reaction for each mixture rounding the average to the nearest second. TABLE 1 MIXTURE SOLUTION A, (ml) SOLUTION B, (ml) ml water ml water ml water ml water

3 CH095 A Study of Reaction Rates Page 3 Part II: The Effect of Temperature There are several different temperatures to be investigated in this part. All the mixtures use the same amount of each solution. You will be assigned one or more temperatures. a. Obtain 30 ml of solution C in a clean dry beaker. b. Place ml of solution C and ml of solution B into separate, labelled test tubes. Warm or cool the solutions to the desired temperature range by placing the test tubes into a water bath. Use ice to cool the water bath below room temperature. Place a thermometer in the tube containing Solution B. Assume Solution C is the same temperature as B. c. After each desired temperature range has been reached, pour Solution C into Solution B and note the time; mix thoroughly. Return the test tube containing the mixture to the water bath. Record the time and temperature when the first sign of blue colour appears. Data for other temperatures can be obtained by using class data. Temperatures should be stated to the nearest degree and times to the nearest second.

4 CH095 A Study of Reaction Rates Page 4 PROCESSING DATA Part I. The Effect of Concentration Mixture Volume of Solution A mmol of IO 3 - (aq) First Trial Second Trial Average Prepare a data table like the one shown above. 2. Make a graph by plotting mmol of iodate ion (Solution A) vs. elapsed time for Mixtures 1-5. Plot elapsed time for reactions along the vertical axis.

5 CH095 A Study of Reaction Rates Page 5 Part II. The Effect of Temperature 1. Prepare a data table of temperature and elapsed time like the one below. You must use class data. Temperature ( 0 C) Time (s) Temperature ( 0 C) Time (s) Make a graph plotting temperature of the mixture vs. elapsed time for all Mixtures. Plot time along the vertical axis.