Effect of Temperature and Concentration on Reaction Rates, Lecture notes of Thermodynamics

Download Effect of Temperature and Concentration on Reaction Rates and more Lecture notes Thermodynamics in PDF only on Docsity! Chem 100 2/26/08 1 THE EFFECT OF TEMPERATURE AND CONCENTRATION ON REACTION RATE INTRODUCTION FACTORS INFLUENCING REACTION RATE: We know that some reactions such as those between ions in solution frequently proceed very rapidly, while others proceed so slowly that the rate is not even detectable. Among the most important factors influencing the rate of a reaction are: temperature, concentration and catalysis. In addition, for solids the condition of the surface is of great importance. COLLISION THEORY Consider the simple reaction: A + B Products A and B are atoms, ions, or molecules (we shall call them molecules in the discussion which follows) and may be in the gas phase or in solution. In order for A and B to react with each other, they must collide with each other. Since molecules are in rapid and continual motion, molecules of A and B will collide with one another at frequent intervals. However, not every collision of A and B will result in the formation of products. Before a reaction can occur, the reactants collide with a certain amount of energy termed the activation energy. This energy comes from the kinetic energy that A and B possess, so that only those collisions which occur with sufficient force will be effective in causing reaction. If the concentration of either A or B is doubled, the number of collisions between A and B per time is doubled. If the temperature is raised, the kinetic energies of both A and B are increased so that there are more collisions per second, and a greater fraction of these will lead to chemical reaction. The rate, therefore, generally increases with increasing temperature. CATALYSIS A catalyst can be thought of as an agent, which alters the speed of a chemical reaction. This results from a decrease in the amount of activation energy necessary for the reaction. When less activation energy is needed, a larger fraction of the collisions will possess the required energy, and the rate will increase. The manner in which the catalyst lowers the activation energy depends upon the type of catalyst. A catalyst which decreases the speed of a reaction is called an inhibitor. CLOCK REACTION In this experiment, the effect of temperature and concentration on the rate of a chemical reaction will be studied. The reaction chosen, frequently termed the “clock reaction”, is actually a series of consecutive reactions represented by the following equations: HIO3 + 3 Na2SO3 _ HI + 3 Na2SO4 (1) HIO3 + 5 HI _ 3 H2O + 3 l2 (2) I2 + Na2SO3 + H2O _ 2 HI + Na2SO4 (3) The iodine that is produced in reaction (2) is immediately used up in reaction (3), so that no appreciable concentration of iodine can build up until all of the Na2SO3 has been used up. When this occurs, the iodine concentration becomes great enough to change the color of a starch indicator to blue. The appearance of the blue color is thus an indication that all of the Na2SO3 has been used up. Experiment EQUIPMENT You will be working on this experiment in pairs. Each pair will fill out a slip (names of both students on slip) to check out 1 timer form the stockroom: Chem 100 2/26/08 2 A. THE EFFECT OF TEMPERATURE ON REACTION RATE TABLE A: Temperature vs. Rate Data Approximate Temperature Run # Measured Temperature (°C) Average Temperature (°C) Run Time (seconds) Average Run Time (seconds) 50°C Room Temp = ____ * 10°C *use this value in the table below Note: You to do two good runs at each temperature, ~10°C, Room temperature, and ~50°C. CAUTION IODIC ACID: CAUSES IRRITATION. HARMFUL IF SWALLOWED. SODIUM SULFITE: HARMFUL IF SWALLOWED. MAY CAUSE ALLERGIC REACTION AND BREATHING DIFFICULTIES. MAY CAUSE IRRITATION TO SKIN, EYES, AND RESPIRATORY TRACT. 1. Set up a water bath in your 250 mL beaker. Heat the water bath to approximately 50°C. To go quicker start with hot water from the tap. 2. In your 10 mL graduated cylinder, measure 10 ml of 0.016 M HIO3 solution into each of two 5” test tubes. Add 5 drops of fresh starch solution to each test tube. Label each test tube with “HIO3”. (You will need 6 total tubes so you may prepare 6 test tubes of HIO3 at the same time.) 3. In a clean 10 mL graduated cylinder measure 10 ml of 0.014 M Na2SO3 into each of two 5” test tubes. (You will need 6 total tubes so you may prepare 6 test tubes of Na2SO3 at the same time.) 4. Place 2 of each of the test tubes you prepared in the water bath that has been heated to the desired temperature. You could start with hot water from the tap and heat it to 50 degrees. Read and record the temperature of the water bath. Leave the samples in the water bath for at least 4 minutes. 5. Take one of the Na2SO3 samples. Pour it into a clean, dry evaporating dish. Immediately add, with stirring, the HIO3 solution. Begin timing when you add the HIO3 solution to the evaporation dish. Stop timing, when the blue color appears and record the time of the reaction. (The color change should be very abrupt. If the change does not occur all at once throughout the solution, you did not mix the two reagents well enough.) Record your observations: 6. Repeat Step 5 with another set of Na2SO3 and HIO3 samples, and time the reaction as before. If the time for this second run is significantly different than the time of the first run, repeat the procedure for a third run. Continue until two consistent values for time are obtained. 7. Now repeat the experiment with water that is room temperature Record the actual temperature = _____ and repeat steps 4 through 6 at that temperature. Chem 100 2/26/08 5 Name:_____________________________ Report Sheet Experiment You must complete the two graphs before you get your report sheet initialed. You must turn in the graphs as part of the report sheet. Graph 1 How the change in temperature affects the time of the reaction 50 40 T e m 30 p e r a t 20 u r e oC 10 0 Time (Sec) C. THE EFFECT OF TEMPERATURE ON REACTION RATE TABLE A: Temperature vs. Rate Data Approximate Temperature Run # Measured Temperature (°C) Average Temperature (°C) Run Time (seconds) Average Run Time (seconds) 50°C Room Temp = ____ * 10°C Chem 100 2/26/08 6 Graph 2 How the change in concentration affects the time of the reaction .0016 .0015 .0014 .0013 .0012 C o .0011 n c .0010 e n .0009 t r .0008 a t .0007 I o .0006 n .0005 M .0004 .0003 .0002 .0001 0 Time (Sec) D. The Effect of Concentration on Reaction Rate Table B: Concentration vs. Rate Data Dilution Run # Volume (mL) 0.016 M HIO3 Volume (mL) water Total Volume (mL) HIO3 solution Molar Concentration HIO3 Solution (after dilution) Time (seconds) Average Time (seconds) none 1 10.00 0.00 10.00 0.016 - * First 10.00 5.00 Second 10.00 10.00 Third 10.00 15.00 Chem 100 2/26/08 7 Answer the following question: 1. Does the reaction rate increase or decrease with decrease in temperature? Explain, giving your reason in terms of Collision Theory 2. Look at graph 1. What is the relationship between the temperature of the reaction and how fast the reaction occurred? 3. Does the reaction rate increase or decrease with dilution of one of the reactants? Give an explanation in terms of collision theory. 4. Look at graph 2. What is the relationship between the concentration of the reaction and how fast the reaction occurred? 5. What did you learn from this experiment?