Temperature, Concentration's Impact on Reaction Rates: Collision Theory & Catalysis, Lecture notes of Chemistry

Download Temperature, Concentration's Impact on Reaction Rates: Collision Theory & Catalysis and more Lecture notes Chemistry in PDF only on Docsity! Chem 112 Procedure 2 1 THE EFFECT OF TEMPERATURE AND CONCENTRATION ON REACTION RATE INTRODUCTION FACTORS INFLUENCING REACTION RATE: The study of chemical reactions is not complete without a consideration of the rates at which these reactions proceed. 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. The practical importance of these rate considerations is difficult to exaggerate. For example, a metal which is exposed to weather will undergo reactions with oxygen and water which result in corrosion. The speed with which a certain reaction will proceed will often determine the industrial usefulness of the material. 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. There are two main theories involved in explaining reaction rates. These are the Activated Complex (Transition State) Theory and the Collision Theory. However, in the lecture part of the course, you will also look at the Activated Complex (Transition State) Theory. 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 come together. Since molecules are in rapid and continual motion, molecules of A and Be 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 collision 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) Chem 112 Procedure 2 2 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 the following four items form the stockroom: 3 5 mL volumetric pipets 1 pipet helper 1 10 mL volumetric pipet 1 timer • During Summer and Fall semesters, students should keep all four items, storing them in their drawers, until they have finished the experiment. • During Spring semester, students must return all four items to the stockroom at the end of the lab period, unless advised otherwise. Before returning items to the stockroom, all glassware is to be rinsed out thoroughly, the final rinse with deionized water. A. THE EFFECT OF TEMPERATURE ON REACTION RATE Lab Notebook: Set up a table in your lab notebook for the data for this part of the experiment. TABLE A: Temperature vs. Rate Data (Make this graph at least ½ a page) Approximate Temperature Run # Measured Temperature (°C) Average Temperature (°C) Run Time (seconds) Average Run Time (seconds) ~50°C ~ RT* = ____ ~10°C *Room Temperature DO NOT RECORD YOUR DATA ON THIS PAGE. COPY TABLE INTO NOTEBOOK!