Part 3: Determining the activation energy ( E a ) for the reaction (a) Temperature(T 2 ) of the mixture is ________________°C (b) Plot a graph of absorbance vs. [CV + ] (6 points) (c) Calculate the initial rate for the reaction at this temperature (3 points) (d) Calculate the rate constant (K 2 ) for the reaction at this temperature (2 points) (e) Calculate the activation energy (E a ) (3 points) 35
UNIVERSITY OF NEW HAVEN – GENERAL CHEMISTRY II LAB EXPERIMENT: DETERMINATION OF INTEGRATED RATE LAW USING RATE OF EVAPRATION Purpose: To experimentally determine the rate law and order of reaction for the evaporation of different alcohol-based hand sanitizers. Introduction: 36
UNIVERSITY OF NEW HAVEN – GENERAL CHEMISTRY II LAB Alcohol-based hand-rubs have become common sanitizer for health care workers before and after contact with patients because they are more effective than traditional soap and water as antibacterial agents. Alcohol-based hand-rub sanitizers typically consist of 60% to 75% ethanol, 20% to 35% water, and small quantities of isopropanol, fragrances, thickeners, and moisturizers. Though the quantity of thickener is a trade secret, some brands must have a small quantity of thickener because, even though they appear more viscous than water or ethanol, they are liquids. Other brands must have a slightly larger quantity of thickener because they are gels. As anyone who has ever used one of these products knows, upon contact with the skin, the sanitizer evaporates. The rate of a chemical reaction (or rate of evaporation, in this lab) may depend on the concentration of one or more reactants, or it may be independent of the concentration of a given reactant. Exactly how the rate depends on the reactant concentrations is expressed in an equation called the rate law. In previous labs you have determined the rate law by mathematically comparing the changes in reactant concentration(s) vs. time (or rate). Rate laws can also be determined by examining graphs of the experimental data. By using an electronic balance to monitor the mass of a thin film (on a glass or metal plate) of either a liquid or a gel alcohol-based hand sanitizer, the mass lost due to evaporation can be determined. The mass lost vs. time data will be plotted in multiple ways to determine the order of the “reaction” In order to effectively predict the order of a reaction from a graph, we have to change the ways we’ve been dealing with more “traditional” rate laws. Our discussion of rate laws thus far has dealt with derived rate laws (i.e. -d[A]/dt, or rate = k[A] x ). The derived rate law really doesn’t tell us how the overall rate of a reaction changes as the reaction proceeds; rather, it is used to describe conditions towards the beginning of an experiment or at any instantaneous point in time throughout the reaction. Using elementary Calculus we can transform the derived rate laws into what’s called the integrated rate laws. Without going into a discussion of what integration actually entails, the integrated rate laws are used to show how the overall rate of a reaction
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