CHEM 1120 1st Edition Lecture 5 Outline of Last Lecture I Colligative Properties of Electrolyte Solutions A van t Hoff Factor II Colloids A Definitions and types B Hydrophillic vs Hydrophobic Colloids Outline of Current Lecture I Reaction Rates II Concentration and Rate III Average Rate vs Instantaneous Rate A Definitions Examples B Reaction Order and Units of Rate Constants Current Lecture I Some reactions like rust building up on an old car take a long time to occur and are very slow Some reactions like an explosion take a short time to occur and are very fast Other reactions like cooking eggs are in between fast and slow I know you are probably wondering why we care and the answer is because sometimes we want to manipulate the reactions speed and maybe make our cars rust way slower than normal or maybe we want to make cement solidify a lot faster than normal Or maybe you want to slow down cancer Ozone depletion Before we look at factors affecting the speed we must look at reaction rates mathematically These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Example Which expression correctly states the rate of the reaction N2 g 3H2 2NH3 g Solution rate NH3 2 T II The rate or speed of a reaction usually depends on the concentration In most cases for a generic reaction aA bB cC dD The rate law looks like Rate k A m B n k rate constant depends on the temperature m order of reaction with respect to A n order of reaction with respect to B m n order of overall reaction m and n are usually integers m and n do not need to be equal to a and b Four factors affect the speed of a reaction 1 Reactant physical state ex fine powder solid block 2 Reactant concentration ex dilute acid concentrated acid 3 Temperature ex boiling an egg 4 Catalysts automobile catalytic converter III When talking abut average rate vs instantaneous rate many tend to get confused because they do not understand the difference between the words average and instantaneous To illustrate these differences better an example is helpful When you drive to New York you can take the total number of miles and divide by the time it took you to get there for the average traveling speed You can also read the speedometer s number for your instantaneous speed at which you are traveling that moment The same deal can be applied to the speed of chemical reactions Average Rate the rate of reaction averaged over a period of time Exercise Compute the average rate of the previous reaction for the first 10 seconds of the reaction Instantaneous Rate the rate of reaction at a particular point in time Exercise Compute the instantaneous rate of the previous reaction 15 seconds into the reaction Here is an example of reaction order The reaction above is 1st order with respect to O3 1st order with respect to NO2 and 2nd order overall 1st 1st 2nd overall The reaction above is 2nd order with respect to NO2 and 2nd order overall The third and final reaction above is 1st order with respect to CH3 3CCl 0 order with respect to water and 1st order overall Units of rate units of rate constant units of concentration order Examples
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