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UA CHEM 152 - Average and Relative Rates
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Lecture 1 CHEM 152Outline of Current Lecture I. Chemical Kinetics II. Influencing FactorsIII. Reaction RatesIV. Reaction Rates and StoichiometryCurrent LectureI. Chemical Kinetics a. Different chemical reactions occur in different time scales. i. For example, an explosion is very quick, whereas rust forms over a longer period of time II. Influencing Factorsa. Temperature: i. Increase temperature, the speed of the particles increase, and therefore the reaction goes faster b. Concentrations of Reactants:i. More reactants means more collisionsii. Also, higher gas pressure means more collisions c. Physical State or Surface Area of a Reactant i. Powders react faster that solids because more area for reactants to be in contactii. Make an increased ability for the reactants to meetIII. Reaction Rates a. Reaction Rate = The change in concentration of a reactant or product with timei. Change in concentration of substance A: [ A ]t2−[A]t1=∆ [ A ]ii. Time Elapsed: t2−t1=∆ tb. Average Reaction Rates:Avg Rate=conc[final]−conc [initial]time(final)−time(initial) orRate=∆[ X ]∆ t=[X ]f−[ X ]itf−tic. What is rate?i. We can relate the reaction rate to the change in the concentration of a reactant or a product with time (M/s or mole/Ls).ii.A → B The rates of A and B together equal the average reaction rate 1. Reaction rate of B= ∆[B ]∆ t ∆[B]=¿ change in concentration B over time period ∆ t2. Reaction rate of A=−∆ [ A ]∆ t ∆[A]=¿change in concentration A overtime period ∆ ta. The rate of A is negative because as the reaction occurs, the concentration of the reactants decrease, and thereforemaking the rate negatived. Example: i. How do you express the “rate” of this reaction? N2O4→ 2 NO21. Reaction rate of N2O4 = - ∆[ N2O4]∆ t a. If 100 N2O4 disappears in 1 second, then the rate, R= -100 molecules/s2. Reaction rate of NO2= ∆[ NO2]∆ ta. If 100N2O4 disappears in 1 second, then 200 NO2 must appear in 1 second R= +200 molecules/sii. Can these rates be related? 1. In the reaction N2O4→ 2 NO2 the stoichiometric coefficients can be directly related to the reaction rates a. In N2O4 the rate is +100 molecules/s and in NO2 the rate is +200 molecules/s b. To find the overall reaction rate, we use the stoichiometric coefficients as fractions so:(1/1) ×100 molecules = (1/2) × 200 moleculesi. This gives us an overall rate of +100 molecules in the reaction because we start with 100 N2O4 and end with 200 NO2IV. Reaction Rates and Stoichiometry a. In general, for the reaction: aA+bB →cC +dDRate=−1a∆ [ A ]∆ t=−1b∆ [B]∆ t=+1c∆[C]∆ t=+1d∆ [D]∆ ti. Negative for reactants; positive for productsReactants are consumed; products formedb. Example:i. Consider the following reaction: N2O5(g)→2 NO2(g)+12O2(g) ii. What is the rate of the reaction if oxygen is formed at a rate of 0.0027 mol L−1S−1?¿11/2∆ [O2]∆ t=0.00271/2=0.0054molel ∙ siii. What is the rate of formation of NO2 if oxygen is formed at a rate of 0.0027 mol L−1S−1?rate of reaction=∆[O2]∆ t=12∆[ NO2]∆ tPlug∈0.0027 for∆[O2]∆ t since we already found it in part (ii) and solve for ∆[ NO2]∆ t 0.0054=12∆[ NO2]∆ t SO: ∆[ NO2]∆ t= 0.011molel∙


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UA CHEM 152 - Average and Relative Rates

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