Chapter 12 - 1 CHEMICAL KINETICS - Study which examines how...

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1 CHEMICAL KINETICS - Study which examines how fast chemical reactions occur and why? NaOH (aq) + HCl (aq) NaCl (aq) + H 2 O (l) - Neutralization happens very fast. 2 Fe (s) + 3 O 2 (g) 2 Fe 2 O 3 (s) - Rusting of iron happens very slow. Reaction rate – speed of a reaction – n (Greek nu) Consider a simple reaction A B ( 29 ( 29 ( 29 ( 29 ν = - - = moles of B final moles of B initial time final time initial molesof B t - higher reaction rate means - more moles of B are formed in a set time - a set number of moles of B forms quicker. As moles of B increases, moles of A decrease. = - ν molesofA t In a constant volume container, the rate can also be defined as the change in concentration of the product divided by the change in time. ν= [ ] B t FACTORS WHICH AFFECT REACTION RATE 1. Concentration of reactants - Reaction with higher amount of starting material will usually proceed faster. 2. Temperature of reaction - Higher temperature usually increases reaction rate. 3. Catalysis - Catalyst is a substance that speeds up a reaction, yet it remains unchanged. - Catalysts are very important in industry. - “Time is money.” - Enzymes are biochemical catalysts. 4. Surface Area - A fine powder reacts quicker than a big crystal.
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2 REACTION RATES Measuring reaction rate - measure concentration of reactant as it reacts to form product Example: C 4 H 9 Cl (aq) + H 2 O (l) C 4 H 9 OH (aq) + HCl (aq) Overhead: Table of [C 4 H 9 Cl] and time Overhead: Graph of [C 4 H 9 OH] vs. time Instantaneous rate: - slope of concentration versus time plot
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3 Reaction Rate and Stoichiometry Consider the reaction 2 Br (g) Br 2 (l) ν = [ ] Br t 2 - note that for every mole of Br 2 formed, 2 moles of Br disappears. [ ] [ ] Br Br = - 2 2 = = - ν [ ] [ ] Br t Br t 2 1 2 For the general reaction: a A + b B c C + d D ν = = = - = - 1 1 1 1 c C t d D t a A t b B t [ ] [ ] [ ] [ ] Example: For the reaction, 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g), a experiment finds that the N 2 O 5 is disappearing at a rate of 0.28 mol/L s. How fast is the NO 2 appearing? How fast is the O 2 appearing? From the given information ν= = - [ ] . N O t mol L s 2 5 028
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4 Reaction Rate Laws - Reaction rates are proportional to initial concentration of products. - Examine the effect of changing concentration on reaction rate to find proportionality. - As an example, examine NH 4 + (aq) + NO 2 - (aq) N 2 (g) + 2 H 2 O (l) Overhead: Initial concentrations and reaction rate - we wish to use this data to find proportionality ν = k [NH 4 + ] m [NO 2 - ] n k – rate constant m – order of reaction with respect to NH 4 + n – order of reaction with respect to NO 2 - To solve for the order of reaction with respect to NH 4 + , the exponent m, find two trials of experiments where the only the initial concentration of NH 4 + has changed. Substitute the data into the above equation and divide each equation into each other
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This note was uploaded on 09/27/2010 for the course CHEM 220 taught by Professor Bates during the Spring '10 term at Skyline College.

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Chapter 12 - 1 CHEMICAL KINETICS - Study which examines how...

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