Lecture+m - Ch 14 – Kinetics Thermodynamics and...

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Unformatted text preview: Ch 14 – Kinetics Thermodynamics and equilibrium deal with state functions ( Δ H, Δ E, Δ S, Δ G, K eq ) which view the reactants and products as two disconnected states. Kinetics can help us see this mysterious connection between the two. E reaction progress (time) reactants products T H E R M O T H E R M O K I N E T I C S “reaction pathway” Ch 14 – Kinetics Rate of reaction The reaction rate measures how fast the reactant or product concentrations are changing in a specific amount of time. Consider the following general reaction: A → B A solution of A is prepared and allowed to degrade over the period of one minute. The concentration of each species is measured every ten seconds and plotted like so: [ ] time [A] [B] Ch 14 – Kinetics The reaction rate is ALWAYS listed as a postive quantity, and expresses the change in concentration of a species in a certain amount of time: rate = − Δ [A] Δ t rate = Δ [B] Δ t ( − sign is there to ensure the rate is positive) Since the stoichiometry is 1:1, these rates will be exactly the same and both will express the rate of the reaction. [ ] time The rate of the reaction is going to depend on what time interval we choose: “fast” “slow” Ch 14 – Kinetics To eliminate the confusion about rate, it becomes helpful to talk about the instantaneous rate , which is loosely defined like so: Δ [x] Δ t dx dt Those of you who are math-savvy will recognize this as the derivative, which also is the tangent line at a specific point on the concentration curve: time rate here, lower slope, slower rate rate here, higher slope, faster rate [ ] lim Δ t → Ch 14 – Kinetics Practice problem #39 Using the reaction and data below, determine the rate of reaction in terms of both NO 2 and O 2 over the first 50 seconds of the reaction. 2 NO 2 (g) → 2 NO(g) + O 2 (g) 0.0018 0.0035 0.0065 100 0.0011 0.0021 0.0079 50 0.01 [O 2 ] [NO] [NO 2 ] Time(s) For NO 2 : rate = − Δ [NO 2 ] Δ t = − (0.0079 − 0.01) 50 = 4.2 × 10 − 5 M s − 1 For O 2 : rate = Δ [O 2 ] Δ t = (0.0011 − 0) 50 = 2.2 × 10 − 5 M s − 1 1/2 of the NO 2-based rate!! Ch 14 – Kinetics The previous problem illustrates the relationship between stoichiometry and the reaction rate. For the general reaction aA + bB → cC + dD, we can show: rate = − Δ [A] Δ t 1 a Δ [B] Δ t 1...
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This note was uploaded on 11/04/2009 for the course CHEM Chem 1C taught by Professor Farmer during the Spring '09 term at UCL.

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Lecture+m - Ch 14 – Kinetics Thermodynamics and...

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