CHM1311 - Lecture(13)

CHM1311 - Lecture(13) - Chemical Kinetics Petrucci, Chapter...

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1 Chemical Kinetics Petrucci, Chapter 15
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2 Practical Significance of kinetics?
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3 Chemical Kinetics Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed? C (diamond) C (graphite) G = - 3 kJ/mol Why don’t diamonds turn into graphite?! Kinetics!
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4 Synthesis of Ammonia An exothermic, spontaneous reaction: N 2 (g) + 3H 2 (g) 2NH 3 (g) G o rxn = -16 kJ H o rxn = -46 kJ Why bother with a Nobel prize?
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5 Reaction rate is the change in the concentration of a reactant or a product with time ( M /s). A B rate = - [A] t rate = [B] t [A] = change in concentration of A over time period t [B] = change in concentration of B over time period t Because [A] decreases with time, [A] is negative . Reaction Rate Reaction Rate However, rate must be a positive number. The rate at which A decreases is equal to the rate at which B increases:
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6 A B rate = - [A] t rate = [ B ] t time
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7 Br 2 ( aq ) + HCOOH ( aq ) 2Br - ( aq ) + 2H + ( aq ) + CO 2 ( g ) time 393 nm light Detector [Br 2 ] α Absorption 393 nm Br 2 ( aq ) Experimental Determination of Reaction Rate Experimental Determination of Reaction Rate
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8 Rate of an electrochemical reaction What do you get when you discharge a battery? What happens when you discharge at high current? What is current?
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9 Experimental Determination of Reaction Rate Experimental Determination of Reaction Rate Average Rate : Rate of disappearance of reactants over a given time interval. Instantaneous Rate : Rate of disappearance of reactants at a specific instant in time Initial Rate : An average rate taken over a very small interval of time at the very onset of the reaction
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10 Br 2 ( aq ) + HCOOH ( aq ) 2Br - ( aq ) + 2H + ( aq ) + CO 2 ( g ) average rate = - [Br 2 ] t = - [Br 2 ] final – [Br 2 ] initial t final - t initial Average Rate Average Rate Average Rate : Rate of disappearance of reactants over a given time interval.
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11 Br 2 ( aq ) + HCOOH ( aq ) 2Br - ( aq ) + 2H + ( aq ) + CO 2 ( g ) average rate = - [Br 2 ] t = - [Br 2 ] final – [Br 2 ] initial t final - t initial Calculate the average rate of reaction over the (a) First 50 seconds of the reaction; (b) First 100 seconds of the reaction.
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12 Br 2 ( aq ) + HCOOH ( aq ) 2Br - ( aq ) + 2H + ( aq ) + CO 2 ( g ) average rate = - [Br 2 ] t = - [Br 2 ] final – [Br 2 ] initial t final - t initial Calculate the average rate of reaction over the (a) First 50 seconds of the reaction; (b) First 100 seconds of the reaction.
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13 Instantaneous Rate : Rate of disappearance of reactants at a specific instant in time Instantaneous Rate Instantaneous Rate Plot the [Br 2 ] as a function of time Instantaneous rate at a specific time is the slope of the tangent to the curve at that time Br 2 ( aq ) + HCOOH ( aq ) 2Br - ( aq ) + 2H + ( aq ) + CO 2 ( g ) How can instantaneous rate be determined?
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14 Usually when we use the term “rate,” we mean “instantaneous rate.” [Br 2 ] (M) t (s) Rate at 100s: 2.96 x 10 -5 M/s Rate at 300s: 1.48 x 10 -5 M/s Rate at 200s: 2.09 x 10 -5 M/s slope of tangent slope of tangent slope of tangent
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15 Initial Rate Initial Rate Instantaneous rates can be difficult to measure, so we use initial rates as an approximation of instantaneous rates.
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CHM1311 - Lecture(13) - Chemical Kinetics Petrucci, Chapter...

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