27 March 2017
L26: Thermodynamics vs. Kinetics
I. Intro to Kinetics
A. Kinetics
1. Beginning Example
0 0
a. H 2(g) Β½ O 2(g) H O2βπΊ(l)βπΊπ(π» π ) π β227 (π)πππ
b. C (s, diamond) (s, graphitπ)β2.9 ππ½/πππ
β’ Diamonds are not really forever, will thermodynamically favorably turn into
graphite
β’ However, thermodynamics tells us how things should go, but nothing about
rates or spontaneity
2. Kinetics
a. The study of reaction rates and mechanisms of reactions
3. Quantifying a rate of reaction
a. For a reaction: aA + bB ο cC + dD (lower case = stoichiometric coefficients)
b. π
ππ‘π = β 1 π π΄, where d[A] is the change in concentration of reaction species
π ππ‘
β’ Negative because we are using up a, so π π΄]< 0, so the negative in front
ππ‘
makes the rate positive
β’ Can be put in terms of B, C, or D
π πππππ’ππ‘]
β’ Note: if in terms of a product, > 0, so no negative
ππ‘
4. Units of Rate of Reaction
a. Rate is always in concentration per time
b. Ex: molL s -1 -1
th
B. Rate Laws (in terms of basics and 0 order)
1. Differential Rate Law
a. Rate β π΄ [ ] [ π΅ ππ΅
β’ Rate is proportional to the concentration of reactants
β’ nAand n Bre the orders of reaction with respect to the reactants, aka the
partial orders
b. In most cases thereβs no relation between the reaction order and stoichiometric
coefficient (different concepts)
c. In special cases, for elementary reactions, the stoichiometric coefficients equal the
orders of reaction
β’ However, if the stoichiometric coefficients equal the orders of reaction, it
doesnβt necessarily mean that it is an elementary reaction
d. The proportionality constant (k) is the rate constant
β’ Rate = π π΄ ] [ π΅ ππ΅
β’ βkβ tells us that only a fraction of collisions lead to reaction
e. Factors that contribute to reaction: orientation of reactants, energy requirement for
reaction, temperature, mechanism (how reaction goes)
f. The units of βkβ depends on the orders of reaction with respect to reactants
β’ Overall order = β πππ(πππππ‘πππ‘π )
2. 0 Order Differential Rate Law
a. If β π

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