Thermodynamics vs. kinetics in chemical reactions


Thermodynamics dictates equilibrium (which says whether this reaction will take place).

bulletSays nothing about what rate (how fast) the reaction will be.
bulletIn a spontaneous reaction, ΔG is negative.  When ΔG is negative, reaction is spontaneous.
bulletΔG (Gibbs free energy) follows this fomula : ΔG=ΔH - TΔS.  It is largely dependent on two concepts :
bulletEnthalpy : ΔH.  Negative values of this (i.e. exothermic) decrease ΔG, making the reaction more favorable. 
bulletEntropy : - TΔS.  The T represents temperature.  The S represents entropy, or a measurement of disorder.  This term in the equation is negative and temperature is always positive.  Therefore, increasing the change in entropy (positive ΔS) makes the reaction more favorable.
bulletThe endothermic or exothermic qualities of a reaction are indicated by ΔH (enthalpy).
bulletNegative enthalpy indicates exothermic reaction
bulletPositive enthalpy indicates endothermic reaction


bulletNote that because ΔG is dependent on both enthalpy and entropy, it is possible for endothermic reactions to proceed spontaneously.
bulletThermodynamics is independent of reaction mechanism (concerned with original and final states only).
bulletThermodynamics concerns the equilibrium constant K (upper-case).

Kinetics involves the rate of the reaction

bulletKinetics is affected by catalysts.  The rate of the reaction involves the activation energy.  Catalysts lower the activation energy, which causes the reaction to speed up.
bulletKinetics concerns the rate constant k (lower-case).


Examples of thermodynamics-kinetics feuds :

bulletCombustion (aka burning) of my physical chemistry book is thermodynamically favored.  Unfortunately, it is not favored kinetically because the activation energy for the combustion reaction is too high for the present conditions.  One could always invest a little energy in the form of a flame and start the reaction.  The energy produced from the combustion would supply the activation energy of the next reaction.
bulletThe conversion of diamond into graphite is thermodynamically favored (negative ΔG.)  The speed of this reaction is just too darn incredibly slow for a diamond to turn into graphite.



  Thermodynamics Kinetics
Determines Equilibrium (product vs. reactant) Speed of reaction
Constant K (uppercase) = equilibrium constant k (lowercase) = rate constant
Determined by ΔG (enthalpy and entropy) activation energy
Buzzwords Entropy, enthalpy, Gibbs free energy, spontaneous, endo- and exothermic, equilibrium Rate, speed, time, catalyst, enzyme, activation energy, reaction order, first-order, second-order, rate-limiting step, bottleneck