Chem%20140A%20Lecture%20Notes%20Winter%202011%20Ch%202

Chem%20140A%20Lecture%20Notes%20Winter%202011%20Ch%202 -...

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Chemistry 140A Winter 2011 (K. Albizati) 1 Chapter 2 Structure and Reactivity; Alkanes Chemical Thermodynamics – concerns the energy changes in chemical reactions; determines the extent to which a reaction will go to completion Chemical Kinetics – concerns the rate (speed) at which chemical reactions occur Thermodynamics - All chemical reactions are reversible - Reactants and products interconvert to varying degrees depending on their structure Consider a typical chemical reaction A + B C + D K eq = [C] [D] [A] [B] K eq is directly related to the Gibb’s Standard Free Energy change via Δ G o =- RTlnK eq generally expressed in kcal/mole If Keq = 0.01 Δ G o = +2.7 kcal/mole If Keq = 1 Δ G o = 0 If Keq = 10,000 Δ G o = - 5.5 So….the extent to which a reaction occurs is dependent on Δ G o Δ G o is directly related to the bond strengths involved in the chemical reaction as well as the energy dispersal of the system
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Chemistry 140A Winter 2011 (K. Albizati) 2 Put another way…… Δ G o = Δ H o - T Δ S o Δ H o is the enthalpy change or heat of reaction and is related mainly to changes in bond strengths in going from Reactants to products Δ S o is the entropy change or degree of disorder of a system; this is more difficult to quantify but in general if one reactant molecule gives rise to 2 product molecules the entropy change is positive leading to a negative contribution to Δ G o and therefore is thermodynamically favorable. The converse is true A + B C + D C A entropically unfavorable entropically favorable Kinetics Basically, how fast does a system reach chemical equilibrium? Dependent on the activation energy of a chemical reaction Consider Paper………….paper is unstable with respect to its combustion products CO 2 and water: PAPER + O 2 CO 2 H 2 O large negative Δ G o Yet it happens so slowly at room temperature so as to be unobservable on our time frame. This is because the activation energy is so high. This can be visualized with a potential energy diagram:
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Chemistry 140A Winter 2011 (K. Albizati) 3 Where does the energy come from to get over the barrier? From collisions between molecules. Collisions transfer energy between molecules. If heat is applied to molecules their kinetic energy increases (they move faster) and hence the collision speed and rate increases. This can provide the energy needed to climb the barrier. Arrhenius Equation Consider A + B C Reaction Rate = k [A] [B] in units of mol per liter per sec
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Chemistry 140A Winter 2011 (K. Albizati) 4 k = the rate constant and is k = Ae –Ea/RT So….if the temperature of a reaction is increased, the rate of the reaction increases. This makes sense because as you raise the temperature of molecules you raise their kinetic energy. A is the Arrhenius constant and each reaction has its own peculiar value of A. An additional factor can influence the rate of a chemical reaction --- the concentration of the reactants
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Chem%20140A%20Lecture%20Notes%20Winter%202011%20Ch%202 -...

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