Chemical Kinetics PE Curves

1. Chemical Kinetics PE Curves Mr. Shields Regents Chemistry U13 L06

2. Energy What are the 2 major forms of energy a molecule can possess? - POTENTIAL - KINETIC These forms of energy are processed by molecules Simultaneously and Since Energy can neither be created nor destroyed … As PE decreases KE increases and vice versa Or stated differently … The energy of a SYSTEM remains CONSTANT

3. Bond breaks Cl Cl Bonds begin To form H H Bond breaks Collision Energy According to Collision theory: - Bonds most be broken when 2 molecules come together so that new bonds can be formed - The energy needed to break bonds comes from the KE of the collision For this reaction to Occur some minimum KE Is req’d. But If the KE of the collision Is too low, bonds will not break Collision Occurs

4. Activation energy In 1887 Svante Arrehenius proposed the idea of a Minimum energy req’d for reaction to occur and called It the ACTIVATION ENERGY (EA). The Activation energy is equivalent to pushing a rock Up to the top of a hill. It takes energy to do that. EA Activation Energy So… Activation Energy is the difference in the energy of the reactants at the peak of the energy curve and their energy at the starting point

5. Activation energy If the Energy applied is not equal to the EA (the energy Necessary to get to the peak) then the stone returns back to where it started. If the Energy applied exceeds the EA then the stone Can easily reach the peak and then easily roll down the other side. E ≥ EA E ≠ EA Starting Energy Of Reactants Activation Energy

6. Activation energy & Catalysts Only a fraction of all collisions have energy sufficiently equal To or exceeding the EA. They return to the starting point. The reactants that do have enough energy to reach the Peak become what is called an “ACTIVATED COMPLEX” Starting Energy of reactants ACTIVATED COMPLEX Activation Energy w/o catalyst Activation Energy with catalyst A Catalyst works by LOWERING the Activation Energy so the Reaction Rate increases.

7. Bond is breaking Cl Cl Bonds begin To form H H Bond is breaking Activation energy • So What exactly is the Activated Complex: • It’s unstable and short-lived • It leads to Product • It’s something in between the process of breaking bonds • andforming new bonds • For example: H2 + Cl2 2HCL The activated Complex for the Above reaction

8. Activated complex PE Activation Energy Reactants Products Time Activation energy The kind of curve we’ve been looking at to describe Activation Energy is what’s called a POTENTIAL ENERGY CURVE or just simply an Energy Curve for Chemical reactions.

9. Reactants Products DH For an exothermic rxn DH is Neg. PE Curve Chemical reactions are either Exothermic or Endothermic. If a chemical reaction gives up heat it is exothermic. In an exothermic process the energy of the products is less Than the energy of the reactants The heat of reaction for An Exothermic reaction is Is designated … - DH H is known as the ENTHALPY of REACTION

10. Products Reactants DH For an endothermic rxn DH is positive PE Curve In an Endothermic reaction the energy of the products is Greater than the energy of the reactants. For endothermic reactions the Enthalpy of Reaction is Positive (+∆H)

11. Enthalpy Summary +DH Reaction is ENDOTHERMIC Products have more energy than Reactants H (products) – H (Reactants) = + -DH Reaction is EXOTHERMIC Products have less energy than Reactants H (products) – H (Reactants) = -