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

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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 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 Bond breaks Cl Collision Occurs Bonds begin To form H

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 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 and forming new bonds For example: H2 + Cl2  2HCL Bond is breaking H Cl Bonds begin To form The activated Complex for the Above reaction

8 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. Activation Energy Activated complex Products Reactants Time PE

9 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 DH For an exothermic rxn DH is Neg. Reactants Products The heat of reaction for An Exothermic reaction is Is designated … - DH H is known as the ENTHALPY of REACTION

10 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) DH For an endothermic rxn DH is positive Reactants Products

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 H (products) – H (Reactants) = -