Graphing ∆H Chapter 5 P171-184

Energy Diagrams Please write Compare the potential energy of the reactants & products as the rxn occurs. The reactants must have enough KE to overcome the repulsive forces between molecules. When they collide with enough KE the reactants form a clump called the activated complex. This is where the energy from the collision breaks the reactant bonds & forms the product bonds. Activation energy is the minimum energy needed for a rxn to occur.

Not all collisions result in a rxn! Please write Not all collisions result in a rxn! Activation Energy (Ea) = the minimum KE needed for the collisions to be effective Activation Energy (EA) ineffective collisions Number of Particles Not enough energy Enough Energy Kinetic Energy available during collisions  effective collisions

Please write Ea ∆H ∆H = products – reactants = positive value! Activated Complex ∆H = products – reactants = positive value! ENDOTHERMIC! Products Ea Activation energy ∆H Heat of Reaction Potential Energy Reactants Progress of the Reaction

Please write Ea ∆H ∆H = products – reactants = negative value! Activated Complex Ea ∆H = products – reactants = negative value! EXOTHERMIC! Reactants ∆H Heat of Reaction Potential Energy Products Progress of the Reaction

Slow, Faster & Spontaneous Reactions Please write Slow, Faster & Spontaneous Reactions Rxns with high Ea. It takes a lot of energy to get started, the rxn will begin slowly, or not at all. Rxns with medium Ea. Will begin more easily & proceed more quickly. Rxns with very low Ea. Spontaneous. Potential Energy Progress of the Reaction

Energy Diagram for a reversible reaction Activated Complex Ea(forward) Reactants Forward rxn Ea(reverse) –∆H(forward) +∆H(reverse) Potential Energy Products of the forward reaction Forward Reaction (Exothermic) Reverse Reaction (Endothermic) Progress of the Reaction

Conclusion from energy diagrams: Please write Conclusion from energy diagrams: The Ea for the forward rxn is smaller. ∴the forward rxn occurs more often than the reverse. ∴ high concentration of products The Ea for the forward & reverse rxns are similar. ∴both rxn occur with high frequency. ∴ concentration of products & reactants are similar.

Mechanism Energy Graphs In a multi-step mechanism, each step will have an activation energy! The activation energy of several small steps is usually lower than a single activation energy would be, therefore these rxns are more common.

Photosyntheis: An extreme example of reaction mechanisms Overall Reaction: 6CO2 + 6H2O  C6H12O6 + 6O2 Mechanism: