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.

Please write Ea ∆H ∆H = products – reactants Activated Complex ∆H = products – reactants Will give a positive value! More energy went in than out! ENDOTHERMIC! Products Ea Activation energy ∆H Heat of Reaction Potential Energy Reactants Progress of the Reaction

Please write Ea ∆H ∆H = products – reactants Activated Complex Ea ∆H = products – reactants Will give a negative value! More energy went out than in! 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 activation energy. It takes a lot of energy to get started, so the rxn will begin slowly, or not at all. Rxns with medium activation energy will begin more easily and proceed more quickly. Rxns with very low activation energy will begin spontaneously. 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

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

Factors affecting rxn rate: Please write Temperature: ↑ T = more molecules that are moving faster = more likely that the collisions reach Ea = higher rate of rxn (rxn occurs more often) Concentration: [↑] = collisions happen more often. = more likely to have a successful collision. = higher rate Bond strength: stronger bonds = higher Ea = less likely to have a successful collision. = slower rate More factors next time …

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 and 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:

Practice reading graphs 2, 3 (direct rxn means forward rxn),