L1.4: Energy Diagrams

• Energy Diagrams

  A graph that shows how the potential energy of a system changes as the reaction progresses from reactants to products.
• Activation Energy (Ea)

  Definition

  ● It is the minimum amount of energy that the reactants must possess for a reaction to occur.

  ● Also called the energy barrier, and the height of this barrier determines the reaction rate.

  High Ea

  ● Few molecules have enough energy to cross the energy barrier.

  ● Resulting in a slow reaction rate.

  Low Ea

  ● Many molecules have enough energy to cross the energy barrier because that barrier is low.

  ● Resulting in a fast reaction rate.
• Change in Enthalpy (ΔH)

  Definition

  ● Tells you how much heat is absorbed or released during a chemical reaction.

  ● Change in heat content (enthalpy) of a system.

  -ΔH

  ● Bonds formed in the product are stronger than bonds broken in the reactant.

  ● Exothermic - gives off or releases heat.

  +ΔH

  ● Bonds broken in the reactants are stronger than bonds formed in the product.

  ● Endothermic - absorbs heat.
• Catalyst

  ● A substance that increases the reaction rate without itself being consumed in the reaction.

  ● Speeds up both forward and reverse reactions.

  ● Provides an alternative reaction pathway that has lower total activation energy than an uncatalyzed reaction.

  ● Does not affect either ΔH or the overall yield for a reaction.