1.4: Potential Energy diagrams

What is enthalpy?

Potential energy that may be given off or absorbed as heat.

What causes the enthalpy stored in the reactants to differ from that in the products of a physical or chemical change?

chemical bonds

What is bond energy?

The chemical potential energy derived from the position of negative electrons relative to the positive nuclei and nearby nuclei in a molecule

What does bond breaking require?

Energy input

What does bond forming require?

Energy release

What symbol denotes the change in energy?

ΔH, delta H

Endothermic reaction

The energy absorbed to break bonds is greater than the energy released during bond formation in an endothermic reactiion — POSITIVE H VALUES

Exothermic reaction

More energy is released during bond formation than is absorbed during bond breaking in an exothermic reaction. Therefore, they have negative H values.

What are the two ways to represent the ΔH associated with a chemical reaction?

• Thermochemical equation —- includes energy as an integral part of the equation

• ΔH notation require the energy change to be written after the equation following a ΔH symbol

What is the activated complex?

An intermediate state which is formed during the conversion of reactants into products.

When does the activated complex form?

When potential energy is at a maximum and kinetic energy is minimized during a reaction.

How long does the activated complex exist for?

less than a nanosecond

What does the horizontal (x) axis of a potential energy diagram represent?

Reaction coordinte/path that the reaction follows

What is the activation energy?

The difference between the potential energy of the activated complex and the total potential energy potential energy of the seperated reactant molecules.

What does activation energy represent?

The amount of energy reactant molecules must gain to form an activated complex: Ea = PE activated complex - PE reactants

What is enthalpy change the difference between?

The difference between the total potential energy of the products and the total potential energy of the reactants.

ΔH : PE products - PE reactants

Why wont altering factors which speed up the reaction not affect the appearance of a potential energy diagram?

Because a potential energy diagram only shows the PE of states, products, and reactants, and not the speed of the reaction itself: remember, the x axis is NOT a time axis, rather a pathway the reaction follows

What does the potential energy diagram of reversible reactions looks like?

There are two graphs, one for the forward reaction and one for the reverse reaction. They are mirror images of each other.

Why is Ea always smaller for the exothermic reaction?

Because in an exothermic reaction, reactants will have more energy than the products —- in an endothermic reaction the reactants will have less energy than the products. As such, there is a smaller difference between the reactants of a exothermic reaction and the activated complex.

What do potential energy diagrams for catalyzed reactions usually look like?

The form more than one activation complex, and therefore one more “peak” in the diagram

How do you calculate the activation energy for a multiple peak diagram?

Its the difference between the highest energy activated complex and the reactants