6.1 Enthalpy

How do electrons achieve a lower energy state?

• Electrons achieve a lower energy state when they occupy a bonding molecular orbital rather than when they are isolated or in nonbonding/anti-bonding orbitals

• Bond formation is favorable in lower energy states

What happens to energy during bond formation?

• energy is released

What happens to energy when bonds are broken?

• energy is required

How is a bond broken?

• To break a bond, electrons in the bonding molecular orbital must absorb energy from nearby molecules. These surrounding molecules transfer kinetic energy to the bond, giving the electrons enough energy to separate the atoms.

What is enthalpy?

• A measure of the exchange of energy between the system and the surrounding

  • Δ𝐻=𝑞 (at constant pressure)

• Δ𝐻= the change in enthalpy (the exchange of kinetic energy)

• q=heat

What determines the ΔH of a bond-breaking reaction?

• It is primarily determined by the amount of energy required to break the bond homolytically (each atom takes one electron).

• More energy needed = larger ΔH (endothermic)

The ΔH of a bond-breaking reaction depends on the sum of all bonds broken and formed.

What is homolytic bond cleavage?

• breaks a bond evenly, with each atom receiving a single electron and generates radicals

What are radicals?

• A chemical entity with an unpaired electron

• Ex:(hydroxyl radical OH and methyl radical CH3)

What is a single-headed arrow (fish hook arrow) used for?

• A single headed curved arrow used in reaction mechanisms to show the movement of one electron

• Used in radical reactions

• Differs from regular (double-headed) arrows which show two electrons mpoving

• Indicates single electron transfer or bond breaking\forming in radical mechanisms

What is a reaction mechanism?

A detailed, step-by-step explanation of how a chemical reaction occurs.

What is a heterolytic bond cleavage

• breaks a bond unevenly, with both electrons going to one of the atoms

  • it is illustrated with a traditional, two headed curved arrow, generating ions

What are ions?

charged species

What is bond dissociation energy?

• The energy required to break a covalent bond via homolytic bond cleavage

• Δ𝐻° = refers to the bond dissociation energy when measured under standard conditions

Homolytically” just means each atom takes one electron when the bond breaks —
and bond dissociation energy measures the energy needed for that specific type of break.

Δ𝐻°: Table values

What is Primary (1°), Secondary (2°), and Tertiary (3°):

Refer to the number of alkyl groups attached directly to a given carbon atom

What is the heat of reaction

• The heat given off during a reaction

• Most reactions involve breaking and forming several bonds

• The sign of ΔH° indicates whether energy is absorbed (+) or released (–) by the system.

What does it mean when something is positive H°

• indicates that the system increased in energy

• it received energy from the surroundings

What does it mean when something is negative H°

• indicates the system decreased in energy

• it gave energy to the surroundings

What does it mean when something is exothermic?

• system gives energy to the surroundings'

• ΔH° is negative

• forming bonds

What does it mean when something is endothermic?

• the system recieves energy from the surroundings

• ΔH° is positive

• breaking bonds

Two graphical representations plot enthalpy (H) on the vertical axis against reaction coordinate on the horizontal axis. In the first graph, a curve begins near three-fourth height of the vertical axis and before one-fourth of the length of the horizontal axis. This region is labeled, starting material. The curve increases in a concave upward manner, and reaches a peak at the top of the vertical axis and toward half the length of the horizontal axis. Thereafter, it gradually drops down and ends slightly above the one-fourth the height of the vertical axis and three-fourth the length of the horizontal axis, and this region is labeled, products. The enthalpy change, delta H superscript 0, equals negative exothermic. In the second graph, a curve begins near one-fourth height of the vertical axis and slightly away from the origin of the horizontal axis. This region is labeled, starting materials. The curve increases in a concave upward manner, and reaches a peak at the top of the vertical axis and half the length of the horizontal axis. Thereafter, it gradually drops down and ends at three-fourth the height of the vertical axis and three-fourth the length of the horizontal axis, and this region is labeled, products. The enthalpy change, delta H superscript 0, equals positive endothermic.