Chem exam 3 study guide

Lewis Base

A substance that can donate a pair of electrons. Many species that are Brønsted-Lowry bases can also serve as Lewis bases.

Anions and neutral molecules with lone pairs (amines, water, etc.) can serve as Lewis bases.

Lewis Acid

A substance that can accept a pair of electrons.

A proton (H+) can serve as a Lewis acid, as well as a Brønsted-Lowry acid, but most Lewis acids are not Brønsted-Lowry acids.

Coupled Equilibria

Two or more separate chemical reactions that share one or more reactants or products.

Thermodynamics

Examines the relationship between heat and work.

Spontaneous Process

One that occurs naturally under certain conditions. A process that is spontaneous in one direction is nonspontaneous in the opposite direction. The spontaneity of a process is not correlated to speed, but it can depend on temperature and pressure. Results in a more uniform dispersal of matter and/or energy.

When ∆H and ∆S are both > 0, then the process is spontaneous at high temperatures. When ∆H and ∆S are both < 0, the process is spontaneous at low temperatures. When ∆H is > 0 and ∆S < 0, the process is always nonspontaneous. When ∆H is < 0 and ∆S > 0, the process is always spontaneous.

Nonspontaneous Process

Only takes place with the continuous input of energy from an external source.

Entropy

The dispersal of matter and energy. Energy flow towards dispersal. Related to the number of possible arrangements, or microstates of the system.

An increase in entropy involves energy dispersal amongst more microstates in the final state than initial state. A state function, represented in J/K.

Exothermic (heat as a product) reactions increase the entropy of their surroundings

Endothermic (heat as a reactant) reactions decrease the entropy of their surroundings.

Free Energy (∆G)

A thermodynamic function that relates enthalpy and entropy to spontaneity, and can also be related to equilibrium constants.

If ∆G < 0, the reaction is spontaneous in the forward direction, if ∆G > 0, the reaction is nonspontaneous in the forward direction. If ∆G = 0, then the system is at equilibrium.

At standard conditions, ∆G˚ < 0; K is large, favors products. ∆G˚ slightly negative, K > 1, slightly favors products. ∆G˚ > 0, K is small, favors reactants. ∆G˚ slightly positive, K > 1, slightly favors reactants

Second Law of Thermodynamics

The total entropy of the universe increases in any spontaneous process. For a spontaneous process, ∆Suniv > 0, or ∆Ssystem + ∆Ssurroundings > 0

Important Entropy Trends

Reminder that S˚ is standard entropy at 1 atm.

standard entropy of a liquid is greater than the standard entropy of a solid

standard entropy of a gas is greater than the standard entropy of a liquid

Standard entropy increases with molar mass - C vs O

Standard entropy increases with molecular complexity - Na vs NaCl

Standard entropy increases with the mobility of a phase (for an element with two or more allotropes)

Standard entropy increases with temperature (at absolute zero, entropy is zero)

Standard entropy increases with volume

Standard entropy increases as the number of particles increases. 1 mol vs 5 mol of NaCl