Chapter 22 - Enthalpy and Entropy

Define lattice enthalpy

Enthalpy change when 1 mole of a substance is made from gaseous ions under standard conditions

Define standard enthalpy of formation

Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions

Define standard enthalpy change of atomisation

Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state under standard conditions.

Define first ionisation energy

Enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions

Define first electron affinity

Enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Define enthalpy change of solution

Enthalpy change that takes place when one mole of a solute dissolves in a solvent

Define enthalpy change of hydration

Enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions

What are the two steps happening in the dissolving process

• Ionic lattice breaks up

• Water molecules are attracted to and surround the ions

General properties of ionic compounds

• High melting and boiling points

• Soluble in polar solvents

• Conduct electricity when molten or in aqueous solution

Factors affecting lattice enthalpy

• Ionic size

• Ionic charge

Effect of ionic size on lattice enthalpy

As ionic radius increases:

• Attraction between ions decreases

• Lattice energy becomes less negative

• Melting points decrease

Effect of ionic charge on lattice enthalpy

As ionic charge increases:

• Attraction between ions increases

• Lattice energy becomes more negative

• Melting point increases

Effect of ionic size on hydration enthalpies

As ionic radius increases:

• Attraction between ion and water molecules decreases

• Hydration energy becomes less negative

Effect of ionic charge on hydration enthalpy

As ionic charge increases:

• Attraction with water molecules increases

• Hydration energy becomes more negative

Define entropy

The dispersal of energy within the chemicals making up the chemical system. Simply, a measure of disorder

2 things affecting entropy

• Physical state - solid < liquid < gas

• Number of molecules -more molecules, especially gaseous molecules, means more entropy

Define standard entropy

Entropy of one moles of a substance under standard conditions

Define feasibility

Describes whether the reaction is able to happen and is energetically feasible

How do you know if a reaction is feasible

When delta G is negative - Reaction is feasible

When delta G is positive - Reaction is not feasible

How can you tell if a reaction is feasible at all temperatures

Delta H will be negative for exo

Delta S will be positive

What is free energy change

The overall change in energy during a chemical reaction

It is made up of two types of energy:

• Enthalpy change

• Entropy change at temperature of reaction

What is Gibbs’ free energy equation

What are some limitations of the predictions for feasibility

They do not take into account:

• The rate of reaction

• The kinetics (change in states)

Equation linking lattice enthalpy, enthalpy of solution and enthalpy of hydration

Enthalpy of solution = Enthalpy of hydration + Lattice enthalpy