Topic 15 HL - Energy and Thermochemistry

Question 1

Born Haber Cycle

Answer 1

An application of Hess’s law which consists of a series of reactions that can be combined to determine the enthalpy of formation of an ionic compound.

Question 2

Lattice Enthalpy

Answer 2

Defined as the standard enthalpy change that occurs on the formation of 1 mol of gaseous ions from the solid lattice. The process is endothermic.

Question 3

Enthalpy of atomisation

Answer 3

The standard enthalpy change that occurs on the formation of 1 mol of separate gaseous atoms of an element in its standard state.

Question 4

Ionisation energy

Answer 4

The standard enthalpy change that occurs on the removal of 1 mol of electrons from 1 mol of atoms or positively charged ions in the gaseous phase.

Question 5

Electron affinity

Answer 5

The standard enthalpy change on the addition of 1 mol of electrons to 1 mol of atoms in the gaseous phase.

Question 6

Variations in lattice enthalpy

Answer 6

The magnitude of the lattice enthalpy for a compound is directly affected by both the charge on the ions and the ionic radii. An increase in the ionic charge will result in a greater attraction between oppositely charged ions, increasing the energy required to break apart the ionic lattice (lattice enthalpy).

Question 7

The standard enthalpy change of solution

Answer 7

The change in enthalpy when 1 mol of a substance is dissolved in a large excess of a pure solvent.

Question 8

The enthalpy change of hydration

Answer 8

For an ion is the enthalpy change when 1 mol of gaseous ion is added to water to form a dilute solution. The term solvation is is used in the place of hydration for solvents other than water.

Question 9

Solvation

Answer 9

Any stabilising interaction of a solute and the solvent or a similar interaction of solvent with groups of insoluble material. Such interactions generally involve electrostatic forces and van der Waals forces as well as chemically more specific effects such as hydrogen bond formation.

Question 10

Water molecules

Answer 10

Orient themselves so that their partial charges surround cations and anions, forming a solvation shell, also known as a hydration shell when the solvent is water.

Question 11

Spontaneous reaction

Answer 11

A reaction is said to be spontaneous when it moves towards either completion of equilibrium under a given set of conditions without external intervention. Reactions that do not take place under a given set of conditions are said to be non-spontaneous.

Question 12

Entropy (S)

Answer 12

A measure of the distribution of total available energy between the particles. The greater the shift from energy being localised to being widespread amongst the particles, the lower the chance of the particles returning to their original state and the higher the entropy of the system. Spontaneous reactions lead to an increase in the total entropy within the system and the surroundings.

Question 13

Second law of thermodynamics

Answer 13

Chemical reactions that result in an overall increase in the entropy of the universe are spontaneous.

Question 14

Total entropy

Answer 14

Entropy of system + entropy of surroundings

Question 15

Entropy change

Answer 15

Total entropy of products - total entropy of reactants

Question 16

Gibbs free energy

Answer 16

G = change in enthalpy (H) - T times change in entropy (S)

A reaction is spontaneous is Gibbs free energy value is negative.

Question 17

Gibbs free energy change of formation

Answer 17

Represents the free energy change when 1 mol of a compound is formed from its elements under standard conditions.