23. Chemical Energetics

Question 1

Define the term enthalpy change of atomization

Answer 1

The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions

Question 2

Define the term lattice energy

Answer 2

The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions

Question 3

Define the term first electron affinity

Answer 3

The first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of gaseous ions each with a single negative charge under standard conditions

Question 4

Define the terms enthalpy change of solution

Answer 4

The enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a very dilute solution

Question 5

Define the term enthalpy change of hydration

Answer 5

The enthalpy change when one mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution

Question 6

What are the factors that affect electron affinity?

Answer 6

Nuclear charge - the greater the nuclear charge the stronger the attractive forces between an incoming electron and the nucleus
Distance - the greater the distance between the nucleus and the outermost shell/orbital where the electron is added the weaker the force of attraction
Shielding - the greater the number of shells, the greater the shielding effect and the weaker the force of attraction

Question 7

What two factors affect lattice energy?

Answer 7

The ionic radius and ionic charge

Question 8

Explain how ionic radius affects the lattice energy

Answer 8

Lattice energy becomes less exothermic (becomes less negative) as the ionic radius of the ions increases. This is because the charge on the ion is more spread out over the ion when the ions are larger. So the electrostatic forces of attraction between the oppositely charged ions increases the lattice are weaker

Question 9

Explain how ionic charge affects lattice energy

Answer 9

It gets more exothermic (more negative) as the ionic charge of the ions increases. The greater the ionic charge the higher the charge density. This results in stronger electrostatic attraction between the oppositely charged ions in the lattice.

Question 10

Explain the effect of ionic charge on the enthalpy of hydration

Answer 10

Ions with large ionic charges have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution.
Therefore more energy is released when they become hydrated and enthalpy change of hydration becomes more exothermic

Question 11

Explain the effect of ionic radius on the enthalpy change of hydration

Answer 11

Smaller ions have a greater charge density resulting in stronger ion-dipole attraction between the water molecules and the ions in the solution
Therefore more energy is released when they become hydrated and enthalpy change of hydration becomes more exothermic