Topic 12: Entropy

Question 1

Explain the solubility trend in group 2 hydroxides (OH-):

Answer 1

• solubility increases down group
• enthalpy of solution becomes more negative
• entropy of surroundings becomes more positive
• entropy of hydrated ions increases (more +)

Question 2

Explain the solubility trend in group 2 sulfates (SO4 2-)

Answer 2

• solubility decreases down group
• enthalpy of solution becomes less negative down group
• entropy of surroundings becomes less positive down group
• entropy of hydrated ions increases (less +)
• however magnitude of entropy of surroundings > entropy of hydrated ions
• so entropy total is NEGATIVE

Question 3

What is lattice energy?

Answer 3

• one mole of an ionic compound is formed from its ions in the gaseous state
• under standard conditions

Question 4

What factors effect lattice energy magnitude?

Answer 4

• size of charge on ions (higher charge, higher LE)
• ionic radii (smaller radii, higher LE)
• no. of covalent interactions between ions (more interactions, higher LE)

Question 5

What is the enthalpy change of atomization?

Answer 5

• when one mole of gaseous atoms are formed from its element in the standard state
• under standard conditions

Question 6

What is electron affinity?

Answer 6

• when each atom in one mole of atoms in the gaseous state gains an electron to form a negative ion

Question 7

What is the first ionization energy?

Answer 7

• the energy required to remove one electron from an atom, in one mole of atoms in the gaseous state

Question 8

What are the assumptions of a perfect ionic model for theoretical lattice energy?

Answer 8

• ions are in contact with each other
• ions are perfectly spherical
• charge on each ion is perfectly distributed

Question 9

What are the reasons for variation in experimental and theoretical lattice energies?

Answer 9

• ionic lattice could have significant covalent character
• if exp. > theoretical
• more energy required to overcome covalent character

Question 10

What is the enthalpy change of solution?

Answer 10

• the enthalpy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution

Question 11

What is the enthalpy change of hydration?

Answer 11

• when one mole of an ion in its gaseous state is completely hydrated by water

Question 12

What are the factors affecting enthalpy change of hydration?

Answer 12

• charge of ion: greater electrostatic forces of attraction between water molecules if charge is high
• ionic radii: electrostatic forces between ions and water are stronger if ionic radii small (more energy released upon hydration)