22.2 - Enthalpy changes in solution

Question 1

What is the standard enthalpy change of solution?

Answer 1

The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions

Question 2

Give the equation of the enthalpy change of solution of NaCl (include charges and states)

Answer 2

Na⁺Cl⁻ (s) + aq -> Na⁺ (aq) + Cl⁻ (aq)
- Na⁺Cl⁻ = solid ionic lattice
- Na⁺ (aq) + Cl⁻ (aq) = aqueous ions

Question 3

Describe the attraction between Na⁺ and Cl⁻ in (s) and (aq)

Answer 3

Solid: Na⁺ and Cl⁻ ions are attracted together in a giant ionic lattice
Aqueous: Na⁺ and Cl⁻ ions are separate, but surrounded by water molecules
- The δ- oxygen atoms are attracted to the positive sodium ions
- The δ+ hydrogen atoms are attracted to the negative chloride ions

Question 4

Why can the enthalpy change of solution be either exothermic or endothermic?

Answer 4

It depends on the balance between the energy required to break the solute’s lattice (endothermic(+ΔH)) and the energy released when ions are hydrated (exothermic(-ΔH).)

If the energy required to break apart the lattice is greater than the energy released by hydration, enthalpy of solution is endothermic (+ΔH).

If the energy required to break apart the lattice is smaller than the energy released by hydration, enthalpy of solution is exothermic (-ΔH).

Question 5

What mass is used in enthalpy calculations?

Answer 5

The mass of of the solution that is changing temperature

Question 6

What are the two processes that take place when a solid ionic compound dissolves in water?

Answer 6

1. The ionic lattice breaks up
2. Water molecules are attracted to, and surround, the ions

Question 7

What are the two types of energy involved?

Answer 7

1. The ionic lattice is broken up forming separate gaseous ions
2. The separate gaseous ions interact with the polar water molecules to form hydrated aqueous ions ( enthalpy change of hydration)

Question 8

What is the standard enthalpy change of hydration?

Answer 8

The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions

Question 9

Give the two enthalpy change of hydration that take place in the dissolving of NaCl(s)

Answer 9

Na+ (g) + aq -> Na+(aq)

Cl-(g) + aq -> Cl-(aq)