Energy Changes In Solution

Question 1

Define lattice enthalpy

Answer 1

The enthalpy change when 1mol of a solid ionic compound is made from gaseous ions.
Eg eq: X(g) + Y (g) => XY(s)

Question 2

What are lattice enthalpys always? And why?

Answer 2

Lattice enthalpy is always negative/exothermic.
Because ionic bonds are being made (bond making is exothermic)

Question 3

What are the factors that affect lattice enthalpy?

Answer 3

1.) **charge on the ions (BIGGER) **, ions with a higher charge attract each other more strongly, so more energy is required to break the lattice
2.) Size of the ion, ions with a small ionic radius can get closer to the opposite ions, giving stronger attractions

Question 4

Define enthalpy of hydration

Answer 4

The enthalpy change when 1mol of gaseous ions are added to water to give 1mol of aqueous ions
Eg eq: X(g) + (aq) => X^+ (aq)

Question 5

What is enthalpy of hydration always? And why?

Answer 5

Enthalpy of hydration is always exothermic.
ion dipole bonds are being made between the polar water molecules and the fully charged ions. The ions are being hydrated.

Question 6

What factors affect enthalpy of hydration?

Answer 6

The enthalpy will be more exothermic when the ions have a higher charge and smaller ionic radius (same as lattice enthalpy)

Question 7

Define the enthalpy of solution

Answer 7

The enthalpy change when 1mol of an ionic solid dissolves in enough water to make an infinitely dilute solution.

Question 8

How do you draw an enthalpy level diagram?

Answer 8

• start near the bottom of the space and draw a line for the solid
• draw a line near the top for the SEPARATE gaseous ions
• join the two lines with a downward arrow, and label it lattice enthalpy
• draw a horizontal line (above for endo, below for exo) for the SEPARATE aqueous ions
• join the gaseous and aqueous lines with a downward arrow and label it enthalpy of hydration
• join the solid and aqueous lines with an upward arrow and label it enthalpy of solution

Question 9

How to calculate the enthalpy change of solution using an enthalpy level diagram?

Answer 9

To calculate the enthalpy of solution, start at the start of its arrow, go around the cycle to the end of its arrow. If you go against the arrow, then swap the sign.

Question 10

What are the three possible solutions for the enthalpy of solution?

Answer 10

• exothermic overall, these substances are almost always soluble (CaCO^3 is an exception)
• endothermic and large, these solids are insoluble (>14KJmol-1)
• endothermic but small, there solids are usually soluble, but they have to take in heat the dissolve, so the solution will feel cold (<14KJmol-1)