Lattice Enthalpy

Question 1

Define enthalpy change of formation

Answer 1

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 2

define lattice enthalpy of formation

Answer 2

The enthalpy change when 1 mole of a solid compound is formed from its gaseous ions under standard conditions.

Question 3

Define enthalpy change of 1st ionisation

Answer 3

The enthalpy change when 1 mole of gaseous 1+ ions are made form 1mole of gaseous atoms.

Question 4

How is lattice enthalpy a measure of ionic bond strength?

Answer 4

Ionic compounds can form giant ionic lattices where positive and negative ions are held together by electrostatic attractions
When gaseous ions combine to make a solid lattice, energy is given out, this is called lattice enthalpy

The standard lattice enthalpy is a measure of ionic bond strength. These active sites more negative the lattice enthalpy, the stronger the bonding.

Question 5

Define enthalpy change of solution

Answer 5

The enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution.

For a substance to dissolve
1) the substance bonds must break (endothermic)
2) new bonds formed between solvent and substance (exothermic)

Question 6

Define enthalpy change of hydration

Answer 6

The enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions.

Question 7

Describe how enthalpy change of solution can be calculated

Answer 7

1) Break the solid lattice structure into its gaseous ions first. (Lattice dissociation enthalpy)
2) dissolve the gaseous ions in water (enthalpy of hydration)

Like Hess’ law

Question 8

What are the 2 things which affects the enthalpy of hydration? Explain them

Answer 8

1) charge
Ions with a higher charge attract eater molecules more strongly as the electrostatic attraction is stronger. This means more energy is released when he bonds are made giving them a more exothermic enthalpy of hydration.
THE LARGER THE CHARGE THE GREATER. THE ENTHALPY OF HYDRATION

2) Size
Smaller ions have a higher charge density than larger ions. They attract weather molecules more strongly hence there is more exothermic enthalpy of hydration.
THE SMALLER THE ION THE GREATER THE ENTHALPY OF HYDRATION.

E.g. Mg2+ ion is smaller and more charge than a Na+ ion so has a bigger enthalpy of hydration.