O - Enthalpies and dissolving

Question 1

What is lattice enthalpy a measure of?

Answer 1

A measure of ionic bond strength.

Question 2

What can ionic compounds form?

Answer 2

Regular structures called giant ionic lattices.

Question 3

What are the positive and negative ions held together by in giant ionic lattices?

Answer 3

Electrostatic attractions.

Question 4

What happens when gaseous ions combine to make a solid lattice?

Answer 4

Energy is given out - this is called the lattice enthalpy.

Question 5

What is the standard lattice enthalpy?

Answer 5

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

Question 6

What are standard conditions (lattice enthalpy)?

Answer 6

298K (25^oC) and 100kPa.

Question 7

Is lattice enthalpy always positive or negative?

Answer 7

Negative.

Question 8

Why is lattice enthalpy always negative?

Answer 8

Because bonds are broken and energy is released.

Question 9

The more negative the lattice enthalpy,…

Answer 9

…the stronger the bonding.

Question 10

What is the value for lattice enthalpy like if the bonds are stronger?

Answer 10

More negative.

Question 11

What does dissolving involve?

Answer 11

Enthalpy changes.

Question 12

What happens when a solid ionic lattice dissolves in water?

Answer 12

1) Bonds between the ions break - this is endothermic and the enthalpy change is the opposite of the lattice enthalpy.
2) Bonds between the ions and the water are made (hydration) - this is the enthalpy change of hydration and it is always exothermic.

Question 13

Is the enthalpy change of hydration always endothermic or exothermic?

Answer 13

Exothermic.

Question 14

What is the enthalpy change of hydration of ions?

Answer 14

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

Question 15

What is the enthalpy change of solution?

Answer 15

The overall effect on the enthalpy when something dissolves. It’s the net effect of the lattice enthalpy and the enthalpy change of hydration.

The enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.

Question 16

What is the enthalpy change of solution the net effect of?

Answer 16

The lattice enthalpy and the enthalpy change of hydration.

Question 17

What is the net effect of the lattice enthalpy and the enthalpy change of hydration called?

Answer 17

The enthalpy change of solution.

Question 18

What can you use an enthalpy cycle to find?

Answer 18

The enthalpy change of solution.

Question 19

What does Hess’s law say?

Answer 19

That the total enthalpy change of a reaction is always the same, no matter which route is taken.

Question 20

What is an infinitely dilute solution?

Answer 20

One which contains so much colvent that when you add more solvent, there is no change in concentration.

Question 21

What can you use an enthalpy level diagram to find?

Answer 21

The enthalpy change of solution.

Question 22

Briefly describe an experiment you can do to measure (and calculate) enthalpy change of solution

Answer 22

Put the reactants in a container, stick a thermometer in and measure the temperature change.

(It is best to use a polystyrene beaker with a lid (with a hole for the thermometer), so that you don’t lose of gain too much heat through the sides.

To calculate the enthalpy change you need to know the number of moles of the stuff that is reacting, the change in temperature, and how much stuff you have.

Question 23

What affects the lattice enthalpy and enthalpy of hydration?

Answer 23

Charge density.

Question 24

What does charge density affect?

Answer 24

The lattice enthalpy and enthalpy of hydration.

Question 25

What is the charge density of an ion equal to?

Answer 25

Its charge divided by its radius.

Question 26

What is the equation for charge density?

Answer 26

Charge density = charge/radius

Question 27

What are ions with a higher charge density better at?

Answer 27

Attracting ions of the opposite charge.

Question 28

Why are ions with a higher charge density better at attracting ions of the opposite charge?

Answer 28

Because the electrostatic attraction between ions with higher charge densities is much stronger than the attraction between ions with lower charge densities.

Question 29

How does charge density affect lattice enthalpy?

Answer 29

Ions with a higher charge density are better at attracting each other in ionic lattices than those with lower charge densities. This means the ionic bonds are stronger, meaning more energy is released when the bonds are made, giving them a more exothermic lattice enthalpy.

Question 30

How does charge density affect the enthalpy of hydration?

Answer 30

Ions with a higher charge density are better at attracting water molecules than those with lower charge densities. This means the intermolecular bonds are stronger, meaning more energy is released when the bonds are made giving them a more exothermic hydration enthalpy.