Enthalpy (22)

Question 1

What is an ionic compound known as and is it strong?

Answer 1

It is known as an ionic lattice. It is very strong because it is composed of positively charged ions that have electrostatic attractions to negatively charged ions which requires a lot of energy to break.

Question 2

What is lattice enthalpy?

Answer 2

A measure of the strength of the ionic bonding in an ionic lattice. It is the enthalpy change when one mole of the ionic compound is formed from its gaseous ions under standard conditions.

Question 3

Are the reactions exothermic or endothermic and what does this mean the enthalpy change is?

Answer 3

The reaction is always exothermic, energy is transferred from the system to the surroundings and the enthalpy change will always be negative

Question 4

What is the Born-Haber cycle and why is it used in lattice enthalpy?

Answer 4

A special type of energy cycle that has two different routes and is used to determine the lattice enthalpy by looking at the indirect enthalpy changes as lattice enthalpy cannot be measured directly.

Question 5

What are the processes required for Route 1?

Answer 5

• The formation of gaseous atoms (endothermic)
• The formation of gaseous ions (endothermic)
• The formation of a lattice (exothermic)

Question 6

What are the processes of Route 2?

Answer 6

This converts elements in their standard states into ionic lattices using one enthalpy change which is exothermic.

Question 7

Standard enthalpy change of formation?

Answer 7

The enthalpy change when ONE MOLE of a compound is formed from its elements under standard conditions with all reactants and products in their standard states

Question 8

Standard enthalpy change of atomisation?

Answer 8

The enthalpy change when ONE MOLE of gaseous atoms is formed from its elements in their standard states under standard conditions.

Question 9

Is the standard enthalpy change of atomisation exothermic or endothermic?

Answer 9

It’s breaking bonds to form gaseous atoms so it’s endothermic.

Question 10

First ionisation energy

Answer 10

The enthalpy change required to remove one electron from one mole of gaseous atoms to form one mole of gaseous positive ions.

Question 11

Is first ionisation energy exothermic or endothermic?

Answer 11

It’s endothermic because energy is required from the surroundings to overcome the attraction between the positive nucleus and negative electron.

Question 12

What is electron affinity?

Answer 12

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous negative ions

Question 13

Is electron affinity exothermic or endothermic?

Answer 13

It is exothermic because electrons are being added and attracted towards the nucleus.

Question 14

Is second electron affinity exothermic or endothermic?

Answer 14

It is endothermic because a second electron is being added to a negative ion which will repel it so extra energy needs to be added to make sure it becomes 2-.

Question 15

What is able to break the giant ionic lattice?

Answer 15

Water can overcome the strong electrostatic attractions between the oppositely charged ions.

Question 16

The standard enthalpy change of solution

Answer 16

This is the enthalpy change when one mole of a solute dissolves in a solution

Question 17

What are the processes of dissolving?

Answer 17

• The ionic compound will break up

- The water molecules will become attracted to the ions and surround them.

Question 18

What types of energy change occur during these processes?

Answer 18

• The ionic compound breaking up is the opposite of the lattice enthalpy so it is endothermic
• The water molecules surround the gaseous ions to become hydrated aqueous ions and the energy change is called the enthalpy change of hydration

Question 19

Enthalpy change of hydration

Answer 19

The enthalpy change when gaseous ions dissolve in water to form one mole of aqueous ions.

Question 20

What are the physical properties of ionic compounds?

Answer 20

They have:

• high melting and boiling points
• soluble in polar solvents
• conduct electricity when molten or in aqueous

Question 21

What factors affect lattice enthalpy?

Answer 21

• The ionic size

- The ionic charge

Question 22

How does ionic size affect lattice enthalpy?

Answer 22

As the ionic radius increases, the melting point will decrease so the lattice enthalpy will be less exothermic (less negative). This is because the attraction between the ions will decrease

Question 23

How does ionic charge affect lattice enthalpy?

Answer 23

The greater the ionic charge, the higher the melting point will be and the more negative the lattice enthalpy will be. This is because the ions become more attracted to each other.

Question 24

What are the supporting and opposing effects of ions in period 3?

Answer 24

The two supporting effects for positive ions are that the charge increases across the period so is more attraction between the ions and also that decreasing the size will give more attraction.
The two opposing effects for negative ions are that the charge increases the inverse way across the period and the increasing size will give less attraction

Question 25

What are factors affecting hydration?

Answer 25

The hydration enthalpies are also affected by ionic charge and size. The bigger the ion is, the smaller the attraction between them so the less negative the lattice enthalpy. The greater the ionic charge, the stronger the attraction between the ions so the hydration energy becomes more negative.

Question 26

How can you predict if a compound should dissolve?

Answer 26

If the total hydration enthalpies add up to more than the lattice enthalpy, the compound will dissolve and will be exothermic