Energy

Question 1

What is lattice enthalpy?

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic lattice from its gaseous ions under standard conditions

Question 2

What are standard conditions?

Answer 2

298K and 1atm (100kpa)

Question 3

What is the standard enthalpy change of formation?

Answer 3

The enthalpy change that accompanies the formation of one mole of a compound from its standard elements under standard conditions

Question 4

What is first ionisation energy?

Answer 4

The energy change that accompanies the removal of one mole of electrons from one mole of gaseous atoms

Question 5

Why do stable elements form ions when it requires a lot of energy?

Answer 5

As oppositely charged ions attract each other to form an ionic lattice which causes a huge lowering of energy, this compensates for the energy that was required to from the ions

Question 6

What does a more negative lattice enthalpy mean?

Answer 6

• Stronger ionic bonds have been formed as making bonds releases energy
• Higher melting and boiling points as more energy is required to overcome the stronger ionic bonds

Question 7

What is the standard enthalpy change of atomisation?

Answer 7

The enthalpy change that accompanies the formation of one mole of gaseous atoms from its element in its standard state

Question 8

What is the second ionisation energy?

Answer 8

The energy change that accompanies the formation of one mole of gaseous 2+ ions from one mole of gaseous 1+ ions

Question 9

What is first electron affinity?

Answer 9

The enthalpy change that accompanies the formation of one mole of gaseous 1- ions from one mole of gaseous atoms

Question 10

What is second electron affinity?

Answer 10

The enthalpy change that accompanies the formation of one one mole of gaseous 2- ions from one mole of gaseous 1- ions

Question 11

Why is first electron affinity exothermic but second electron affinity is endothermic?

Answer 11

• First electron affinity is exothermic because the electron is attracted to the nucleus and forms a bond which releases energy
• Second electron affinity is endothermic as the first electron has already been added to the atom to form a 1- ion, the 1- ion repels the electron that is being added, the energy overcomes this repulsion hence the second electron affinity is endothermic

Question 12

What is the standard enthalpy change of solution?

Answer 12

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

Question 13

What is the standard enthalpy change of hydration?

Answer 13

The enthalpy change that takes place when one mole of gaseous ions dissolve in water

Question 14

What is the breakdown of an ionic lattice equal to?

Answer 14

-ΔLEH

Question 15

What dictates the size of the lattice enthalpy?

Answer 15

• The size of the ions involved
• The charge on the ions involved
• The ionic bond strength which is dependent on size and charge

Question 16

What is the trend in lattice enthalpy as ions increase in size?

Answer 16

• As ionic radius increases, ions can’t pack as closely together meaning they can’t attract each other as strongly
• This causes lattice enthalpy to become less negative as ions increase in size, weaker ionic bonds are formed which means less energy is released when the ionic lattice is formed

Question 17

What is the trend in lattice enthalpy as the charge on ions increases?

Answer 17

• As the charge on ions increases eg. 2+ instead of 1+ or 2- instead of 1-, the ions can attract each other more strongly
• This causes lattice enthalpy to become more negative/more exothermic as the ions form stronger bonds meaning more energy is released when the ionic lattice is formed

Question 18

Describe the process of hydration of ions

Answer 18

• When an ionic solid has broken down into its constituent ions, they have to become part of solution
• Positive ions are attracted to the delta negative oxygen in the water molecules
• Negative ions are attracted to the delta positive hydrogens in the water molecules
• The water molecules surround the ions, therefore dissolving them

Question 19

What dictates enthalpy change of hydration?

Answer 19

• The size of the ions

- The charge on the ions

Question 20

What is the trend in enthalpy change of hydration as ionic size decreases?

Answer 20

As ionic size decreases, the ions can get closer to the water molecules and can attract them more strongly, this means there is a stronger bond formed and more energy is released making ΔhydH more negative

Question 21

What is the trend in enthalpy change of hydration as ionic charge increases?

Answer 21

As ionic charge increases, the ions can attract the water molecules more strongly therefore making a stronger bond and releasing more energy. This makes ΔhydH more negative/more exothermic

Question 22

How is enthalpy change of solution calculated from enthalpy change of hydration and lattice enthalpy?

Answer 22

• ΔsolH + ΔLEH = ΔhydH
• ΔsolH = ΔhydH - ΔLEH
• When an ionic compound has a greatly exothermic lattice enthalpy, it will also have a greatly exothermic enthalpy change of hydration
• Lattice enthalpy affects ΔsolH more than ΔhydH does
• This means that when a compound has a more exothermic lattice enthalpy than another, it will have a more positive ΔsolH