AC15: Lattice Enthalpy

Question 1

Why is the enthalpy of neutralisation for weak acids/alkalis less exothermic compared to strong acids/alkalis

Answer 1

Weak acids/alkalis require more energy to dissociate and release H⁺ or OH⁻ as they only partially dissociate, and so less heat released overall

Question 2

What do Born-Haber cycles show, what are they used to calculate and why are they used?

Answer 2

They show the enthalpy changes in the formation of ionic compounds from their elements, used to calculate the value of lattice enthalpy, because this cannot be determined experimentally

Question 3

Define standard enthalpy of atomisation of an element and give examples for chlorine and sodium

Answer 3

The standard enthalpy change when 1 mole of gaseous atoms of an element are formed from the element in its standard state.
½Cl₂ (g) -> Cl (g)
Na (s) -> Na (g)

Question 4

Define bond enthalpy and give an example for hydrogen

Answer 4

the enthalpy change required to break 1 mole of bonds between two atoms in the gaseous state
H₂(g) -> 2H (g)

Question 5

Define first electron affinity of an element and give an example for oxygen

Answer 5

the enthalpy change when one electron is added to each atom in a mole of gaseous atoms of an element
O (g) + e⁻ -> O⁻ (g)

Question 6

Define second electron affinity by using an example for oxygen

Answer 6

O⁻ (g) + e⁻ -> O²⁻ (g)

Question 7

Define lattice enthalpy and give an example for sodium chloride

Answer 7

the standard enthalpy change when 1 mole of a solid ionic lattice forms from its separate ions in the gas phase
Na⁺ (g) + Cl⁻ (g) -> NaCl (s)

Question 8

Define negative lattice enthalpy (lattice dissociation enthalpy)

Answer 8

the standard enthalpy change when 1 mole of a solid ionic lattice is converted to its separate ions in the gas phase
NaCl (s) -> Na⁺ (g) + Cl⁻ (g)

Question 9

Define enthalpy of hydration

Answer 9

the standard enthalpy change when 1 mole of gaseous ions dissolve to form 1 mole of hydrated ions
Na⁺ (g) + water ->Na⁺ (aq)

Question 10

Define enthalpy of solution

Answer 10

the enthalpy change when 1 mole of a compound dissolves in a large excess of water
NaOH (s) + water -> NaOH (aq)

Question 11

Define charge density

Answer 11

The ratio of the charge of an ion compared to its volume

Question 12

Describe and explain the difference in charge density between a calcium ion and a potassium ion

Answer 12

charge density of Ca²⁺ ion is greater than that of K⁺ ion, because Ca²⁺ has a bigger ionic charge but similar ionic radius to K⁺

Question 13

Describe and explain the difference in charge density between a chloride ion and a bromide ion

Answer 13

Charge density of Cl⁻ ion is greater than that of Br⁻ ion because Cl⁻ and Br⁻ have same ionic charge but Cl⁻ has a smaller ionic radius

Question 14

Why does calcium chloride have a more exothermic lattice enthalpy than potassium bromide? (3 marks)

Answer 14

charge density of Ca²⁺ ion is greater than that of K⁺ ion, because Ca²⁺ has a bigger ionic charge but similar ionic radius to K⁺.
Charge density of Cl⁻ ion is greater than that of Br⁻ ion because Cl⁻ and Br⁻ have same ionic charge but Cl⁻ has a smaller ionic radius
Therefore attraction between Ca²⁺ ions and Cl⁻ ions is greater than that between K⁺ ions and Br⁻ ions therefore more energy is needed to overcome attraction between Ca²⁺ and Cl⁻ ions

Question 15

Explain why the second electron affinity enthalpy change of oxygen is positive whereas the first electron affinity enthalpy change of oxygen is negative

Answer 15

The charges on O⁻ and e⁻ repel and so energy is required