thermodynamic exam questions

Question 1

in terms of electrostatic forces suggest why the electron affinity of fluorine has a negative value

Answer 1

• there is an attraction between the nucleus and electrons
• energy is released when the electron is gained

Question 2

Explain why the theoretical enthalpy of lattice dissociation is higher than the value from the Born-haber cycle

Answer 2

• has some covalent character
• theoretical lattice enthalpy assumes its perfectly ionic

Question 3

the theoretical enthalpy of lattice dissociation for silver chloride is 770klmol-1. Explain why this is less than the value for silver fluoride

Answer 3

• chloride ion is larger than the fluoride ion
• attraction between Ag+ and Cl- weaker

Question 4

H2O(s)->H2O(L)
use your knowledge of bonding to explain why the enthalpy change is positive

Answer 4

• hydrogen bonds need to be broken
• so energy is released

Question 5

two properties that influence the value of a lattice enthalpy calculated using the perfect ionic model

Answer 5

• size of the ions
• charge of the ions

Question 6

suggest why the first electron affinity of oxygen is an endothermic process

Answer 6

making bonds

Question 7

equation of enthalpy of solution of MgCl2

Answer 7

MgCl2 -> Mg2+(aq) + 2Cl-(aq)

Question 8

Why is the enthalpy of hydration of Ca2+ less exothermic than Mg2+

Answer 8

• Ca2+ larger charge compared to size
• higher electron density

Question 9

why is the standard entropy of CO2 greater than carbon

Answer 9

gas is more disordered than a solid

Question 10

Explain why the enthalpy of hydration becomes less exothermic from Li+ to K+

Answer 10

• size of ion increases
• the electrostatic attraction between metal ion and o- of water decreases attraction between lone pair o and + ion decreases

Question 11

why is the enthalpy of lattice formation of AgI a smaller numerical value than the value calculated

Answer 11

• AgI contains covalent character
• forces are stronger

Question 12

why is the value for enthalpy of hydration for the chloride ion more negative than the bromide ion

Answer 12

• Cl- are smaller
• force of attraction between Cl- and water is stronger
• Cl- attraction +H of water

Question 13

explain why gibbs energy change for dissolving KCl in water is -ve, even though enthalpy is +ve

Answer 13

• entropy change is +ve
• 1mol solid -> 2 mol aq

Question 14

explain why entropy is 0 when the temperature is zero kelvin

Answer 14

• at 0K particles are stationary
• no disorder

Question 15

explain why Mg+ attract water molecules

Answer 15

• water is polar
• Mg+ is attracted to delta - on O-

Question 16

why is the enthalpy of solution of MgO not found in any data books

Answer 16

reacts with water

Question 17

Explain why fluoride ion is larger than a sodium ion

Answer 17

• fluoride ion has a lower nuclear charge
• weaker attraction between fluoride ions outer electron and nucleus

Question 18

why could the lattice dissociation be more or less than the value obtained from the perfect ionic model

Answer 18

• greater
• has covalent character

Question 19

how is the lattice formation of NaCl compare with that of NaF

Answer 19

• less exothermic
• f- smaller ions
• so attraction of halide ion to Na+ is weaker

Question 20

why is the second electron affinity an endothermic process

Answer 20

negative 1- ion, repels electrons being added

Question 21

why is the electron affinity of chlorine an exothermic change

Answer 21

net attraction between the chlorine nucleus and extra electron

Question 22

what would the value of enthalpy of lattice dissociation of silver chloride be compared to silver bromide

Answer 22

• greater than
• bromide is a larger ion
• attracted more strongly to the silver ions