[22.1-2] lattice enthalpy, enthalpy changes in solution

Question 1

pressure, temperature, concentration

what are the standard conditions for enthalpy?

Answer 1

• standard pressure: 100kPa (1 atm = 101kPa)
• standard temperature: 298 K (25 °C)
• standard concentration: 1 mol dm⁻³ (only relevant for solutions)

Question 2

what is standard state?

Answer 2

the physical state of a substance under standard conditions (100kPa and 298 K)

Question 3

lattice enthalpy definition

Answer 3

the enthalpy change when one mole of an ionic compound is formed from its constituent gaseous ions under standard conditions

Question 4

is lattice enthalpy exo or endo?

Answer 4

always exothermic (forming bonds)

Question 5

how is lattice enthalpy calculated?

Answer 5

indirectly using known energy changes in a born-haber cycle

Question 6

what is the enthalpy of atomisation?

Answer 6

enthalpy change when one mole of gaseous atoms is produced from an element in its standard state under standard conditions

enthdothermic

Question 7

define first ionisation energy (endo)

Answer 7

the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 8

what is the difference between ionisation energy and electron affinity?

Answer 8

• electron affinity measures the energy to gain electrons
• ionisation energy measures the energy to lose electrons

Question 9

define first electron affinity (exo)

Answer 9

enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 10

are second electron affinities exo or endo and explain why

Answer 10

• endothermic
• a second electron is being gained by a negative ions, which repels the electron away
• energy must be put in to force the negatively-charged electron onto the negative ion

Question 11

define enthalpy of hydration (exo)

Answer 11

enthalpy change when one mole of gaseous ions are dissolved in water to form one mole of aqueous ions

Question 12

define enthalpy of solution (exo or endo)

Answer 12

enthalpy change when one mole of an ionic solid is completely dissolved in water under standard conditions

Question 13

what does whether an enthalpy change of solution is exo or endo depend on?

Answer 13

• lattice enthalpy of the solid
• hydration enthalpy of the ions

Question 14

describe what happens at a particle level when an ionic compound dissolves in water?

Answer 14

• δ+ and δ- partial charges in the water molecules are attracted towards the positive and negative ions
• δ- oxygen atom is attracted to positive ion
• δ+ hydrogen atoms are attracted to negative ions