Enthalpy And Entropy

Question 1

Is lattice enthalpy endothermic or exothermic?

Answer 1

Exothermic - value for enthalpy change will always be negative

Question 2

What 3 processes occur in the long route of a Born-Haber cycle? Are they exothermic or endothermic? And what enthalpy change names do they involve?

Answer 2

1. Formation of gaseous atoms (endothermic, enthalpy change of atomisation)
2. Formation of gaseous ions (endothermic overall, ionisation energy and electron affinity)
3. Lattice formation from gaseous ions (exothermic, lattice enthalpy)

Question 3

Are electron affinities exothermic or endothermic? Why?

Answer 3

1st electron affinity - exothermic because the electron being added is attracted in towards the nucleus
2nd electron affinity - endothermic because a 2nd electron is being gained by a negative ion, which repels the electron away. So energy must be put in to force the negatively-charged electron into the negative ion.

Question 4

What is the procedure for the experimental determination of the enthalpy change of solution?

Answer 4

1. Weight out sample of ionic compound
2. Using a measuring cylinder, pour 25cm^3 of distilled water into the polystyrene cup in the beaker. Measure the temperature of the water, to the nearest 0.5 degrees C.
3. Quickly tip all of the ionic compound into the water in the polystyrene cup. Stir the mixture with the thermometer until all of the ionic compound has dissolved and the temperature no longer changes. Record this value to the nearest 0.5 degrees C.

Question 5

What are the 2 routes involved in a dissolving born-haber cycle?

Answer 5

Route 1: lattice enthalpy and enthalpy change of solution
Route 2: enthalpy change if hydration

Question 6

What are the general properties of ionic compounds?

Answer 6

1. High melting and boiling points
2. Soluble in polar solvents
3. Conduct electricity when molten or in aqueous solution

Question 7

What is average bond enthalpy?

Answer 7

The energy needed to break one mole of a bond in the gas phase, averaged over the different compounds that the bond is found in