Unit 5 thermodynamics

Question 1

By describing the nature of the attractive forces involved, explain why the value for the enthalpy of hydration for the chloride ion is more negative than that for the bromide ion. (3)

Answer 1

Forces of attraction between chloride ions and water are stronger
Chloride ions attract the delta+ on the H of water

Question 2

The enthalpy of solution for potassium chloride is +17.2kj mol-1
Explain why the free energy for the dissolving of potassium chloride in water is negative, even though the enthalpy change is positive. (3)

Answer 2

Increased disorder when dissolving in water
Entropy change is positive
One mole of solid makes two moles of aqueous
T🔺S > 🔺H

Question 3

An accurate value for the enthalpy of formation of ammoni is -46
Suggest why your answer was different from this value. (1)

Answer 3

Average bond enthalpy da are from a range of compounds

Question 4

Suggest two properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model. (2)

Answer 4

Size of ion

Charge of ion

Question 5

Define bond dissociation enthral pay applied to chlorine (2)

Answer 5

Enthalpy change to break the bond in 1 mole of chlorine

To form gaseous chlorine atoms

Question 6

Explain why the Enthalpy of atomisation of chlorine is exactly half the bond dissociation Enthalpy of chlorine (1)

Answer 6

Only one mile of chlorine atoms are formed

Question 7

Explain why the Enthalpy of formation of ClF3 you calculated is likely to be different from a data book value (1)

Answer 7

The Enthalpy is different in different compounds

Question 8

Suggest why a value for the Na-Cl bond Enthalpy is NOT found in any data book. (1)

Answer 8

NaCl is ionic, not covalent

Question 9

Write an equation including state symbols, for the reaction that has an Enthalpy change equal to the lattice dissociation Enthalpy of magnesium chloride (1)

Answer 9

mgCl2(s) –> Mg2+(g) + 2Cl-(g)

Question 10

Explain why the lattice dissociation Enthalpy of magnesium chloride is greater than calcium chloride (2)

Answer 10

Magnesium ion is smaller/smaller radius

Attraction between ions is stronger

Question 11

Explain why the lattice dissociation Enthalpy of magnesium oxide is greater this that of magnesium chloride (2)

Answer 11

Oxide ion has a higher charge than chloride ion

So attracts the magnesium more strongly

Question 12

Explain why magnesium ions attract water molecules (2)

Answer 12

Water is polar/ delta negative on the O

Mg2+ ion attracts the delta negative on the oxygen

Question 13

Suggest why a value for the Enthalpy of solution of magnesium oxide is NOT found in any data books. (1)

Answer 13

Magnesium oxide reacts with water