Energy Changes

Question 1

Define enthalpy change of lattice formation

Answer 1

The enthalpy change that occurs when one mole of an ionic compound is formed from the ions of the element in the gas phase
E.g. Na+(g) + Cl-(g) –> NaCl(s)

Question 2

Define enthalpy change of lattice breaking

Answer 2

The enthalpy change that occurs when one mole of an ionic compound is broken up into ions of the elements in the gaseous phase. It is the reverse of lattice formation and hence has the opposite sign
E.g. NaCl(s) –> Na+(g) + Cl-(g)

Question 3

Define enthalpy of hydration

Answer 3

The enthalpy change that occurs when one mole of ions in gaseous form is surrounded by water molecules to make a solution
E.g. Na+(g) + aq –> Na+(aq)

Question 4

Define enthalpy change of solution

Answer 4

The enthalpy change that occurs when one mole of an ionic compound dissolves in water to form a solution
E.g. NaCl(s) + aq –> NaCl(aq)

Question 5

What does the solubility of a compound depend on?

Answer 5

It depends mainly on the enthalpy of hydration being exothermic. This means that enthalpy of hydration for the ions must be greater than the enthalpy of lattice breaking

Question 6

How do we determine the stability of a compound?

Answer 6

One way is to use the enthalpy of formation, in general, a compound will be more stable the more negative that this value is

Question 7

What does Hess’s law state?

Answer 7

The enthalpy change accompanying a chemical reaction is the same regardless of the route taken from reactants to products

Question 8

What can we say about the reaction if the Gibbs free energy value is negative?

Answer 8

Reaction will be spontaneous

Question 9

What can we say about a reaction with a positive Gibbs free energy value?

Answer 9

It will not occur spontaneously

Question 10

Define enthalpy of atomisation

Answer 10

The enthalpy required to form one mole of atoms in the gaseous phase from the element in its standard state under standard conditions
E.g. Na(s) –> Na(g)
1/2 Cl2(g) –> Cl(g)