Entropy

Question 1

Explain the difference between thermodynamic stability and kinetic inertness.

Answer 1

Thermodynamic stability is when delta S total is negative and the reaction will not occur. Energy of reactants is greater than products.
Kinetic inertness is when the Ea is high so the reaction rate is slow. Energy input is required.

Question 2

Define entropy.

Answer 2

A measure of disorder; the number of arrangements of quanta in a system.

Gas, liquid, Solid

Question 3

Define enthalpy of hydration and lattice enthalpy.

Answer 3

Hydration: the enthalpy change when one mole of gaseous ions dissolve in sufficient solvent to form an infinitely dilute solution.
Lattice: the enthalpy change when one mole of solid ionic compound is formed from its constituent gaseous ions.

Question 4

Define the enthalpy change of solution.

How would you calculate it?

Answer 4

The enthalpy change when one mole of solid is dissolved in sufficient solvent to give an infinitely dilute solution.
Delta H of solution = -delta H [Lattice enthalpy] + delta H [hydration enthalpy]

Question 5

What does delta H solution indicate about solubility?

Answer 5

The more negative delta H, the more likely a solute will dissolve. If delta H is small and positive; there is sufficient increase in entropy, it may dissolve.
The magnitude between delta H lattice and hydration determines the sign of the enthalpy of solution.

Question 6

What factors affect lattice and hydration enthalpy?

Answer 6

1. Ionic radius - smaller radius = higher charge density and more exothermic lattice energy
2. Ionic charge - larger charge = more exothermic lattice energy
3. Degree of covalency

Question 7

What is the difference between hydration and hydrolysis?

Answer 7

Hydration: only involves bond making
Hydrolysis: is bond breaking and making

Question 8

State the three laws of thermodynamics.

Answer 8

1. Energy cannot be created or destroyed
2. Entropy always increases in the transformation of energy (order to disorder)
3. Entropy of a perfect crystal at absolute zero is zero (delta S = 0)

Question 9

Why is CoCl3 known, but MgCl3 is not? Consider Born-Haber cycle data.

Answer 9

Mg3+ is unstable. MgCl3 has a highly endothermic enthalpy of formation. High ionisation energy is required to form it. 3rd IE requires breaking into noble gas configuration. The lattice energy of formation is negative, but does not compensate for the ionisation energy needed to form ions from atoms.

Question 10

How does a cold pack work?

Is the reaction endothermic or exothermic?

Answer 10

A pack contains separate sections of ammonium nitrate and water. When it is squeezed, the two components mix together. Water molecules dissolve the NH4NO3 by separating the ions NH4+ and NO3-. This is an endothermic process. The ions are then hydrated by water molecules. (This is exothermic but the energy required to separate the ions outweighs this, so overall it is endothermic).