Energetics II

Question 1

What is the standard enthalpy of atomisation?

Answer 1

The enthalpy change when one mole of gaseous atoms are produced from an element in its standard state.

Question 2

What is first electron affinity?

Answer 2

The enthalpy change when one mole of electrons is added to a mole of gaseous atoms to form a mole of gaseous 1- ions.

Question 3

What is the second electron affinity?

Answer 3

The enthalpy change when one mole of electrons is added to one mole of gaseous 1- ions to form one mole of gaseous 2- ions.

Question 4

What is the lattice enthalpy of formation?

Answer 4

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.

Question 5

What is the lattice enthalpy of dissociation?

Answer 5

The enthalpy change when one mole of a solid ionic compound is dissociated into its gaseous ions

Question 6

What is the enthalpy of hydration?

Answer 6

The enthalpy change when water molecules surround one mole of gaseous ions

Question 7

What is the enthalpy of solution?

Answer 7

The enthalpy change when one mole of a solute dissolves completely in a solvent to form a solution in which the ions are far enough apart not to interact with one another

Question 8

Are electron affinity values generally endothermic (positive) or exothermic (negative)?

Answer 8

First electron affinity is negative as energy is released when attractions are formed between the electron and the nucleus. Subsequent electron affinities are positive as there is repulsion between the electron and the negatively changed ion, which needs to be overcome. This requires energy.

Question 9

What is a Born-Haber cycle?

Answer 9

Similar idea to a Hess cycle but is generally used for lattice energies. The species at each stage are written on lines with arrows between lines indicating enthalpy changes ti get between the species. State symbols and electrons must be used and species must balance. The enthalpy changes required for the formation of NaCl would be enthalpy of atomisation for both Na and 1/2Cl2 (in separate stages), first ionisation energy of Na, first electron affinity of Cl, lattice enthalpy of formation of NaCl, enthalpy of formation of NaCl.

Question 10

How would you use a Born-Haber cycle?

Answer 10

Follow the arrows around the cycle to find any gap, reversing the sign of the enthalpy changes when you have to go against the direction of an arrow. Alternatively, the magnitudes of the enthalpy changes on both sides (left and right) of the cycle are the same, so you can use that, combined with the direction of the arrow (up is endothermic, down is exothermic) to work out any particular enthalpy change, provided you know all other relevant values.

Question 11

If one ionic compound has a more exothermic (more negative) lattice enthalpy of formation, what does this indicate about the strength of the ionic bonding in the structure?

Answer 11

The compound with the more exothermic lattice enthalpy of formation has stronger ionic bonding

Question 12

What factors affect the lattice enthalpy of an ionic compound?

Answer 12

• polarising power (cations) and polarisability (anions)
• these are determined by charge and ionic radius (and therefore charge density)
• these affect the degree of covalent character present within the lattice
• lattices with the greatest degree of covalent character have more polarisable anions and cations with higher polarising power
• having a greater degree of covalent character means a more exothermic lattice energy

Question 13

What makes a cation have greater polarising power?

Answer 13

Higher charge density, so highly charged and small ionic radius

Question 14

What makes anions more polarisable?

Answer 14

Larger ionic radius and higher charge means a more polarisable anion

Question 15

What is the perfect ionic model?

Answer 15

The perfect ionic model assumes a lattice where all bonds are purely ionic and all the ions are perfect spheres, in which the charge is evenly distributed (there is no covalent character). This is most likely to happen when the ions have similar charge densities and similar electronegativities.

Question 16

If there is a big difference between the theoretical lattice energy and the experimental lattice energy, what does this indicate?

Answer 16

That the compound has a high degree of covalent character

Question 17

Which other enthalpy changes is enthalpy of solution the sum of?

Answer 17

Lattice dissociation enthalpy, hydration of cations, hydration of anions

Question 18

Is enthalpy change of solution generally endothermic or exothermic?

Answer 18

The lattice dissociation is an endothermic process as bonds are being broken, but the hydration of the ions is exothermic, as electrostatic attractions are being formed. Therefore, the enthalpy of solution is generally either exothermic or slightly endothermic if a compound dissolves. Insoluble compounds usually have very endothermic solution enthalpies as this will generally not be feasible.

Question 19

What factors affect enthalpy of hydration?

Answer 19

Charge density - ions with a higher charge density have more exothermic hydration enthalpies

Question 20

What is entropy?

Answer 20

Entropy is a measure of the disorder of a system (which is the number of ways particles can be arranged and the number of ways that energy can be shared between them).

Question 21

What makes a compound more energetically stable?

Answer 21

Compounds with more entropy are more energetically stable, so all spontaneous reactions/processes will result in an increase in total entropy (second law of thermodynamics).

Question 22

How does entropy change with physical state?

Answer 22

Solids have low entropy as the particles are touching and can only move by vibrating about a fixed position. Liquids have more entropy than solids as only some of the particles are touching and the particles can move over each other. Gases have much higher entropy than solids and liquids as the particles are much further apart and can move freely.

Question 23

How does the number of moles present in a reaction mixture affect entropy?

Answer 23

The greater the number of moles of any species present, the higher the entropy if all species are in the same state.

Question 24

What is the equation for the entropy change of a system?

Answer 24

Entropy change of the system = entropy of the products - entropy of the reactants

Question 25

Do entropy changes determine whether or not a reaction can occur?

Answer 25

They do not determine this, but they do give a good indication of feasibility. A reaction with a positive entropy change may not occur due to enthalpy, temperature or kinetic factors. A reaction with negative entropy could occur (though this is unlikely - and it likely won’t occur spontaneously).

Question 26

What is the equation for the total entropy change?

Answer 26

Total entropy change = entropy change of the system + entropy change of the surroundings

Question 27

What is the equation for the entropy change of surroundings?

Answer 27

Entropy change of surroundings = - enthalpy change / temperature
Temperature is in K and enthalpy change must be converted to J mol -1 (as entropy changes are generally given in J K-1 mol -1 but enthalpy changes are generally in kJ mol-1)

Question 28

What is the equation for Gibbs free energy?

Answer 28

Gibbs energy = enthalpy change - (temperature x entropy change of the system)
Units are usually (but can be in kJ): deltaG - J mol -1 deltaH - J mol-1 T - K deltaSsys - J K-1 mol-1

Question 29

What does Gibbs energy tell you about the feasibility of a reaction?

Answer 29

If the Gibbs energy is negative, the reaction is feasible at the given temperature. If it is zero, it is just feasible and if it is positive, it is not feasible.

Question 30

Does having a negative Gibbs value guarantee a reaction?

Answer 30

No, just because a reaction is theoretically feasible doesn’t mean it will occur. Kinetic factors, such as having a high activation energy, may prevent the reaction from occurring.

Question 31

What is an alternative equation for Gibbs energy?

Answer 31

Gibbs energy = -T x total entropy change
This can be obtained by rearranging and substituting other equations and means that -T x total entropy change is also a way of determining feasibility.

Question 32

How does Gibbs energy relate to the equilibrium constant?

Answer 32

DeltaG = -RTlnK