Energetics II

Question 1

Define enthalpy of lattice formation:

Answer 1

Enthalpy change when 1 mole of ionic lattice is formed from its gaseous ions (exothermic)
Eg/ Na+(g) + Cl- → NaCl(s)

Question 2

Define enthalpy of solution:

Answer 2

Enthalpy change when 1 mole of ionic substance dissolves in a large enough amount of water to ensure that the ions are well separated

Question 2

What is the equation for enthalpy of solution:

Answer 2

• (ΔH lattice) + ΣΔH hydration

Question 3

Define enthalpy of hydration:

Answer 3

ΔH when 1 mole of gaseous ions become hydrated

Question 4

What enthalpies are always exothermic?

Answer 4

Lattice enthalpy: bonds are being formed between ions, this releases energy
Hydration: bonds are being formed between ions + H2O molecules (hydrogen bonding)
Enthalpy of solution: either endothermic/exothermic

Question 5

Define first electron affinity:

Answer 5

The energy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous ions (with a single negative charge)
Eg/ S(g) + 2e- → S2-(g) - first and second electron affinity

Question 6

Why is ionisation energy always exothermic but electron affinity can be either?

Answer 6

This is because oxygen needs to gain electrons to get a full outer shell, so doing so should release energy. However the second electron being gained would be repelled by the already negative ion, so this process will require energy

Question 7

How does the charge of the ion affect lattice enthalpy?

Answer 7

The greater the charge, the stronger the attraction between the oppositely charged ions
Because the bond formed is stronger, more energy is released on forming the lattice (so ∆H latt is more exothermic)

Question 8

How does the size of the ion affect lattice enthalpy?

Answer 8

• As the size of the ions increases, the attraction gets weaker
• The ions cannot approach as closely, so the bond formed is weaker so less energy is released on forming the lattice (so ∆H latt is less exothermic)

Question 9

What is meant by polarisation?

Answer 9

Polarisation is where the electrons within a negative ion can be pulled to one side of the ion by the presence near by of an ion of the opposite charge.

Question 10

What factors affect how polarising a cation is? (polarising power)

Answer 10

The smaller the cation is, the more polarising it is of the negative anion. This is because the smaller ion can approach more closely and so attract the electrons better.

A more positively charged ion will also be more polarising as it will attract the electrons better

Question 11

What factors affect how easily polarised the anion is?

Answer 11

It depends on the size (radius) and charge. The larger the anion is, the further the outer electrons will be from the nucleus, so the more easily they are polarised and pulled towards the cation.

The more highly charged the negative ion is, the stronger the attraction will be for the positive ion, so the more polarisable it will be.

Question 12

What is meant by the ‘covalent character’ of the ionic substance?

Answer 12

The greater the covalent character the more polarised the anion is.

Question 13

What effect does covalent character have on the lattice enthalpy?

Answer 13

If the substance has some covalent character, it is not really 100% ionic. This means that the lattice enthalpy value will be slightly different than that calculated theoretically from the Born-Haber cycle.

In fact, the value will be slightly greater than that calculated (because covalent bonds are stronger and release more energy when made than ionic bonds do)

Question 14

Define entropy:

Answer 14

Measure of disorder

Question 15

What does it mean when there is a high entropy?

Answer 15

The more disordered the particles are, the higher the entropy is. A large, positive value of entropy shows a high level of disorder

Question 16

What is the entropy change equation:

Answer 16

∆S = ΣS products - ΣS reactants

Question 17

What is Gibbs free energy?

Answer 17

∆G = ∆H - T∆S

Question 18

Define spontaneous

Answer 18

Is one that has a natural tendency to occur without being driven by any external influences
∆G ≤ 0 for spontaneous change

Question 19

When are reactions feasible?

Answer 19

When ∆H is negative and ∆S is positive, ∆G will always be negative and the reaction is feasible

When ∆H is positive and ∆S is negative, ∆G will always be positive and the reaction is not feasible

If ∆G equals zero, the system is at equilibrium (no change)

Question 20

What is entropy of surroundings?

Answer 20

The entropy of the universe is increasing all the time but the reactions which actually decrease the entropy of the system can occur if the temperature is low and the reaction is exothermic (∆H is less than zero).
This is because the Gibbs free energy will be negative overall.

Question 21

Equation for entropy of surroundings?

Answer 21

∆S surr = -∆H/T
Units: kJ K-1 mol-1

Question 22

Equation for overall reaction:

Answer 22

∆S total = -∆H /T + ∆S sys

Question 23

What is the equation for ∆G?

Answer 23

-RT ln k

Question 24

Why might a reaction might not take place?

Answer 24

Because ∆S total is negative, ∆G is positive or E cell is negative - this would mean that the reactants were thermodynamically stable over the products

Question 25

Why might a reaction not take place despite the fact that ∆S total is positive and ∆G is negative?

Answer 25

E cell is positive, this might be because the reactants are kinetically stable (there is a high activation energy for this reaction, so a very small proportion of particles collide with enough energy to react, at the temperature used)