Energetics II Flashcards
Define enthalpy of lattice formation:
Enthalpy change when 1 mole of ionic lattice is formed from its gaseous ions (exothermic)
Eg/ Na+(g) + Cl- → NaCl(s)
Define enthalpy of solution:
Enthalpy change when 1 mole of ionic substance dissolves in a large enough amount of water to ensure that the ions are well separated
What is the equation for enthalpy of solution:
- (ΔH lattice) + ΣΔH hydration
Define enthalpy of hydration:
ΔH when 1 mole of gaseous ions become hydrated
What enthalpies are always exothermic?
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
Define first electron affinity:
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
Why is ionisation energy always exothermic but electron affinity can be either?
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
How does the charge of the ion affect lattice enthalpy?
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)
How does the size of the ion affect lattice enthalpy?
- 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)
What is meant by polarisation?
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.
What factors affect how polarising a cation is? (polarising power)
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
What factors affect how easily polarised the anion is?
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.
What is meant by the ‘covalent character’ of the ionic substance?
The greater the covalent character the more polarised the anion is.
What effect does covalent character have on the lattice enthalpy?
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)
Define entropy:
Measure of disorder
What does it mean when there is a high entropy?
The more disordered the particles are, the higher the entropy is. A large, positive value of entropy shows a high level of disorder
What is the entropy change equation:
∆S = ΣS products - ΣS reactants
What is Gibbs free energy?
∆G = ∆H - T∆S
Define spontaneous
Is one that has a natural tendency to occur without being driven by any external influences
∆G ≤ 0 for spontaneous change
When are reactions feasible?
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)
What is entropy of surroundings?
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.
Equation for entropy of surroundings?
∆S surr = -∆H/T
Units: kJ K-1 mol-1
Equation for overall reaction:
∆S total = -∆H /T + ∆S sys
What is the equation for ∆G?
-RT ln k
Why might a reaction might not take place?
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
Why might a reaction not take place despite the fact that ∆S total is positive and ∆G is negative?
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)
