Topic 13 Energetics II Flashcards
What is the definition of lattice enthalpy formation?
Enthalpy change when 1 mole of solid ionic compound is formed from it’s gaseous ions under standard conditions.
Ca2+(g) + 2cl- (g) = Cacl2(s)
What is the definition for Enthalpy change of atomisation?
It is the change when 1 mole of gaseous atoms are made from an element in its standard state.
1/2 F2 (g) = F(g)
What is the 1st Electron Affinity?
This is the enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous atoms
O(g) = O- (g)
What is the second electron affinity?
This is when 1 mole of gaseous 2- ions are made from 1 mole of gaseous 1- ions
O-(g) = O2-(g)
What is ionic bonding?
It is the electrostatic attraction between oppositely charged ions
What affects the strength of an ionic bond?
The size of the charge on the ion
The size of the ion (ionic radii)
How does the size of the charge affect the strength of the ionic bond?
The bigger the charge on an ion, the stronger the electrostatic charge between the ions
More energy is required to overcome the forces, so they have a high MP and BP
KCL - K+ and cl- = 770 degrees
CaO - Ca2+ and O2- = 2572 degrees
How does the size of the ion affect the strength of the ionic bond? (ionic radii)
The smaller the ion, the stronger the electrostatic attraction between ions
Smaller ions can pack together more closely and more energy is required to overcome the stronger forces, so MP and BP are higher
NaCl has a higher BP than KCl as Na+ is a smaller ion than K+
What are born Haber cycles used to do?
They are used to calculate lattice enthalpies.
What are the steps for a born Haber cycle?
You start with your solid compound at the bottom
1) There is the enthalpy change of formation which the arrow is pointing down as it is ALWAYS exothermic
2) Then there are the enthalpy changes of atomisation - this is where the solids are turned into gas. They are Endothermic
3) Then there is the 1st IE. This is at the top of the cycle and it is endothermic
4) Then you bring an arrow down for the 1st Electron Affinity. It is exothermic.
5) Then there is the lattice enthalpy of formation all the way down to the solid at the start. This is also exothermic
Ps if there is a second EA - It is endothermic and should have an upwards arrow
How can theoretical lattice enthalpies be calculated?
They can be calculated from data assuming a perfectly ionic modelW
What is a perfectly ionic model?
- Ions that are perfectly symmetrical
- The charge is evenly distributed in the sphere
Why is the experiment value different to the theoretical value?
This is not uncommon at all. It just tells us that the compound doesn’t follow the perfectly ionic model and has some covalent characteristics. Most of the time we say that the + ion distorts the charge distribution in the - ion. We say the positive ion polarises the negative ion. The more polarisation we get, the more covalent character there will be.
Experimental value always shows a higher lattice enthalpy than the purely ionic theoretical value. This shows some covalent character is being displayed
Polarisability
Smaller cations are more polarising than larger ones. They have a high charge density as the charge is concentrated in a smaller area. The cation pulls electrons toward itself more readily.
Large anions with a large charge are polarised much more easily than small lower charged anions. This is as the electrons are further away from the nucleus and there will be more repulsion between the electrons in the ions.
What is electronegativity?
It is the ability of an atom to attract electrons towards itself in a covalent bond
The higher the difference in EN, the more ionic the compound will be. a difference of 0 is purely covalent
