Chemical Bonding

Question 1

What are the steps for forming an ion pair?

Describe what is happening in this energy level diagram.

Answer 1

1. Sublimation enthalpy ↑
2. Ionization enthalpy ↑
3. Atomisation enthalpy ↑
4. Electron affinity ↓
5. Lattice enthalpy ↓
   1) Working with naturally occurring lithium and fluorine. But we can’t just blow flourine gas onto the lithium and make stuff. So first you make your lithium into a gas. Solid directly to gas is called sublimation, which takes energy, endothermic, which is why the energy is going up. (Going from a more condensed phase to a less condensed phase is always endothermic.)
   2) Then you make lithium into a cation, the ionization enthalpy.
   3) atomisation breaks fluorine into a single F atom.
   4) Electron affinity - add an electron to fluorine to make it fluoride. Negative down arrow because it’s more stable to have fluroide than F with an electron hanging around.
   5) The last down arrow is the Coulomb energy, which is actually the coulomb energy of all the extended interactions in the lattice (since it is solid).

Question 2

What is lattice enthalpy?

Answer 2

• This extended structure gives significant stability
• Calculate using coulomb’s law
  
  • E_coulomb=k(charge)(charge)/d (d is the internuclear distance, k is a constant we’re given)

Question 3

Tips for Lewis structures:

Answer 3

• Carbon tends to like to have 4 bonds.
• Nitrogen tends to like 3 bonds and a lone pair
• Oxygen likes to have 2 bonds and 2 lone pairs.
• When you have a hydrocarbon that is saturated with hydrogens, put them all around.

Question 4

What is formal charge, how do you calculate it?

Answer 4

• Formal charge is a way of counting the electrons around one of the symbols.
• You calculate a formal charge by taking the # of VE in the atom (group number) and subtract the # of electrons in lone pairs and the # of bonds.
• Want the formal charge on each individual atom to be 0 if possible, or really low and cancel out.

Question 5

What are resonance structures, equivalent and nonequivalent?

Answer 5

• Equivalent resonance structure - Same type of bonding. The bond just moves around.
  
  • Ex: the double bond is just on another oxygen atom. Same formal charge
• Nonequivalent structure. Different way to represent where electrons could be
  
  • Different formal charge. Either the charge is on a different atom, or there is a different number of formal charges.

Question 6

Lewis structure for CO₂

Answer 6

Question 7

Lewis structure for NH₃

Answer 7

Question 8

Lewis structure for CH₄

methane

Answer 8

Question 9

Resonance structures for C₆H₆ (benzene)

Answer 9

Question 10

Trend for electronegativity

Which element is the most electronegative?

Answer 10

• Fluorine is the most electronegative
• Increases across and up (just like electron affinity and ionization energy)

Question 11

ionization energy vs. electron affinity vs. electronegativity

Answer 11

• They follow the same trend on periodic table
• Ionization energy: the energy required to remove an electron from a neutral gaseous atom and make it a positive ion
  
  • Reflects how tightly the e- is held by the atom
• Electron affinity: the energy change associated with adding an electron to a gaseous form of an atom. Forms an anion.
  
  • Usually exothermic ↑ (endothermic if noble gas, since it’s hard to add another e-)
• Electronegativity: the tendency of an atom in a molecule to draw bonding electrons to itself.
  
  • For covalent compounds, one element more greedy
  • Assign it more formal charge in a Lewis structure

Question 12

Energy of a bond diagram

Describe what’s going on!

Answer 12

• At some partial overlap of the orbitals, that is the most stable bond. Attraction, negative energy, stable.
• If you tried to overlap even more, you’d have repulsion between protons in the nuclei, more positive energy, less stable.
• This also relates to coulomb’s law

Question 13

What is the relationship between bond order, strength and length?

Answer 13

• Bond order comes from whether it is a single (1) double (2) or triple (3) bond
• Higher bond order means stronger bond, shorter bond length
• The stronger the bond, the higher the bond enthalpy

Reminder:

• The stronger the bond, the more energy is released during the bond formation process, and the more energy it takes to break the bond (higher bond enthalpy)
  
  • Bond formation = exothermic
  • Bond breaking = endothermic

Question 14

Lewis structure for acetic acid

CH₃COOH

Answer 14