Bond Strength And Electronegativity Flashcards
What is bond dissociation enthalpy and what is it always?
It is a measure of the strength of a chemical bond
It is always endothermic and always in gaseous state
Why can’t exact bond dissociation enthalpy changes only be used for molecules greater than 2 atoms?
Because in the case of water, the two successive dissociations have different enthalpies
Averages have to be used instead
What relation does the bond dissociation enthalpy and enthalpy of atomisation of water have in common?
The average bond dissociation enthalpy is = to half the enthalpy of atomisation - 924 / 2 = 462
(Splitting water into its constituent atoms - gaseous)
Define the enthalpy of atomisation
Refer to the enthalpy change that accompanies the separation of 1 mole of compound into its constituent atoms in the gas phase
How do you find the number of bonds per atom in a compound / lattice?
Divide the number of bonds by the number of atoms it links.
What are the bonding trends down groups in the p-block?
Bond distance increases down groups because atomic radii increases
Bond enthalpy decreases down the groups because of decreasing orbital overlap - weaker bonds
What experimental data can be used to see the leap in atomic radii between periods 2 and 3?
A graph showing bond enthalpies of group 14 elements - all with diamond-type structure.
There’s a large distance between C and Si which reflects this leap in atomic radii between periods 2 and 3 - bonds are weaker with less overlap
Why does metallic character of p-block element increase down the groups?
Because valence electrons are less tightly held
Hence bonding becomes less directional and electrons more delocalised
Why does pi bonding mainly occur between atoms of the first row of the p-block?
Because the atoms are sufficiently small so that their p-orbitals can overlap effectively
Why are O-O and N-N single bonds considerably weaker than a C-C bond?
Because the small atoms bring their lone pairs in close contact - so repel each other away
What’s a consequence of the considerably N-N and O-O single bonds?
A consequence of this is that compounds containing these bonds are particularly reactive
Example - hydrogen peroxide
H - O — O - H which is prone to decombustion
What drives the decombustion of hydrogen peroxide?
The formation of the O=O double bond - being more than twice as strong as a O-O single bond
Hence exothermic
Why is the formation of ethylene exothermic?
Because a C=C double bond is less than twice as strong as a C-C single bond
Hence the polymerisation of ethylene is exothermic
Why is nitrogen a gas but phosphorus a solid at ambient conditions?
Because nitrogen forms very strong N=N triple bonds - hence nitrogen consists of small non-polar N2 molecules
In contrast, pi bonds in phosphorus is weaker due to less pi overlap between the larger atoms - hence phosphorus atoms tend to make 3 single bonds to other P atoms
This results in a polymeric solid.
What does the enthalpy of reaction of HCl being exothermic suggest?
It indicates that the polar HCl bond must be stronger than the average strength of the non-polar H2 and Cl2 bonds.
