Spring 2011 Lec 1 A # 12 Physical Properties & Chemical Properties: how to analyze them, what do they mean, and how not to get confused as to what influences each property.

Well first, the question is asking you to give two` distinct reasons why HOCOOH is a stronger acid than water.

or So I think I understand the trouble you are running into. This seems like it could be an easy mistake for me or anyone else, but we dont want to get how we determine non-covalent molecular forces from structure and determine their effect on the overall strength of intermolecular with how structure controls acidity or basicity. I made this visual down below it is trying to show you what influences the physical properties which are the forces between the atoms in the molecule (intra) and the forces between neighboring molecules (intermolecular) Then I have tried to go over what controls acidity and basicity. Before reading this visual thing, here are some important things to consider (in my opinion) When talking about chemical reactions we are now dealing with how molecules react ~ what they form, products vs. reactants And how they interact with each other, what might be the stronger acid or base? In which direction does the equilibrium lie? What products are most stable and why? So when we are asked like in this question why HOCOOH is a stronger acid than water we must consider what is produced in the reaction of each and then relate this to strength of acid which is directly related to stability as shown in this picture.

produce any more less stable

Strong acid

So in the backwards reaction we dont

less stable acid

the conjugate base is more stable can acoomadate electrongs better

has a weak conjugate base

Structure of a molecule ( usually given ) What can we say about the intial molecule, we arent changing anything ? look at the :hybridization, structure , planarity anything of that sort , maybe look if there is some resonancePhysical Properties :Melting point, boiling point, solubitiy, ect This tells us strength of molecules interacting NOTE: while a highly stable molecule might have a low boiling point this doesn't mean it has a low or high bascity. Don't mix that up

First, Intramolecular forces or forces between atoms in the molecule : Cation-anion (ionic bonds ) & Covalent bonds

Intermolecular forces : These include Dipole-dipole , hydrogen bonding, ion-dipole, cation-pi, pi stacking and london forces. Please note that this concept of intermolecular forces dominating strucural properties is dominated by the idea of electrostatic attractive forces and their strength.Resonance: Then in relation to chemical reactions Resonance results in electron deolocatization which leads to minimization of electron-electron interactions. Some times in a chemical reaction there is stability or improved resonance maybe even a possibiltuy of attaining conjugation from losing a H or better said acting as an electrophole in the case of an acid base chemical reaction So more resoannce if beneficial for electron accomidation = stronger acid, less stable acid, higher Keq lower PKa produces more products while is a weaker more stable conjugate base

1) Resonance Ask your self does this molecule have resonance? Yes ..... Is it beneficial to the molecule to lose a atom ( such as hydrogen )? (or gain an atom in some cases very rare though) How does the molecule accomodate electrons?

Chemical properties :How a molecule will interact, to what extent, what will the products be Lets talk acid and bases

2) Atomic radiusWhen analyzing atomic radius, this only applies to the atoms in the molecule/ strucure that are directly attached to where the proton, or when electrons will be transfered to . So, Larger atomic radius better electron density accomidation due to sheilding of pull from nucleus by other electrons , hence good electron accomidation . The larger the atomic radius ( of atom which would lose or gain proton or electron withdrawing group) the more likley the molecule that the atom in question is in would be a stronger acid , producing a weaker but more stable conjugate base ( this base is able to accomodate electrons ) .

3) electronegativity effects :The more electronegative an atom is, the better it accomodates more electrons around it. (don't forget we must also firstly consider atomic radius) Keeping this in mind, consider our structure and we are looking at , consider the acidicty and basicity the more electronegative the atom in the molecule that supplies the electron pair to the proton it is less likely that it will need that proton attached to the acid to accomodate electron . Therefore the higher the EN of that atom it is more likely for the molecule to lose that proton up and function as a stronger acid and produce higher amounts of a WEAKER but MORE stable conjugate base.

Acids :Stronger acids, lessstable, produce more conjugate base,

Conjugate base of strong acid: accomodates electtons well, is a WEAK base, but has good stability The oppisite goes for Bases Resonance