pH Curves in 60 seconds

pH curves are inherently hard things to get your head around but once you do getting it is very worthwhile, so here is my 1 minute guide to what working them out...

1. Shape - Think of each curve as made up of 2 half curves and there are 4 possible half curves (weak acid, strong acid, strong base, weak base), the shapes will either be almost L shaped (Strong) or almost fallen over S shaped (weak) and either high (base) or low (acid). So, when drawing (or describing) a curve just think, what is in the conical flask before I started titrating and draw the half curve for that first, then think what am I adding to it from the burette and draw that sort of half curve next.
2. Intial pH - is the pH in the conical flask before you added anything
3. Estimate of final pH - this will be very close to the pH of whatever was in the burette but reduced a bit (if its a base) or increased (if its an acid)
4. Equivalence volume - Volume it gets v steep
5. Equivalence pH - Middle of v steep section
6. Getting Ka from graph - At 1/2 equivalence volume the Ka = H+ (or pKa = pH)
7. Where is there a buffering effect? - If you have a weak acid or a weak base, in the middle of the half curve for this solution the curve flattens for a bit, this is the buffering effect, this happens because there is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid)

59, 60. Done

Naming Multifunctional Organic Molecules

If you are naming molecules with multiple functional groups your first priority is to keep the numbers low and then when you write the name you put them in alphabetic order.

So for example this molecule...

...is called,  3-Chloro-1-Iodo butane

This is right because

- it keeps the numbers low (the alternative is "2-chloro-4-Iodo", i.e. 1,3 is lower than 2,4)

- and we have put it in alphabetic order

There is a more complicated rule that groups have different priority based on their Mr but you don't really need to know that,

The other fact it is worth knowing is that if you have a functional group that must be on the end, mainly aldehydes and carboxylic acids, you wouldn't number these because they have to be on the end (obvs) but then when you put other functional groups on you can't start numbering these "end only" functional groups because that would make no sense, it which case you always assume that the "end only" functional group is at 1 and number from there and this overrides the rule about keeping the numbers low.

So for example... 

...is called, 4-Chloro-3-Methyl Butanoic Acid

NOT

1-Chloro-2-Methyl Butan-4-oic Acid

Even though the numbers are kept lower on the alternative name.

The only other thing worth remembering is that of you have more than one of a functional group it becomes, for example, 1,3-dimethyl or 1,3,3-trimethyl and then when you are alphabetising them you go by the "d" or "t" not "m" e.g. it would be "1,3-dimethyl-4-ethyl....." not "4-ethyl-1,3-dimethyl-....."

Isn't It Ionic?

Ionic or Covalent? What is electronegativity?

This is a fairly confusing area of A level Chemistry (Alanis Morissette certainly found it hard) so I thought I would put together a blog post to explain it.

Think of it this way, we need to measure the ability of atoms to draw electrons towards them (i.e. electronegativity) the only way of doing this is on a comparative scale, that is, compared to other atoms. Then all somebody did (the man who did it was called Linus Pauling) was to give this scale values. The reactivity series from GCSE is the same principle except that nobody has ever bothered to give reactivities values so it is still just a list.

Anyhow, if you are going to compare atoms electronegativity (and therefore assign values) you need to get in to a situation where two atoms are both trying to pull electrons towards them and see which one has the bigger pull.

The only place this happens is in a covalent bond. In a covalent bond the electrons are shared between the two atoms but the electrons will spend more time with the atom that has the greater pull and therefore we say that atom is more electronegative (than the other one it is bonded to).

So, by comparing the covalent bonds between every possible pair of atoms you get a hierarchy of pulling ability (electronegativity) which Linus Pauling assigned numbers to. On Pauling's scale, Fluorine is the highest (best at pulling electrons) and is assigned a value of 4.0 and Francium is the lowest (poorest at pulling electrons) at 0.7. All of this measurement is done by comparing pulling ability in covalent bonds.

So the definition has to be something along the lines of...

"Electronegativity is a chemical property that describes the tendency of an atom to attract electrons towards itself in a covalent bond".

So, "How can compounds that form ionic bonds have electronegativities?" I hear you ask.

Electronegativity is a property of an element, if the value is high (e.g. Chlorine, 3.2) the element will form covalent bonds with other atoms of high electronegativity, because they are both pulling really hard on the electrons and so end up sharing them, but will form ionic bonds with atoms of low electronegativity (e.g. Na, 0.9) because the chlorine has so much of a bigger pull on the electrons compared to the sodium that it ends up pulling the electrons towards itself completely and forming a negative ion, whilst the sodium loses its electron and ends up being a positive ion.

So, don't think of electronegativity as happening "in bonds". It is a property that controls what sort of bonds an atom will form.

The only loose end to tie up is how do we know sodium's electronegativity if it never forms covalent bonds (pure sodium forms metallic bonds and sodium compounds have ionic bonds). The answer to this is that even though sodium doesn't naturally form covalent bonds, in a lab it can be made to share electrons (i.e. form a covalent bonds) with other atoms so that its electronegativity can be measured.

My analogy for all this is tug of war competitions, I won't expand on it now but it does work pretty well.

....and in future Alanis, just buy less spoons.