Chemistry

Chem

I check my previous email today and i found this email :))
teacher saya nak tny soalan may june 2007
soalan no 12
Which chlorine compound that can be described as ionic with some covalent character?
A.NaCl
B.MgCl2C.AlCl3D.SiCl4
the answer is B.. kenape bukan C teacher??saya confius?? @_@
soalan 36

The element astatine lies below iodine in Group VII of the Periodic Table
What will be the properties of astatine

1.It forms diatomic molecules which dissociate more readily than chlorine molecules.
2.It reacts explosively with hydrogen
3.It is a good reducing agent.
the answer is 1 only.. knape astatine is more readily dissociate teacher???

then my teacher answer me this

Q1. AlCl3 is principally covalent meaning its covalent nature > its ionic nature.The question ask for ionic compound with some covalent nature that is a compound which is more ionic than covalent in nature,that's why the answer is MgCl2


Q2. Reactivity of group 7 elements decreases down the group. So astatine being lower than I2 in the group would not react violently (explode) with H2.
As the atoms get bigger,the covalent bonds between the atoms in At2 will be weaker that's why the molecules At2 can dissociate readily
Astatine cannot be a reducing agent (readily donates electrons). It is the astatide ions that behave as reducing agent that can reduce Br2 to bromide ion and it itself is oxidised to At2

I hope that it'll help u.. :D







Today i'll write about titration

specifically about indicator

Choosing indicators for titrationsRemember that the equivalence point of a titration is where you have mixed the two substances in exactly equation proportions. You obviously need to choose an indicator which changes colour as close as possible to that equivalence point. That varies from titration to titration.


Strong acid v strong base

From the graph it shows that:

-a Sharp fall in graph line between pH 10.5 and pH 3.5

The sharp pH change occurs over wide pH range,so there are MANY indicators can be used. such as Phenolpthalein, methyl orange, Bromothymol blue



Strong acid v weak base

From the graph it shows that:

-a Sharp fall in the graph line between pH 7.5 and pH 3.5
-Choose indicator with ACIDIC working range eg. methyl orange

This time it is obvious that phenolphthalein would be completely useless. However, methyl orange starts to change from yellow towards orange very close to the equivalence point.
You have to choose an indicator which changes colour on the steep bit of the curve.




Weak acid v strong base

From the graph it shows that:

-A sharp fall in the graph between pH 11 to pH 7.5
-Choose indicators with BASIC working range eg. phenolphthalein

This time, the methyl orange is hopeless! However, the phenolphthalein changes colour exactly where you want it to.

Weak acid v weak base
The curve is for a case where the acid and base are both equally weak - for example, ethanoic acid and ammonia solution. In other cases, the equivalence point will be at some other pH.

From the graph it shows that:
-the equivalence point of the graph is not clear

You can see that neither indicator is any use. Phenolphthalein will have finished changing well before the equivalence point, and methyl orange falls off the graph altogether.
It may be possible to find an indicator which starts to change or finishes changing at the equivalence point, but because the pH of the equivalence point will be different from case to case, you can't generalise.
On the whole, you would never titrate a weak acid and a weak base in the presence of an indicator.