Friday, February 11, 2011

Here is the list of most recent questions concerning Periodicity of Elements. This relates to syllabus sections A6.1 - A6.4.

Thursday, January 20, 2011

Electronegativity And Its Effect On Bond Type

The summary at your level is that:

1.       No electronegativity difference between two atoms results in a non polar covalent bond.
2.       Slight electronegativity difference between two atoms results in a polar covalent bond.
3.       Large electronegativity difference between two atoms results in an ionic bond.

In general, situation 1 above involves two identical non metal atoms, situation 2 involves two non identical non metals and situation 3 involves a metal and a non metal.

References differ in regard to what bond types are denoted by electronegativity differences:

Reference 1 electronegativity difference ranges:

EN = 0.0                                -              Non polar covalent
0.0 < EN < 2.0                      -              Polar covalent
EN > 2.0                                -              Ionic

Reference 2 electronegativity difference ranges:

0.0 < EN < 0.6                     -              Non polar covalent
0.6 < EN < 1.8                     -              Polar covalent
EN > 1.8                               -              Ionic

Reference 2 is more in line with what you will have to know at this point.
So the question that will arise, is do you have to learn electronegativity values for CSEC exams. The answer is no. It will be sufficient to know that:

1.       The most electronegative elements are non metals on the top right hand side of the periodic table.
2.       The least electronegative elements are metals on the bottom left hand side of the periodic table.
3.       Electronegativity increases from left to right across a period.
4.       Electronegativity increases as you ascend a group.

These are, of course general rules. Exceptions will be met at CAPE (See group 4).

Friday, January 14, 2011

Salting Out - Soap Manufacture

This one is for people who want a little more details on the use of salt in soap making. Many students had trouble explaining it in the June 2010 exam. Below is the resason why addition of salt causes soap to be deposited from the reaction mixture. Solubility product is not required at CSEC, but shows up in Module 2 of Unit 1 CAPE Chemistry.

Salting out is a term often used in industry for removing salt from a solution. An example occurs in the manufacture of soap. The chief constituent of soap is sodium stearate, C17H35CO2Na – otherwise known as sodium octadecanoate. It is salted out by adding a concentrated solution of sodium chloride. This causes the product of the stearate and sodium ion concentrations to exceed the solubility product of sodium stearate and sodium ion concentrations to exceed the solubility product of sodium stearate. In this case the salting out is due to the common ion effect. The common ion is sodium, Na+.

Tuesday, January 11, 2011

Handling Bonding Questions


Dot and cross diagrams for ionic compounds.

Do I show the actual electron transfer process?
If you have the space, do so, because usually CXC does not ask for it but want it anyway.

Do I show the inner electrons of ions?

For a cation, do I show the 8 outer electrons or do I show none?
Any one is fine, but CXC likes the version with no outer cation electrons.

How do I represent multiple cations or anions, e.g. in MgCl2?
Either with a number in front of the ion or by drawing the ion multiple times.

Dot and cross diagrams for covalent compounds.

These diagrams require that:
1.       Outer electrons only must be shown.
2.       Shared electrons are in the region of overlap.
3.       Circles to represent outer shells are to be used.
Note that in many textbooks, the circles are not shown. Although omission of the circles to represent outer shells will not result in lost marks, please include them in all of your covalent dot and cross diagrams.

Deciding what type of bonding occurs in different substances.

When given different named elements:

Metal and non metal                             :               ionic bonding e.g. Na and Cl to form NaCl .
Non metal and non metal                      :               covalent bonding e.g.  H and Cl to give HCl.

If elements are the same:
Metal                                                  :               metallic bonding e.g. Na.
Non metal                                           :               covalent bonding e.g. Cl and Cl to give Cl2 and C to give
                diamond or graphite.

Note that if we have two different metals they do not undergo metallic bonding e.g. Na and Mg. This question should not even arise at the Form 4 to Form 5 level.
If we are given unnamed elements, e.g. ‘Element X’ or A the first step is to deduce whether they are metals or non-metals. This is achieved by writing the electronic configuration of that element from either the number of electrons, or protons (which are both the same for an atom not yet bonded). Remember that metals will have 1 to 3 outer electrons. Non metals have 4 and more outer electrons.
After this has been done, we can proceed with the method above to determine type of bonding, and even formulae. While working on scrap, it is usually possible to figure out what actual element is represented by the unknown. For example, element A, having a proton number of 11 or an electronic configuration of 2.8.1 is sodium. This knowledge simplifies the rest of the question.

To read even more on this topic, go to:

Monday, January 10, 2011

Bonding Comes Almost Every Year

The topic of Structure and Bonding which runs from 4.1 to 4.8 in Section A of the current syllabus is one which comes almost every year in the written papers. In fact, in the last 13 years of June exams, it has come 11 times. Here is a list of questions on this topic for you to work out:

Thursday, January 6, 2011

June 2009 P2 Q2 - Solution

2(a)(i)    Solid, liquid and gas.
2(a)(ii)   Melting point – A.
              Boiling Point – B.
2(a)(iii)  A mixture of water and ice.
2(a)(iv)  Water vapour to water – condensation.
              Iodine (s) to iodine(g) – sublimation.
2(b)(i)   An allotrope is a crystalline solid form of an element having a different   structure from
             another solid form of the same element.
2(b)(ii)  Conducts electricity – Graphite.
             Explanation – There are delocalised electrons in graphite.
2(c)(i)  Crude oil.
2(c)(ii)  Structural isomerism is the existence of compounds with the same   molecular formula 
             but different structural formulae.