Wednesday, January 20, 2016

Chlorine Bleach

Hypochlorous acid, HClO, forms when chlorine dissolves in water. In this reaction, half of the chlorine is oxidized to hypochlorite ion and half is reduced to chloride ion in a disproportionation reaction.

Cl2(g) + 2H2O(l) H3O+(aq) + HClO(aq) + Cl(aq)

If Cl2 is dissolved in cold aqueous NaOH instead of in pure water, hypochlorite ion and chloride ion form.

Cl2(g) + 2OH(aq) ClO(aq) + Cl(aq) + H2O(l)

Under basic conditions, the equilibrium lies far to the right. The resulting alkaline solution is the “liquid bleach” used in home laundries. The bleaching action of this solution is a result of the oxidizing ability of ClO. Most dyes are colored organic compounds, and hypochlorite ion oxidizes dyes to colorless products.

When calcium hydroxide is combined with Cl2, solid Ca(ClO)2 is the product. This compound is easily handled and is the “chlorine” that is sold for swimming pool disinfection.
This information is relevant to the January 2016 P2 and can be used to answer Q 2(a).

Wednesday, October 28, 2015

Sources Of Lead Pollution

Yesterday in class, we worked the January 2008 P2. Question 5(b)(ii)(a) asked for two sources of lead as a pollutant.

In the past, lead was cited as coming from:

1. Combustion of leaded fuels.

2. Lead pipes.
3. Paints containing lead.
4. Lead batteries (improper disposal/recycling/manufacture).

Since 1 - 3 have been all but phased out in the region, what would a student write as a second source of lead.

Currently, lead pollution worldwide comes from include smelters and electric utilities. Three years ago, a study in Japan added plastics to the list. The material which follows is taken from:

Quantification of Toxic Metals Derived from Macroplastic Litter on Ookushi Beach, Japan

Center for Marine Environmental Studies, Ehime University, 2-5, Bunkyo-cho, Matsuyama, 790-8577, Japan
Environ. Sci. Technol.201246 (18), pp 10099–10105
DOI: 10.1021/es301362g
Publication Date (Web): August 23, 2012
Copyright © 2012 American Chemical Society

"The potential risk of toxic metals that could leach into a beach environment from plastic litter washed ashore on Ookushi Beach, Goto Islands, Japan was estimated by balloon aerial photography, in situ beach surveys, and leaching experiments in conjunction with a Fickian diffusion model analysis. Chromium (Cr), cadmium (Cd), tin (Sn), antimony (Sb), and lead (Pb) were detected in plastic litter collected during the beach surveys. Polyvinyl chloride (PVC) fishing floats contained the highest quantity of Pb. Balloon aerial photography in conjunction with a beach survey gave an estimated mass of Pb derived from plastic litter of 313 ± 247 g. Lead leaching experiments on collected PVC floats showed that Pb in the plastic litter could leach into surrounding water on the actual beach, and that plastic litter may act as a “transport vector” of toxic metals to the beach environment. Using the experimental data, the total mass of Pb that could leach from PVC plastic litter over a year onto Ookushi Beach was estimated as 0.6 ± 0.6 g/year, suggesting that toxic metals derived from plastic beach litter are a potential “pathway” to contamination of the beach environment due to their accumulation in beach soil over time."

The highest metal concentrations were of lead, which they estimated made up more than 300 g of the 500 kg of plastic trash on Ookushi Beach. Most of the lead came from PVC; manufacturers often use the metal to stabilize PVC.

So now, we can safely use PVC as 

a source of lead pollution.

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: