Monday, 9 May 2011

Filtration

Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Oversize solids in the fluid are retained, but the separation is not complete; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size and filter thickness). Filtration is also used to describe some biological processes, especially in water treatment and sewage treatment in which undesirable constituents are removed by adsorption into a biological film grown on or in the filter medium.

Diagram of simple filtration: oversize particles in the feed cannot pass through the lattice structure of the filter, while fluid and small particles pass through, becoming filtrate.

Procedure for standard gravity filtration

1) Select and fold the filter paper

Select the size of filter paper that, when folded, will be a few millimeters below the rim of your glass funnel. Fold the paper into a cone by first folding it in half, and then in half again, as shown.

2) Filter the solution

Support the glass funnel in a ring or place it in the neck of an Erlenmeyer flask. Wet the filter paper with a few milliliters of the solvent to be used in the following procedure. Wetting the paper holds it in place against the glass funnel. Pour the mixture to be filtered through the funnel, in portions if necessary.

limiting reagent

In a chemical reaction, the limiting reagent, also known as the "limiting reactant", is the substance which is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent since the reaction cannot proceed further without it. The other reagents may be present in excess of the quantities required to react with the limiting reagent.

The limiting reagent must be identified in order to calculate the percentage yield of a reaction, since the theoretical yield is defined as the amount of product obtained when the limiting reagent reacts completely.

Given the balanced chemical equation which describes the reaction, there are several equivalent ways to identify the limiting reagent and evaluate the excess quantities of other reagents.

shielding effect

The screening effect, or shielding effect, is how electrons in the same atom interact with each other. In a single-electron atom (in isolation), the electron is only interacting with the proton; in a multielectron atom, the electrons are both interacting with the proton(s), but also with each other. While electrons are attracted to the protons in the nucleus, they are repelled by the other electrons. This electron-electron repulsion decreases the attractive force of the protons on the electrons.

The shielding effect changes the effective nuclear charge -- effectively decreasing the true nuclear charge. This effect causes atoms to get smaller as you across a period (row) of the periodic table, as well as many other periodic trends observed in the periodic table.

Saturday, 7 May 2011

Mole Fraction

The mole fraction of a solution component X_i is the fraction of moles of component i of the total number of moles of all components in solution.

moles of component i

X_i

------------------------

total moles of solution

Example problem
What is the mole fraction of each component in a solution in which 3.57 g of sodium chloride, NaCl, is dissolved in 25.0 g of water?
Solution
First, convert from mass of NaCl to moles of NaCl.

		1 mole NaCl
3.57 g NaCl	x	---------------------	=	0.0611 mole NaCl
		58.44 g NaCl

Next, convert from mass of water to moles of water.

		1 mole H₂O
25.0 g H₂O	x	---------------------	=	1.39 mole H₂O
		18.02 g H₂O

Substitute these two quantities into the defining equation for mole fraction.

		0.0611 mol NaCl
X_NaCl	=	--------------------	=	0.0421
		(0.0611 + 1.39) mol solution

		1.39 mole H₂O
X_water	=	--------------------	=	0.958
		(0.0611 + 1.39) mol solution

Note that within the limits of the significant figures that X_NaCl + X_water = 1. The sum of the mole fractions for a solution will equal 1. In our example above, after we have calculated one mole fraction we could have subtracted it from 1 to obtain the other.

Normality

When you need to compare solutions on the basis of concentration of specific ions or the amount of charge that the ions have, a different measure of concentration can be very useful. It is called normality.

We will deal with normality more completely in acid-base titration.The normality of a solution is simply a multiple of the molarity of the solution. Generally, the normality of a solution is just one, two or three times the molarity. In rare cases it can be four, five, six or even seven times as much. The symbol for normality is N or N.

CaCl₂

Ca²⁺ + 2 Cl^-

1 M CaCl₂ = 2 N CaCl₂

Molality

Molality (m) of a solution is a ratio giving the number of moles of solute per kilogram of solvent.

Molarity

Molarity is the number of moles of solute dissolved in one liter of solution. The units, therefore are moles per liter, specifically it's moles of solute per liter of solution.

atoms and moleucles

Atoms

An atom is the smallest particle of an element that has the properties of that element.

There have been various models that have been proposed to describe the structure of the atom.

Billiard ball model - John Dalton
Plum pudding model - J.J. Thomson
Nuclear model - E. Rutherford
Shell model of the atom - N. Bohr
The quantum model of the atom - various scientists.

The atom consists of three subatomic particles. The proton, the neutron and the electron.

In a chemical reaction all the action takes place in the outer or valence shell of electrons.

Molecules

What is a molecule? A molecule is a group of atoms that joined together by a covalent bond.

chemical formula

The use of chemical formula is a shorthand way of showing

the number and type atoms in a compound and
the number of atoms in a molecular element The chemical symbols of the elements are shown in the Periodic table. For example the element hydrogen is given the chemical symbol H and the element oxygen the chemical symbol O.
In a chemical formula the chemical symbol of each element is shown with subscript numbers which tell us the numbers or ratio of atoms in the compound or molecular element.
For example the compound water has the chemical formula H₂O. This tells us water is made up of two elements, hydrogen and oxygen. The subscript 2 in H₂O tells us there are two parts hydrogen to one part oxygen. Note: The subscript 1 is never but taken for granted. That is why water has the formula H₂O and not H₂O₁
Some elements also exist as molecules. Molecules are groups of atoms joined together by chemical bonds. Molecular elements like compounds also have a chemical formula. Examples of molecular elements include hydrogen, H₂ and ozygen O₂.
Hydrogen atoms are too unstable to exist by themseleves as free atoms. They pair up to with other hydrogen atoms to form hydrogen molecules which are much more stable. The chemical formula of the hydrogen molecule is therefore H₂. This means two hydrogen atoms are joined together. The molecule O₂ is also much more stable than single oxygen atoms.
The combining power or valency of an element determines the ratio that atoms will combine with one another. A knowledge of valencies can help us predict the chemical formula of a substance. Atoms achieve a stable outer shell of electrons by transferring or sharing electrons. They always combine with one another n simple whle number ratios.

radioactivity

Radioactive decay is the process by which an excited, unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves, thereby transitioning toward a more stable state.

The atomic nucleus comprises certain combinations of protons and neutrons held in a stable configuration through a precise balance of powerful forces: The strong force holding the protons and neutrons together is powerful but very short range; the electrostatic repulsion of the positively charged protons is less powerful but long range; the weak force makes the neutron inherently unstable and will turn it into a proton if given the chance. This balance is very delicate: a uranium-238 nucleus has a half-life of 4.5 billion years while uranium-237 with just one less neutron has a half-life of 1.3 minutes.

If there is an imbalance in these forces, the system will eventually shed the excess by ejecting radiation in some combination of particles and wave energy. The most common radioactive decays occur in response to one of three possible types of imbalance. If the nucleus has too many neutrons, one of its neutrons decays (through beta decay) into one proton plus two fragments ejected from the nucleus, a neutrino and an electron (called a a beta particle). If the nucleus has too many protons, it undergoes alpha decay by ejecting two protons and two neutrons as an alpha particle. If the nucleus is excited (has too much energy) it ejects a gamma ray.