Distillation Process

Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points.

Therefore, when this vapour is cooled and condensed, the condensate will contain more volatile components. At the same time, the original mixture will contain more of the less volatile material.

Distillation columns are designed to achieve this separation efficiently.

Although many people have a fair idea what “distillation” means, the important aspects that seem to be missed from the manufacturing point of view are that:

	distillation is the most common separation technique
	it consumes enormous amounts of energy, both in terms of cooling and heating requirements
	it can contribute to more than 50% of plant operating costs

The best way to reduce operating costs of existing units, is to improve their efficiency and operation via process optimisation and control. To achieve this improvement, a thorough understanding of distillation principles and how distillation systems are designed is essential.

voltaic cell

Voltaic Cells

A Voltaic Cell (also known as a Galvanic Cell) is an electrochemical cell that uses spontaneous redox reactions to generate electricity. It consists of two separate half-cells. A half-cell is composed of an electrode (a strip of metal, M) within a solution containing Mⁿ⁺ ions in which M is any arbitrary metal. The two half cells are linked together by a wire running from one electrode to the other. A salt bridge also connects to the half cells. The functions of these parts are discussed below. 

Half Cells

Half of the redox reaction occurs at each half cell. Therefore, we can say that in each half-cell a half-reaction is taking place. When the two halves are linked together with a wire and a salt bridge, an electrochemical cell is created.

Electrodes

An electrode is strip of metal on which the reaction takes place. In a voltaic cell, the oxidation and reduction of metals occurs at the electrodes. There are two electrodes in a voltaic cell, one in each half-cell. The cathode is where reduction takes place and oxidation takes place at the anode. The figures below illustrate a cathode and an anode.

Through electrochemistry, these reactions are reacting upon metal surfaces, or electrodes. An oxidation-reduction equilibrium is established between the metal and the substances in solution. When electrodes are immersed in a solution containing ions of the same metal, it is called a half-cell. Electrolytes are ions in solution, usually fluid, that conducts electricity through ionic conduction. Two possible interactions can occur between the metal atoms on the electrode and the ion solutions.

1. Metal ion Mⁿ⁺ from the solution may collide with the electrode, gaining "n" electrons from it, and convert to metal atoms. This means that the ions are reduced.
2. Metal atom on the surface may lose "n" electrons to the electrode and enter the solution as the ion Mⁿ⁺ meaning that the metal atoms are oxidized.

When an electrode is oxidized in a solution, it is called an anode and when an electrode is reduced in solution. it is called a cathode.

Anode

The anode is where the oxidation reaction takes place. In other words, this is where the metal loses electrons. In the reaction above, the anode is the Ag(s) since it increases in oxidation state from 0 to +1.

Cathode

The cathode is where the reduction reaction takes place. This is where the metal electrode gains electrons. Referring back to the equation above, the cathode is the Cu(s) as it decreases in oxidation state from +2 to 0. 

Remembering Oxidation and Reduction

When it comes to redox reactions, it is important to understand what it means for a metal to be “oxidized” or “reduced”. An easy way to do this is to remember the phrase “OIL RIG”.

OIL = Oxidization is Loss (of e^-)

RIG = Reduction is Gain (of e^-)

In the case of the example above Ag⁺(aq) gains an electron meaning it is reduced. Cu(s) loses two electrons thus it is oxidized.

Salt Bridge

The salt bridge is a vital component of any voltaic cell. It is a tube filled with an electrolyte solution such as KNO_3(s) or KCl_(s). The purpose of the salt bridge is to keep the solutions electrically neutral and allow the free flow of ions from one cell to another. Without the salt bridge, positive and negative charges will build up around the electrodes causing the reaction to stop.

Flow of Electrons

Electrons always flow from the anode to the cathode or from the oxidation half cell to the reduction half cell. In terms of E^ocell of the half reactions, the electrons will flow from the more negative half reaction to the more positive half reaction.

Cell Diagram

A cell diagram is a representation of an electrochemical cell. The figure below illustrates a cell diagram for the voltaic shown in Figure 1 above.

Figure 2 Cell Diagram

When drawing a cell diagram, we follow the following conventions. The anode is always placed on the left side, and the cathode is placed on the right side. The salt bridge is represented by double vertical lines (||).  The difference in the phase of an element is represented by a single vertical line (|), while changes in oxidation states are represented by commas (,).

Constructing a Cell Diagram

When asked to construct a cell diagram follow these simple instructions.
Consider the following reaction:

2Ag⁺_(aq) + Cu_(s) ↔ Cu²⁺_(aq) + 2Ag_(s)

Step 1: Write the two half-reactions.

Ag⁺(aq) + e^- ↔ Ag_(s)

Cu_(s) ↔ Cu²⁺_(aq) + 2e^-

Step 2: Determine the cathode and anode.

Anode: Cu_(s) ↔ Cu²⁺_(aq) + 2e-     

Cathode: Ag⁺(aq) + e^- ↔ Ag(s)

Cu_(s) is losing electrons thus being oxidized. Oxidation happens at the anode. Ag⁺ is gaining electrons thus is being reduced. Reduction happens at the cathode.
Step 3: Construct the Diagram.

Cu_(s) | Cu²⁺_(aq) || Ag⁺_(aq) | Ag(s)

The anode always goes on the left and cathode on the right. Separate changes in phase by | and indicate the the salt bridge with ||.

Cell Voltage/Cell Potential

The readings from the voltmeter give the reaction's cell voltage or potential difference between it's two two half-cells. Cell voltage is also known as cell potential or electromotive force (emf) and it is shown as the symbol E_cell. 

Standard Cell Potential: E^o_cell = E^o_{right(cathode)} - E^o_left(anode)

The E^o values are tabulated with all solutes at 1 M and all gases at 1 atm. These values are called standard reduction potentials. Each half-reaction has a different reduction potential, the difference of two reduction potentials gives the voltage of the electrochemical cell. If E^ocell is positive the reaction is spontaneous and it is a voltaic cell. If the E^ocell is negative, the reaction is non-spontaneous and it is referred to as an electrolytic cell.

This is a link that shows the standard reduction potentials of all common half-reactions: http://butane.chem.uiuc.edu/cyerkes/.../standpot.html

Practice Problems

Consider the following two reactions:

a) Cu_²⁺(aq) + Ba_(s) --> Cu_(s) + Ba²⁺_(aq)

b) Al_(s) + Sn²⁺_(aq) --> Al³⁺_(aq) + Sn_(s)

1. Split the reaction into half reactions and determine their E^o value. Indicate which would be the anode and cathode.

2. Construct a cell diagram for the following each reactions.

3. Determine the E^o_cell for the voltaic cell formed by each reaction.

*Solution are given below.

Solutions

1.a)    Ba²⁺_(aq) + 2e- --> Ba_(s)     E^o = -2.92 V        Anode

  Cu_²⁺(aq) + 2e- --> Cu_(s)     E^o = +0.340 V     Cathode

1.b)    Al³⁺_(aq) 3e^- --> Al_(s)           E^o = -1.66 V        Anode

          Sn²⁺_(aq) --> Sn_(s) +2e^-      E^o = -0.137 V     Cathode

2.a)    Ba²⁺_(aq) | Ba_(s) || Cu_(s) | Cu_²⁺(aq)

2.b)    Al_(s) | Al³⁺_(aq) || Sn²⁺_(aq) | Sn_(s)

3.a)    E^o_cell = 0.34 - (-2.92) = 3.26 V

3.b)    E^o_cell = -0.137 - (-1.66) = 1.523 V

What happen when mix bromine with alcohol

Substitution, Addition and Elimination Reactions

Introduction:

Substitution, addition, and elimination reactions are of great importance in a major branch of chemistry known as Organic Chemistry, which covers the chemistry of compounds of carbon. These reactions, which generally involve covalently bonded molecules, are also found, to a much more limited extent with other compounds.

Substitution reactions:

A substitution reaction is a reaction in which an atom (or group of atoms) in a molecule is replaced by another atom or group of atoms:

Example 1:

The gas ethane, CH₃CH₃ reacts with bromine vapour in the presence of light to form bromoethane, CH₃CH₂Br and hydrogen bromide, HBr. In the process, a hydrogen atom in ethane has been substituted for a bromine atom:

Example 2:

Ethanol, CH₃CH₂OH, reacts with hydrogen iodide, HI, to form iodoethane and water. Here, a group of atoms, OH, has been replaced by an iodine atom:

Example 3:

Benzene, C₆H₆, reacts with bromine in the (presence of iron bromide as catalyst) to form bromobenzene, C₆H₅Br. This results in a hydrogen atom being replaced by a bromine atom:

Addition reactions:

An addition reaction is a reaction whereby a molecule reacts with another molecule having one or more multiple covalent bonds so as to form a molecule whose molecular mass is the sum of the molecular masses of the reacting molecules:

Example 3:

Ethene, CH₂=CH₂ has a double bond joining the two carbon atoms. This substance can add a hydrogen molecule (in the presence of platinum as catalyst) to form ethane, CH₃CH₃:

Example 5:

Ethyne, C₂H₂ has a triple bond joining the two carbon atoms. Hydrogen bromide adds onto this triple bond to form 1,1-dibromoethane, CH₃CHBr₂:

Elimination reactions:

An elimination reaction is a reaction whereby a multiple covalent bond is formed in a molecule by the removal of another, usually smaller molecule:

Example 6:

Ethanol, CH₃CH₂OH, when treated with concentrated sulphuric acid, H₂SO₄, loses 2 hydrogen atoms and one oxygen atom, forming ethene, CH₂=CH₂ and water (the atoms that have been eliminated are shown in red):

When an elimination reaction removes the elements of water from a compound, as in the reaction above, the reaction is called a DEHYDRATION REACTION.

Example 7:

Bromoethene, CH₂=CHBr, when treated with potassium hydroxide dissolved in ethanol, loses one hydrogen atom and one bromine atom, forming ethyne, CH≡CH (the atoms that have been eliminated are shown in red):

When an elimination reaction removes the elements of a halogen acid (HCl, HBr, HI) from a compound, as in the reaction above, the reaction is called a DEHYDROHALOGENATION REACTION.

Formation of ionic bond

Cleansing action of soap.......

How does soap clean.........................

Soap is a part of our daily life, it has different shapes and perfumes, it can be solid or liquid... But have you ever thought about how this common substance works? Here is a little explanation. I've used a minimum number of scientific terms, so I hope that everybody can understand it.

There are substances which can be dissolved in water (salt for example), and others that can't (for example oil). Water and oil don't mix together, so if we try to clean an oily stain from a cloth or from the skin, water is not enough. We needsoap.

Soap is formed by molecules with a "head" which likes water (hydrophilic) and a long chain which hates it(hydrophobic).

Because of that dualism, soap molecules act like a diplomat, improving the relationship between water and oil. How? When soap is added to the water, the hydrophilic heads of its molecules stay into the water (they like it!), while the long hydrophobic chains join the oil particles and remain inwards (escaping from the water). In that way, they form circular groups named micellas, with the oily material absorbed inside and trapped.

An emulsion of oil in water is then formed, this means that the oil particles become suspended and dispersed into the water. Thus, those oil particles are liberated from the cloth or the skin, and the emulsion is taken away with the rinsing.

In summary, soap cleans by acting as an emulsifier. It allows oil and water to mix so that oily grime can be removed during rinsing. There are more things involved in this process, but this is the general idea.

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Titration Uses in Everyday Life

1.                               Titration is a process or method to determine the concentration of a substance in an unknown solution, in which a known reagent is added to an unknown solution to produce a known reaction such as a change in color or electrical measurement. While it is more commonly associated with high school chemistry labs, medicine and the science industry, many common items use titration to test for the presence of various substances.

Blood Sugar Testing

2.                               Diabetics can measure the amount of glucose in their blood by using a small portable machine called a blood glucose meter. To use the machine, a small sample of blood is applied to a test strip and mixed with reactants, then a small electrical current is applied to the sample. The current is affected by the concentration of the reactants and can then be used to measure the amount of glucose present in the blood.

Pregnancy Testing

3.                               Home pregnancy testing kits detect the presence of human chorionic gonadotropin (hCG) in a subject's urine applied to a test strip or solution. The application of urine causes a color change that will indicate either a positive or negative test result as early as two weeks after conception. A more comprehensive pregnancy test applies titration to a blood sample to measure the specific amount of hCG present in the blood. This test can also detect possible problems with the pregnancy and determine how long the subject has been pregnant and can only be performed by a health care provider.

Aquarium Water Testing

4.                               The water in a home aquarium constitutes a small and delicately balanced environment for fish. If the properties of aquarium water change too drastically, disease and death of pets can result. In order to monitor changes in water conditions, home test kits are sold containing chemicals to test the pH level of water as well as the levels of ammonia, nitrites, nitrates and phosphates by placing a measured amount of the test chemical in a water sample and observing the extent, if any, of the resulting color change.