What Is a Second Order Reaction?

A chemical reaction can be classified either according to its molecularity or according to its order. The order is a description of the number of concentration terms multiplied together in the rat equation. Order is the number of concentration factors in the rate equation. The order of a reaction can only be found by experiment and cannot be worked out from the equation of the reaction.

“A reaction whose rate depends upon the second power of the concentration of the reactant is called second order reaction.”

In the second order reaction
A + B -->  Products

The rate equation is
Rate = - d[A]/dt = - d[B]/dt = k[A][B] or rate(for one type of reactants) = [A]^2

For the second order reaction, the rate of reaction increases with the concentration of reactants. In second order reaction the rate of reaction is proportional to the concentration of reactant. When a reaction is second order in a reactant, the doubling its concentration increases rate by a factor 22 and tripling the concentration increases rate by 32 and so forth.

The rate of a reaction depends on concentration but rate constant is independent of concentration. The rate as well as the rate constant are affected by temperature and catalyst.

For example, saponification of ethyl acetate, the chemical equation for this reaction is
CH_3COOC_2H_5 + NaOH --> CH_3COONa + C_2H_5OH

The molecules (CH_3COOC_2H_5 + NaOH) are involved in this reaction and hence the molecularity of the reaction is two. This rate of this reaction has been found to obey the following rate.

-d[CH_3COOC_2H_5]/dt a [CH_3COOC_2H_5][NaOH]

Hence the order to this reaction is two.

What Oxidation Means in Chemistry?

In our daily life we come across processes like rusting of objects made of iron, fading of the color of clothes, burning of the combustible substances such as cooking gas, wool, coal etc. What is happening behind these changes? These process fall in the category of specific type of chemical reactions called oxidation-reduction reactions.

Oxidation is the most important high temperature reaction. Oxidation refers to an increase in oxidation state, which means a loss of electrons. In organic chemistry it is often easier to view oxidation as increasing the number of bonds to oxygen or other heteroatoms. Simply oxidation is a process of addition of oxygen or removal of hydrogen to a substance.

An oxidation reaction between a metal (M) and the oxygen (O_2) can be written as 

M(s) + O_2 (g) \rightarrow MO_2(s).
Some of the examples of oxidation reaction are listed below.

1. Rusting of iron - In rusting, iron does not combine directly with oxygen but involves the oxidation of iron in an electrochemical process.
2. Digestion of food in our body - When foods are broken down inside the body during the course of normal digestion oxidation also takes place, the vital difference is that the food energy is made available for our use.
3. Burning of fuels - The combustion of fuels is an oxidation reaction. The combustion of hydrocarbon fuels is a vital source of energy in our economy. Daily we make use of combustion of fuels. In respiration the combustion of foods provides us with energy.
4. Rancidity - The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.

Why Veins Look Blue?

Vein Looks Blue

Are veins blue in real life? If you look at the palm side of your wrist, you can see the veins, they too are blue. The vessels that bring blood toward the heart are veins. Veins carry deoxygenated blood back to the heart and their pressure is significantly lower than in arteries. Veins have valves that help keep the blood moving toward the heart in the one way circuit.

Red blood cells are important because they carry oxygen to other cells. They do this with the help of a protein called hemoglobin. This protein contains iron and can capture tiny particles or molecules of oxygen. Hemoglobin proteins that contain oxygen make red blood cells red. When hemoglobin molecules don’t contain oxygen, red blood cells are blue. This is why vein looks blue.

A simpler test can confirm that though your eyes and brain perceive blue, the vein is actually a pinkish flesh color. The test requires two pieces of standard gray duct tape. Place one piece of the duct tape on each side of the vein, covering the adjacent skin. Once you see the vein all by itself, you can see its real color, a shade of pink.

It is considered that the oxygenated blood in the arteries is red, and that the deoxygenated blood in the veins is blue. While it is true that the oxygenated blood in the arteries is a bright red color, the blood in the veins is also a red color not blue. Why does the blood trickling out of a cut vein look bright rad? Because the moment the dark purplish blood hits the air, it mixes with oxygen and changes color.

How to Find pOH

The pH and pOH of a neutral solution at 25oC are equal to 7. The ion concentrations are small and the negative exponents make them tedious to work. Soren Peer Lauritz Sorenson, a Danish biochemist devised the pH concept in 1909 to express hydrogen ion concentration. The pOH of a solution can be defined as

pOH = - log[OH^-]
The [H+] and [OH-] are inversely related and consequentially pH and pOH are inversely related. We can picture pH and pOH as sitting on opposite sides of a seesaw as one goes up the other always goes down. The product of the hydrogen and hydroxide concentrations will be equal to 1.0 x 10-14, while the sum of the pH and pOH will be equal to 14.

The relationship between the type of solution, pOH and ion concentration is shown in the table below.

The pH and pOH scales are based on logarithms, a change in 1 pH or pOH unit represents a change in ion concentration of a factor of ten. For example, coffee with a pH of 5 has approximately 100 times the hydronium ion concentration as tap water with a pH of 7.

The concept of alkalinity is much less commonly used; it is defined as the logarithm of the reciprocal of the concentration of hydroxyl in a solution. The following are the formulas used to determine the pOH.

pOH = 14 – pH
pOH = - log [OH^-]

What an Electron Domain Means in Chemistry

An electron domain is any region of space around the central atom in which there is a high probability of finding electrons. A domain of electrons, two electrons in a nonbonding domain sometimes called a lone pair, two electrons in a single bond domain, four electrons in a double bond domain, six electrons in a triple bond domain tends to repel other domains of electrons. Domains of electrons around a central atom will orient themselves to minimize the electron-electron repulsion between the domains.

The number of atoms that a central atom is attached to plus the number of unshared electron pairs that an atom may have. The atom bonded to the central atom by a single or a double or a triple bond is still counted as one electron domain. The geometry of the molecule can then be predicted on the basis of the repulsion of electron domains and the relative magnitudes of these repulsions.

Bonding electron domain is of three types. Single bond (1 pair, 2 electrons); double bond (2 pairs, 4 electrons); triple bond (3 pairs, 6 electrons). Non-bonding electron domains is the pair of valence electrons not involved in a bond.

For example, take NH3. Here the nitrogen has five valence electrons and these five electrons are placed in four hybrid atomic orbitals. N has attached to it three H atoms by three shared electron pairs and therefore N has one unshared electron pair or non-bonded electron pair left. Thus the central atom nitrogen N has a total of four electron domains that is three bonded pairs and one non-bonded electron pair.

Why Do We Smell When We Sweat?

When people exposed to heat, through experiencing elevated air, temperature, exercise or emotional stress, they often sweat. Sweating is the body’s way of cooling itself off, whether a person is stressing over a big test. Sweat comes from the 2.6 million sweat glands in the skin. However, the degree to which people sweat and the resulting smell that is produced varies greatly.

The skin has two types of sweat glands: eccrine and apocrine. The skin contains millions of eccrine glands distributed all over the body, and the fluid that they produce consists mainly of salt and water and has no smell. Appocrine glands meanwhile tend to locate in areas where there are lot of hair follicles and the sweat they produce is fatty.

Sweat only begins to smell and cause the unpleasant odors when the bacteria living on our skin and clothes deign to break it down to produce the fatty acids they feed on. Sweat is almost completely of water with tiny amounts of other chemicals like ammonia, urea, salts and sugar. When the sweat hits the air, the air makes it evaporate. As the sweat evaporates off the skin and cool down.

Sweat itself is odorless whether it comes from the armpits or other areas of the body. The smelliness begins when sweat mixes with bacteria that occur naturally on the surface of the skin. This distinctive odor is called bromhidrosis -foul smelling sweat.

How pH and pKa Are Related?

pH vs pKa

pH is a measure of the concentration of hydrogen ions in an aqueous solution. pKa (acid dissociation constant) is related, but more specific, in that it helps you predict what a molecule will do at a specific pH. Essentially, pKa tells you what the pH needs to be in order for a chemical species to donate or accept a proton.

• The lower the pH, the higher the concentration of hydrogen ions, [H+]. The lower the pKa, the stronger the acid and the greater its ability to donate protons.
• pH depends on the concentration of the solution. This is important because it means a weak acid could actually have a lower pH than a diluted strong acid. For example, concentrated vinegar (acetic acid, which is a weak acid) could have a lower pH than a dilute solution of hydrochloric acid (a strong acid). On the other hand, the pKa value is a constant for each type of molecule. It is unaffected by concentration.
• Even a chemical ordinarily considered a base can have a pKa value because the terms "acids" and "bases" simply refer to whether a species will give up protons (acid) or remove them (base). For example, if you have a base Y with a pKa of 13, it will accept protons and form YH, but when the pH exceeds 13, YH will be deprotonated and become Y. Because Y removes protons at a pH greater than the pH of neutral water (7), it is considered a base.

Relating pH and pKa With the Henderson-Hasselbalch Equation

If you know either pH or pKa you can solve for the other value using an approximation called the Henderson-Hasselbalch equation:

pH is the sum of the pKa value and the log of the concentration of the conjugate base divided by the concentration of the weak acid.

At half the equivalence point:

pH = pKa

It's worth noting sometimes this equation is written for the Ka value rather than pKa, so you should know the relationship:

pKa = -logKa

Assumptions That Are Made for the Henderson-Hasselbalch Equation

The reason the Henderson-Hasselbalch equation is an approximation is because it takes water chemistry out of the equation. This works when water is the solvent and is present in a very large proportion to the [H+] and acid/conjugate base. You shouldn't try to apply the approximation for concentrated solutions. Use the approximation only when the following conditions are met:

• -1 < log ([A-]/[HA]) < 1
• Molarity of buffers should be 100x greater than that of the acid ionization constant Ka.
• Only use strong acids or strong bases if the pKa values fall between 5 and 9. Continue reading..