Quantum numbers (animated with exercises)

Electronegativity and electropositivity trends in periodic table

How to assign the formal charge exercise

Video link: https://youtu.be/4NPok41FwRM

How to assign formal charge?

Acid dissociation constant and position of equilibrium in acid-base reactions

Amino acids

Amino acids:

Amino acids are compounds that contain amine and carboxylic groups. Each unit of amino acid (monomer) contains positive and negative group thus the amine group and the carboxylic group form what is known as a zwitterion (dipolar ion or internal salt). In the zwitterion, the carboxylic group loses hydrogen ion (COO^-) to the amine group (NH₃⁺).

The formation of the internal salt gives the amino acid the distinguished properties of salt such as; forming crystalline solids and having high melting points. The solubility of amino acids is associated with the R side chain. More polar side chain results in more soluble amino acids. Still, amino acids are insoluble in nonpolar organic solvents such as; ether and carbon tetra-chloride (hydrocarbon solvents). Amino acids are classified into polar and non-polar according to the R side chain:

Amino acid 

a. polar amino acids:  

There are 20 natural amino acids that protein is driven form. The polar amino acids are divided into;

a. Acidic side chain amino acids such as; Aspartic acid, glutamic acid, cysteine, and tyrosine. Those amino acid contain side chains that end with COO^-, OH or SH groups.

b. Basic side chains amino acids are; arginine, histidine, and lysine.

c. Unionized amino acids: Asparagine, glutamine, serine, and threonine.

For non-polar amino acids, there are 8 amino acids: alanine, glycine, isoleucine, leucine, methionine, phenylalanine, proline, tryptophan, and valine

The 20 natural amino acids divided into two groups polar and non-polar amino acids

From the above figure, it can be seen that there are some amino acids with aromatic side chains (phenylalanine and tyrosine) and others with only aliphatic side chains (valine and leucine).

Except for glycine all amino acids contain chiral carbon atom (carbon atom that is connected to different groups). The natural amino acids all contain alpha (α) carbon atom.

The amino group in 19 of the 20 amino acids is a primary (connected to two hydrogen and one carbon atoms) except for proline it is secondary (connected to two carbon and one hydrogen atoms).

The polymerization of amino acids by condensation reaction results in either peptide (short series of amino acids) or longer chains which are proteins. Amino acids that contain alpha group their polymerization result in the formation of beta sheets of α-helix.

Next: Amino acids reactions

The three types of carbohydrates

Carbohydrates definition and classification:

Carbohydrates, which comprise one of the three basic classes of foodstuffs, contain carbon, hydrogen, and oxygen atoms. Their general formula, C_n(H₂O)_m, is the basis for their name. They can be classified as

a. Monosaccharides:

Which cannot be broken down chemically to simpler carbohydrates. The most familiar monosaccharides contain either six carbon atoms per molecule (glucose, fructose, galactose,..) or five (ribose, arabinose,….).

b. Disaccharides: which are dimers formed when two monosaccharide units combine with the elimination of H₂O. The monosaccharides may be the same (two glucose units in maltose) or different (a glucose and fructose unit in sucrose).

c. Polysaccharides, which are condensation polymers containing from several hundred to several thousand monosaccharide units. Cellulose and starch are the most common polysaccharides.

Covalent bonds (polar covalent and pure covalent bonds)

Covalent bonds and electronegativity:

A covalent bond is a bond that forms due to the sharing of electrons, mostly between two non-metals. If the sharing atoms are of the same type of element the electrons will be shared equally. This means the electrons will spend equal times around each atom and there will be no difference in the electronegativity between sharing atoms (the electronegativity is the measure of the strength of an atom to draw the bonding electrons towards itself). The molecules of the similar atoms are considered to be non-polar. Example, the bond between H-H in H₂ is non-polar covalent and the bond to be a pure covalent bond.

If the atoms are from different elements the electrons will spend more time around the more electronegative atom and the molecule is considered to be polar covalent

Example of polar covalent bonds is the bond between H and Cl in HCl. One electron is shared between the two atoms, this electron spend more time around the chlorine atom than around hydrogen atom.

The strength of polar covalent lies between ionic and non-covalent bonds (ionic bond, such as the bond between NaCl-metals and non-metals). For that, the bond strength of HCl (polar covalent) is stronger than between H2 (non-polar covalent) and weaker than between NaCl (ionic).

If the difference in the electronegativity between the two bonded atoms is more than 2, the bond considered to be ionic. If the difference in the electronegativity less than 2, the bond is polar-covalent. If there is no difference in the electronegativity, the bond considered to be non-polar covalent.

The elements and periodic table

Balancing Chemical Equations:

When we write the chemical reaction we have to keep in mind that matter is neither created nor destroyed. 

This means the number of atoms on both sides of equation should be the same.

In general, we can balance a chemical equation by the following steps:

1. Identify all reactants and products and write them correct.

2. Begin balancing the equation by trying different coefficients (numbers written before formula) to make the number of atoms of each element the same on both sides of the equation.

3. First, we try coefficients to balance the element that appeared only once on each side of the equation (the formulas containing these elements must have the same coefficient).

Next, balance the elements that appear only once on each side of the equation but in unequal numbers of atoms.

Finally, balance elements that appear in two or more formulas on the same side of the equation.

4. Check your balanced equation to be sure that total number of atoms on both sides are the same on both sides of equation.

For example, the producing of oxygen for heating potassium chlorate (KClO₃).

KClO₃ → KCl + O₃

All three elements (K, Cl, and O) appear only once on each side of the equation, but only for K and Cl  have equal numbers of atoms on both sides. Thus, KClO₃ and KCl must have the same coefficient.

The next step is balance the O atoms, because there are three O atoms on the left and two O atoms on the right of the equation, we can balance the O atoms by placing a 2 in front of KClO₃ and a 3 in front of O₂:

2KClO₃ → KCl + 3O₃

Finally, we balance the K and Cl atoms by multiplying the same coefficient on both sides "2":

2KClO₃ → 2KCl + 3O₃

For final check,

Number of K atoms on the reactants = products = 2

Number of Cl atoms on the reactants = products = 2

Number of O atoms on the reactants = products = 6

Now let us consider the combustion of the natural gas component ethane (C₂H₆) in presence of  oxygen or air to yield carbon dioxide (CO₂) and water. The unbalanced equation is

C₂H₆ + O₂ → CO₂ + H₂O

First, we look at the elements that appeared only once on each side of the equation (C and H).

To balance the C atoms, we place a 2 in front of CO₂:

C₂H₆ + O₂ → 2CO₂ + H₂O

To balance the H atoms, we place a 3 in front of H₂O:

C₂H₆ + O₂ → 2CO₂ + 3H₂O

To balance O atoms we multiply left hand side of the equation by the number of atoms on the right hand side of the equation and divide over the number of atoms on the left hand side of the equation (meaning we multiply by 7 and divide by 2).

C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O

However, we normally prefer to express the coefficients as whole numbers. Therefore, we multiply the entire equation by 2 to convert 7/2 to 7:

2C₂H₆ +7 O₂ → 4CO₂ + 6H₂O

For final check,

Number of C atoms on the reactants = products = 4

Number of H atoms on the reactants = products = 12

Number of O atoms on the reactants = products = 14

What is the acid-base properties of water?

The acid-base properties of water:

Water, as we know, is a unique solvent. One of its special properties is its ability to act either as an acid or as a base. Water functions as a base in reactions with acids such as HCl and CH₃COOH, and it functions as an acid in reactions with bases such as NH₃. Water is a very weak electrolyte and therefore a poor conductor of electricity, but it does undergo ionization to a small extent:

This reaction is sometimes called the autoionization of water. To describe the acid-base properties of water in the Brønsted framework, we express its autoionization as follows:

The acid-base conjugate pairs are (1) H₂O (acid) and OH^- (base) and (2) H₃O⁺ (acid) and H₂O (base).

Brønsted acid and Brønsted base

Brønsted acid: A substance capable of donating a proton.

Brønsted base: A substance capable of accepting a proton.

Conjugate acid-base pair: an acid and its conjugate base or a base and its conjugate acid.

Every Brønsted acid has a conjugate base, and every Brønsted base has a conjugate acid. For example, the chloride ion (Cl₂) is the conjugate base formed from the acid HCl, and H₃O⁺ (hydronium ion) is the conjugate acid of the base H₂O.

Similarly, the ionization of acetic acid can be represented as;

The subscripts 1 and 2 designate the two conjugate acid-base pairs. Thus, the acetate ion (CH₃COO₂) is the conjugate base of CH₃COOH. Both the ionization of HCl and the ionization of CH3COOH are examples of Brønsted acid-base reactions. The Brønsted definition also enables us to classify ammonia as a base because of its ability to accept a proton:

In this case, NH⁺₄ is the conjugate acid of the base NH₃, and the hydroxide ion OH^- is the conjugate base of the acid H₂O. Note that the atom in the Brønsted base that accepts a H⁺ ion must have a lone pair. 

What are the classifications of matter?

Matter is a substance that has a mass and occupies a space.
Matter is classified into:

1- Element: contains one type of atoms. Examples: Hydrogen gas, copper element.

2- Compound: made from atoms of different elements bonded together chemically with fixed ratios

chemical and physical properties of compound different from parent elements. Example: Sodium chloride.

3- Mixture: contains more than one compound or element mixed physically without chemical bond with any ratios. Example: Sea water, air and brass alloy.

  

Element	Compound	Mixture
One or more atoms	Two or more atoms	Two or more compounds or elements joined together physically (mixing only)
Same type of atoms	Different type of atoms	Different type of atoms
Iron (Fe) Copper (Cu)  Hydrogen gas H2	Water (H2O)    Carbon dioxide (CO2) Table salt (NaCl)	Sea water   Air (air is a mixture of different gases like oxygen, hydrogen and nitrogen