Naming ionic and molecular compounds

A chemical compound has a unique and defined chemical structure in which fixed ratios of atoms are held together in a defined spatial arrangement by chemical bonds. Basically there are two types of chemical compounds in chemistry, namely

Ionic compounds: Any substance which consists of two or more ionically-bonded atoms is called ionic compounds. A good example of an ionic compound is sodium chloride (table salt). In ionic compound, ions are held together in a lattice structure by ionic bonds. Positively charged ions are called cations and negatively charged ions are called as anions.


Molecular compounds: A molecule is the basic unit of a molecular compound. A molecule is defined as an object containing two or more atoms bonded together by covalent bonds. A molecule is the smallest particle of a compound which defines the properties of that compound. When a molecule is dissolved, it never dissociates.

naming ionic compounds

Ionic compounds dissolve in polar solvents which ionize, such as water and ionic liquids. They are also capable of dissolving in other polar solvents like alcohols, acetone and dimethyl sulfoxide. Ionic compounds do not tend to dissolve in non polar solvents.
According to International Union of Pure and Applied Chemistry IUPAC names of any ionic compound is written in two words. First name contains cation with oxidation number written in parentheses followed by the name of the anions. For example, the name of Fe₂ (so₄)₃  is written as iron(III) sulfate.

naming molecular compounds

We use chemical formula to describe the constituents of any chemical compound. Basically, there are three types of chemical formula namely, simple formula, graphic formula, and structural formula.

Simple formula:
In this formula, the elements are represented in symbolic form with subscripts to describe their ratio. Consider the simple formula of water which is H₂o. It shows the presence of two parts hydrogen and one part of oxygen.

Graphic formula:
In graphical formula, the element is represented in such a way to show the physical orientation of the constituent atoms to one another. The graphic formula of water is HOH. Water molecule has an atom of hydrogen to either side of an atom of oxygen.

Structural formula:
In structural formula, each and every atom is two dimensionally represented. The structural formula for water is given by H-O-H.

An example of a compound

An example of a compound is the common salt in pure form.
The molecular formula or the chemical formula of this compound is NaCl and it's a pure homogeneous compound.

The general physical appearance of this compound is :
it is white , crystalline in nature.The taste of this compound as we are all  familiar with:  it is salty.
The given compound is a salt and  is called sodium chloride.
This  chemical species is  called a compound, because of the following reasons:
It cant be separated into its constituent elements by any physical means and
the properties of this compound are totally different from the properties of its constituent elements.

why NaCl is a compound

     1) NaCl is a compound,because it is made up of Na and Cl atoms in a fixed ratio of 1:1
    2)   In the formation of the compound  NaCl, some amount of energy transformations took place and made this formation of NaCl.
      a permanent change ..... which means we cannot revert to the elemental state of Na and Cl from   NaCl  by any physical means.
      3)Example: the properties of Na and the properties of Cl are totally different than the properties of NaCl compound.

Other properties of a compound.

For example Na is a soft metal with grey colour and a metallic lustre and it is a solid , highly reactive  in presence of water( so its kept in kerosene always ), it can form an oxide as Na₂O.  when in presence of air and this oxide readily dissolves in water to form NaOH,....... but the salt has none of these properties.... the salt is crytalline , white in colour , dissolves readily in water but does  not form any base or alkali in water ,  is not so reactive in air or water  and has no metallic lustre.
The other element that forms NaCl is chlorine and that we know is a gas and is acidic ,,, it is also suffocating and acidic to moist  litmus.

Properties of compounds are thus not the same.....

But the salt is totally neutral and is not a gas but is a solid .In other words the compound has properties far different from the properties of any of the constituent elements.
 Further we cannot separate the constituent elements by any physical means.That is why we can differentiate between a compound and a mixture and study of compounds  becomes more interesting.

De broglie atomic model

Introduction :
Bohr assumed that an electron is a particle and postulated that it revolves around the nucleus in an orbit in which the angular momentum ( mvr ) of the electron is an integral multiple of   `(h)/(2pi)` .
                      m v r  =  n`(h)/(2pi)`   .
But according to de-Broglie an electron in motion with high speed is associated with wave nature. For an electron moving around the nucleus in the circular path, two cases of electron waves of different wavelengths are possible .

In one case the circumference of the electron orbit ( 2`pi`r ) is an integral multiple of the wavelength ( `lambda` ) of the electron. This is shown in the figure (1). In the second case, the circumference of the electron orbit is not an integral multiple of the wavelength . This is shown in figure (2) .



In the first case of atomic model above, the two ends of the electron wave meet to give a regular series of crests and troughs. In this case the electron waves are said to be in-phase. This means that there is constructive interference of the electron - waves. In such a case, the electron is considered to behave as a standing wave or non-energy radiating wave. This extends round the nucleus in a circular orbit.
In the second case of atomic model above, here the circumference of the Bohr's orbit ( 2`pi`r ) is bigger or smaller than n`lambda` , the electron waves are said to be out of phase. This is shown in the figure (2). In this case, a destructive interference of waves occur causing radiation of energy.
Therefore the necessary condition to get an electron - wave in phase is that the circumference of the Bohr's orbit ( 2`pi`r ) is equal to integral ( whole number ) multiple of the wave length (`lambda`) of the electron - wave  
                       n`lambda`   =   2`pi`r    
                        `lambda`    =    `(2pir)/(n)`     ;   n   =   integer or whole number
According to de Broglie theory   `lambda`   =   `(h)/(mv)`    
               `:.`    `(2pir)/(n)`   =   `(h)/(mv)`
        or           m v r   =   n  *  `h/(2pi)`  
This is Bohr's postulate that stipulates the angular momentum of an electron moving round the nucleus is an integral multiple of `h/(2pi)` . In other words, de Broglie theory and the Bohr's theory are in agreement with each other. In case the circumference of the Bohr's orbit (2`pi`) is bigger or smaller than n`lambda`  , destructive interference of waves takes place and the orbit cannot exist.

Solved Problem on De broglie atomic model:

Q ) Find the momentum of a particle whose de Broglie wavelength is 1A.
Solution :  de Broglie Wavelength , `lambda`  =  1 A   =   1  *   10^-10  m
As per de Broglie's equation   `lambda`  =   `h/p`    
`:.`  Momentum of particle ,  p   =    `h/lambda`     
                                                          =    `(6.6256*10^-34kgm^2s^-1)/(1*10^-10m)`
Momentum of particle, p  =  6.6256*10^-24 kgms^-1 .

Another problem on De broglie atomic model:

Q ) A 8 kg bowling ball rolls with a velocity of 9 metre per second. a ) What is de Broglie's wavelength of the bowling ball. b) Why does the bowling ball exhibit no observable wave behaviour ?
Solution :  Given mass of the ball m = 8 kg   and velocity  v  =  9 m / s

a ) de Broglie's Wavelength is given by   `lambda` =  `h/p`   
where   p  =  m v     and    h  is Planck's constant .
In this case    `lambda` =  `(6.6256*10^-34)/(8*9)`
                          `lambda` =  0.092 * 10^-34

b ) Wave  cannot be seen for macroscopic objects like bowling balls. The de Broglie wavelength for this object is smaller than the atomic nucleus and impossible to detect .

Ionisation energy and electron affinity

Introduction
Ionisation Energy Ionisation energy is one of the important properties of elements . If energy is supplied to an atom , electrons are promoted to higher energy levels . If sufficient energy is supplied , an element in the outer most  shell can be completely removed from the atom , resulting in the formation of a positive ion .

The minimum energy required to remove the most loosely bound electron from an isolated gaseous atom is called ionisation energy . It is also called the first ionisation energy . ( I₁ )
      M _(g)    +   I₁    `|->`   M⁺ _(g)   +  e^-

The minimum energy required to remove another electron affinity from the uni positive ion is called second ionisation energy ( I₂ ) .
     M _(g)    +   I₂    `|->`   M²⁺ _(g)   +  e^-

The second ionisation energy ( I₂ ) is greater than the first ionisation energy  . On removing an electron from an atom , the uni positive ion formed will have more effective nuclear charge than the number of electrons .

This decreases the repulsions between the electrons and increases the nuclear attraction on the electrons . As a result , more energy is required to remove an electron from the uni positive ion . Hence the second ionisation energy ( I₂ ) is greater than the ionisation energy ( I₁ )  .

Similarly the third ionisation energy ( I₃ ) is greater than the second ionisation energy . An atom has as many ionisation energies as the number of electrons present in it . The order of ionization energies
     I₁  <   I₂   <   I₃   <  ........I_n
where n is the number of electrons in the atoms . Ionisation energies are determined from spectral studies as well as from discharge tube experiments . They are measured in electron volts (eV) atom^-1 .
    1 eV   =  1.602 * 10^-19 J ,  hence 1 mol of eV has energy  1.602 *10^-19 * 6.023*10²³
               = 96.45 k j mol^-1 .

The discharge tube is filled with gas whose ionisation energy is required . At low voltages , there is no flow of electricity . But , on increasing the voltage between anode and cathode , the gas ionizes at a particular voltage , which is indicated by sudden increase in the flow of electricity . That particular voltage is called ionization energy  .

The magnitude of ionisation of an atom depends on the following factors

• Atomic radius
• Nuclear charge
• Screening or shielding effect on the outer most electrons from the attraction of the nucleus .
• Completely filled or half filled nature of sub shells .

Ionisation potentials of some elements

Electron Affinity

Electron Affinity
Electron affinity is another important property of elements like ionisation energy is required to remove an electron from an atom . Conversely , when electron is added to an atom , energy is released .

Electron affinity of an element is the energy released when an electron is added to a neutral gaseous atom of that element .
Energy is released when only one electron is added to an  atom forming a uni negative ion . The negative ion prevents entry of further electrons due to repulsive forces between negative charges . Thus energy is needed to overcome these repulsive forces between uni-negative ion ( X^- ) and electron ( e^- ) and to add one more electron energy is required . Hence , second electron affinity value usually has a positive value .

Electron affinity depend on the size and the effective nuclear charge of atom . They cannot be determined directly , but are obtained indirectly using the Born-Haber cycle . Electron affinity are measured in kj mol^-1 .
Halogens have high values for electron affinity due to their small atomic sizes and requirement of only one electron to get the nearest inert gas configuration . Depending on thermodynamic notation that energy liberated is shown by negative sign , the electron affinity values are mentioned with numerals carrying negative sign before them . If energy is absored during the addition of an electron to an atom , then it is shown by  positive sign .

Variation of electron affinity in a group and in a period .

Variation of Electron affinity in a group

From top to bottom as the atomic size increases , the electron affinity decreases , in a period . But the electron affinity of the second element in the group is greater than the first one . For example , in halogens , the electron affinity of fluorine is -333 kj mol^-1 while the electron affinity of chlorine is -348 kj mol^-1 . It is because fluorine atom is smaller in size than chlorine atom and has strong inter repulsions . During the addition of electrons of fluorine atom the electronic repulsions are overcome at the expense of some liberated energy and hence , the overall energy liberated is less than that of chlorine atom .
Similarly phosphorus has bigger electron affinity value than nitrogen and sulphur has greater electron affinity than oxygen in V and VI groups .

Variation of electron affinity in a period

In a period , as we move from left to right , the atomic size decreases and the nature of the element changes from metallic to non metallic and this results in an increase in the electron affinity values . In a given period halogen has the highest electron affinity . Since the outer shells of zero group elements are filled with electrons to attain octet structure sp²sp⁶ , they do not accept electron and their electron affinities are treated as zero .

Net Ionic Equation Example

On the basis of type of chemical bonds in compounds, they can be mainly two types; ionic and covalent compounds. In ionic compounds, cations and anions are bonded by electrostatic force of attraction while in covalent compounds; elements are bonded with covalent bonds. Due to different types of bonds, these compounds show different type of reactions.

In covalent compounds, molecules involve in reactions while in ionic compounds, ions take part in reaction. Therefore chemical reactions of ionic compounds can be represents in three different types; molecular equation, ionic equation and net-ionic equation.

Molecular equation represents reacting substance in its molecular form while ionic and net-ionic equation shows in ionic form.

Let’s take a Net Ionic Equation Examples of reaction of silver nitrate with potassium chloride to form white precipitate of silver chloride and potassium nitrate. The molecular equation for this reaction would be;

AgNO_3(aq) + KCl_(aq)   AgCl () + KNO_3(aq) 

While the ionic equation will show all ions of reacting substances excluding precipitate as it exists in solid state.

Ag⁺_(aq)  + NO₃^-_(aq)  + K⁺_(aq)  + Cl^-_(aq)   AgCl()+ K⁺_(aq)  + NO₃^-_(aq) ..........(1)

You can observer in equation (1) only silver ion and the chloride ion takes part in the reaction to form AgCl precipitate while potassium ion and nitrate ion remains unchanged. These ions are known as spectator icons. Therefore we can eliminate from the ionic equation. Hence net ionic equation helps to understand the involvement of reacting substances in given reaction and net ionic equations examples show only those ions which really take part in reaction or get some change.

Ag⁺ + Cl^-  AgCl () .......(2)

Let’s take some more examples of Net Ionic Equations and practice Net Ionic Equations from their molecular equation.

• Silver nitrate and sodium bromide to form silver bromide :

1. Molecular equation:   

AgNO₃ (aq)   +   NaBr (aq)         AgBr (s)   +   NaNO₃ (aq)

2. Ionic equation:    

Ag⁺ (aq)+ NO₃^¯ (aq) +  Na⁺ (aq)+ Br¯ (aq)      AgBr (s)    +   Na⁺ (aq)  +  NO₃¯ (aq)

3. Net-Ionic equation:    Ag⁺ (aq)   +   Br¯ (aq)       AgBr (s)

• Potassium carbonate and calcium nitrate to form calcium carbonate :

1.  Molecular equation:  Ca(NO₃)₂ (aq)    +   K₂CO₃ (aq)       CaCO₃ (s)   +    2 KNO₃ (aq)

2. Ionic equation:  

Ca⁺ (aq)  +  2 NO₃¯ (aq)  +  2 K⁺ (aq)  +  CO₃^2- (aq)    CaCO₃ (s)  +  2 K⁺ (aq) +  2 NO₃¯ (aq)

3. Net-Ionic equation:  Ca²⁺ (aq)    +   CO₃^2- (aq)      CaCO₃ (s)

• Barium bromide and potassium sulphate to form barium sulphate :

1. Molecular equation:   BaBr₂ (aq)  +  K₂SO₄ (aq)         BaSO₄ (s)   +   2 KBr (aq)

2. Ionic equation:  

Ba²⁺ (aq) +  2 Br¯ (aq) +  2 K⁺ (aq) + SO₄^2- (aq)    BaSO₄ (s) +  2 K⁺ (aq) +  2 Br¯ (aq)

3. Net Ionic equation:  Ba²⁺ (aq)  +  SO₄^2- (aq)       BaSO₄ (s)

Newtons law of heating and cooling

Newton's Law of Cooling states that the rate of change of the temperature of an object is proportional to the difference between its own temperature and the ambient temperature (i.e. the temperature of its surroundings).

Newton's Law makes a statement about an instantaneous rate of change of the temperature. We will see that when we translate this verbal statement into a differential equation, we arrive at a differential equation. The solution to this equation will then be a function that tracks the complete record of the temperature over time. Newton's Law would enable us to solve the following problem.

 Example 1: The Big Pot of Soup As part of his summer job at a restaurant, Jim learned to cook up a big pot of soup late at night, just before closing time, so that there would be plenty of soup to feed customers the next day. He also found out that, while refrigeration was essential to preserve the soup overnight, the soup was too hot to be put directly into the fridge when it was ready. (The soup had just boiled at 100 degrees C, and the fridge was not powerful enough to accommodate a big pot of soup if it was any warmer than 20 degrees C). Jim discovered that by cooling the pot in a sink full of cold water, (kept running, so that its temperature was roughly constant at 5 degrees C) and stirring occasionally, he could bring that temperature of the soup to 60 degrees C in ten minutes. How long before closing time should the soup be ready so that Jim could put it in the fridge and leave on time ?


Solution: Let us summarize the information briefly and define notation for this problem.
Let

= Temperature of the soup at time t (in min).

= Initial Temperature of the soup =100 deg.

= Ambient temperature (temp of water in sink) = 5 deg .

Given: The rate of change of the temperature , is (by Newton's Law of Cooling equation) proportional to the difference between the temperature of the soup and the ambient temperature This means that:



Here a bit of care is needed: Clearly if the soup is hotter than the water in the sink , then the soup is cooling down which means that the derivative should be negative. (Remember the connection between a decreasing function and the sign of the derivative ?). This means that the equation we need has to have the following sign pattern:


where is a positive constant.
This equation is another example of a differential equation. The independent variable is for time, the function we want to find is , and the quantities are constants. In fact, from Jim's measurements, we know that , but we still don't know what value to put in for the constant . 

Net Ionic Equations

An ionic chemical reaction involves the interactions of ionic species to form a new compound. For example, if we add a solution of barium chloride (BaCl2) with a solution of sodium sulphate (Na2SO4), it forms an insoluble solid (precipitate) of barium sulphate (BaSO4). The molecular and ionic equation for the given reaction would be;

Na₂SO₄ + BaCl₂  2NaCl + BaSO₄

2Na⁺_(aq) + SO₄^2-_(aq) + Ba²⁺_(aq) + 2Cl^-_(aq)  2Na⁺_(aq) + 2Cl^-_(aq) + BaSO_4(s)

A total ionic equation indicates all soluble ionic materials like ions with subscript (aq). For insoluble ionic solids, (s) stands for their solid state. Those ions which are remain same before and after reaction are called as spectator ions. For writing a net ionic equation, subtract these spectator ions from ionic equation. Let’s see what is a net ionic equation for is given reaction. Here sodium and chloride ions are common and have to eliminate from net-ionic-equation;

2Na⁺_(aq) + SO₄^2-_(aq) + Ba²⁺_(aq) + 2Cl^-_(aq)  2Na⁺_(aq) + 2Cl^-_(aq) + BaSO_4(s)

Ba²⁺_(aq) + SO4^2-_(aq)  BaSO_4(s)

Let’s see how to writing net ionic equations of reaction of sodium phosphate and calcium chloride to form an insoluble white solid form of calcium phosphate. The molecular equation for this reaction is as follow;

2Na₃PO₄(aq) + 3 CaCl₂(aq)  6 NaCl(aq) + Ca₃(PO₄)₂(s)

First write complete ionic equation with the use of chemical reaction. It shows all ions involve in reaction in their physical state and also indicate the correct formula, charge and number of each ion.

Hence ionic equation would be;

6 Na⁺ (aq) + 2 PO₄^3- (aq) + 3 Ca²⁺ (aq) + 6 Cl^- (aq) 6 Na⁺ (aq) + 6 Cl^- (aq) + Ca₃(PO₄)₂ (s)

Now subtract spectator ions (Na+, Cl-) from reaction to get net ionic equation.

6 Na⁺ (aq) + 2 PO₄^3- (aq) + 3 Ca²⁺ (aq) + 6 Cl^- (aq) 6 Na⁺ (aq) + 6 Cl^- (aq) + Ca₃(PO₄)₂ (s)

2 PO₄^3- (aq) + 3 Ca²⁺ (aq)  Ca₃(PO₄)₂(s)

Overall net ionic equations show soluble, strong electrolytes reacting in the form of ions. Now days many net ionic equation Solver are available to solve equation from molecular equation. For writing ionic equations, you must remember that most alkali metal compounds, ammonium compounds, halide, nitrates, chlorate compounds are soluble in nature. Carbonate, phosphate, oxalates, sulphide, chromate compounds are insoluble in nature. Except group-1, oxides are insoluble in water and form acids or bases. Only HCl, HBr, HI, HNO3, H2SO4, HClO4 are strong acids and ionize completely while all other are weak acids.