Chemistry Test - 6

Other Concepts :

Concept 1 :
Atomic radius trends• An atom gets larger as the number of electronic shells increase; therefore the radius of atoms increases as you go down a certain group in the periodic table of elements.

• In general, the size of an atom will decrease as you move from left to the right of a certain period.

EXCEPTIONS: Because the electrons added in the transition elements are added in the inner electron shell and at the same time, the outer shell remains constant, the nucleus attracts the electrons inward. The electron configuration of the transition metals explains this phenomenon. This is why Ga is the same size as its preceding atom and why Sb is slightly bigger than Sn.

Other Concepts :

Concept 1 :
Solution of weak electrolyte

Weak electrolytes are electrolytes that do not fully dissociate into ions in solution. These substances only partially ionize in solution (roughly 1-10%).

Think of a weak electrolyte as a substance that is quite stubborn. When added to a solution, there is a 1-10% chance that it will either completely break apart into its respective ions or remain stubborn in its ways and not dissociate.

When it does dissociate, it is those ions that can contribute to carrying an electrical charge in solution. The table shown below lists some examples of weak electrolytes.

.

Based on the family tree of electrolytes, there are two broad types of weak electrolytes: weak acids and bases. These substances are classified as weak electrolytes given their similar behavior in solution. For example, when you place a weak acid or base in solution they also have a 1-10% chance of dissociating in solution. This similarity in partial dissociation is what classifies a weak acid or base as a type of weak electrolyte.

Hematite $\left({\text{Fe}}_2{\text{O}}_3\right)$ is an oxide ore of iron.
 
Main Concept :
Minerals & OresElement which have low chemical reactivity generally in occur native or free or metallic state. e.g. Au,Pt noble gas etc. Element which are chemically reactive, generally occur in the combined state. e.g. halogens, chalcogens etc. The natural materials in which the metals occur in the earth are called minerals. The mineral from which the metal is conveniently and economically extracted is called an ore. All the ores are minerals but all minerals cannot be ores. 

Ketones are not oxidized by Tollens’ reagent, so the treatment of a ketone with Tollens’ reagent in a glass test tube does not result in a silver mirror.

This option is not correct because transition metal get rendered passive by coating of the oxide.
Main Concept :
Catalytic properties of Transition metals & their compoundsCatalytic properties : Most of the transition metals and their compounds particularly oxides have good catalytic properties. Platinum, iron, vanadium pontoxidc, nickel, etc., are catalysts. Platinum is a general catalyst. Nickel powder is a good catalyst for hydrogenation of unsaturated organic cornpound such as, hydrogenation of oils some typical industrial catalysts are,

(i) Vanadium pentoxide (V₂,O₅,) is used in Contact for the manufacture of sulphuric acid.

(ii) Finely divided iron is used in the Haber's process for the synthesis of ammonia,

Explanation : Most transitbn elements act as good catalyst of,

(i) The presence of vacant d-orbitals.

(ii) The tendency to exhibit variable oxidation states.

(iii) The tendency to form reaction intermediates with reactants.
Other Concepts :

Concept 1 :
Examples on Characteristics of d-block elements

General Characteristics of d-Block elements:

All the elements of the d-block show similar properties due to the presence of similar electronic configuration of the outermost shell. The outermost shell configuration is ns². Here is a list of general properties such as the atomic and ionic radii, electronic configuration and the ionisation potentials observed among the d-block elements.

1. Metallic nature: as the number of electrons in the outermost shell is very less i.e. All the transition elements are metals. They show the characteristics of metals such as malleability and ductile in nature and form alloys with several other metals. They also serve as good conductors of heat and electricity. Except for Mercury which is liquid and soft like alkali metals all the transition elements are hard and brittle unlike the non-transition elements. The hard and brittle nature of these elements indicates the presence of covalent bond which is due to the presence of unfilled d-orbitals. However, their property such as good conductivity is an indication for the presence of metallic bonding. Hence, they are said to form covalent bonding as well as the metallic bonding.

2. Melting and boiling points: They show very high melting and boiling points. This can be attributed to the presence of strong metallic bonding due to the overlapping of (n-1) d orbitals and covalent bonding of the unpaired d orbital electrons. Since Zn, Cd and Hg have completely filled (n-1)d orbitals they are not expected to form covalent bonds. Hence, they show comparatively lower melting point than other d-block elements.

3. Atomic radii: a great degree of variation is seen in the atomic radii across each transition series. The atomic radii of the d-block elements within a given series decreases with increase in the atomic number. This is due to the increase in the nuclear charge that attracts the electron cloud inwards resulting in decrease in size. However, the decrease a uniform decrease in atomic radius is not observed across a period. The decrease in atomic radii is small compared to the S and P block elements. This is due to the screening effect caused by the electrons of the (n-1)d subshell on the outermost shell. As a result the nucleus cannot pull the outermost electrons. Thus, the size of the atom does not alter much in moving from Cr to Cu.

The atomic radius increases on descending the group. In a given series, the atomic radius decreases to a minimum for the group VIII elements and then it increases towards the end of the series. This increase in radius towards the end of the series is due to the force of repulsion among the added electrons. A close similarity is observed in the radii of the elements of the second and third transition series due to the filling of 4f subshells.

4. Ionic radii: The ionic radius is similar to the pattern of atomic radii. Thus, for ions of a given charge the radius decreases slowly with increase in atomic number.

5. Atomic volume and Densities: the atomic volume of transition elements is much lower than those of S and P block elements. This is because of the filling of the (n-1)d orbitals that cause an increase in the nuclear charge and pulls the electrons inward. This results in decrease in atomic volume. With the decrease in the atomic volume, the atomic density of these elements increases. Osmium is having a maximum density.

In a given transition series, the density increases in moving across the period and reaches a maximum value at group VIII.

The density increases as we move down the group. The atomic sizes of elements of the second and third transition series are nearly same but their atomic weights increase nearly two fold and the densities of elements of third transition series are generally twice of the corresponding second transition series.

6. Ionization potentials: Transition elements have high ionization energy due to their small size. Their ionization potentials lie between those of S and P block elements. Thus, they are less electropositive than the s-block elements. Hence, they do not form ionic compounds readily like the alkali and alkaline earth metals. They also have the ability o form covalent compounds. The ionisation potentials of d-block elements increase as we move across each series from left to right. However, the increase is not as much as in case of S and P blocks elements. This is due to the screening effect caused by the new electrons that are added into the (n-1) d subshell.

The second ionisation energies of the first transition series also increases with the increase in atomic number. However, Cr and Cu are sufficiently higher than those of their neighbours. This is due to their stable electronic configuration.

7. Electronic configuration: the outer electronic configuration remains constant. But, a electron is added to the penultimate shell till the d-sub shell reaches its full capacity. There are three series of elements depending on the (n-1)d orbital that is being filled. The orbitals are filled in order of their increasing energy i.e. an orbital of lower energy is filled first. Thus 4s orbital with lesser energy is filled first to its full extent then the 3d orbital with higher energy is filled. The exactly half-filled and completely filled d-orbitals are extra stable.

The electronic configuration of the first series is given as,

Transition elements also show variable oxidation states, tendency to form complexes, magnetic nature and other properties.

Each contributing structure can be represented by a Lewis structure, with only an integer number of covalent bonds between each pair of atoms within the structure. Several Lewis structures are used collectively to describe the actual molecular structure, which is an approximate intermediate between the canonical forms called a resonance hybrid. Contributing structures differ only in the position of electrons, not in the position of nuclei.

Electron delocalization lowers the potential energy of the substance and thus makes it more stable than any of the contributing structures. The difference between the potential energy of the actual structure and that of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy.

Resonance is distinguished from tautomerism and conformational isomerism, which involve the formation of isomers, thus the rearrangement of the nuclear positions.

General characteristics of resonance

Molecules and ions with resonance (also called mesomerism) have the following basic characteristics:

Contributing structures of the carbonate ion

•  They can be represented by several correct Lewis formulas, called "contributing structures", "resonance structures" or "canonical forms". The real structure is an intermediate of these structures represented by a resonance hybrid.

•  The contributing structures are not isomers. They differ only in the position of electrons, not in the position of nuclei.

•  Each Lewis formula must have the same number of valence electrons (and thus the same total charge), and the same number of unpaired electrons, if any.

•  Bonds that have different bond orders in different contributing structures do not have typical bond lengths. Measurements reveal intermediate bond lengths.

•  The real structure has a lower total potential energy than each of the contributing structures would have. This means that it is more stable than each separate contributing structure would be.

Misconception

It is a common misconception that resonance structures are actual transient states of the molecule, with the molecule oscillating between them or existing as an equilibrium between them. However these individual contributors cannot be observed in the actual resonance-stabilized molecule. Any molecule or ion exists in only one form – the resonance hybrid. Due to confusion with the physical meaning of the word resonance, as no elements actually appear to be resonating, it has been suggested that the term resonance be abandoned in favor of delocalization. Resonance energy would thus become delocalization energy and a resonance structure becomes a contributing structure. The double headed arrows would be replaced by commas to illustrate a set of structures rather than suggesting that there is a reaction that converts among them.
Other Concepts :

Concept 1 :
Stability of carbocationStructure and properties

The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore, carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge. One could reasonably assume a carbocation to have sp³ hybridization with an empty sp³ orbital giving positive charge. However, the reactivity of a carbocation more closely resembles sp² hybridization with a trigonal planar molecular geometry. An example is the methyl cation, 

Main Concept :
Equilibrium constant expression

According to law of mass action “The rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants at a constant temperature at any given time.”

Consider a simple reversible reaction

b (At a certain temperature)

According to law of mass action

Where, K_C is called equilibrium constant. 
The important characteristics of equilibrium constant are discussed below:

1. The equilibrium constant has a definite value for every reaction at a particular temperature.

2. The value of equilibrium constant is independent of the original concentration of reactants.

3. The value of equilibrium constant tells the extent to which a reaction proceeds in the forward or reverse direction. If the value of K is larger, then the equilibrium concentration of the components on the right hand side of the reaction will be greater than the components on the left hand side of the reaction. Hence the reaction proceeds to a greater extent and vice versa.

4. The equilibrium constant is independent of the presence of catalyst. This is because the catalyst affects the rate of forward reaction and backward reactions equally.

5. For a reversible reaction, the equilibrium constant for the forward reaction is inverse of the equilibrium constant for the backward reaction i.e.
K forward reaction> = 1 / Kbackward reaction

6. Equilibrium constant is dependent on the temperature which is given as:

Where K₁ and K₂ are the equilibrium constants at temperature T₁ and T₂ respectively and ∆H is the enthalpy change for the reaction. It is assumed that ∆H is independent of the temperature.

Solved Examples

Q1) Consider the following reaction:

2 NO₂ (g) ⇌ N₂ (g) +2 O₂ (g)

The equilibrium constant for the reaction at 298 K is: