Thursday, 3 October 2019

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Saturday, 22 December 2018

Rate law and molecularity of reaction in chemical kinetics

Rate Law and Molecularity Law

Molecularity of reactions and Rate law of reactions, Molecularity Fully Explained,Comparison of molicularity and order of reaction Molecularity

Rate law

                Rate law gives the actual rate of reaction. In case of a complex reaction means which processed in two or more steps and the Rate-law corresponds to the slowest step of the reaction.
We see that for complex reactions law of mass action and rate law give different expressions for the reaction rate.
Important
For simple reaction: the law of mass action and rate law generally give the same expression for the reaction rate.
The slowest step in complex reactions gives the actual rate of reaction.
Rate law expression is useful in finding the order of reaction “rate law is the mathematical expression which denotes the experimentally observed rate of reactions in terms of the molar concentration of the reactant species at a given temperature.

Molecularity of reactions

Molecularity of a reaction is defined as the number of reactant species which collide simultaneously to brings about a chemical reaction.

Characteristics of molecularity

Molecularity of pertains to a single step reaction or a single step of a complex reaction which involves a number of steps for its completion. Molecularity of a reaction as a whole has no meaning.
Molecularity is always a whole number and is never infraction or zero.
The molecularity of a single step reaction or a step of a complex reaction does not exceed three.
Reactions with Molecularity equal to three are also rare. The simultaneous collision of the molecules means that the third molecule must collide the other two molecules at the same time when they are in the process of collision. The chances for the occurrence of such collisions are very rare. It has been found that there are certain reactions which involve a large number of species but still, they are very fast.
Such a reaction cannot take place in one step and are said to proceed through a sequence of two or more consecutive steps and thus called complex reactions.

Order of reaction

It is defined as the sum of the exponents to which the molar concentration terms in the rate law are raised to express the observed rate of a reaction.

Consider a single step reaction : mA+ nBžproduct
For this, the order of reaction = m+n

Comparison of molecularity and order of reaction
Molecularity

Its value does not exceed three.
Depends upon the conditions of a reaction
Molecularity is the number of molecules of the reactant species taking part in a single step chemical reaction.
It is always a whole number.
Molecularity pertains to one step reaction which involves several steps for its completion.
It can be obtained from a single balanced chemical equation.
It does not change with the change in temp.  and pressure.
Order of reaction
It also does not exceed three.
It also depends upon conditions of a reaction.
Order of reaction is the sum of exponents to which molar concentrations of the reactants are raised in the rate law equation of a chemical reaction.
Order of reaction pertains to reactions as a whole irrespective of the intermediate steps involved for its completion.
It cannot be obtained from a single balanced chemical equation and can be determined only experimentally.
It changes with the change in temperature and pressure.









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Rate Of Reactions and how factor are affecting

The Rate of Reactions in Chemical Kinetics easy To Understand 

When we listen to the term Rate what we thought normally absolutely we thought about the speed of something but in Chemistry concept many reactions take place and those have some Rate(speed) to complete the process there are three types of Rate Of Reactions like fast reaction, moderate reaction, and slow reaction.

Rate Of Reactions and how factor are affecting,affect the rate of reaction,activation energy,increase the concentration of reactant,rate of a chemical reaction

The Rate of Reaction definition

"The quantity of a reactant species consumed or the quantity of a product species formed in unit time in a chemical reaction is called the rate of reaction"  
There are three types of reactions

  1. Fast Reactions or Instantaneous Reactions
  2. Moderate Reactions
  3. Slow Reactions

Fast Reactions

These reactions are completed immediately. These reactions are mostly ionic in nature. It is difficult to determine the Rate of these reactions.
The reactions which are taken a very short time period for completion is called the fast reaction. Here Rate of Reaction cannot be measured accurately.
Ex- 
AgNO3  (aq) +  NaCl (aq) → NaNO3 (aq) + AgCl (solid)

Moderate Reactions

These Reactions are completed in a measurable time period. These are mostly molecular reactions.
The Reactions which takes measurable time period for completion is called Moderate Reaction. There are Rates of reaction can be measured accurately.
Ex-
CH3COOC2H5  +  NaOH → CH3COONa  + C2H5OH

Slow Reactions

These Reactions Consume much time to be completed.
The Reaction which takes a very long time period for completion is called the slow reaction.
Ex-
Fixation of atmospheric nitrogen, Oxidation of copper in the presence of moisture to form basic copper carbonate, Rusting of iron in the presence of moisture.
Fe  +  H2O + CO2 → Fe(OH)(HCO3)

Average Rate Of  Reactions

Also, we can determine the average rate of reaction as follows
It is defined as the rate of chemical reaction change of Concentration per unit time 
= Change of concentration in given time Per  Total Time Taken = Δx/Δt
Rate Of Reactions and how factor are affecting,affect the rate of reaction,activation energy,increase the concentration of reactant,rate of a chemical reaction

Instantaneous Rate

In case of a reaction, the average rate has no meaning since the rate of reaction is always changing with time. As it is well known from the law of mass action that the rate of a reaction at any instance is directly proportional to the concentration or active mass of the reactants at that time. This rate is called Instantaneous rate and can be defined as the rate of change of concentration of any one of the reactants or products at a specific instant of time.
If during an infinitesimally (very very short) small interval of time, During which rate must have remained constant and dx should be a very small change in concentration, then,
    [    Instantaneous rate of reaction = Δx/Δt =LtΔt→0 ×  Δx/Δt    ]                 

Some Factor affecting the rate of reaction

The concentration of the reaction: 

We know that the molar concentration of the reactants is maximum to start with and thus, the rate of reactions is also maximum in the beginning. As the reaction proceeds, the molar concentration of reactant goes on decreasing and hence the rate also decreases. Clearly, the rate of reaction is directly proportional to the molar concentration of the reactant for example if a candle is lighted in a cylinder, the glow of light diminishes as the oxygen in the cylinder is consumed.
Rate Of Reactions and how factor are affecting,affect the rate of reaction,activation energy,increase the concentration of reactant,rate of a chemical reaction

Nature of the reactant:

The rate of reaction varies from one reaction to the other even if the concentration and temperature are kept the same
The reason for this is that reactant with weak bonds react quickly. Also, the reactant having strong chemical bonds break with difficulty and hence reaction rate is slow.

Temperature:

The rate of reaction greatly depends upon temperature. It has been found that the rate of reaction is almost doubled by an increase of just 100 C, experimentally it has been found that for each 100 C rise in temperature, the rate of decomposition of H2O2 increase by some factor.

Catalyst:

A catalyst is defined as a substance whose presence increases the rate of the chemical reaction. The catalyst does not itself undergo any change in the chemical composition at the end of the reaction.
Radiation:
                The rate certain reactions are considerably increased in pressure of radiations. The photons of these radiations process sufficient energies to break bonds in the reactant species. We know that H2 and Cl2 react very slowly in the absence of light but take place rapidly in presence of light.
                H 2 + Cl2 ž 2HCl
Such reactions are called photochemical reactions.

Surface area:

In heterogeneous reactions, the surface area of reactant plays an important role. With the increase in surface area or with the decrease in the particle size of reactants, the rate of reaction increases. The reason is that for the same mass of the substance, a larger surface is provided for the reaction. For example (i) coal dust burns brilliantly than a lump of coal (ii) wood chips burn rapidly than a log of wood.
Characteristics of the rate constant:
(1)Different reactions have different values of the rate constant at a given temperature.
(2)The higher value of K faster is the unity.
(3)The value of K at a given temperature is most characteristic of the reaction.
(4)For a reaction, the value of rate constant (K) changes with temperature.
(5)The unit of the rate constant depends upon the order of reaction.

How is the variability in oxidation states of transition metals different forming that of non-transition metals? illustrate with example.


How is the variability in oxidation states of transition metals different forming that of non-transition metals?  illustrate with an example.
                The number of variable oxidation states in transition metals is due to the participation of (n-1) d electrons in addition to ns electrons in the bond formation. Thus, they exhibit a large number of variable oxidation states. But non-transition metals generally of s-block do not show variable oxidation states because, by the loss of s-electrons, they acquire stable configuration.
Why do transition elements form colored compounds?
                Most of the d-block elements form colored compounds because of the presence of unpaired electrons in these. The electrons absorb radiations of one colour from white light for excitation from one energy level to another within the same d-subshell. Thus, the transmitted light appears coloured. If the compound absorbs blue light, then it appears orange and vice versa. Zn, Cd, and Hg are white because of the absence of unpaired electrons in the d-orbital’s.
Why Zn, Cd, and Hg are not regarded as transition elements?
                These metals do not have any partially filled orbital but are still regarded as transition elements. The reason is that these are quite similar to other transition elements in their chemical properties.
Why do transition elements show similarities along the horizontal period as well as down a vertical colour?
                The properties of an element mainly depend upon its valence shell configuration. In a period of d-block, the electrons are filled in the inner (n-1) d sub-shell while the valence shell remains unaffected. As a result, transition elements show similar behavior along the horizontal period. These elements also showed similar behavior down a vertical column due to the same valence shell configuration.
Most of the d-block elements are paramagnetic but zinc, cadmium, and mercury are diamagnetic. Why?
                Most of the d-block elements and their compounds are paramagnetic i.e. they are attracted by the magnetic field. It is because of the unpaired electrons in them. The spin movement of those unpaired electrons generates a magnetic field which can interact with the applied field. Zn, Cd, and Hg are diamagnetic due to the absence of unpaired electrons in them.
Explain why d-block elements are hard metals but Zn, Cd, and hg are soft.
                All the d-block elements are metals due to their low ionization potentials and the presence of vacant orbitals. These elements are harder and non-brittle than s-block elements. The reason is that these can form covalent bonds by the overlap of their partially filled d-orbitals. Zn Cd and hg are soft due to the absence of partially filled orbitals in them.

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Why are noble gases in general chemically inert?


Why are noble gases in general chemically inert?
Noble gases are generally chemically inert because.
(i)                  All these elements have stable ns2 np5 configuration. Helium has 1s2 configuration.
(ii)                They have high ionisation energies and thus have no tendency to lose electrons.
(iii)               They have zero electron affinity due to which they have no tendency to gain electrons.   
Why ease to liquefy noble gases increases with increase in atomic number?
                The ease of liquefaction of a gas depends upon the magnitude of attractive forces in its molecules. The attractive forces in such gases are Vander Waal’s forces and their magnitude increases with increase in atomic size. Since atomic size increases down the group, so the ease of liquefaction increases down the group.
Besides from atmosphere, how is helium obtained?
                Helium can also be obtained from natural gas which contains 2-7% helium. It can also be obtained by heating cleveite which is an ore of uranium.
Why XeF2 is linear in spite of the fact that Xe is sp3 d hybridised?
                The reason is that xe atom has three lone pairs of electrons and after various repulsions, it acquires linear structure.
Which noble gas has the highest boiling point?
                Xe is largest in size and the magnitudes of Vander Waal’s forces in it are maximum.  Hence it has the highest boiling point.
Give reason which prompted Berthelot to prepare first noble gas compound.
                The preparation of first noble gas compound, XePtCl6 was based on the chance discovery of O2PtCl6. Bartlett predicted that Xenon should react with PtCl6 to form the ionic compound because.
(i)                  The ionisation energies of O2 and Xe are comparable
(ii)                The molecular diameter of O2 and Xe are also similar.
He obtained a crystalline compound of composition, XePtCl6 on just mixing Xe and PtCl6 at room temperature.
                        Xe+PtCl6 (g) ž Xe+(PtCl6)-(s).
Which noble gas can be used in botanical gardens?
        Neon lamps are used in botanical gardens and also in green houses. This gas stimulates growth and is also effective in the formation of chlorophyll.
Which inert gas/gases do not form clatrate compounds?
        The small sized helium and neon do not form clathrate compounds whereas Ar, Kr and Xe do form.
Which noble gas can form interstitial compounds with metals?
        Only helium forms interstitial compounds with metals.
What is the utility of clathrate formation?
Clathrate formation help in the separation of noble gases.
For,  example, Neon can be separated from Argon, krypton and Xenon by forming clathrate with quinol. Neon does not form clathrate with quinol.
How will you justify the inclusion of noble gases in the zero group of the periodic table?
        The long form periodic table is based upon filling of last electrons in various shell of element. Noble gases due to their ns2 np6 configuration are expected to be placed in the p-block of the periodic table after the members of the halogen family. These elements due to their stable configuration do not have normally any tendency to either lose gain or share electrons. As a result, there valence is zero and therefore, they are said to belong to zero group of the periodic table.
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