Hack tech fact

 Order of complex reactions    The order of a complex reaction, i.e., a multi step reaction is determined by taking its slowest step known as rate determining step.         For example, in the reaction  2NO² + F² --> 2NO²F, the number of molecules involved is three, but the rate of the reaction determined experimentally is [NO²][F²]. Thus it's order is 2.                     Thus possible mechanism for the reaction, may be written as  NO² + F² --> NO²F + F ( Slow)  NO²F + F --> NO²F²     (Fast)  Example:  Reaction between NO and H² 2NO + H² -slow-> N² + H²O² H²O² + H² -fast-> 2H²O  

 Fraction order reaction The reaction whose order is a fraction is called a fractional  order reaction.  Example:       (1) - Thermal decomposition of acetaldehyde                   ∆ CH³CHO ---> CH⁴ +CO, rate = k[CH³CHO]¹.⁵ (2)- Reaction between H² and Br² to form HBr  H²(g) + Br²(g) --> 2HBr, rate = k[H²] [Br²]¹/²

Third order reaction:  In this reaction, the rate of the reaction depends upon the concentration of three reacting species. Example:  (1) Reaction between nitric oxide and oxygen 2NO +O --> 2NO²; rate k[NO]²[O²]  (2) Reaction between nitric oxide and bromine   2NO + Br² --> 2NOBr; rate = k[NO]²[Br²]

Second order reaction:  In this reaction, the rate of the reaction depends upon the concentration of two reacting species.  Example:  (1) Decomposition of nitrogen peroxide  2NO² --> 2NO + O², rate = K [NO²]² (2) Reaction between H²(g) and I²(g) to from HI(g) H²(g) + I²(g) 2HI(g), rate =  k[H²] [I²]  (3) Alkali hydrolysis of ester  CH³COOC²H⁵ + NaOH--> CH³COONa + C²H⁵OH, rate = k[CH³COOC²H⁵] [NaOH].

First order reaction: The reaction in which the rate of the reaction depends on the concentration of one reacting species is called a first order reaction.  Example: (1) Decomposition of nitrogen pentoxide in CCl⁴ solution                                      1 N²O⁵ -CCl⁴-> 2NO² + --- O², rate = k[N²O⁵] (2) Decomposition of ammonia nitrite in aqueous solution NH⁴NO² ---> N² + 2H²O, rate = k[NH⁴NO²] (3) Decomposition of hydrogen peroxide in the presence of pt catalyst                                 1 H²O² -Pt-> H²O + ---   O², rate = k[H²O²]  (4) Hydrolysis of methyl acetate in aqueous solution  CH²COOCH³ + H²O --H+--> CH3OOH + CH³OH  (5) Inversion of Cane sugar  C¹²H²²O¹¹ + H²O --H+--> C⁶H¹²O⁶+H²O Note: In the example 4 and 5, the concentration of H²O does not...

Order of a reaction:   Order of a reaction may be defined as - the sum of the power (coefficient) of the concentration terms of the reactants occurring in the rate determining step of a chemical reaction. Consider the following general reaction.                         aA + bB ---> products                        Rate = k [A]a × [B]b  Order of the reaction = a + b           The order of a reaction can only be determined from experiment. Thus, order of reaction is defined as the sum of powers raised on concentration terms in order to write rate expression from experimental evidences. 

Zero order reaction: The reaction in which the rate of the reaction is independent of the concentration of the reacting species is called a zero order reaction. Most of these reactions are heterogeneous in nature and take place on the surface of the crystalyst. Example:  (1)  photo chemical reaction between hydrogen and chlorine. H²(g)+Cl²(g)--hv-->2HCl(g), rate = K[H²]°[Cl²]° (2) Thermal decomposition of Hl on the surface of gold as catalyst                  ∆ 2NH³(g) -----> H²(g) +I²(g), rate                  Au = k[HI]° (3) Thermal decomposition of NH³ on the surface of pt catalyst at high pressure.                  ∆ 2NH³(g) -----> N²(g), rate = k[NH³]°                  Pt

Secondary cells : These are also called as storage cells or accumulators. In these cells the chemicals can be brought back to their initial state by charging the exhausted cell with electric current from an external source. Thus, the secondary cells can be used again and again by changing them. The electrical energy can be stored in these cells and these cells are called storage cells accumulators. The lead storage battery (or Acid storage cell) nickel - cadmium storage cells, and alkali storage cell (or, the Edison accumulator) are the secondary cells.

Primary cells:  In these cells, the chemicals used up during the supply of electrical energy cannot be regenerated by passing current into the used up cell from external source. In other words, the cell reaction is not reversible. It is, therefore, after sometimes, the battery becomes dead. Such primary cells are called dry cells. The dry cells are used in torches, radio receivers, electronic calculators, hearing aids, etc.

 Measurement of electrode potential:    The absolute electrode potential of an electrode can not be measured due to the following reasons :  (1) No half cell either oxidation or reduction can take place independently and can work only when both are connected. (2) The electrone releasing or accepting tendency of an electrode is only relative tendency and not absolute tendency.     Therefore, in order to measure the standard electrode potential of a half call, a reference electrode is required and an arbitrary electrode potential is assigned to it. The commonly used reference electrode is standard hydrogen electrode (SHE) also called normal hydrogen electrode (NHE) and it's standard electrode potential (oxidation or reduction) is taken as zero.

 Concentration of ions in the solution :  In the case of zinc in contact with Zn²+ ions in solution, Zn(s) <=> Zn²+(aq) + 2e an increase zinc ion concentration (i.e, oxidation) will tend to shift the equilibrium to the left. I.e., will decrease the electrode potential. Similarly, a decrease in zinc ion concentration (reduction) will decrease in zinc ion concentration (reduction) will increase the electrode potential. In other  words, the reduction potential of an electrode (E red) is proportional to the concentration of ions, and the oxidation potential of an electrode (E ox) is inversely proportional to the concentration of ions.

Single electrode potential:  In the preceding section, we knew that the potential of a single electrode in a half cell is called the single Electrode potential. In a Daniell cell in which the electrodes are not connected extremely, the anode, Zn(s)/Zn²+(aq). Cu²+(aq)/Cu(s) develops a nagative charge and develops a positive charge, respectively. The amount of charge produced on an individual electrode determines its single electrode potential.      It should be stated here that it is not possible to measure the single electrode potential experimentally. It can be determined in conjunction with a reference electrode thus constituting a complete electrochemical cell. Measuring the potential difference between the two electrodes of the complete cell and from the known potential value of the reference electrode, the potential of the required electrode can be determined.

Electrolytes :  These are the substances which conduct electricity in fused state or molten state or in aqueous solutions and undergo chemical decomposition. Their conduction are due to the presence of free ions. As solid state donot contain any free ions, thus, these substances are bad conductor of electricity in the state. The conductance of the electrolyte solution is called Electrolytic conductance or Electrolytic conduction.            Example : NaCl, HCl, H²SO⁴ etc.       An electrolyte in its fused state or in aqueous solution contains no detectable concentration of electrons. It conducts electricity not by virtue of the flow of electrons but as a result of the movement of ions which are electrically charged. Is why electrolytes are also called ionic conductors.

 Function of semi-permeable:  It may be recalled that when a solvent and a solution, or two solutions of different concentrations are separated by a semipermeable membrane (abbreviated as SPM), osmosis takes place resulting in the flow of solvent molecules from the solvent (or from a less concentrated solution) side to the solution (or to the more concentrated solution) side. Now the question is - How does the SPM work in osmosis?                    It is considered that the solvent (water) molecules get absorbed on on the surface of the SPM. The adsorbed layer on SPM leads to interaction between the solvent molecules on both side of the SPM. As a result, the solvent molecules can permeate through the SPM. Since the solute molecules don't get absorbed on the membrane, they cannot permeate.

 Effects of temperature:  In general, the solubility of a gas in a liquid decrease with increase of temperature. The process of solubility of gas in a liquid is exothermic in nature. It is because during the dissolution, a gas contract in volume and as a result, energy is released.                         F Gas + liquid  ---> Dissolved gas, ∆H _ ve                       <---                         B As per Le Chatelier's principal, the increase of temperature favours the reverse process and thus solubility of a gas decrease with rise of temperature.

 Nature of gas and solvent:  The solubility of gases in liquids depends both upon the nature of gas and the liquids (solvent).  (1) Generally gases which are easily liquefiable are more soluble in common solvents. For example, CO² is more soluble in water than H² and O².  (2) The solubility of a gas also depends upon the chemical nature of the solvents. For example, nitrogen and oxygen are much more soluble in ethyl alcohol than in water at the same temperature and pressure. It is because of the chemical similarity between the gas and the solvent. (3) The gases which are capable of forming ions in aqueous solution are much more soluble in water than in other solvents. For example, gases like HCL and NH³ are highly soluble in water but not in organic solvents in which they do not ionise.

 Colligative properties of dilute solutions :    In the preceding section, we described physical properties of pure liquids. In a binary solution behaving ideally, it was considered that the vapour pressure of the solution is the sum of the crystal partial pressure of the components constituting the solution. But, when a non-volatile solute, (e.g., sugar) is added to a volatile solvent, it is found that the vapour pressure of the solvent is lowered. In other words, the addition of a non-volatile solute to a volatile solvent alters or modifies the properties of the solution. In such cases the properties of the solution depends on the number of solute particles ( atoms, molecules or ions) and not on the nature of the solute particles. Now, we use the term colligative to  describe those properties of a solution which may be determined by taking simple arithmetic averages of the properties of solute and solvent.         The colligative properties of s...

 Why ferromagnetic substances make better permanent magnets? Ans- This is because the metal ions in a ferromagnetic substances are grouped into small regions called domains and each domain acts at a tiny magnet. These domans are randomly oriented, but when a ferromagnetic substances is placed in a magnetic field all the domains get oriented in the duration of the magnetic field which produces a strong magnetic field. This permanent magnetism persists even when the external magnetic field is removed. Therefore, ferromagnetic substances make better permanent magnet.

 Application of n -type and p - type semiconductors : n-type and p-type semiconductors have wide applications in the field of electronics. Some of its applications are described below.  1- A diode is a combination of n-type and p-type semiconductors which is used used as a rectifier. 2- The solar cell is an efficient photo-diode used for transformation of light energy into the electric energy.  3- n-type and p-type semiconductors are used detect radio or audio signal. 4- Due to fast response of Galinium arsenide it is widely used in semiconductor device.

 Thermal conductivity :  Metals are good conductor of heat. This is due to transfer of heat energy from one end to other by the free electrons. Tensile strength : Metals have high tensile strength, i.e. they can be stretched without breaking. This is due to the relatively strong electrostatic forces of attraction between the +ve Kernels and mobile valence electrons.

Anti-ferroelectricity: When the alternate dipole are aligned in opposite direction, the net dipole moment becomes zero and in such cases the crystal is called anti- ferroelectric and the phenomena is called anti-ferroelectricity. Examples of such solids lead zirconate  (PbZrO³)

Malleable and ductile: Metals are malleable and Ductile. The metals can be easily drawn into wires and can be twisted. This is due to the non-directional nature of metallic bond. When a deformation force is applied, the kernels of metals can slip over each other. Thus the crystal lattice gets deformed by the slippage of adjacent layers.

Hardness of metals: The metals are generally hard. The hardness of metals is due to strong metallic bond present in them. More is the strength of metallic bond, harder is the metal. The strength of metallic bond depends upon the number of valence electrons and size of kernel. More is the number of valence electrons for delocalisation, stronger is the metallic bond. Similarly, smaller is the size of +ve Kernels, greater is the force of attraction for delocalised electrons and stronger is the bond. For examples, alkali metals due to the presence of only one electron in their valey shell and larger size of atom form weak metallic bond and thus are soft metals.

Electrical conductivity of metal : Metals are good conductor of electricity which is due to the presence of mobile electrons in the crystal lattice of the metals. It has been found that the increase of temperature decreases the electrical conductivity of metals. This is because increase of temperature increases the Kinetic energy of the metal atoms which start vibrating the Kernels of metal vigorously. This interacts with the movement of electrons and make its movement slow and thus conductivity decreases.

 Application of super conductors :   1- In the field of electronics as super conducting cables, electronic devices. 2- In power transmission sector and in levitation transportation, i.e., trains which move in air without rail. 3- In cryogenic gyro operated instruments. 4- In lazer technology and computers. 5- In building supermagnets. 

Piezoelectricity : The crystals in which the dipoles align themselves in the same direction leading to resultant dipoles when certain external mechanical stress or pressure is applied on it are called piezoelectric and this phenomenon is called piezoelectricity. When some pressure (stress) is applied on such crystals there is displacement of ions in the crystal which result in the formation of electric current. Therefore, such crystals are used as mechanical electrical transducers. For example, such crystals are used as pickups in record players as they produce electric signals when pressure is applied on them. Piezoelectric crystals are also used in microphones, ultrasonic generation, sonar detectors etc.        The reverse effect may also produce in these crystal when placed in an external electric field leading to the deformation .

Ferroelectricity: The dipoles in certain solids are spontaneously aligned, in a particular direction, even in the absence of electric field. Such substances are called ferroelectric substance and the phenomenon is called ferroelectricity. However, the direction of polarisation in these substances can be changed on applying electricity. Examples of ferroelectric solids are (BaTiO³), Rochelle salt (sodium potassium tartarate), potassium dihydrogen phosphate) (KH²PO⁴) etc.

Paramagnetic substances:   The substances which are weakly attracted by magnatic lines of force are called paramagnetic substances and this phenomenon is called paramagnetism. These substances possesses permanent magnetic dipoles due to the presence of unpaired electrons. These substances lose their magnetic behaviour in the absence of an external magnetic field i.e. their magnetic behaviour is temporary. Examples, many transition metals and their ions i.e. TiO², VO², CuO, FeSO⁴ etc.

Ferromagnetic substances :  These are the substance which are strongly attracted by the magnetic field and exhibit permanent magnatic behaviour even when magnetic field is removed. Examples, Fe,Co,Ni,CrO².      The properties of ferromagnetism arises due to the alignment (orientations) of magnatic moment in the direction of the external magnetic field and as a result a strong magnetic field is produced and the substance behaves like a permanent magnet. For example, CrO² is used to make magnetic tapes in audio and video recorders.

Frenkel defect: This defect was discovered by russian scientist Frenkel in 1926 in ionic crystals. This defect arises when certain ion found missing from its normal sites and occupy positions elsewhere in the interstitial sites in the crystal lattice. In this case, the crystal remain electrically neutral as the number of cation and anions remain same. This defect is shown below where a cation A+ is found missing from its normal site and present in one of the interstitial sites in the crystal lattice.

About Schottky defect:   This defect arises in the ionic and was discovered by German scientist Shottky in 1930. This defect arises when equal number of +ve and -ve ions are found missing from their normal lattice sites. Taking example of an ideal crystal A+B- the arrangement of ions in an ideal crystal and Schottky defect crystal are shown below   An ideal crystal Schottky defect in crystal In case of (I) Strongly ionic compounds having high co-ordination number and (ii) Cation the anions having almost equal sizes.