Chemistry Test - 12

For 75% completion we need first and second t_1/2 = 16 + 11.3 = 27.3 min

Key Concept

n^th order reaction

Consider the reaction:

Tin & Lead have low melting points and hence can be purified by liquation process.

Main Concept :

Liquation process (Bi, Sn, Pb) This process is used for refining easily fusible metals like lead and tin. The impure metal is heated on the sloped hearth of a reverberatory furnace. The metal melts and flows down leaving the impurities.

Liquation is a metallurgical method for separating metals from an ore or alloy. The material must be heated until one of the metals starts to melt and drain away from the other and can be collected. This method was largely used to remove lead containing silver from copper, but it can also be used to remove antimony minerals from ore, and refine tin.

At constant temperature PV constant On doubling P and V with constant T, the equilibrium constant (K) will remain constant. equilibrium constant (K) depends only on temperature so does not alter on changing P & V.

KEY CONCEPTS

Characteristics of equilibrium constant (K_C)

Characteristics of equilibrium constant:

​1) Equilibrium constant has a value at a given temperature but changes with change in temperature.

2) At a given temperature and pressure, its value remains constant irrespective of concentration or pressure of the reactants and products.

3) If the concentration ot pressure change, the equilibrium shifts but equilibrium constant does not change.

4) The value of the equilibrium constant does not change with the presence of catalyst.

5) At a given temperature, the equilibrium constant denotes to which extent the reaction proceeds and in which direction. If K_c > 1; forward reaction takes place more than backward reaction ; if K_c < 1 backward reaction takes place more than forward reaction. If K_c = 1 both forward and backward reactions proceed equally.

6) Equilibrium constant (K_c) and its characteristics

Some main characteristics of equilibrium constant are described below:

•  The value of the equilibrium constant of a reaction is the same, at constant 'T' and 'P'. However, if either temperature 'T' or pressure 'P' or both are changed, the value of the equilibrium constant may also change. For example, the value of K_p (equilibrium constant in terms of partial pressures) for the reaction,

When the transition element absorb violet light it emits yellow green light

Main Concept :
Examples on Colour of coordination complex ion

Description
Colours of Coordination Complexes:
Crystal Field Splitting :When ligands attach to a transition metal to form a coordination complex, electrons in the d orbital split into high energy and low energy orbitals. The difference in energy of the two levels is denoted as Δ, and it is a characteristic property of both of the metal and the ligands. This is illustrated in the "o" subscript on the Δ indicates that the complex has octahedral geometry.

If  Δ₀  is large, and much energy is required to promote electrons into the high energy orbitals, the electrons will instead pair in the lower energy orbitals, resulting in a "low spin" complex; however, if  Δ₀  is small, and it takes little energy to occupy the higher orbitals, the electrons will do so, and remain unpaired (until there are more than five electrons), resulting in a “high spin” complex. Different ligands are associated with either high or low spin—a "strong field" ligand results in a large  Δ₀  and a low spin configuration, while a "weak field" ligand results in a small  Δ₀  and a high spin configuration. For more details, see the Crystal Field Theory (CFT) page.

A photon equal to the energy difference  Δ₀  can be absorbed, promoting an electron to the higher energy level. As certain wavelengths are absorbed in this process, subtractive colour mixing occurs and the coordination complex solution becomes coloured. If the ions have a noble gas configuration, and have no unpaired electrons, the solutions appear colourless; in reality, they still have a measured energy and absorb certain wavelengths of light, but these wavelengths are not in the visible portion of the EM spectrum and no colour is perceived by the eye.

In general, a larger Δ indicates that higher energy photons are absorbed, and the solution appears further to the right on the EM spectrum shown. This relationship is described in the equation  where h and c are constants, and λ is the wavelength of light absorbed

Using a colour wheel can be useful for determining what colour a solution will appear based on what wavelengths it absorbs. If a complex absorbs a particular colour, it will have the appearance of whatever colour is directly opposite it on the wheel. For example, if a complex is known to absorb photons in the orange range, it can be concluded that the solution will look blue. This concept can be used in reverse to determine  Δ  for a complex from the colour of its solution.

Relating the Colours of Coordination Complexes to the Spectrochemical Series

According to the Crystal Field Theory, ligands that have high spin are considered "weak field" and ligands that have low spin are considered "strong field." This relates to the colours seen in a coordination complex. High spin ligands induce the absorption of longer wavelength (lower frequency = lower energy) light than complexes with low spin ligands since their respective  Δ₀  values are smaller than the electron pairing energy.

The energy difference,  Δ₀  , determines the colour of the coordination complex. According to the spectrochemical series, the high spin ligands are considered "weak field," and absorb longer wavelengths of light (weak  Δ₀  ), while complexes with low spin ligands absorb light of greater frequency (high  Δ₀  ). The colour seen is the complementary colour of that of the wavelength absorbed. To predict which possible colours and their corresponding wavelengths are absorbed, the spectrochemical series can be used:

KEY CONCEPT

Haloform reactions of aldehydes and ketones

The Haloform Reaction

Methyl ketones typically undergo halogenation three times to give a trihalo ketone due to the increased reactivity of the halogenated product as discussed above. This trihalomethyl group is an effective leaving group due to the three electron withdrawing halogens and can be cleaved by a hydroxide anion to effect the haloform reaction. The product of this reaction is a carboxylate and a haloform molecule. Overall the haloform reaction represents an effective method for the conversion of methyl ketones to carboxylic acids. Typically, this reaction is performed using iodine because the subsequent iodoform (CHl₃) is a bright yellow solid which is easily filtered off.

Aldehydes carry a hydrogen atom next to their carbonyl group. This hydrogen is activated by the carbonyl group and is readily oxidised to OH. Aldehydes are therefore readily oxidised to carboxylic acids.

Ketones are not readily oxidised at all. They have no effect on mild oxidising agents. This is because they do not have an oxidisable hydrogen atom joined to the carbonyl group.

a) When warmed in Fehlings solution, an alkaline solution of Cu²⁺ ions (a red precipitate of copper (I) oxide) is produced with an carboxylic acid.

This problem is based on conceptual mixing of equilibrium constant and pH of solution.

Main Concept :
Ionisation constants of weak acid and weak base

The strength of an acid or a bas is experimentally measured by determining its dissociation or ionisation constant.

When acetic acid (a weak electrolyte) is dissolved in water, it dissociates partially into H⁺ or and H₃O⁺ and 

ions and the following equilibrium is obtained,

CH₃COOH+H₂O⇌CH₃COO⁻+H₃O⁺

Applying law of chemical equilibrium,

The sulphur containing organic compounds when fused with sodium metal give Na₂S which react with lead acetate and forms black ppt. of PbS.

Conjugated dienes (2 & 3) are more stable than non-conjugated dienes. Also more substituted double bonds are more stable due to hyper conjugation

stability 3 > 2 > 4 > 1

KEY CONCEPTS

Hyperconjugation - Stability of alkenes

•   The stability of unsaturated hydrocarbons like nitriles, alkenes effects with hyper conjugation. The more possible contributing structures in hyper conjugation increase the stability of molecule.

•  Since hyper conjugation mainly involves alpha carbon-hydrogen sigma bond and π-electrons, therefore as the number of α, σ bonds increases, hyper conjugation increases.

•  For example, 2-butene consists of six alpha carbon-hydrogen sigma bonds while there are only 2 carbon-hydrogen bonds next to double bonded carbon atom in 1-butene. Hence 2-butene shows six contributing structures while 1-butene shows only two which make 2-butene more stable compare to 1-butene.

•  This rule is applicable on other alkenes also. Hence as the number of alkyl group on double bonded carbon atoms increases, hyper conjugation increases which stabilized the molecule.
Or more substituted alkenes are more stable than less substituted alkenes.

When an alkali metal halide heated in presence of vapours of alkali metal. The metal vapours enter the crystal lattice for cation & anion vacancy generated, Na⁺ of Metal atom occupies cation vacancy and e^- will occupy the anion vacancy of Cl^-. This entrapped electron is called F centre and is responsible for colour.
Main Concept :
Metal excess defectsMetal Excess Defects:

Metal excess defects are of two types:

(a) Metal excess defects due to anionic vacancies:

These type of defects seen because of missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound. Hole occupied by electron is called F-centre and responsible for showing colour by the compound.

This defect is common in NaCl, KCl, LiCl, etc. Sodium atoms get deposited on the surface of crystal when sodium chloride is heated in an atmosphere of sodium vapour. In this process, the chloride ions get diffused with sodium ion to form sodium chloride. In this process, sodium atom releases electron to form sodium ion. This released electron gets diffused and occupies the anionic sites in the crystal of sodium chloride; creating anionic vacancies and resulting in the excess of sodium metal.

The anionic site occupied by unpaired electron is called F-centre. When visible light falls over the crystal of NaCl, the unpaired electron present gets excited because of absorption of energy and impart yellow colour.

Because of similar defect if present, crystal of LiCl imparts pink colour and KCl imparts violet.

(b) Metal excess defect due to presence of extra cations at interstitial sites:

Zinc oxide loses oxygen on heating resulting the number of cations (zinc ion) become more than anions present in zinc oxide.

The excess cations (Zn++ ions) move to interstitial site and electrons move to neighbouring interstitial sites. Because of this zinc oxide imparts yellow colour when heated. Such defects are called metal excess defects.

Metal excess anionic and cationic:

ZnO heated in Zn vapour ⟶ ZnyO (y > 1)

The excess cations occupy interstitial voids.

The electrons (2e^-) released stay associated to the interstitial cation.

Other Concepts :

Concept 1 :
Non-stoichiometric defectsNon-stoichiometric defects: The defects which disturb the stoichiometry of the compounds are called non-stoichiometry defects. These defects are either due to the presence of excess metal ions or deficiency of metal ions.

(a) Metal excess defects due to anion vacancies: A compound may have excess metal anion if a negative ion is absent from its lattice site, leaving a ‘hole’, which is occupied by electron to maintain electrical neutrality. This type of defects are found in crystals which are likely to possess Schottky defects. Anion vacancies in alkali metal halides are reduced by heating the alkali metal halides crystals in an atmosphere of alkali metal vapours. The ‘holes’ occupy by electrons are called F-centres (or colour centres).

(b) Metal excess defects due to interstitial cations: Another way in which metal excess defects may occur is, if an extra positive ion is present in an interstitial site. Electrical neutrality is maintained by the presence of an electron in the interstitial site. This type of defects are exhibit by the crystals which are likely to exhibit Frenkel defects e.g., when ZnO is heated, it loses oxygen reversibly. The excess is accommodated in interstitial sites, with electrons trapped in the neighborhood. The yellow colour and the electrical conductivity of the non-stoichiometric ZnO is due to these trapped electrons.

Consequences of Metal excess defects

The crystals with metal excess defects are generally coloured due to the presence of free electrons in them. The crystals with metal excess defects conduct electricity due to the presence of free electrons and are semiconductors. As the electric transport is mainly by “excess” electrons, these are called n-type (n for negative) semiconductor.

The crystals with metal excess defects are generally paramagnetic due to the presence of unpaired electrons at lattice sites.

When the crystal is irradiated with white light, the trapped electron absorbs some component of white light for excitation from ground state to the excited state. This gives rise to colour. Such points are called F-centres. (German word Farbe which means colour) such excess ions are accompanied by positive ion vacancies. These vacancies serve to trap holes in the same way as the anion vacancies trapped electrons. The colour centres thus produced are called V-centres.

(c) Metal deficiency defect by cation vacancy: In this a cation is missing from its lattice site. To maintain electrical neutrality, one of the nearest metal ion acquires two positive charge. This type of defect occurs in compounds where metal can exhibit variable valency. e.g., Transition metal compounds like NiO, FeO, FeS etc.

(d) By having extra anion occupying interstitial site: In this, an extra anion is present in the interstitial position. The extra negative charge is balanced by one extra positive charge on the adjacent metal ion. Since anions are usually larger it could not occupy an interstitial site. Thus, this structure has only a theoretical possibility. No example is known so far.

Consequences of metal deficiency defects

Due to the movement of electron, an ion A⁺ changes to A⁺² ions. Thus, the movement of an electron from A⁺ ion is an apparent of positive hole and the substances are called p-type semiconductor.