Tiêu đề CHEMICAL KINETICS.pdf ✅

 Digital www.allendigital.in [ 41 ] Introduction Chemical Kinetics is the branch of physical chemistry which deals with the study of rate of reactions, the mechanism by which the reactions proceed and factors affecting rate of reaction. On the basis of rate, chemical reaction are broadly divided into three categories: (A) Very fast or instantaneous reactions: Generally these reactions involve ionic species and known as ionic reactions. These reactions take about 10-14 or 10-16 seconds for completion. So, it is almost impossible to determine the rate of these reactions. Examples: AgNO3 + NaCl → AgCl  + NaNO3 (white ppt) BaCl2 + H2SO4 → BaSO4  + 2HCl (white ppt) HCl + NaOH → NaCl + H2O (B) Very slow reactions: These reactions proceed very slowly, may take days or months to show any measurable change at room temperature. Examples: • Rusting of iron. • Reaction between H2 and O2 to form H2O at ordinary temperature in absence of catalyst. • CO + 2H2 ⎯⎯⎯⎯⎯⎯→ at room temperature CH3OH (C) Moderate or slow reactions: This type of reactions proceed with measurable rates at normal temperature and we can measure the rate of these reactions easily. Mostly these reactions are molecular in nature. Examples: • Decomposition of H2O2 2H2O2 → 2H2O + O2 • Decomposition of N2O5 2N2O5 → 4NO2 + O2 • Hydrolysis of ester CH3COOC2H5 + NaOH → CH3COONa + C2H5OH • Inversion of cane sugar in aqueous solution • Reaction of NO with chlorine NO + Cl2 → NOCl2 Very fast Or Instantaneous reactions Moderate Or Slow reactions Very Slow reactions Chemical Reactions 02 Chemical Kinetics
NEET : Chemistry [ 42 ] www.allendigital.in  Digital Rate of Reaction Rate of reaction is defined as the change in concentration or pressure of reactant or product per unit time. It is always a positive quantity. Where C = change in concentration in a small interval t [+] sign is used when we refer for product concentration. [–] sign is used when we refer for reactant concentration. For gaseous reactions r =  P t   (unit of rate = pressure time–1) and r = 1 RT × P t         (unit of rate = M time–1) Types of Rate of Reactions: (A) Average Rate of Reaction The rate of reaction over a certain measurable period of time during the course of reaction is called average rate of reaction. It is denoted by r . For a reaction A → B raverage = 2 1 2 1 [A] [A] [A] t t t   −    =   −  Where [A]1 = Concentration of reactant A at time t1, [A]2 = Concentration of reactant A at time t2. (B) Instantaneous Rate of Reaction The rate of reaction at any particular instant during the course of reaction is called instantaneous rate of reaction. For a reaction A → B Mathematically ; Instantaneous rate = t lim →0 (Average rate) inst t 0 t 0 [A] [B] r lim lim t t  →  →       = − =           or Hence, Slope of the tangent at time t in plot of concentration with time gives instantaneous rate of reaction. Instantaneous rate of reaction = t 0 C dC lim t dt  →       =     Rate of reaction = Change in concentration of reactant of product Time taken in change Types of Rate of reactions Average Rate of Reaction Instantaneous Rate of Reaction rinst = ሺ−ሻ dሾAሿ dt = ሺ+ሻ dሺBሻ dt Product Reactant Time Concentration t(time) [A] dt d[A] 
Chemical Kinetics  Digital www.allendigital.in [ 43 ] Rate of Reaction in the Form of Stoichiometry of a Chemical Reaction Let us consider a reaction: m1A + m2B → n1P + n2Q Where, Rate of disappearance of A = – d[A] dt Rate of disappearance of B = – d[B] dt Rate of appearance of P = d[P] dt Rate of appearance of Q = d[Q] dt 1 2 1 2 1 d[A] 1 d /[B] 1 d[P] 1 d[Q] Rate of Reaction m dt m dt n dt n dt     = − = − = =         • Rate of reaction is always positive; negative sign represents decrease in concentration of reactant. Units of Rate of Reaction Unit of rate of reaction = mol L–1 time–1 i.e. (mol L–1 s –1 or mol L–1 min–1 or mol L–1 h–1) Illustration 1: For the reaction N2 + 3H2 ⎯⎯→ 2NH3, if rate of appearance (ROA) of NH3 = 2×10–4 mol L–1 s –1 Then calculate rate of disappearance (ROD) of N2 & H2 and also calculate rate of the reaction ? Solution: N2 + 3H2 → 2NH3 ROR = 1 × ROD of N2 = 1 3 × ROD of H2 = 1 2 × ROA of NH3 ROD of N2 = 1 2 × 2 × 10–4 = 10–4 ms–1 ROD of H2 = 3 2 × 2 × 10–4 = 3 × 10–4 ms–1 ROR = 1 2 × ROA of NH3 = 1 2 × 2 × 10–4 = 10–4 ms–1 Illustration 2: If the reaction 2NO2 → 2NO + O2, the rate of formation of NO is 6 g min–1, calculate rate of disappearance of NO2 in g min–1 . Solution: 2NO2 → 2NO + O2 ROR = 1 2 × ROD of NO2 = 1 2 × ROF of NO = 1 × ROF of O2 ROD of NO2 = 2 2 × ROF of NO ROD of NO2 = 1 × 6 30 mol min. = 0.2 mol min–1 (molar mass of NO = 30) 2 1 gmin (ROD of NO ) − = 0.2 × 46 = 9.2 g min–1 (molar mass of NO2 = 46)
NEET : Chemistry [ 44 ] www.allendigital.in  Digital Illustration 3: A gaseous hypothetical chemical equation 2A ⇌ B+C is carried out in a closed vessel. The concentration of B is found to increase by 5 × 10–3 mol L–1 in 10 second. The rate of appearance of B is : Solution: Increase in concentration of B = 5 × 10–3 mol L–1 Time = 10 sec Rate of appearance of B = increase of conc. B Time taken − −  = 3 1 5 10 molL 10 sec = 5 × 10–4 mol L–1 sec–1 1. The rate of a reaction is expressed as :                 + = = − = −             1 C 1 D 1 A B 2 t 3 t 4 t t Then reaction is (1) 4A + B → 2C + 3D (2) B + 3D → 4A + 2C (3) A + B → C + D (4) B + D → A + C 2. In the reaction, A + 2B → 6C + 2D if   −  A t is 2.6 × 10–2 M s–1, what will be the value of   −  B t ? (1) 8.5 × 10–2 M s–1 (2) 2.6 × 10–2 M s–1 (3) 5.2 × 10–2 M s–1 (4) 7.5 × 10–2 M s–1 3. In the following reaction, how is the rate of appearance of the underlined product related to the rate of disappearance of the underlined reactant ( ) ( ) ( ) ( ) ( ) − − + + + → + 1 BrO aq 5Br aq 6H aq 3Br 3H O aq 3 2 2 (1) ( )   −       − =   BrO3 Br2 t t (2) ( )   −      − =   1 BrO3 Br2 3 t t (3) ( )   −       − =   BrO3 1 Br2 t 3 t (4) None of these Rate Law The experimental expression of rate of reaction in terms of concentration of reactants is known as rate law. In this expression the rate of a reaction is proportional to the product of molar concentration of reactants with each term raised to the power or exponent that has to be found experimentally. In a chemical reaction: aA + bB → Product The rate law is: The values of exponents x and y are found experimentally which may or may not be same as stoichiometric coefficients. Above relationship can be written as :- Rate = k[A]x[B]y Where k is a proportionality constant known as rate constant. BEGINNER’S BOX-1 Rate  [A]x [B]y