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Physical chemistry 2
Physical chemistry 2 ۲ كيمياء فيزيائية Dr. Tahani M Bawazeer BSc, MSc, PhD Room: 372/256 E-mail: tmbawazeer@uqu.edu.sa Umm Al-Qura University College of Applied Science Chemistry Department Outline ² Pseudo order reaction ² Half-life of a Reaction ² Calculation of Time for Completing any Fraction of the Reaction ² Comparison between the different order reactions Umm Al-Qura University College of Applied Science Chemistry Department Pseudo Order Reaction A+B p • The experimental order which is not the actual one. Hydrolysis of an ester : rate = k[A][B] rate = k`[A] CH3COOC2H5 + H2O CH3COOH + C2H5OH Rate = k'[CH COOH] Umm Al-Qura University College of Applied Science Chemistry Department Half-life of a Reaction The time required for the concentration of a reactant to decrease to half its initial value. First order: (1) half-life for a first order reaction is independent of the initial concentration. (2) it is inversely proportional ot k, the rate-constant. Umm Al-Qura University College of Applied Science Chemistry Department Half-life of a Reaction Concentration of A as a function of time for a 1st reaction. Umm Al-Qura University College of Applied Science Chemistry Department Concentration of A as a function of time for a 2ed reaction. Calculation of Time for Completing any Fraction of the Reaction Umm Al-Qura University College of Applied Science Chemistry Department Example: Umm Al-Qura University College of Applied Science Chemistry Department PROBLEM 1. Compound A decomposes to form B and C the reaction is first order. At 25°C the rate constant for the reaction is 0.450 s–1. What is the half-life of A at 25°C ? SOLUTION • We know that for a first order reaction, half-life t1/2, is given by the expression t1/2 = 0.693 /k where k = rate constant • Substituting the value of k = 0.450 s–1 • t1/2 = 0.693/ 0.450s-1 = 1.54s-1 Umm Al-Qura University College of Applied Science Chemistry Department PROBLEM 2. The half-life of a substance in a first order reaction is 15 minutes. Calculate the rate constant. SOLUTION • For a first order reaction • t1/2 = 0.693/ k • Putting t1/2 = 15 min in the expression and solving for k, • k = 0.693/t1/2 = 0.693/ 15mins = 4.62 × 10–2 min–1 Umm Al-Qura University College of Applied Science Chemistry Department Comparison between the different order reactions Umm Al-Qura University College of Applied Science Chemistry Department Examples of First order Reactions • From the following data for the decomposition of N2O5 in CCl4 solution at 48°C, show that the reaction is of the first order t (mins) 10 15 20 ∞ Vol of O2 evolved 6.30 8.95 11.40 34.75 Umm Al-Qura University College of Applied Science Chemistry Department Examples of First order Reactions Umm Al-Qura University College of Applied Science Chemistry Department Examples of second order Reactions • Hydrolysis of ethyl acetate by NaOH using equal concentration of the reactants, was studied by titrating 25ml of the reaction mixture at different time intervals against standard acid. From the data given below, establish that this is a second order reaction. t (mins) 0 5 15 25 ml acid used 16 10.24 6.13 4.32 Umm Al-Qura University College of Applied Science Chemistry Department Examples of second order Reactions Umm Al-Qura University College of Applied Science Chemistry Department Examples of second order Reactions Umm Al-Qura University College of Applied Science Chemistry Department End of Lecture 4 Umm Al-Qura University College of Applied Science Chemistry Department