Unit 1 Mod 2 Rates of Reaction PDF

Unit 1 Mod 2 Rates of Reaction page 1 of 8 RATES OF REACTION Collision theory 1. For a reaction to occur, particles MUST collide 2. Particles must collide with at least a certain minimum amount of energy (activation energy) 3. Particles must...

Unit 1 Mod 2 Rates of Reaction page 1 of 8 RATES OF REACTION Collision theory 1. For a reaction to occur, particles MUST collide 2. Particles must collide with at least a certain minimum amount of energy (activation energy) 3. Particles must collide with the correct orientation All three conditions can be collectively called EFFECTIVE COLLISIONS Rate of reactions are generally measured in mol dm-3 s-1 Rate equation A rate equation shows the effect of concentration of the reactants to the initial rate of reaction. For a general reaction A + B 🡪 product general rate equation 🡺 initial rate =k[A]m[B]n initial rate = rate of reaction at the beginning of the reaction k = rate constant m = order of reaction with respect to reactant A n = order of reaction with respect to reactant B total order of reaction = m + n Orders of reaction (shows how the initial rate is affected by any one particular reactant) Zero order implies that the rate of reaction is not affected by concentration. i.e. the rate does NOT change even if the concentration increases or decreases. First order implies that the rate of reaction increases proportionally with the change in concentration. e.g. if concentration of a reactant doubles, the rate ALSO DOUBLES, if the concentration triples, the rate ALSO TRIPLES etc Second order implies that the rate of reaction increases with the SQUARE of the change in concentration i.e. if the concentration doubles, the rate QUADRUPLES, if the concentration triples, the rate increase NINE FOLD. Unit 1 Mod 2 Rates of Reaction page 2 of 8 You also need to know both zero AND first order reactions using concentration vs time graphs. Sample calculation on determining orders of reaction, the value and units of the rate constant and the initial rate of a reaction The following results were obtained for a reaction between A and B Expt # Concentration /mol dm-3 Initial rate / mol dm-3 s-1 [A] [B] 1 0.5 1 2 2 0.5 2 8 3 0.5 3 18 4 1 3 36 5 2 3 72 6 2 4 ? Unit 1 Mod 2 Rates of Reaction page 3 of 8 a) to find the order of reaction with respect to B, you would compare expt # 1 and 2 as [A] is constant and [B] varies As [B] doubles, the initial rate increases 4 fold, therefore rate increase with the SQUARE of the change in concentration i.e. 2nd order therefore n=2 b) to find the order of reaction with respect to A, you would compare expt # 4 and 5, [B] is constant and [A] varies As [A] doubles, initial rate doubles, therefore it is a proportional change, which means 1st order i.e. m = 1 Total order of reaction = 2 + 1 = 3 c) To determine the rate of reaction for expt #6 You can use ANY expt # from 1 to 5 e.g. expt #3 Specific rate equation Initial rate = k[A]1[B]2 For expt #3 🡺 18 = k[0.5]12 18 = k =4 0.5 x 4 To determine the units of k 🡺 mol dm-3 s-1 = k x mol dm-3 x (mol dm-3)2 🡺 mol dm-3 s-1 = k x mol3 dm-9 🡺 mol dm-3 s-1 = k mol3 dm-9 therefore units of k = mol-2 dm6 s-1 Unit 1 Mod 2 Rates of Reaction page 4 of 8 Checkpoint A a) What is the order of reaction with respect to:- i) A2 ii) B2 iii) The overall order of the reaction ……………………………………. b) Write the rate equation for the reaction ……………………………………………………. c) Calculate the value of the rate constant and its units d) If an experiment was conducted with the [A] as 0.4 and the [B] as 0.3, calculate the initial rate of the reaction. …………………………………………………………… Unit 1 Mod 2 Rates of Reaction page 5 of 8 Rate determining step and reaction mechanism All chemical reactions occur as a series of small steps from beginning to end. Reaction mechanisms can only be determined experimentally. All mechanisms have a rate determining step which is the slowest step in the reaction. Rate determining steps (RDS) would have an order of either 1 or 2. The FAST step in any reaction mechanism would have an order of zero. Example Calculate the orders of reaction with respect to A and B, determine the rate equation and propose a mechanism consistent with the kinetics. order wrt A = 1 order wrt B = 0 rate equation 🡺 initial rate = k[A][B]0 🡺 initial rate = k[A] proposed mechanism A2 🡪 2A (slow) 2A + B2 🡪 2AB (fast) CHECKPOINT B a) Use the data on the left to determine the order of reaction with respect to i) S2O82- and ii ) I- b) The total order of reaction Unit 1 Mod 2 Rates of Reaction page 6 of 8 c) the rate equation for the reaction Half-life The time taken for a reaction to go to half completion, it is called the half-life. First order reactions have CONSTANT half lives. equation (1) kt = ln (A0 / A) equation (2) kt1/2 = 0.693 k = rate constant t = time A0 = initial concentration or original percentage (usually 100%) A = current concentration or current percentage t1/2 = time of half life Sample calculation of half life The half life of radium is 1590 years. How long will it take for a sample of radium to decay to 10% of its original radioactivity? This means t1/2 = 1590 First using equation (2) kt1/2 = 0.693 🡺 k x 1590 = 0.693 🡺 k = 4.36 x 10-4 year-1 Then using equation (1) 4.36 x 10-4 x t = ln (100/10) 🡺 t = 5280 years Checkpoint C 1. Calculate the percentage of radioactivity remaining after 280 years if its half-life is 28 years (will show this one in class) 2. A sample decays by 50% in 45 minutes. What is the half life of the sample? 3. An isotope of caesium has a half -life of 30 years). If 4.2 mg of cesium-137 disintegrates over a period of 90 years, how many mg of cesium-137 would remain? Unit 1 Mod 2 Rates of Reaction page 7 of 8 Effect of temperature and catalysts on rate of reaction Maxwell-Boltzman distribution curve The effect on temperature The effect of a catalyst Unit 1 Mod 2 Rates of Reaction page 8 of 8