Chemistry 1011 Slot 51 Chemistry 1011 Introductory Chemistry II

Chemistry 1011 Slot 52 Chemistry 1011 TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11

Chemistry 1011 Slot Meaning of Reaction Rate YOU ARE EXPECTED TO BE ABLE TO: Define the average rate and instantaneous rate of a chemical reaction in terms of reactant and/or product concentration. Sketch a concentration vs time curve for a reactant of product in a reaction given experimental data. Calculate average rate or instantaneous rate of reaction from concentration vs time data. State the qualitative effects of changes in concentration, temperature, surface area of solid reactants and the presence of a catalyst on rate of reaction.

Chemistry 1011 Slot 54 Rate of Reaction – What is it? Some exothermic reactions proceed very quickly at room temperature P 4(s) + 5O 2(g)  P 4 O 10(s) Other exothermic reactions proceed very slowly at room temperature C 6 H 12 O 6(s) + 5O 2(g)  6CO 2(g) + 6H 2 O (l) Other reactions, such as the burning of a match after striking, require an initial input of energy

Chemistry 1011 Slot 55 Rate of Reaction – What is it? The rate of a reaction is a measure of how fast a reaction proceeds The rate of a reaction is a measure of how fast a reactant is used up or how fast a product is formed The rate is a measure of the change in concentration of reactant or product with time

Chemistry 1011 Slot 56 Determining the Average Rate For the reaction A  B Time (min) [A] (mol.L -1 ) [B] (mol.L -1 ) Average rate = Change in concentration of product Time elapsed = (0.60 – 0.00) mol.L -1 (3.0 – 0.0) min = 0.20 mol.L-1.min-1 for 1 st 3 minutes

Chemistry 1011 Slot 57 Determining the Average Rate For the next three minutes: Average rate = Change in concentration of product Time elapsed = (0.75 – 0.60) mol.L -1 (6.0 – 3.0) min = 0.05 mol.L-1.min-1 for 2 nd 3 minutes

Chemistry 1011 Slot 58 Determining the Average Rate – Some Notes The average rate for the second period is less that that for the first period. The average rate depends on when you take the measurements. The decreasing concentration of reactant A could also be used to determine the rate. In this case, by convention, change in concentration of a reactant is negative so that the overall rate is positive.

Chemistry 1011 Slot 59 Decomposition of N 2 O 5 N 2 O 5(g)  2NO 2(g) + 1 / 2 O 2(g) When 1 mole of N 2 O 5 decomposes, 2 moles of NO 2 and ½ mole of O 2 are formed The rate of decomposition of N 2 O 5 is ½ the rate of production of NO 2 and 2x the rate of production of O 2. In any given time,  [ N 2 O 5 ] =  [NO 2 ] =  [O 2 ] 2 1 / 2  [ ] refers to the change in concentration in moles per litre

Chemistry 1011 Slot 510 Concentration of N 2 O 5, 2NO 2, and 1/2 O 2 over time

Chemistry 1011 Slot 511 Decomposition of N 2 O 5 For the time interval  t, Average rate = -  [ N 2 O 5 ] =  [NO 2 ] =  [O 2 ]  t 2  t 1 / 2  t

Chemistry 1011 Slot 512 Average Rate for N 2 O 5 Choose time interval to be 0.0min to 4.0min  t = 4.0min For N 2 O 5  [ N 2 O 5 ] = 0.04mol/L mol/L = -0.12mol/L Rate = -  [ N 2 O 5 ] = -(-0.12mol/L) = 0.03mol/L/min  t 4.0min Repeat for NO 2 and O 2 In each case, average rate is the same

Chemistry 1011 Slot 513 Average Rate in General In general, for the reaction: aA + bB  cC + dD Rate = -  [A] = -  [B] =  [C] =  [D] a  t b  t c  t d  t

Chemistry 1011 Slot 514 Instantaneous Rate Instead of the average rate over a period of time, it is possible to determine the instantaneous rate. This can be done by determining the slope of the tangent to the concentration vs time curve at the desired point For N 2 O 5 at t = 2 minutes, the rate is 0.28 mol/L/min

Chemistry 1011 Slot 515 Determining the rate of decomposition of N 2 O 5

Chemistry 1011 Slot 516 Sketching a Concentration vs Time Curve Textbook p332 Q7 For A  B + C Time (sec) [B] Sketch the curve Find instantaneous rate at t = 4 seconds

Chemistry 1011 Slot 517 Factors Affecting the Rate Concentration of reactants –Higher concentrations  faster rates Surface area of solid reactants –More surface area  faster rates Temperature –Higher temperatures  faster rates Catalysts –Speed up reactions without being consumed