Chemistry 1011 Introductory Chemistry II http www mi

Chemistry 1011 Introductory Chemistry II http: //www. mi. mun. ca/~pfisher/chemistry. html Chemistry 1011 Slot 5 1

Chemistry 1011 TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11 Chemistry 1011 Slot 5 2

11. 1 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 5 3

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

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 5 5

Determining the Average Rate • For the reaction A B Time (min) 0. 0 3. 0 6. 0 [A] (mol. L-1) 1. 00 0. 40 0. 25 [B] (mol. L-1) 0. 00 0. 60 0. 75 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 5 6

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 5 7

Determining the Average Rate – Some Notes • The average rate for the second period is less 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 5 8

Decomposition of N 2 O 5(g) 2 NO 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 2 x the rate of production of O 2. In any given time, D[N 2 O 5] = D[NO 2] = D[O 2] 1/ 2 2 D[ ] refers to the change in concentration in moles per litre Chemistry 1011 Slot 5 9

Concentration of N 2 O 5, 2 NO 2, and 1/2 O 2 over time Chemistry 1011 Slot 5 10

Decomposition of N 2 O 5 For the time interval Dt , Average rate = -D[N 2 O 5] = D[NO 2] = D[O 2] Dt 2 Dt Chemistry 1011 Slot 5 1/ 2 Dt 11

Average Rate for N 2 O 5 • Choose time interval to be 0. 0 min to 4. 0 min Dt = 4. 0 min • For N 2 O 5 D[N 2 O 5] = 0. 04 mol/L - 0. 16 mol/L = -0. 12 mol/L Rate = -D[N 2 O 5] = -(-0. 12 mol/L) = 0. 03 mol/L/min Dt 4. 0 min Repeat for NO 2 and O 2 • In each case, average rate is the same Chemistry 1011 Slot 5 12

Average Rate in General In general, for the reaction: a. A + b. B c. C + d. D Rate = -D[A] = -D[B] = D[C] = D[D] a Dt b Dt c Dt d Dt Chemistry 1011 Slot 5 13

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 5 14

Determining the rate of decomposition of N 2 O 5 Chemistry 1011 Slot 5 15

Sketching a Concentration vs Time Curve • Textbook p 332 Q 7 • For A B + C Time (sec) [B] 0 2 4 6 8 10 0 0. 100 0. 130 0. 150 0. 165 0. 175 • Sketch the curve • Find instantaneous rate at t = 4 seconds Chemistry 1011 Slot 5 16

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 Chemistry 1011 Slot 5 17