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Chemistry 1011

Chemistry 1011. Introductory Chemistry II http://www.mi.mun.ca/~pfisher/chemistry.html. Chemistry 1011. TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11. 11.1 Meaning of Reaction Rate. YOU ARE EXPECTED TO BE ABLE TO:

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Chemistry 1011

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

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

  3. 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

  4. Rate of Reaction – What is it? • Some exothermic reactions proceed very quickly at room temperature P4(s) + 5O2(g) P4O10(s) • Other exothermic reactions proceed very slowly at room temperature C6H12O6(s) + 5O2(g) 6CO2(g) + 6H2O(l) • Other reactions, such as the burning of a match after striking, require an initial input of energy Chemistry 1011 Slot 5

  5. 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

  6. Determining the Average Rate • For the reaction A  B Time (min) [A] (mol.L-1) [B] (mol.L-1) 0.0 1.00 0.00 3.0 0.40 0.60 6.0 0.25 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 1st 3 minutes Chemistry 1011 Slot 5

  7. 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 2nd 3 minutes Chemistry 1011 Slot 5

  8. 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 5

  9. Decomposition of N2O5 N2O5(g) 2NO2(g) + 1/2O2(g) When 1 mole of N2O5 decomposes, 2 moles of NO2 and ½ mole of O2 are formed The rate of decomposition of N2O5 is ½ the rate of production of NO2 and 2x the rate of production of O2. In any given time, D[N2O5] = D[NO2] = D[O2] 2 1/2 D[ ] refers to the change in concentration in moles per litre Chemistry 1011 Slot 5

  10. Concentration of N2O5, 2NO2, and 1/2 O2 over time Chemistry 1011 Slot 5

  11. Decomposition of N2O5 For the time interval Dt , Average rate = -D[N2O5] = D[NO2] = D[O2] Dt 2 Dt 1/2Dt Chemistry 1011 Slot 5

  12. Average Rate for N2O5 • Choose time interval to be 0.0min to 4.0min Dt = 4.0min • For N2O5 D[N2O5] = 0.04mol/L - 0.16mol/L = -0.12mol/L Rate = -D[N2O5] = -(-0.12mol/L) = 0.03mol/L/min Dt 4.0min Repeat for NO2 andO2 • In each case, average rate is the same Chemistry 1011 Slot 5

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

  14. 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 N2O5 at t = 2 minutes, the rate is 0.28 mol/L/min Chemistry 1011 Slot 5

  15. Determining the rate of decomposition of N2O5 Chemistry 1011 Slot 5

  16. Sketching a Concentration vs Time Curve • Textbook p332 Q7 • For A  B + C Time (sec) 0 2 4 6 8 10 [B] 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

  17. 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

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