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Rates of Chemical Reactions

13. Rates of Chemical Reactions. 13.1 Rates of Chemical Reactions 13.2 Expressions of Reaction Rates in Terms of Rates of Changes in Concentrations of Reactants or Products 13.3 Methods of Measuring Reaction Rates 13.4 Factors Affecting Reaction Rates. Chemical Kinetics. A study of

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Rates of Chemical Reactions

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  1. 13 Rates of Chemical Reactions 13.1 Rates of Chemical Reactions 13.2 Expressions of Reaction Rates in Terms of Rates of Changes in Concentrations of Reactants or Products 13.3 Methods of Measuring Reaction Rates 13.4 Factors Affecting Reaction Rates

  2. Chemical Kinetics A study of (1) reaction rates (2) the factors affecting reaction rates (3) reaction mechanisms (the detailed steps involved in reactions)

  3. Explosive reactions 2H2(g) + O2(g)  2H2O(l)

  4. Potassium reacts with water vigorously Vigorous reactions 2K(s) + 2H2O(l)  2KOH(aq) + H2(g)

  5. Formation of insoluble salts Very rapid reactions Ag+(aq) + Cl−(aq) AgCl(s)

  6. Formation of insoluble bases Very rapid reactions Fe3+(aq) + 3OH−(aq) Fe(OH)3(s)

  7. Very rapid reactions Acid-alkali neutralization reactions H+(aq) + OH−(aq) H2O(l)

  8. Q.1 Ag+(aq) + Cl−(aq) AgCl(s) Fe3+(aq) + 3OH−(aq) Fe(OH)3(s) H+(aq) + OH−(aq) H2O(l) All involve oppositely charged ions

  9. Rapid or moderate reactions Displacement reactions of metals : - Zn(s) + 2Ag+(aq)  Zn2+(aq) + 2Ag(s)

  10. Rapid or moderate reactions Displacement reactions of metals : - Zn(s) + 2Ag+(aq)  Zn2+(aq) + 2Ag(s) Displacement reactions of halogens : - Cl2(aq) + 2Br(aq)  2Cl(aq) + Br2(aq)

  11. Slow reactions Fermentation of glucose C6H12O6(aq)  2C2H5OH(aq) + 2CO2(g)

  12. Slow reactions 2MnO4(aq) + 5C2O42(aq) + 16H+(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l)

  13. Very slow reactions Rusting of iron 4Fe(s) + 3O2(g) + 2nH2O(l)  2Fe2O3 · nH2O(s)

  14. After corrosion Before corrosion Extremely slow reactions CaCO3(s) + 2H+(aq)  Ca2+(aq) + CO2(g) + H2O(l)

  15. Two Ways to Express Reaction Rates 1. Average rate 2. Instantaneous rate (rate at a given instant)

  16. Amount is usually expressed in Concentration mol dm−3 Mass g Volume cm3 or dm3 Pressure atm

  17. Q.20.36 g of magnesium reacted with 50.0 cm3 of 1.0 M hydrochloric acid to give 360 cm3 of hydrogen under room conditions. The reaction was completely in 90 seconds. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) (a)

  18. Q.2 0.36 g of magnesium reacted with 50.0 cm3 of 1.0 M hydrochloric acid to give 360 cm3 of hydrogen under room conditions. The reaction was completely in 90 seconds. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) (b)

  19. 2.(c) Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) Mg is the limiting reactant Decrease in concentration of HCl(aq) in 90 s

  20. Rate of reaction w.r.t. HCl(aq) Rate of reaction w.r.t. MgCl2(aq) = 2  2.(d) Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) Increase in concentration of MgCl2(aq) in 90 s

  21. The rate at a particular instant of the reaction is called the instantaneous rate. 2. Instantaneous rate For the chemical reaction aA + bB  cC + dD [X] = molarity of X

  22. The rate at a particular instant of the reaction is called the instantaneous rate. 2. Instantaneous rate For the chemical reaction aA + bB  cC + dD Units : mol dm3 s1, mol dm3 min1, mol dm3 h1…etc.

  23. A rate curve is a graph plotting the amount of a reactant or product against time. Graphical Representation of Reaction Rates – Rate curves

  24. Consider the reaction A  B + C(reactant) (product)

  25. At any time t, the instantaneous rate of the reaction equals the slope of the tangent to the curve at that point. The greater the slope, the higher the rate of the reaction.

  26. -ve slope of curve of reactant A  [A]  with time

  27. +ve slope of curve of product B  [B]  with time

  28. The rate at t0 is usually the fastest and is called the initial rate. The curve is the steepest with the greatest slope at time t0.

  29. The rate of the reaction gradually  as the reaction proceeds. Flat curve  reaction completed

  30. C Concentration of product Z (mol dm−3) B A Time of reaction (min) Q.3 X + Y  2Z

  31. C Concentration of product Z (mol dm−3) B A Time of reaction (min) X + Y  2Z

  32. C Concentration of product Z (mol dm−3) B 1.6 A Time of reaction (min) X + Y  2Z

  33. C 5.1 Concentration of product Z (mol dm−3) B 2.7 A Time of reaction (min) X + Y  2Z

  34. C Concentration of product Z (mol dm−3) B A Time of reaction (min) X + Y  2Z

  35. Methods of Measuring Reaction Rates A. Physical measurements 1. Continuous measurements 2 Initial rate measurements (Clock reactions) B. Chemical measurements (Titration)

  36. 1. Continuous measurements Experiment is done in ONE take. The reaction rates are determined by measuring continuously a convenient property which is directly proportional to the concentration of any one reactant or product of the reaction mixture. Properties to be measured : – Gas volume / Gas pressure / Mass / Color intensity / Electrical conductivity

  37. 1.1 Measurement of large volume changes Examples: (1) CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g) (2) Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g) (3) 2H2O2(aq)  2H2O(l) + O2(g)

  38. A typical laboratory set-up for measuring the volume of gas formed in a reaction 1.1 Measurement of large volume changes Temperature is kept constant

  39. Volume of gas formed (cm3) Time of reaction (min) Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g)

  40. Volume of CO2 Q.4 (2) Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g) H2(g) is sparingly soluble in water while CO2 is quite soluble in water. Rate  Rate  Sigmoid curve

  41. Capillary tube Liquid phase reaction mixture 1.2 Measurement of small volume changes - Dilatometry CH3COOH(l) + CH3CH2OH(l)  CH3COOCH2CH3(l) + H2O(l)

  42. 1.3 Measurement of mass changes CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)

  43. stopwatch cotton wool plug limestone pieces of known mass measured volume of standard hydrochloric acid electronic balance The cotton wool plug is to allow the escape of CO2(g) but to prevent loss of acid spray due to spurting.

  44. Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g)  CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g) Which reaction is more suitable to be followed by mass measurement ? Hydrogen is a very light gas. The change in mass of the reaction mixture may be very small. The electronic balance used in the school laboratory may not be sensitive enough to detect the small change.

  45. mfinal = total mass loss mfinal = mfinal – m0 mfinal - mt time Loss of mass (m) time (∵ m0 = 0) =  rate 2

  46. 1.4 Colorimetry ∵ colour intensity  [coloured species]

  47. H2O2(aq) + 2H+(aq) + 2I(aq)I2(aq)+ 2H2O(l) colour intensity  as reaction proceeds CH3COCH3(aq) + I2(aq) CH3COCH2I(aq) + H+(aq) + I(aq) Br2(aq) + HCOOH(aq) 2H+(aq) + 2Br(aq) + CO2(g) 2MnO4(aq) + 16H+(aq) + 5C2O42(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l) colour intensity  as reaction proceeds

  48. cuvettes A colorimeter

  49. Yellow filter Blue solution Complementary colours Yellow light

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