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Chemical Kinetics

Chemical Kinetics. Definition: the study of reaction rates Rate of Reaction: The change in concentration , ∆[ ], of a reactant or a product per unit of time can be determined from a [ ] vs time graph

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Chemical Kinetics

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  1. Chemical Kinetics

  2. Definition:the study of reaction rates Rate of Reaction:The change in concentration, ∆[ ], of a reactant or a product per unit of time • can be determined from a [ ] vs time graph Average Rate of Rxn: from the slope of the secant drawn between 2 points on the curve over a given time interval. Instantaneous Rate of Rxn: from the slope of the tangent to the curve at a specific moment in time.

  3. Reaction Rates are given as: • as rates are positive and as the rxn proceeds, [reactants]  • the rates of the different reactants and products are linked by the balanced chemical reaction. In General, for aA + bB  cC + dD Rrxn req’d = rate of rxn given x mole ratio of req’d given

  4. eg. For: 2 C2H6 + 7 O2 4 CO2 + 6 H2O The rate of reaction with respect to C2H6 is 4.0 x 10-4 mol/L•s. State the Rrxn with respect to each product and reactant. Similarly,Rrxn (CO2) = 8.0 x 10-4 mol/L•s and Rrxn (H2O) = 1.2 x 10-3 mol/L•s

  5. Measuring Raterxn • requires an observable, measurable change that doesn’t disturb the rxn: • gas pressure or volume • conductivity due to change in ion concentration • reactions that change colour intensity

  6. Factors Affecting the Raterxn

  7. CaCO3 (s) + 2 HCl (aq)  CaCl2 (aq) + CO2 (g) + H2O (l)

  8. Factors Affecting the Raterxn • Nature of the Reactant eg. aqueous, gas, liquid, reactivity • depends on the strength of the inter- and intra- particle forces. • Concentration as Rate  as [ ]  • Temperature as Rate  as T  • Surface Area as Rate  as S.A.  • Catalyst

  9. Born Haber Cycles

  10. Mg2+ (g) + 2 Cl (g) + 2e- Mg (s) + Cl2(g) MgCl2 (s) 2 E.A of Cl atom= -696 kJ +2194 kJ -696 kJ 1st + 2nd I.E. Mg = +2194 kJ Mg2+(g) + 2 Cl1-(g) Mg (g) + 2 Cl (g) +239 kJ BE (Cl-Cl) = +239 kJ Mg (g) + Cl2(g) Lattice energy = -2526 kJ -2526 kJ Mg (s) + Cl2(g) +148 kJ Hvap of Mg metal= + 148 kJ MgCl2 (s) Hf° of MgCl2 = -641 kJ

  11. S (g) + 2 Cl (g) 1/8 S8(s) + Cl2(g) SCl2 (l) BE (Cl-Cl) = +239 kJ +239 kJ S (g) + Cl2(g) -510 kJ 2BE (S-Cl) = -510 kJ +280 kJ BE (S-S) = +280 kJ SCl2(g) 1/8 S8(s) + Cl2(g) Hvap of SCl2= -56 kJ -56 kJ = -47 kJ Hf° of SCl2 SCl2(l)

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