2._Chapter15_Kinetics_post - Chemical Kinetics Reaction...

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Chapt. 15: Kinetics 1 Chemical Kinetics Reaction Rates The goal of Kinetics is to deduce information about the sequence of molecular processes involved in the conversion of reactants into products Factors that affect reaction rates All factors are specific for each reaction. Reactant concentrations Ions in solution generally react very quickly H + + OH H 2 O One of the fastest reactions known N 2 + 3H 2 2NH 3 Reacts slowly and requires high temperature and pressure Molecules with strong bonds or complicated (sterically hindered) shapes tend to react slowly. Nature of the reactants Generally the reaction rate increases as the concentration increases because the number of collisions between reactant molecules increases and increases the probability that a reaction will occur. The relationship between concentration is specific for each reaction.
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Chapt. 15: Kinetics 2 Temperature Factors that affect reaction rates For many reactions taking place at room temperature, a 10°C increase in temperature will the cause the reaction rate to double. The rate increases because more molecules have sufficient kinetic energy to surmount the energy barrier ( G ). T 1 < T 2 At T 2 more molecules have sufficient kinetic energy to surmount the energy barrier or the free energy of activation ( G ). Catalyst A catalysts is a substance that speeds up a reaction by providing a lower energy pathway for converting reactants into products Physical State of the reactants A solid block of wood burns quite slowly in oxygen, however sawdust burns explosively because of the increased surface area or the increased number of collisions
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Chapt. 15: Kinetics 3 For example: We have the following Chemical reaction 2 Fe 3+ (aq) + Sn 2+ → 2 Fe 2+ (aq) + Sn 4+ (aq) When you mix a solution of Fe 2+ and Sn 2+ you observe an oxidation-reduction reaction. After the 38.5 s the change in Fe 2+ has gone from zero to 0.0010M. Which represent the change in concentration i.e. Fe 2+ . Reaction Rate: The change in the concentration of a reactant or a product with time (M/s of mol/s or L/s or g/s or atm/s). Reactant Products aA bB Rate =- [ A ] t Rate = [ B ] t Form textbook
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Chapt. 15: Kinetics 4 0.0010 M 38.5 s 2 Fe 3+ (aq) + Sn 2+ → 2 Fe 2+ (aq) + Sn 4+ (aq) t Fe = + ] [ Rate 2 = Rate of formation of Fe 2+ = 2.6 x 10 -5 M s -1 Reaction Rates How has the concentration of Sn 4+ changed during the 38.5 s?
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Chapt. 15: Kinetics 5 What is the rate of change in concentration of Fe 3+ and Sn 2+ 2 Fe 3+ (aq) + Sn 2+ → 2 Fe 2+ (aq) + Sn 4+ (aq) The rates determined are average rates, not instantaneous rates
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Chapt. 15: Kinetics 6 Consider the decomposition of N 2 O 5 (dinitrogen pentoxide) to give NO 2 (nitrogen dioxide) and O 2 : 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) Example Unfortunately, the definition of rate of reaction depends on how we write (and balance) the reaction. To avoid any possible confusion, we will avoid the use of the term “rate of reaction” and instead use terms such as the “rate of consumption or decomposition of A” or the rate of production or formation of X”
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