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a supplemental resource for students

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    Lecture21222 Lecture21222 Presentation Transcript

    • Kinetics: the Rate of Chemical Reaction Lecture 21
    • It is now time to learn
      • How fast the reaction is proceeding at a given moment;
      • What the reactant and product concentrations will be when the reaction is complete;
      • Whether the reaction will proceed by itself and release energy or it will require energy to proceed.
    • Chemical equilibrium in a chemical process is the state in which the chemical activities or concentrations of the reactants and products have no net change over time.
    • Chemical thermodynamics studies the effects of changes in temperature, pressure, and volume on chemical systems at the macroscopic scale by analyzing the collective motion of their particles.
    • Chemical kinetics is the study of rates of chemical processes, the changes in concentrations of reactants (or products) as a function of time.
    • Key factor of reaction rate: concentration
      • Molecules must collide to react.
      • Reaction rate is proportional to the concentration of reactants.
      • Rate ~ collision frequency ~ concentration.
    • Key factor of reaction rate: physical state
      • Molecules must mix to collide.
      • The more finely divided a solid or liquid reactant, the greater its surface area per unit volume, the more contact it makes with the other reactant, and the faster the reaction occurs.
    • Key factor of reaction rate: temperature
      • Molecules must collide with enough energy to react.
      • At a higher temperature, more collisions occur in a given time.
      • Raising the temperature increases the reaction rate by increasing the number and, especially, the energy of the collisions.
      • Rate ~ collision energy ~ temperature.
    • Key factor of reaction rate: catalysis
      • A catalyst effects a lower activation energy, which in turn makes the rate constant larger and the rate higher.
      • In more detail - on Wednesday.
    •  
    • Rate is a change in some variable per unit of time.
    • Rate of motion = change in position x 2 — x 1 ∆x ------------------------- = -------------- = -------- change in time t 2 — t 1 ∆t
    • Rate of growth = change in height l 2 — l 1 ∆ l ------------------------- = -------------- = -------- change in time t 2 — t 1 ∆t
    • Reactant concentrations decrease while product concentrations increase. Consider A  B
    • The change in concentration of reactant A in A  B is always negative.
    • The change in concentration of product B in A  B is always positive.
    • Rate of concentration = change in concentration of A = — --------------------------------------- = change in time concentration A 2 — concentration A 1 = — ----------------------------------------------- = t 2 — t 1 ∆ (conc A) = — -------------- ∆ t .
    • Use square brackets to express concentration in moles per liter: ∆ [A] ∆ [B] Rate = — ------------ or ------------- ∆ t ∆ t
    • The rate of the reaction itself varies with time as the reaction proceeds. The rate decreases during the course of the reaction.
    •  
    •  
    • Reaction rates: different approaches
      • Average rate;
      • Instantaneous rate;
      • Initial rate.
    • A sample problem on expressing rate in terms of changes in concentration with time.
    • THE END