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Reaction Rates

Learn about reaction rates in chemical kinetics and how reactants transform into products over time. Discover how to calculate average and instantaneous rates of reaction, and explore the relationship between stoichiometry and rates.

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Reaction Rates

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  1. Reaction Rates Chapter 14 part I

  2. Chemical Kinetics Reactants –> Products • A reaction is defined by its reactants and products. • Spontaneity refers to the inherent tendency for the process to occur. • Spontaneous does not mean fast. ∆G = ∆H -T∆S C diamond –> C graphite

  3. Reaction Rates • 2NO2(g)–> 2NO(g) + O2(g) • The above reaction is the decomposition of nitrogen dioxide a form of air pollution which contributes to acid rain. • Sidebar: acid anhydrides • Note the rate of change in concentration changes over time.

  4. The reaction rate is defined: Rate = (concentration of A at time t2-concentration of A at time t1) (t2-t1) aka: ∆ [A] /∆ t • Reaction Rate is the change in concentration of a reactant or product per unit time

  5. 2NO2(g)–> 2NO(g) + O2(g)

  6. 2NO2(g)–> 2NO(g) + O2(g)

  7. 2NO2(g)–> 2NO(g) + O2(g) • Calculate the average rate at which the concentration of NO2 changes over the first 50 seconds . • Note that NO2 decreases over time, so the ∆[NO2] is a negative quantity. So: • Rate = - ∆[NO2]/∆t

  8. Instantaneous Rate • Notice that over time the rate of reaction changes. • The rates in the tale to the left are average rates. • The value of the rate at a particular time is called the instantaneous rate.

  9. Instantaneous Rate cont. • To find the instantaneous rate one must find the tangent of the curve. • Slope of the tangent line = ∆ y/∆x (rise/run) • Rate = - ∆[NO2]/∆t • Or: Rate = -(slope of the tangent line)

  10. Reaction Rates • Rates of reactions may also be applied to products, not just reactants. • As NO2 is decomposed NO and O2 are produced, the rates are related but not necessarily the same. • The rates are different based on stoichiometry.

  11. Stoichiometry and Rates • Rate of consumption of NO2 = • Rate of production of NO = • 2 X’s the Rate of production of O2 Rate = - ∆[NO2]/∆t Rate = ∆[NO]/∆t Rate = 2(∆[O2]/∆t)

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