Unit 6

1. Unit 6 Kinetics

2. Why do some reactions occur quickly (air bags) and others take a long time (metal rusting)? How do scientist describe and compare the rates of reactions? What factors affect the rates of reaction? Can chemist control the rate of reactions? Reaction rates (rates of reaction) ---> the change in the amount of reactants or products over time. Just like a runner’s velocity can be expressed as the change in displacement over the change in time, reaction rates can be expressed as a change ion concentration over a change in time. rate = - Δ[A] [A] expressed a mol/L Δ t time expressed in s (see handouts)

3. Collision Theory • Collision Theory provides an explanation for why reactions proceed at different rates. • Most chemical reactions involve the transfer of atoms from one molecule to another. This requires contact or COLLISION. • Reactants require energy to start the reaction (activation energy - the minimum amount of energy colliding particles must have in order to react). The activation energy is needed to break existing bonds and form new bonds. • In every reaction an activation complex (transition state) is formed at the peak of the activation energy curve. • See overhead • Explain why the following would affect rate: p 467 • Temperature • Concentration • Surface Area • Nature of Reactants • Catalysts -

4. Examples of factors affecting rate • Nature of reactants - Na is more reactive than Ca (it more easily loses its outer electron) • Surface Area - Flour burns faster as a fine powder because oxygen surrounds it (heterogeneous reaction?) • The concentration of the reactants - reaction d has a higher H2SO4 (reactant) concentration, and thus reacts faster; flour + pure O2 • The temperature of the system - reaction b is faster due to an elevated temperature • The presence of catalysts - manganous sulfate (reaction c) is a catalyst for this reaction

5. Measuring Reaction Rates • What units are associated with concentration? • What units are associated with reaction rate? • What do the square brackets in [HI] indicate Explain how the rate of a reaction is determined • Plot this data (include title, axes labels):

6. Measuring Reaction Rates • For the data, determine the rate of reaction at i) 25 s, ii) 175 s, and iii) 325 s. Show your work and calculations on the graph. • How does the rate at the beginning of a reaction compare to the rate later in a reaction? Explain why this makes sense according to one of the 5 factors that affect reaction rates (from 18.2)? • Explain how the rate of reaction of 2HI  H2 + I2 is determined experimentally. • Do PE 2 (pg. 742)

7. Measuring Reaction Rates • Concentration: mol/L or mol•L–1 • Rate = concentration/time: (mol/L)/s or mol•L–1•s–1 • The square brackets "[ ]" is the symbol for concentration (mol/L) • The rate of reaction is measured by: Instantaneous slope; rise over run; slope of the tangent at any point.

8. 4, 5, 6 Slope (rise/run) is the reaction rate in (mol/L)/s… rise run

9. Measuring Reaction Rates cont • i) rate  (0.100-0.072)/50 = 0.00056 (mol/L)/s = 5.6 x 10-4 mol•L-1•s-1 ii) rate  1.4 x 10-4 mol•L-1•s-1 iii) rate  0.8 x 10-4 mol•L-1•s-1 • As the reaction proceeds, the rate decreases because reactants are being used up (recall, concentration of reactants affects rate) • HI concentration is measured indirectly by measuring the production of I2(g) (purple) – likely via a spectrometer • N2O4 2NO2, NO2 forms at 0.010 mol•L-1•s-1, thus N2O4 decomposes at 0.005 mol•L-1•s-1 For more lessons, visit www.chalkbored.com

10. http://www.youtube.com/watch?v=mjcDxrBGLHQ&feature=c4-overview-vl&list=PLA11A67B3AF749AA1http://www.youtube.com/watch?v=mjcDxrBGLHQ&feature=c4-overview-vl&list=PLA11A67B3AF749AA1 • http://www.youtube.com/user/richthornley