Rates of Reaction

# Rates of Reaction

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## Rates of Reaction

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1. Rates of Reaction

2. 8H+ + C2O42- + MnO4- Mn2+ + 4H2O + CO2 H2SO4, H2C2O4 (oxalic), KMnO4 provide ions • Read 18.1 pg. 737-738 • Demonstration of variations in reaction rates: Ca vs. Na vs. Li in water to produce H2 gas Flour burning in air • From the list predict the fastest (1) to slowest (4)

3. Examples of factors affecting rate • Read 18.2. For each of the 5 factors give one example from today’s demonstrations • Nature of reactants – • Ability of the reactants to meet – • The concentration of the reactants – • The temperature of the system – • The presence of catalysts –

4. Examples of factors affecting rate • Nature of reactants - Na is more reactive than Ca (it more easily loses its outer electron) • Ability of the reactants to meet - 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 Read 18.3 (740 – 42). Answer these questions: • What units are associated with concentration? • What units are associated with reaction rate? • What do the square brackets in [HI] indicate (see figure 18.2, pg. 741) • Explain how the rate of a reaction is determined (see fig. 18.2)? • 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 rise Slope (rise/run) is the reaction rate in (mol/L)/s… 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