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Chapter 11

Chapter 11. Chemical Kinetics. Chemical Kinetics. Speed and rate at which reactions occur. Concentration. Greater concentration = faster reaction (most of the time). Temperature. Increased temperature = increased rate of reaction rate

theodore-pallas
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Chapter 11

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  1. Chapter 11 Chemical Kinetics

  2. Chemical Kinetics • Speed and rate at which reactions occur

  3. Concentration Greater concentration = faster reaction (most of the time)

  4. Temperature • Increased temperature = increased rate of reaction rate • Food spoils in warm temperatures more than in the refrigerator

  5. Catalyst • Increases the rate of a reaction without being consumed in the reaction • Enzymes are catalysts in many biochemical reactions

  6. Surface Area • Greater surface area = faster reaction

  7. Reaction Rate • Speed of a chemical reaction • Measures how quickly a substance is produced or consumed For A →B • Average rate = Δ(moles of B) Δ Time Measures the rate of appearance of B

  8. Reactions are: • Dependent on concentration • Reaction rates are usually expressed as M/s

  9. As a reaction proceeds: • Note the concentration vs time • Slope of the tangent gives you the instantaneous rate at a given time

  10. For the reaction A → B • The rate of disappearance of A is equal to the rate of appearance of B • But…if it’s not a 1:1 ratio • 2HI (g)→ H2(g) + I2 (g) 2 moles of HI disappear for each 1 mole of H2 and I2 formed So, to equate the rates, you must divide the rate of disappearance of HI by 2 (its coefficient) Rate = -½ Δ[HI] = Δ[H2] = Δ[I2] Δt Δt Δt

  11. Concentration • Rate of reaction depends on concentration of reactants • As concentration of reactants decreases, the rate of reaction slows • As you increase concentration, reaction rate increases

  12. Rate Laws • Rate laws show how rate is dependent on concentration of reactants

  13. For Most Reactions • Rate Laws are: m n Rate = k [reactant 1] [reactant 2] Exponents ‘m’ and ‘n’ are reaction orders and their sum is the overall reaction order

  14. Zero Order Reaction • For a zero order reaction, changing the initial concentration has no effect on the reaction rate (as long as there is some of the reactant present)

  15. First Order Reaction • For a first order reaction, changing the concentration has a proportional change in rate • In other words, doubling the concentration would double the rate

  16. Second Order Reaction • In a second order reaction, doubling the concentration of the reactant increases the rate by a factor of 4 (2 squared) • Tripling the concentration increases the rate by a factor of 9 (3 squared), etc.

  17. NH4(aq) + NO2(aq)¯→ N2(g) +H 2O (l) Exp. Initial Initial observed initial rate # [NH4+] [NO2¯] M/s 1 0.0100 0.2000 5.4 x 10 -7 2 0.0200 0.2000 10.8 x 10 -7 3 0.0400 0.2000 21.5 x 10 -7 4 0.0600 0.2000 32.3 x 10 -7 5 0.2000 0.0202 10.8 x 10 -7 6 0.2000 0.0404 21.6 x 10 -7 7 0.2000 0.0606 32.4 x 10 -7

  18. Rate Law NH4(aq) + NO2(aq)¯→ N2(g) +H 2O (l) The rate law for this reaction would be: Rate = k [NH4+] [NO2¯] The rate law shows how reaction rate is dependent on the concentration of the reactants.

  19. However: • Reaction orders don’t necessarily correspond with the coefficients in an equation….they must be determined experimentally!!!

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