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Kinetics

Kinetics. State of reactions can be described a couple of ways: Equilibrium – overall reactions (our study so far) Kinetics – specific reaction pathways, times for those reactions, and equilibrium along the way. Overall reactions – no kinetic information

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Kinetics

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  1. Kinetics • State of reactions can be described a couple of ways: • Equilibrium – overall reactions (our study so far) • Kinetics – specific reaction pathways, times for those reactions, and equilibrium along the way

  2. Overall reactions – no kinetic information • Magnitude and sign of free energy (DG) don’t give specific information about rates • Only describes tendency to proceed any particular direction • Rates might be determined from distance from equilibrium

  3. kinetic vs equilibrium control? • Is reaction fast and reversible? – then can be considered equilibrium controlled • Is reaction homogeneous or heterogeneous? • Homogeneous – only one phase (gas, liquid or solid) - more often equilibrium controlled • Heterogeneous – multiple phase. Often slow and kinetic control • Are there sufficient reactants for equilibrium be obtained?

  4. Slow reversible, heterogeneous, and irreversible reactions typically kinetically controlled • Kinetics difficult to apply in natural systems • Biologically mediated (catalyzed) • Often faster than estimated lab rates

  5. Heterogeneous reactions sensitive to surface control • faster than laboratory estimates • Depend on type and density of crystal defects, impurities

  6. Elementary and Overall Reactions • Elementary reactions – describe exact reaction mechanism or pathway H+ + OH- = H2O CO2 + OH- = HCO3- H4SiO4º = SiO2(qtz) + 2H2O

  7. Overall Reactions – don’t include reaction mechanism or pathway • Really is sum of multiple reactions CaCO3(cal) + CO2 + H2O = Ca2+ + 2HCO3-

  8. Steps in calcite dissolution • Detachment of Ca2+ and CO32- from surface • Diffusion (or transportation) of Ca2+ and CO3- from surface • Conversion of CO3- to HCO3- and H2CO3 to HCO3- in solution • Conversion of CO2(aq) to H2CO3 • Dissolution of CO2(g) • Slowest step is “Rate Limiting Step”

  9. Order of reaction • Expression of dependence of reaction rate on concentrations of species involved • Zeroeth order reactions don’t depend on concentration of any species • It depends only on concentration of A or B, then 1st order • If depends on mA2, second order A + B = AB

  10. If depends on mA and mB • Second order overall • First order with respect to A and B • Reaction order can be higher • These are rare

  11. Rate of reaction: • Can be written: • k+ represents the reaction rate coefficient • Order • First order with respect to A, • Second order with respect to B • Third overall – very rare A + 2B = C

  12. Units of rate constants (k’s): • Zeroeth order – mole/cm3 sec • First order – 1/sec • Second order – cm3/mol sec

  13. Reaction rates – on board

  14. Zeroeth order Rate is slope of A/t Ao Rate independent of A A=Ao-kt

  15. Temperature dependence • Most rates relate to Arrhenius equation • Where: • R = gas constant • T = temperature • A = coefficient (usually empirical) • DE = activation energy Rate = Ae-DE/RT

  16. Examples: • Organic C and calcite dissolution

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