Kinetics State of reactionscan 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 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

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?

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

Heterogeneous reactions sensitive to surface control faster than laboratory estimates Depend on type and density of crystal

defects, impurities

Elementary and Overall Reactions

Elementary reactions – describe exact reaction mechanism or pathway

H+ + OH- = H2OCO2 + OH- = HCO3

-

H4SiO4º = SiO2(qtz) + 2H2O

Overall Reactions – don’t include reaction mechanism or pathway

Really is sum of multiple reactions

CaCO3(cal) + CO2 + H2O = Ca2+ + 2HCO3-

Steps in calcite dissolution Detachment of Ca2+ and CO3

2- 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”

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

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

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

2BAmmk

dtdC

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

Reaction rates – on board

Zeroeth order

Rate is slope of A/t

A=Ao-ktRate independent of AAo

ktoeAA

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

Examples: Organic C and calcite dissolution