Kinetic and Equilibrium

Rate determining step

A rate-limiting step (or rate-determining step) is the slowest step in a multi-step reaction mechanism, process, or metabolic pathway that dictates the overall speed of the entire sequence.

Collision theory model

Reaction rate is determined by:

How frequently the reactant molecules collide

The orientation of the colliding molecules

Their energy

Increase reactant concentration

More collisions

Increase temp

Faster reaction

Activation energy

The lower the activation energy the faster the reaction rate

Catalysts can lower activation energy

Average Reaction rate

The greater concentrations of reactants means faster reaction rate

Higher temperature of reaction mixture, the faster the reaction rate

Reaction rate can be characterized by changes in reactants or products over time. In the generic reaction aA → bB, the reaction rate = –(1/r) Δ[r]/Δt*c = (1/p )Δ[p]/Δt*c

coefficients on bottom

p=product

r=reactant

Arrhenius equation

No need to memorize

k=Ae^-(Ea/RT)

k=rate constant

bigger k=faster

Want lower Ea and faster temp

Rate determing step

Slowest

Smaller K

Biggest Ea for RDS

Rate law instantaneous

Rate law of RDS/slowest step=for overall reaction!

rate law=k[reactants]^r

r=coefficients

For slowest step!

What impacts reaction rate

Ignore solids!

Intermediates

Products of individual steps that then get consumed to produce final product

Catalysts

In reactants in a slower step sometimes RDS and then get regenerated as a product later

Reaction order and rate law

Rate data:

Can use rate data when do not have individual steps

First order reactants conc. and rate change proportional. A doubled, k doubled

0th order no matter reactants conc. rate change does not change. B double, k same

Everything else tends to be second order C doubles and then rate does something different (squared).

Rate law=k[A]^1[C]²

Use coefficients and add them together for overall=2+1 =3

0: rate=k M/S

1: rate=k[A] 1/S

2: rate=k[A]² 1/MS

3: 1/M²S

4: 1/M³S

Can find k or rate constant using data

Exponent rules

Thermo vs kinetic

Delta G has no impact on rate of reaction kinetic part

Thermodynamic and kinetic factors do not impact one another

Equilibrium expression

Keq=Products/reactants

Or Kc=[products]/[reactants]

Exponents=coefficients

IGNORE SOLIDS

Can do partial pressures if gases involved

Q and K

If reaction not at equilibrium then use Q instead of K

Products/reactants

Q=Keq

At equilibrium

Q<Keq

Excess reactant, less than supposed to be

Reaction proceeds in the forward direction to produce more PRODUCTS Q increase and gets closer to Keq

Q>Keq

Excess product, greater than supposed to be

Reaction proceeds in the reverse direction to produce more REACTANTS Q lowers to get closer to Keq

K and Q throughout

K stays the same throughout, the overall ratio

Q changes

Only temp can change Keq

Le Chatliers Principle

If you add more reactant to a system that is at equilibrium the system will react favoring the forward reaction in order to consume reactant and return to equilibrium

Minimize stressors

Volume changes impact partial pressures, adding inert gas to a flexible/movable container:

Decrease in volume, less space, shift towards less moles of gas

Increase in volume, more space, shift to more moles of gas

Increasing pressure favors side with FEWER moles of gas

Thermo and equlibrium

deltaG=deltaG’+RTlnQ

When at equilibrium: delta G=0

deltaG’=-RTlnKeq

Keq and deltaG’

deltaG’<0 spontaneous exergonic, products favored at equilibrium Keq>1

deltaG’=0, Keq=1 products and reactants are present in equal amounts at equilibirum

deltaG’>0 Keq<1 reactants favored at equilibrium

Endothermic: Treat heat as a reactant

Exothermic: Treat as a product. Temp goes up, products increase shift to left more reactants

K now diff because of temps

Hyperventilation example

Lose tons of CO2, shift to left less H2CO3=less [H^+] blood pH increases becomes less acidic

Rateforward=Rreverse

Kf/Kr=Keq

Equilibrium constants

Bigger K= more favored

Reverse/flip equilibrium inverts (reciprocal) of equilibrium constant

Solubility product/constant=Ksp

Ca3(PO4)2 (s)—3Ca²+ + 2PO43-

-S +3S +2S

Ksp=Do think about solids

Ksp=[Ca2+]³[PO43-]²

No in terms of molar solubility (S)

Ksp=[3S]³[2S]²

Qsp and Ksp comparisons

Ksp=Qsp fully saturated

Ksp>Qsp more salt can be dissolved, Qsp can get bigger

Ksp<Qsp excess salt will precipitate

(Solubility Product Constant Ksp) represents the maximum equilibrium ion concentration for a saturated solution at a specific temperature.

Ksp at equilibrium fully saturated

Qsp don’t have to be at equilbrium

Adding a common ion…

Decerases solubility

CaCO3(s)—Ca²++CO32-

In solution of CaCl2, Ca²+ increases shift towards solid so decrease solubility

Phase solubility rules

Solubility of solids in liquids increases with increasing temps

Solubility of gases in liquids decrease with increasing temps

Solubility of gases in liquids increase with increasing pressure

Increase temp less soluble O2 (gas) decreases

Soluble salts

All group I: Li, Na, K, Rb, Cs, and NH4+

NO3-, ClO4- and C2H3O2-

Insoluble

Silver Ag, lead Pb, and mercury, Hg