Chem1aa3 Test 2

Zero-order processes

• evaporation/sublimation with constant surface

• decrease in solvent volume does not impact rate of evaporation

• Rare

Pseudo zero-order reactions

reactions where catalyst or enzyme is saturated with reactants

Nucleophilic substitution

An electronegative atom or group bonded to a carbon atom is replaced by another

Can be first or second order overall depending on structure

First order reactions (SN1)

• I- + Cl-C(CH3)3 —| I-C(CH3)3 + Cl-Reactions that depend on the concentration of one reactant, where the rate is directly proportional to that concentration.

• Step 1 where slow c-cl bond cleavage forms carbocation (This is the RDS!)

• Step 2 is fast attack of I on carbocation

• I doesnt enter until AFTER

• Integrated Rate Law: At=A0*e^-kt

• Kt is unitless in this case only!

• Have constant half lives depend only on k and not A0

Second order reactions SN2

• I- + Cl-CH3 —-| 1-CH3 + Cl-

• Is first ordrr regarding I- and Me-Cl

• Second order bc single concentrated step w both reactants present in RDS

• Two types either A² or AB

• Rate law is 1/At=kt+1/A0

• For reactions that are AB, can be turned into pseudo first order by swamping (vast excess of one reactant) i.e sucrose inversion

Determing order of reaction from the graph

Compare A vs lnA vs 1/A all against t to see which one is linear

What do reaction rates depend on

• Collision densiy (colisions per volume per unit per time)

• Fraction of molecules with enough kinetic energy

• Proper orientation of molecules

Most collisions do not result in a reaction

Transition state

• unstable liftime

• highest energy point on the lowest energy path b/w reactants and prods

A from Arrhenius equation

• Ifnormation abt collision density and geometry

T2=

(1/t1 -Rln(k2/k1)/Ea)^-1

k2=

k1r^ea/R(1/t-1/t1)

Ea=

-Rln(k2/k1)/1/t2-1/t1)

Catalysis

• Accelerate reactions w/o being altered

• Allow for a different reaction pathway with a reduced activation energy

  Homogenous catalysts: same phase as reactants, catalytic activity proportional to the catalysts concentration

  Heterogenous catalysts: Different phase as reactions, concentration means number of active sites or surface area

  Involves reactant absorption, diffusion along surface, at active site, product desporption

Elementary Steps

• Unimolecular (single species dissociates) or bimolecular two species

• Are reversible but may not reach equilibrium

Reaction Intermediates

• Are species produced in one elementary process and consumed in a later step

• Do not appear in overall reaction or rate law

Enzymes

• At low {s} reaction is 1st order in S0

• At high S is 0 order in S0

Michaelis Menten Equation

• kcatEoS/kM+S

• Plot v0/E0 vs S

• kcat is maximal rate of enzyme corresponding to the experimentally observed limting step \

1st assumption: S0 is much greater than E0 results in S is much greater than E*S and S=S0

2nd assumption: (always true) V0=k2(E*S)

Third assumption: steady state approximation where d(ES)/dt = 0

Km has units of concentration, while kcat has units of rate constant

km is the value of (s) where the rate is ½ maximal

Reaction order changes with (s)

Intermolecular Forces

• Play significant role in solids and liquids but not in gases

• Affects melting point, boiling point, vapour pressure, surface tension, enthalpy change of phase transitions

• Boiling point increases with IMF strength

IMFs in pure substances

London dispersion forces, Dipole-Dipole forces, Hydrogen bonding forces, hydrogen bonding forces NOT ion-dipole forces which are in solution

London dispersion forces

• Non polar atom or molecule

• Experiences an instantenous dipole which induces a dipole in its neighbour

• Strength depends on polarizability (atomic number and size of the molecule)

• Branched structures have increased intermolecular contact

Dipole-Dipole forces

• Occur in polar molecules

• Permanent dipole: bond dipoles and asymmetric shape

• Polar molecules are stronger than NP even when similar polarizability

Hydrogen

Between an H atom in a polar bond HN HO HF

Range of strength of IMFs

1. Dispersion

2. Dipole-Dipole

3. H bonding

4. Ionic/Covalent

Vaporization

• Molecules at the surface of a liquid have enough energy to overcome intermolecular forces of attraction and escape to the gas phase

• Occurs more readily with incresed temp, surface arrea, decreased IMF strength

vapour pressure

The equilibrium partial pressure of the vapour in the space above the liquid k=Pa

deltag=-RTlnk

pvap influenced by IMF strength inverse rltnship

Temp is not linearly related but proportional

When equal to atmospheric pressure is when liquid boils

Phase Changes

Solid—Liquid—Gas (Endothermic)

Gas—Liquid—Solid (Exothermic)

Triple point

Three phases in equilibrium

Critical point

A temperature and pressure beyond which liquid and gas are indistinguishable

Density of liquid and vapour are zero

Surface tension of liquid approaches zero

Interface b/w l and g dissapears

Substrate Concentration

• When very low most of enzyme is unbound, thus efree=enot and adding substrate will rapidly increase

• When already saturated will not significantly impact the rate

When is the SSA invalid

When rate at which something is being produced is significantly less than it is disappearing or if there is build up

Are k1 and k-1 equal at equilibrium

No, RATES are the same, constants may differ