5. Chemical Kinetics

Question 1

Know how to read a rxn diagram

Answer 1

Ea for forward and reverse, delta G, y-axis = free energy and x-axis = reaction coordinate. Intermediates at valleys, transition states at peaks

Question 2

Endergonic vs exergonic

Answer 2

pos deltaG vs neg deltaG

Question 3

Know the formula for rate laws

Answer 3

R = k[A]^x[B]^y if rxn was A + B –> C

THE EXPONENTS ARE COMPLETELY UNRELATED TO COEFF; THEY’RE FOUND EXPERIMENTALLY. INIT CONC = REACTANTS, NOT PRODUCTS

Question 4

Know how to find individual orders and overall order

Answer 4

Compare 2 experiments, find the same conc of 1 reactant (ignore that one) and compare other conc and initial rxn rate of other reactant to find exponent. THIS IS HOW YOU FIND EXPONENTS; EXPONENTS ARE NOT COEFF, THEY’RE FOUND EXPERIMENTALLY

Question 5

What are the units for R and [conc] in rate laws? What are the units for k in rate laws?

Answer 5

R = M/s, [conc] = M; k varies depending on rate law formula. If you have 2 conc –> M^2

Question 6

What’s the eqn for boiling pt elevation and freezing pt depression?

Answer 6

DeltaT = k*i*m
k = given constant, i = van Hoff factor (how many particles a cmpd will dissolve into), m = molality (mol solute/kg solvent)

Question 7

Arrhenius eqn

Answer 7

k = Ae^(-Ea/RT)

Know the relationships b/w these variables

A=freq factor aka attempt freq; measures how often molec in a certain rxn collide, units s^-1
e=2.71...
Ea=activation energy 
R=ideal gas constant 
T=temp in K

Question 8

Transition state/activation complex

Answer 8

Has greater energy than both reactants and products; there just a theoretical construct as opposed to real life identities. Located at peaks in rxn coordinate diagram

Question 9

Factors affecting rxn rate

Answer 9

[reactants] (higher conc —> higher collisions, except for zero order rxns); temp (higher temp —> higher collisions); mediums (better in aq slns or polar solvents > nonaq slns; states of matter); catalysts (presence of catalyst —> dec Ea)

Question 10

Homogenous vs heterogenous catalysis. What happens if you have saturated catalyst?

Answer 10

Catalyst = same phase (liq, solid, gas) as reactants vs in distinct phase. You already have max amount of catalyst activity and inc [reactants] won’t change anything

Question 11

Know rate expressions

Answer 11

In the rxn A —> 2B: R = (-deltaA/deltat) = (1/2deltaB/deltat)

A is neg b/c you’re losing it; coefficients = denom

Question 12

Zero vs first vs second vs broken order vs mixed order rxn

Answer 12

When forming product C = indep from reactants A and B (ie. Conc of reactants don’t affect the rxn) —> rate = k[A]^0[B]^0 = k vs when forming product = DIRECTLY PROPORTIONAL to reactant —> rate = k[A]^1 or k[B]^1 vs when forming product = proportional to conc of 2 reactants or square of 1 reactant —> rate = k[A]^2 or k[B]^2 or k[A]^1[B]^1 vs having fraction or noninteger orders vs when rate order varies throughout entire rxn

Question 13

Does the rate law apply to entire rxn?

Answer 13

NO, Only the rate limiting step

Question 14

How do catalysts affect k?

Answer 14

They inc rate constants for both forward and reverse rxn

Question 15

Kinetic vs thermodynamic product

Answer 15

Lower Ea needed —> forms faster, get it at low temps, ends with higher deltaG compared to thermodynamic product —> less stable product; IRREVERSIBLE vs high Ea needed —> forms slower, get it at high temps, ends with lower deltaG than kinetic product —> more stable product; REVERSIBLE