Chem 102 Exam 2

Ch. 17

What is the goal of kinetics?

to deduce the rate of reaction an d how it depends on the concentration of reactants

Instantaneous rate of concentration change

change in concentration at some particular instant of time

slope of tangent line at t

general rate equation

aA + bB —> cC + dD

rate of rxn = -1/a (Δ [A]/Δt) = -1/b (Δ[B]/Δt) = 1/d(Δ[D]/Δt)

rate law

rate of reaction = k[A]₀^x[B}₀^y

second order reaction rate law

rate of run 1/rate of run 3 = (k[A]₁/k[A]₃ )

rate equation for first order reaction + slope

- Δ[A]/Δt = k[A]

ln [A] = [A]₀ - kt

• y= b + mt, m= -k, b = [A]₀

• y-axis: ln [ ]

[A] = [A]_oe^-kt

rate equation + slope for 2nd order reaction

1/[A] = 1/[A]₀ + kt

y = b + mt, m=k

to graph:

y-axis = 1/[A]

x-axis = time

half life

time required for 50% of initial reactant to react

[A] = (1/2)ⁿ[A₀]

• n = number of half lives

nuclide

single type of nucleus, each element can have multiple (isotopes are a type)

band of stability

nuclei w/ 1:1 or higher ratio of neutrons to protons

• 1;1 stable below Z = 20

most stable isotopes, compare to periodic table mass

nuclear decay

spontaneous emission of particles

mass number

number of neutrons + protons

isobar

different elements with the same mass number

isotopes

atoms of an element with different number of neutrons and therefore different masses

ions

atoms can gain or lose electrons to change charge

radioactive decay

directly proportional to the amount of radiactive material (N) present: first order

ln N/N₀ = -(0.693/t_1/2)t

carbon-dating

carbon-14 used ofr radiodating archeological finds less than 30,000 yrs due to its ½ life:

t_1/2 = 5,730 yrs

half life of second order reactions

t_1/2 = 1/k[A]₀

mechanism

sequence of steps called “elementary reactions” that add up to the overall reaction

catalysts

substance that increases the rate of a chemical reaction w/o being consumed

enzymes = biological ones

lowers E_a

in beginning and end of reaction — is not consumed (?) used up then reformed in products of overall reaction

can be included in rate law

bimolecular interaction

elementary reactions w/ 2 reactants

monomolecular reactions

elementary reactions with only 1 reactant

ex/ radioactive decay

3 requirements for a reaction

rate of reaction = (collision frequency)(fraction of collisions w/ the required energy)(fraction of collisions in which molecules have the required relative orientation)

• ONLY collision frequency depends on the concentration of reactants

activation energy

amount of energy required for reactants to react

fraction of collisions w/ enough energy to react increases with temperature

activated complex

state with the least amount of additional energy needed to pass from reactants to products

cannot be isolated

as soon as have activated complex — have product

arrhenius equation

k = Ae^-E_a/RT

ln k₂/k₁ = E_a/R (ΔT/T₁T₂)

rate-determining step

overall rate = rate slowest step

equilibrium

rate forward reaction = rate reverse reaction

homogeneous catalyst

catalyst is in the same phase as the reaction mixture

heterogeneous catalyst

catalyst is in a different phase than the reaction mixture

enzyme

protein molecules that catalyze specific biochemical reactions

substrate

reactant molecule enzyme acts on

active site

region of protein where the reaction takes place

• only a small portion of the whole molecule

rate law of enzyme-catalyzed reaction

rate of reaction = R = Δ[P]/Δt = k[S]/[S] + K_m

[S] = concentration of substrate

[P] = concentration of product

k and K_M are constants

steady-state approximation

ES is consumed as fast as it is formed

k₁[E][S] = (k_-1 + k₂)[ES]

dynamic equilibrium

reactions aren’t being paused/frozen, no net change in [products] and [reactants] because being formed + reacted at same rate

for any reaction at chemical equilibrium…

[product]^x_eq /[reactant]^y_eq = K_c

k_c =. equil. constant in terms of concentrations

decay particles, ordered in terms of mass

alpha - emission of helium nucleus

• minus 4 at the top (mass number), 2 from atomic number

beta - emission of electron from the nucleus

gamma - high energy electromagnetic photon emission by nucleus - photon has no mass

positrons: anti-matter of electrons, particle with negligible mass and positive charge, beta + particles emitted by nucleus

Reaction quotient, Q_c. What does relationship between Q_c and k_c tell you about direction in which reaction should shift?

same expression as Kc, but for any state that is not at equilibrium

Q >K shift to reactants

Q<K shift to products

homogeneous catalyst

catalyst is in the same phase as the mixture

heterogeneous catalyst

catalyst is in a different phase than the reaction mixture

lineweaver-burk plot

When R = 1/2R_max, [S] = K_m

if 1/R is plotted against 1/[S], we get a straight line

Le Chatlier’s Principle

if reaction at equilibrium is subject to change in conditions that displaces it from equilibrium, then the reaction adjusts toward a new equilibrium state. The reaction proceeds in the direction that offsets the change in conditions

at equilibrium, high pressure and low temperature is best

what quantities affect equilibrium?

concentration of reactant or product

reaction volume or applied pressure

temperature — only temperature can change the value of k

if there is the same number of moles on both sides of the reaction, volume wont change equilibrium

affect of temperature on k

increase in temperature (exothermic) —> shift from right to left, k decreases

endothermic- left to right as sustem absorbs heat, increasing concentration of products

consider which direction reaction has to go in order to absorb heat