rates and equilibria Flashcards
rate equation
rate = k [A]^m [B]^n
where [ ] is the equilibrium concentration and m/n is the order
order of 0
increase in concentration has no effect on rate of reaction
order of 1
increase in concentration and rate are directly proportional
order of 2
concentration and rate have a squared relationship
half-life
the time taken for the concentration of a reactant to decrease by half
k in terms of t1/2
k = ln 2 / t1/2
rate-determining step
slowest step of a reaction mechanism of a multi-step reaction
how to find the rate-determining step
the rate equation gives the species and their quantities in the rate-determining step
activation energy
minimum energy required to start a reaction by breaking bonds in the reactants
use of arrhenius equation on a graph
ln k = -Ea/R x 1/T + ln A
y-intercept gives ln A
gradient gives -Ea/R
le chatelier’s principle
when a system in dynamic equilibrium is subjected to a change in conditions, the position of equilibrium shifts to minimise the effect of the change
the equilibrium constant, Kc
Kc = [C]^c[D]^d / [A]^a[B]^b
where the lower case letters are the balancing numbers
homogenous equilibrium
all species are in the same state
heterogenous equilibrium
species are in different states
significance of Kc values
Kc < 1 equilibrium shifts left
Kc > 1 equilibrium shifts right
the equilibrium constant, Kp
Kp = p(C)^c x p(D)^d / p(A)^a x p(B)^b
where p is the partial pressure
mole fraction
mole fraction of A = moles of A / total moles
partial pressure
partial pressure = mole fraction x total pressure
rate of reaction
change in concentration of reactant or product per unit time
collision theory
molecules must collide with each other with energy greater than or equal to activation energy, with the correct orientation
effect of increasing pressure on rate of reaction
gas molecules are pushed closer together
more gas molecules per unit volume
more frequent successful collisions
rate increases
effect of increasing temperature on rate of reaction
kinetic energy of molecules increases
a higher proportion of molecules have energy greater than or equal to activation energy
more frequent successful collisions
rate increases
types of catalysis
heterogenous catalysis - catalyst is in a different physical state than the reactants
homogenous catalysis - catalyst is in the same physical state as the reactants
effect of a catalyst
increases the rate of reaction by providing an alternate reaction pathway with a lower activation energy
dynamic equilibrium
rate of forward and reverse reactions are equal
system is closed
concentration of reactants and products does not change
effect of pressure on position of equilibrium
if pressure is increased, the position of equilibrium will shift to the side with the fewest moles of gas (and vice versa)
effect of temperature on position of equilibrium
in de
exothermic | L | R
endothermic | R | L
effect of catalyst on position
catalyst increases the rate of the forward and reverse reaction equally so has no effect on the position of equilibrium
