Rates and equlilbria Flashcards
(51 cards)
Rate of reaction definition and equation
the rate of a chem reaction is the change in concentration of a reactants or product per unit if time
reactants used/ time
products produced/time
simple collision theory
for a chem reaction to occur:
molecules must collide
must have greater energy or equal to activation energy
must collide at correct orientation
if one or more of these conditions isn’t meant the no reaction occurs the molecules bounce off one another. An ineffective collision
effect of conc on rate
conc of reactants molecules increases
rate of reaction increases
at higher conc there are more molecules in given volume
more frequent successful collisions
effect of pressure on rate (gases only)
pressure if gas increases and gas molecules pushed closer together
no of gas molecules in given volume increases
more frequent successful collisions occur
same effect as increase conc but that is solids and this is gases
effect of temp on rate
as temp of reaction increases
rate of reaction increases
higher temp the average energy of molecules increases
a greater proportion if molecules have energy greater than or equal to the activation energy
more frequent successful collisions occur
activation energy
is the minimum energy required to start a reaction by breaking of bonds in the reactants
Catalysts and why they are used
increase r of r but not consumed by overall reaction they do this by providing a different pathway for the recation with lower A energy
(if drawing enthalpy profile for this just lower height of curve)
heterogenous- catalyst has diff physical state from the reactants
homogeneous- catalyst and reactants are the same physical state
great economic and environmental benefits-
lower temp
reduce energy demand
less combustion of fossil fuels
and therefore less cost due to lower energy demand and less CO2 emissions
increased sustainability- higher atom economy and less waste
using enzyme also generates specific products no side products
operate close to room temp+ pressur
enzymes commonly in solution with reactants
examples of catalyzed reactions
ammonia( Haber process)- N2 (g) + 3H2 (g) —-> 2NH3 (g) iron is catalyst
polythene- ethene + polythene ziegler-Natta catalyst
catalytic converter( to remove toxic gas)-
2NO (g) + 2CO (g)——–>N2 (g) + 2CO2 (g)
Rh/Pd/Pt (alloy of all 3 metals )
The boltzmann distribution curve
shows the distribution of molecules-
very few have high energy
more have low to mid
x axis- energy
y axis- no of molecules
must start at 0
and can’t be symmetrical
can’t curve up at the end
greatest no of molecules have the mode/average energy, this is peak of curve
total area under curve= total no of molecules
shaded area or area after Ea line is proportion of molecules have > or = Ea
(these can react/ have enough energy)
asymmetrical shape shows more molecules have lower energy
effect of temp- boltzmann curve
higher temp= more molecules have higher energy
average energy increases
curve flattens and shifts right
shaded area/ above Ea area is greater
larger proportion of molecules with > or = Ea
more frequent successful collisions
r of r increases
area under curve= same as same no of molecules
effect of catalyst- boltzmann curve
catalyst increases r of r by lowering Ea
greater proportion of molecules have energy > or = Ea
shaded area/ Area past Ea Is greater
more frequent successful collisions
r of r increases
here the Ea line just move back along curve slightly
procedures to study r of r
reactants used
products formed
measure change in conc of either at regular intervals
diff methods-
measuring Mass of R or P at regular intervals
extracting sample from reaction mixture + analysing by titration at regular intervals (often acid is either R or P
use colourimeter/ spectrophotometer at regular time intervals
measure vol of gas evolved at regular time intervals
measure change in Ph (PH probe) or elec conductivity at reg time intervals
For gases either:
1.vol of gas produced measured at reg time
beaker, reactants, bung + delivery tube and gas syringe
graph increases then levels off
x- time Y-vol of gas
2.measure decreasing mass of R using mass balance at reg time intervals.
beaker, reactants, stopper and balance
graph decreases and levels off
x-time Y-mass
R of R time graphs (conc) + tangents
time graph shows chance in conc of R or P with time
conc- plotted on Y axis from continuous measurements during reaction
can be used to calc R of R at given time
overall rate= gradient of graph over all
total change in Y/ total change in X
initial rate= draw tangent from t=0
change in Y/ change in X
Rate at specific time= draw a tangent at specific time
change In Y/ change in X
if it asks for after Xs just draw target at Xs
when plotting a graph draw smooth curve for line of best fit
Reversible reaction
where both forward and backwards reaction can occur at same time
dynamic equilibrium + how it is reached
exists in a closed system when rate of forward reaction is equal to rate of reverse reaction
doesn’t mean the concentration of P and R are equal but they don’t change
macroscopic properties don’t change e.g. temp/pressure
reaching it:
r of r at beginning of reaction of R is fastest as R have high conc so frequent successful collisions
once sufficient P is made, reverse reaction begins
r of r of reverse starts slow and builds
eventually rate of Forward and backward are =
dynamic equilibrium
can be detected when things like colour or Ph become constant
le chatelier’s principle + equilibrium position
often used to predict the effect of a chage in Temp, pressure or conc of R or P on dynamic equilibrium
Catalyst also effect it
percentage of P to R in a mixture when at dynamic equilibrium shows position of equilibrium
more product = lies to right
more reactants= lies to left
desirable to be as far right as possible for high product yield
effect of conc on dynamic equilibrium
when you increase the conc of one reactants/products
equilibrium shifts to the other side, to minimize the effect of the change in conc
to produce more products/reactants
mention:
where
why
composition
colour change
rember this might be slight cause there doesn’t have to be equal conc of R and P just equal rates of reaction
effects of pressure of dynamic equilibrium
pressure- force exerted by gas particals when they collide with the walls of their containers
only consider gases in system
when pressure increases equilibrium shifts to the side with less moles of gas to minimize the increase in pressure
when pressure decreases
equilibrium shifts to the side with more moles of gas
to minimize the decrease in pressure
if same no of moles then there is no change/effect of equilibrium position
mention?
Where
why
comp
and colour change
effect of tempreatue on dynamic equilibrium
exo- released heat to surrounding
endo- absorb heat
one direction will be endo and the other exo
if the temp is increases
equilibrium will shift to the endo reaction side
to minimize the effect of an increase in temperature by absorbing heat energy
as other reaction is exo (forward or backward reaction)
if temp decreases the opp happens
shifts to exo
to minimize effect of decrease by releasing heat energy
as the other reaction Is endo (foward/backward)
effect of catalyst on dynamic equilibrium
DOESNT AFFECT POSITION OF EQUILIBRIUM
but does increase the rate of dynamic equilibrium
as it increases the rate of both the forward and reverse reactions in equilibrium by the same amount
so unchanging position of equilibrium
equilibrium and industrial processes e.g. Haber process
want to achieve the highest percentage yeild possible of product (far right equilibrium)
also high r of r
sometimes need to compromise between equilibrium and r of r
Fe(s)
N2(g)+3H2(g) <——>2NH3(g) -92kjmol
need to be pure gases to avoid side reactions , lowering percentage yield
N can be obtained from fractional distillation of liquid air
iron catalyst lowers Ea and raises r of r
finely divided for high Sa so more frequent collision and higher r of r
lower temp is best as forward reaction is exo and shift equilibrium to the right
but if too low then r of r is too slow
high pressure is best as shift equilibrium right for higher yield
also increases r of r
but has high running cost plus is dangerous
low pressure - too low too slow
Best comp between rate, yield and safety is 400-500 ⁰c and 200 atmospheres (20000 Kpa)
even with this only 15% of H and N converted rest passes through again
things to remeber when answer why might actual conditions may be diff
High pressure- dangerous + expensive
low pressure+ low temp- too low too slow r of r
high temp- expensive
need a compromise between equilibrium yield + rate of reaction + safety + expense
most talk about r of r and yield
Equilibrium constant, Kc
quatative measure of the proportion (ratio) of product to reactants
Kc indicates where equilibrium position lies in equilibrium
expression = products conc/ reactants conc
R and P are raised to the power of their balancing numbers
units are canceled out to decide the final units, if on the bottom then you need to swap the positives and negative so mol-1 dm3 instead of mol dm-3
Extra=
Kc value depends only on temp
only refers to 1 specific temp
and is unaffected by conc pressure or catalyst at set temp (new equ value will restore og Kc value)
if temp change shifts right- Kc increases
if temp change shifts left-Kc decreases
The significance of the value of Kc/ Kp
the value of Kc indicates the extent of a chemical equilibrium
Kc > 1 equilibrium lies to the right
Kc < 1 equilibrium lies to the left
Kc = 1 equilibrium lies halfway between
if Kc is a large number (e.g. 1000) equilibrium position lies far to the right
and the higher yield of product is achieved
if Kc is small number ( e.g. 1x10-3) equilibrium position lies far to the left
and a lower yield of product is achieved
so the larger Kc value further position of equilibrium lies to right and higher conc of products compared to reactants
