Rates and equlilbria

Question 1

Rate of reaction definition and equation

Answer 1

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

Question 2

simple collision theory

Answer 2

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

Question 3

effect of conc on rate

Answer 3

conc of reactants molecules increases

rate of reaction increases

at higher conc there are more molecules in given volume

more frequent successful collisions

Question 4

effect of pressure on rate (gases only)

Answer 4

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

Question 5

effect of temp one rate

Answer 5

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

Question 6

activation energy

Answer 6

is the minimum energy required to start a reaction by breaking of bonds in the reactants

Question 7

Catalysts and why they are used

Answer 7

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

Question 8

examples of catalyzed reactions

Answer 8

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 )

Question 9

The boltzmann distribution curve

Answer 9

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

Question 10

effect of temp- boltzmann curve

Answer 10

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

Question 11

effect of catalyst- boltzmann curve

Answer 11

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

Question 12

procedures to study r of r

Answer 12

reactants used
products formed

measure change in conc of either at regular intervals

diff methods-
measuring Mads if 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 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

Question 13

R of R time graphs (conc) + tangents

Answer 13

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

Question 14

Reversible reaction

Answer 14

where both forward and backwards reaction can occur at same time

Question 15

dynamic equilibrium + how it is reached

Answer 15

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

Question 16

le chatelier’s principle + equilibrium position

Answer 16

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

Question 17

effect of conc on dynamic equilibrium

Answer 17

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

Question 18

effects of pressure of dynamic equilibrium

Answer 18

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

Question 19

effect of tempreatue on dynamic equilibrium

Answer 19

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)

Question 20

effect of catalyst on dynamic equilibrium

Answer 20

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

Question 21

equilibrium and industrial processes e.g. Haber process

Answer 21

want to achieve the highest percentage yearly 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

Question 22

things to remeber when answer why might actual conditions may be diff

Answer 22

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

Question 23

Equilibrium constant, Kc

Answer 23

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

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

Question 24

The significance of the value of Kc

Answer 24

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