Transcript Presentation by class of 2013

KINETICS!
IS
A BRANCH OF CHEMISTRY CONCERNED WITH THE RATE OF CHEMICAL
REACTIONS
By: Maria Fernanda Arango and Sara Urbina
WHAT IS KINETICS?
It is a branch of chemistry that
focuses on the rate of change
of the concentration of
reactants and products in a
chemical reaction.
COLLISION THEORY
Explains the theory of a reaction taking place due to the
particles colliding,
 Once collided the particles undergo a reaction.

The rate of reaction can be made increased by:
1) Increasing the amount of times they collide
2) Increasing hte amount of successful collisons.
WHAT IS ACTIVATION ENERGY?
This is the MINIMUM energy required by a particle to
produce a successful reaction
 (All particles have kinetic energy, although the activation
energy is the minimum amount reguired.)


Symbol: Ea

Collision theory can be taken a step further by
evaluating what affects the rate of reactions.
RATES OF REACTION
The rate of a reaction is: how quickly a reaction reaches
a certain point.
 Can be defined as:

Decrease in
concentration
of reactants
per unit time
(s)
or
An increase
in
concentration
of the
products
per unit time
(s)
What affects the rate of a reaction and
how it is measured?

A rate of a reaction is measured by the amount of the
product produced in a particular period of time.

This is affected by different factors:
Temperature
Concentration
Catalyst
Particle
size

TEMPERATURE!
By increaseing the
temperature, you also
increase the rate of
the reaction.
*The animation shows that
the hotter the solution the
more often
particles collide and so the
reaction takes place more
quickly.
PARTICLE SIZE!

If the reactants have
a larger surface area
(i.e. are a powder).
They will react more
ofter then if they
had a smaller
surface area (i.e. a
big rock). Meaning
that if the surface
area is larger the
rate of the reaction
is faster. Because it is
easier for the
reactants to collide
against each other
IS CONCENTRATION!

* As we can see the green particle will
collide more easily in the purple solution
and the concentration of purple molecules
is higher than in the in solution. Meaning
that reaction will be faster in the purple
solution.
The higher the
concentration of a
reaction the more of
that reaction there is
and so the more often
the particles collide.
This also speed up the
reaction. If a solution
have a highre
concetration, it will
allow the reaction to
take place more
quickly.
CATALYST!

The graph shows
that a catalys lowers
the activation
energy, meaning
that more particles
now have enough
energy to react. It
consequently
accelerates the rate
of the reaction.
*This occures because the catalyst provides
the right conditions for the reaction to take place more quickly.
By allowing the products and reactants to come together faster etc.
SUMMARY!
MATH STUFF..

The rate of a reaction can be measured though the change
in concentration.
change in
concentration
Rate =
time
Units= moldmˉ³sˉ¹
The [ ] mean it´s CONCENTRATION
EXAMPLE!
FIRST! We need a blanaced equation like the one below:
The co-efficient tell you how much faster something is
being made of used up compared to something else.
i.e. Iodine and hydrogen are being used up 2 times as fast
as H₂O₂.
FILL IN THE GAP!
Water is being produced ………………. as ............, compared
to Iodide.
CONTINUED…

Let us assume that the reaction went on for two
minutes, do you REMEMBER how to find the rate of
reaction??
Pssstttt, if you
don´t maybe
the next slide
can help you!
HOW TO FIND THE RATE OF REACTION WITH MATH!
Using the same equation.
1) As water is being produced twice as fast you multiply by a HALF
Make sure you are aware this it is two in THIS CASE, although you would
multiply by the reciprical of the co-efficient .
2) Write it like this:
3) Given that the concentration is 0.005moldmˉ³ and it took 2 minutes can
you calculate the rate of the reaction?
Procedure:
ANSWER!
2.083 X 10ˉ²
INTRODUCTION TO EQUILIBRIUM:
CHEMICAL CHANGE
Some reactions will only go to completion if:
A) the activation energy (Ea) is low enough
B) the products are more stable than the reactants
 Some reactions don’t happen at all because either:
A) activation energy is too high
B) products are less stable than the reactants
 Sometimes the reactants and products have similar
energies, meaning the reaction is reversible; meaning it can
occur in either direction – forward or backward. This is
indicated by means of a double arrow:

DYNAMIC EQUILIBRIUM
For any reversible reaction, at some point the rate of the
forward and backward reaction will be equal. This
balance between the two opposite reactions is dynamic
equilibrium; meaning:
A) equilibrium = balance between the forward and
backward rate of reaction.
B) dynamic = there are changes taking place – the
reaction still continues.
 HOWEVER, the concentrations of reactants and products
remains constant – but NOT equal.

CHEMICAL EQUILIBRIUM
Chemical equilibrium is the state of dynamic
equilibrium that takes place in a closed system when
the rate of the forward and backward reaction is equal.
 A closed system means no atoms of the products or
reactants can escape to the outside environment (i.e. a
closed vessel).

A chemical reaction is in equilibrium when the concentrations of
products and reactants remains constant (but not equal) and the
rate in the opposite directions are equal. There is no net change
in the quantities of products and reactants.
A+B
C+D
 At first, the forward reactions occurs rapidly, but the
concentrations of the reactants A and B will fall;
decreasing the rate.
 At the start, the reverse reaction cannot occur.
However, as soon as products C and D start to form,
the rate of the backward reaction increases.
 Eventually, both rates will be equal and the
concentrations will be maintained constant; meaning
there is a state of equilibrium.
EXAMPLE: THE HABER PROCESS
N2 + 3H2
2NH3 When nitrogen and
hydrogen react, only 15%
of these reactants is
converted to ammonia
and as the reaction is
reversible, the ammonia
can react and turn back
into nitrogen and
hydrogen. However, the
position of equilibrium is
shifted to the right by
using:
-A low temperature
(compromise)
-A high pressure – but
not too expensive
AMMONIA!
CHANGE OF CONCENTRATION WITH TIME ESTABLISHING
CHEMICAL EQUILIBRIUM
When the reaction starts, the
concentration of the reactants
will start to decrease, as they
are formed into products. Due
to this, the concentration of the
products will start to increase
as more reactants react.
Finally, when equilibrium is
reached – the forward and
backward rates are equal – the
concentrations will remain
constant.
However , as the graph shows ,
the concentrations of products
and reactants are not the same.
RATE OF REACTION WITH TIME ESTABLISHING
CHEMICAL EQUILIBRIUM
When the chemical reaction is
initiated, the rate of the forward
reaction will be very high. However,
the reverse reaction cannot take
place as there are no products
present that can be turned to
reactants.
As the reaction progresses, the
rate of the forward and reverse
reaction will start to equalize:
a) The forward reaction rate will
decrease while
b) The reverse reaction rate will
increase
In the end, the forward and reverse
reaction will coincide; reaching
equilibrium.
If the concentrations are constant, then
observable properties such as pH, colour, density
and viscosity (among others) will remain constant.
The relative concentrations of reactants and
products depends on the relative rates of forward
and backward reactions.
Even though concentrations are
constant, any molecule from
either the reactants or products
may react. However, the system
tries to oppose this change and
equilibrium is restored once
again.
THE EQUILIBRIUM CONSTANT

The rate at which a reaction occurs depends on the concentrations of the
chemicals present. In the following reversible reaction:

The rate expressions for the forward and backward reactions are:
Forward rate = kf.[A] [B]
Reverse rate = kr.[C] [D]

When equilibrium is reached, these rates are equal, therefore:
kf.[A] [B] = kr.[C] [D] this rearranges to give

As Kf and Kr are constants at a certain temperature, then their ratio should also
be equal to a constant. This is then the equilibrium constant (Kc).
The equilibrium constant is given by the concentration of
the products raised to the power of their stoichiometric
coefficients divided by the concentrations of the reactants
also raised to these powers.
 For
the following reaction:
aA + bB + cC +...
pP + qQ + rR +...
 The equilibrium constant is:
 Kc has
no fixed units and these units must be
calculated from the equation of the equilibrium
constant, given the fact that the concentrations of
the products and reactants have units of mol dm-3.
THE IMPORTANCE OF STATE SYMBOLS

When calculating Kc, it is important to take into account state symbols:
(aq), (s), (l) or (g), because certain concentrations of chemicals will be
omitted depending on the state they’re in.
State symbol
Omitted?
Reason
S (solid)
Yes
As all solids have a constant
concentration and a fixed
density.
l (liquid)
Yes
The concentration of a pure
liquid is constant and it also
has a fixed density.
aq (aqueous)
No
Concentrations can vary;
therefore, they should be
included in the calculation.
g (gas)
No
Always taken into account,
as concentration can
change during the reaction.
 Nevertheless,
water is a special case.
 The concentration of water in a liquid phase is taken as
constant and it is omitted for calculations of the
equilibrium constant in dilute aqueous solutions.
 If the reaction is not in aqueous solution, then the
concentration of water must be included even if it is in
the liquid phase; as the concentration of water can vary.
 However, the concentration of water must be included if
it is in the gas phase.
State symbols dictate when to take the products or
reactants into account when calculating the
equilibrium constant Kc.
LE CHATELIER’S PRINCIPLE
If the conditions under which the equilibrium was established
are changed, then the rates of the backward and forward
reaction will no longer be equivalent.
 The equilibrium has been disturbed and the concentrations of
the species will alter until equilibrium is reached once again.
 Le Chatelier’s principle is a way of predicting the direction in
which the position of equilibrium will change once the
conditions have been varied. It manifests:

“If a change is made to the conditions of a chemical equilibrium,
then the position of equilibrium will readjust so as to minimise the
change made.”
THE EFFECT OF CHANGES IN CONDITIONS ON THE
POSITION OF EQUILIBRIUM
Change
Effect on equilibrium
Change in Kc?
Increase concentration
Shifts to the opposite side
No
Decrease concentration
Shifts to that side
No
Increase pressure
Shifts to the side with the
least moles of gas
No
Decrease pressure
Shifts to the side with most No
moles of gas
Increase temperature
Shifts in endothermic
direction
Yes
Decrease temperature
Shifts in exothermic
direction
Yes
Add a catalyst
No change
No
SUMMARY
BIBLIOGRAPHY









http://dl.clackamas.edu/ch105-03/dynamic.htm
http://www.chemistryexplained.com/Di-Fa/Equilibrium.html
http://www.gcsescience.com/h4.htm
http://www.chem1.com/acad/webtext/chemeq/Eq01.html#SEC1
http://www.chemguide.co.uk/physical/equilibria/haber.html
http://www.chemguide.co.uk/physical/equilibria/kc.html
http://www.chem1.com/acad/webtext/chemeq/Eq-03.html
http://www.blobs.org/science/article.php?article=42
http://ibchem.com/IB/ibnotes/brief/kin-sl.htm#rat
Wow! My
forehead grew
from all that
chemisty!