Core & Extension - Chemical Rxns - Reversible Rxns I.pptx

© 2004-05 Dorje Gurung
Core & Extension:
Reversible Reactions

Core & Extension Chemical Rxns: Reversible Reactions
Slide 2 of 27
Learning Objectives
Concepts:
Core
– Reversible reaction,
Extension
– Equilibrium, dynamic equilibrium, static equilibrium, closed system, open
system, isoloated system, rate of forward reaction, rate of reverse
reaction, pressure, temperature

Slide 3 of 27
Learning Objectives
Skills:
Core
– describe the idea that some chemical reactions can be reversed by
changing the reaction conditions (Limited to the effects of heat on
hydrated salts. Concept of equilibrium is not required.)
Extension
– Define and describe the concept of equilibrium
– Predict the effect of changing the conditions (temperature and pressure)
on other reversible reactions

Slide 4 of 27
Chemical Equilibriums – OH YES
Most chemical reactions happen only one way:
A + B C + D
methane (g) + oxygen (g) carbon dioxide (g) + Water (l)
reactants products
Under certain conditions, A and B can be made to react and produce C
and D.
But, under a different set of conditions, C and D can be combined so as
to form A and B.
Sorry mate – no you can’t get the oxygen back – its gone.
Some chemical reactions are reversible can go forward & backward.

Slide 5 of 27
Example
For example, when hydrated salts are heated the salt undergoes
dehydration evolving water vapor and leaving behind the
anhydrous salt.
For example, when blue crystals of hydrated copper sulfate is heated
it decomposes to water vapor and white anhydrous copper
sulfate.
CuSO4•5H2O(s)  CuSO4 (s) + 5H2O(g)
Blue White
But the reaction is reversible; that is, we can reclaim the hydrated
copper sulfate.
All that is required is to add water to the white anhydrous copper
sulfate

Slide 6 of 27
Example
CuSO4 (s) + 5H2O(l)  CuSO4•5H2O(s)
Notice that while for the first reaction, heat was required, while the
second, the reverse did not.
We indicate a reversible reaction by replacing the single headed arrow
with .
CuSO4•5H2O(s)  CuSO4 (s) + 5H2O(g)
All hydrated salts undergo this kind of reaction.

Slide 7 of 27
Chemical Equilibrium
As soon as C and D (the products) are made they chemically react to
remake A and B.
The starting concentrations of A B and C D initially change.
(reactants) (products)
Why do the starting concentrations change – how do they change?
After a short amount of time the concentrations stay the same.
A reaction has been started and left to take place for a few minutes.
A + B C + D

Slide 8 of 27
Chemical Equilibrium: Molecular Level View
What is happening every second inside the reaction container:
A B
A B
A B
A B
A B
Initially, only 5 A’s and 5 B’s.
There are no C’s and D’s.

Slide 9 of 27
After some time, two pairs of A and B have converted to two pairs
of C and D.
A B
C D
A B
C D
A B
A B
C D
A B
C D
A B
A B
A B

Slide 10 of 27
Subsequently, one pair of C and D have recombined to produce A
and B.
But at the same time another pair of A and B form C and D.
The conversion doesn’t stop there.
A B
C D
A B
C D
A B
A B
C D
A B
C D
A B
A B
C D

Slide 11 of 27
Interconversion continues and when the conversion between the two are
taking place at the same speed (one each second for example) the
reaction is in equilibrium.
There will always be three pairs of A and B and two pairs of C and D.
A B
C D
A B
C D
A B
A B
C D

Slide 12 of 27
Dynamic equilibrium
The two chemical reactions will continue at the same rate (speeds).
rate of forward reaction = rate of drawkcab reaction
How many A and B particles will always be in the reaction container?
How many C and D particles will always be in the reaction container?
We say the reaction has reached “equilibrium” – a balance.
There will now be no change in the concentration of all the particles.
It is a “DYNAMIC EQUILIBRIUM” - why?
The 2 chemical reactions will keep taking place - DYNAMIC and the
concentrations of the 6 particles will stay balanced - at EQUILIBRIUM.

Slide 13 of 27
Dynamic equilibrium of a man
on the escalator
The man is running up the escalator at the same
rate the escalator is moving down
What happens to the position of the running man?
It does not change.
We say the system is in “DYNAMIC
EQUILIBRIUM” because both the man and the
escalators are still moving.

Slide 14 of 27
Chemical equilibrium
The state where the concentrations of all reactants and products
remain constant with time.
Properties of a chemical equilibrium are:
1. properties such color, concentration, mass and volume remain
constant.
2. it is a highly dynamic situation, both forward and reverse reactions
are taking place at the same rate,
3. it is reversible—state of equilibrium can be approached from either
direction, and
4. it is closed (or isolated)

Slide 15 of 27
Physical equilibrium
Bromine vapor (right) in equilibrium with liquid bromine.
As long as the cap on the bottle remains, the
concentration of the bromine vapor above the liquid
will remain constant though at the molecular level
vapor will be condensing as fast as liquid bromine
evaporate.
– One evidence for the constancy of the concentrations
would be the color of the vapor.
Water (left) in a corked up flask (a)
will eventually come to a state of
equilibrium with its vapor (b).

Slide 16 of 27
Effect of Conditions on Equilibrium
Various conditions affect equilibrium.
We will consider the effects of two: pressure and temperature.

Slide 17 of 27
Effect of pressure on equilibrium
Pressure affects reversible reactions that involve gases.
Increasing the pressure causes the reaction to proceed in the direction
that reduces the overall number of molecules.
– That is so because increasing the pressure reduces the volume of the
system which is like you moving to a smaller room, much smaller room
– Being given a smaller room will force you to reduce the number of stuff
you have in your new room.
N2 (g) + 3H2 (g)  2 NH3 (g) Exothermic
Increasing the pressure would shift equilibrium in the direction of the
products because 4 molecules combine to form 2 molecules of gas.

Slide 18 of 27
Effect of pressure on equilibrium: Example
(a) A mixture of NH3(g), N2(g),
and H2(g) at equilibrium
contains 4 N2 molecules, 2
NH3 molecules and 8 H2
molecules, a total of 14
molecules.
(b) The external
pressure is
suddenly
increased.
(c) After pressure is increased, the
number of molecules of NH3 is 6,
H2 2, N2 2, a total of 10 molecules,
4 less than the original.
6 H2 and 2 N2 molecules have reacted
and produced 4 NH3 molecules.
N2 (g) + 3H2 (g)  2 NH3 (g)

Slide 19 of 27
Effect of pressure: Another Example
Imagine that the reaction involving NO2 and N2O4 is at equilibrium in a
container with a moveable piston.
The equation for the reaction is:
N2O4  2NO2
This reaction is different from the previous one in that when the reaction
proceeds in the forward direction, the total number of molecules
increases.
– Every molecule of N2O4 that decomposes produces 2 of NO2.
The reverse direction of course dicreases it.
Remember that increasing the pressure causes the reaction at
equilibrium to proceed in the direction that reduces the total number
of molecules.

Slide 20 of 27
Effect of pressure: Another Example
For convinence we’ll start with
only 2 molecules of N2O4
and 6 of NO2 at
equilibrium, giving us a
total of 9 molecules. Here let’s assume that the
pressure has been
increased.
The reaction should therefore
proceed in the reverse
direction.
N2O4  2NO2
The end result is that the
total number of
molecules decreases.
Has it?
Total number of molecules is
7, 2 down from the
original

Slide 21 of 27
Effect of Temperature
In general, increasing the temperature causes reversible reaction to
proceed in the direction that absorbs heat.
Treat heat as a reactant in an endothermic reaction while in an
exothermic reaction as a product.
Endothermic: Reactants + heat  products
Exothermic: Reactants  products + heat
Adding heat (i.e. heating the vessel) favors the direction which consumes
heat:
– If endothermic, increasing temperature causes reversible reaction to
proceed in the forward direction using up the excess energy added
Reactants + heat  products

Slide 22 of 27
Effect of Temperature: Endothermic
Reaction
Consider
[Co(H2O)6]2+
(aq) + 4Cl
(aq)  [CoCl4] 2
(aq) + 6H2O(l) Endothermic
pale pink blue
In other words,
Heat + [Co(H2O)6]2+
(aq) + 4Cl
(aq)  [CoCl4] 2
(aq) + 6H2O(l)
pale pink blue
So, if the temperature is increased in a mixture at equilibrium, then the
reaction should proceed in the forward direction turning the solution
blue.
And that’s exactly what happens. (See next slide.)

Slide 23 of 27
Effect of Temperature: Endothermic
Reaction
Mixture at room temperature.
Purple in color.
Mixture after being heated.
More blue than at room temp.
Heat + [Co(H2O)6]2+
(aq)  [CoCl4] 2
(aq)
pale pink blue

Slide 24 of 27
Effect of Temperature: Exothermic Reaction
– If exothermic, increasing the temperature causes reversible reaction to
proceed in the reverse direction.
Reactants  products + heat
• For example, in the following reaction which is exothermic
N2 (g) + 3H2 (g)  2 NH3 (g) Exothermic
• The Reaction can be visualized as
N2 (g) + 3H2 (g)  2 NH3 (g) + Heat
• Increasing the temperature increases the heat which forces ammonia to
decompose reducing the ammount of ammonia at equilibrium.

Slide 25 of 27
Effect of Temperature: Exothermic Reaction
The percentage by Mass of NH3 at equilibrium in a mixture of N2, H2 and
NH3 as a function of Temperature.
Notice that the percentage of ammonia decrease with temperature which
means that as the temperature is increased, the reaction goes in the
reverse direction reducing the percentage.
N2 (g) + 3H2 (g)  2 NH3 (g) + Heat

Slide 26 of 27
Effect of Temperature
Reactants + heat  products
– if exothermic, cooling favors the forward reaction,
Reactants  products + heat
Likewise, removing heat (i.e. cooling the vessel) favors the direction which
generates heat:
– if endothermic, cooling favors the reverse reaction

Slide 27 of 27
Effect of various factors on equilibrium
position
i) Pressure
– a) Increasing it forces reversible reaction (a reaction at equilibrium)
in the direction that decreases the number of molecules of gas
– b) Decreasing it forces reversible reaction (a reaction at equilibrium)
the direction that increases the number of molecules of gas
ii) Temperature
– a) Increasing it forces reversible reaction (a reaction at equilibrium)
in the direction in the direction that consumes heat energy
– b) Decreasing it forces reversible reaction (a reaction at equilibrium)
in the direction that produces heat energy

Core & Extension - Chemical Rxns - Reversible Rxns I.pptx

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