WOLFRAM|DEMONSTRATIONS PROJECT

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exothermic reaction plot
endothermic reation plot
exothermic reaction heat flux
endothermic reaction heat flux
temperature (arbitrary units)
0
This Demonstration shows the effect of temperature on chemical equilibrium by changing temperature in an exothermic and in an endothermic reaction, specifically:
H
2
+
I
2
⇌2HIΔH<0
and
N
2
O
4
⇌2
NO
2
ΔH>0
.
The initial concentrations
[X]
0
of all species are fixed.
The final concentrations in a reaction are related to initial concentrations by the equation for the equilibrium constant [1]:
2
(
[HI]
0
+2x)
(
[
H
2
]
0
-x)(
[
I
2
]
0
-x)
=
K
c
and
2
(
[
NO
2
]
0
+2x)
(
[
N
2
O
4
]
0
-x)
=
K
c
[M]
.
This allows us to obtain the final concentrations for the exothermal reaction using:
[HI]
eq
=
[HI]
0
+2x
,
[
H
2
]
eq
=
[
H
2
]
0
-x
and
[
I
2
]
eq
=
[
I
2
]
0
-x
; for the endothermal reaction using:
[
NO
2
]
eq
=
[
NO
2
]
0
+2x
and
[
N
2
O
4
]
eq
=
[
N
2
O
4
]
0
-x
.
Changing the temperature in an equilibrated system will stimulate the establishment of a new equilibrium. In exothermic reactions, heat can be included among the products of reaction. Le Chatelier's principle states that lowering the temperature can be seen as subtracting products, so it will cause a shift toward the product. Endothermic reactions work in the exact opposite way: as heat can be considered one of the reactants, lowering the temperature causes a shift toward the reactants.
Opacity of the arrows is proportional to the forward and backward reaction rate. Iodine in the gas phase is colored purple, while the products are colorless. For the endothermic reaction, nitrogen dioxide assumes a brown-yellow coloration. Thus the reaction rate can be monitored by the intensity of the color.