Application of Rates and Equilibria

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  • Created on: 25-04-14 00:02
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Applications of Rate and Equilibrium ­ Answers
1. When both reactants and products are present in a vessel at a stated temperature, the system is either at
equilibrium or not. The situation that exists can be determined by calculating the value of the concentration term
and comparing it with the value of the equilibrium constant at that temperature:
If Kc equals the concentration term, the system is at equilibrium. There is no change in the relative amounts of
the reactants and products.
If the concentration term is smaller than Kc the system is not at equilibrium. The system will react to form
more of the products until the concentration term equals Kc. This means that the position of equilibrium
moves to the right.
If the concentration term is greater than Kc, the system is not at equilibrium. The system will react to form
more of the reactants until the concentration term equals Kc. This means that the position of equilibrium
moves to the left.
2. The only factor that alters the value of the equilibrium constant of a particular reaction is temperature. The value
of the equilibrium constant depends on the temperature because of the relationships:
Stotal = RlnK where R is the gas constant
Stotal = Ssystem + Ssurr = Ssystem + (-H/T)
Exothermic (-H/T is positive): If the temperature is increased, the bottom line gets bigger, which makes
-H/T smaller. Thus Stotal becomes smaller, making lnK and hence K smaller.
Endothermic (-H/T is negative): If the temperature is increased, the bottom line gets bigger, which makes
-H/T less negative. Thus Stotal becomes bigger. This makes lnK and hence K bigger.
3. In exothermic reactions, an increase in temperature causes the value of Stotal to become smaller and hence Kc to
become smaller. Therefore, the concentration term is bigger than the new value of Kc. The system reacts to make
the concentration term smaller ­ which it does by forming more of the reactants ­ until the value of the
concentration term equals the new value of Kc. Thus, the position of equilibrium shifts to the left.
4. In endothermic reactions, an increase in temperature causes the value of Stotal to become bigger and hence Kc to
become larger. Therefore, the concentration term is smaller than the new value of Kc. The system reacts to make
the concentration term larger ­ which it does by forming more of the products ­ until the value of the
concentration term equals the new value of Kc. Thus the position of equilibrium shifts to the right.
5. An increase in temperature always results in an increase in the rate of reaction. It happens because the molecules
possess greater average kinetic energy, so more collisions have energy that is greater than the activation
energy, meaning that a greater proportion of the collisions result in a reaction. In a reversible reaction, an
increase in temperature increases the rate of the forward and back reactions, but does not do so equally. The
endothermic reaction has higher activation energy and so its rate is increased more than that of the exothermic
reaction.
6. Altering the pressure of a gaseous system or the volume of the container has no effect on the value of either the
total entropy change or the equilibrium constant.
7. The equilibrium involving hydrogen, iodine and hydrogen iodide is an example of a reaction in which the number
of gas moles on the left equals the number on the right. The pressure has been doubled by halving the volume of
the container. This will cause the concentrations of all species to double:
2
[HI]eq
K c = [H2]eq[I2 ]eq
An increase in pressure has no effect on the value of Kc
The concentration of HI rises by a factor of 2, and so [HI]2 increases by a factor of 4
The concentrations of both H2 and I2 rise by a factor of 2. Therefore, [H2] multiplied by [I2] increases by a
factor of 4
Both the top and the bottom lines of the concentration term rise by the same factor. Therefore, it value does
not change
Neither Kc nor the concentration term has altered, thus Kc still equals the concentration term. This means that
the system is still in equilibrium, so there is no change to the position of equilibrium
8. The equilibrium between nitrogen, hydrogen and ammonia is an example of a reaction in which the number of gas
moles on the left is more than the number on the right:

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NH3]eq
K c = [N ] [ H 3
2 eq 2]eq
An increase in pressure has no effect on the value of Kc
The concentration of ammonia doubles, so [NH3]2 rises by a factor of 4
[N2] multiplied by [H2]3 rises by a factor of 2 x 23. This is a greater increase than that of the top line of the
concentration term.
The concentration term becomes smaller.…read more

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In this way, almost all the hydrogen is eventually converted to ammonia.
21. The contact process is the crucial step in the manufacture of sulfuric acid:
2SO2 (g) + O2 (g) 2SO3 (g) H = - 196kJ mol-1
The conditions are:
Temperature of 698K
Pressure of 2 atm
Catalyst ­ vanadium (V) oxide, V2O5
22. A temperature higher than 698K would mean a lower yield and a lower temperature would mean an uneconomic
rate.…read more

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