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Module 1: Equilíbrio químico

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Constantes de Equilíbrio

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XSIQ
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Chemistry - Equilibrium constants

Equilibrium constants

Equilibrium constants are always quoted at a particular temperature. An
increase in temperature causes an increase in the rate of the vast majority
of chemical reactions. However, the extent of a reaction may either
increase or decrease, depending on the thermochemical nature of the
particular reaction, with an increase in temperature.

If the forward reaction of a system at equilibrium is favoured by a change
in temperature, the K value will be higher when equilibrium is established
at the new temperature. If the reverse reaction is favoured the K value
will be lower.

NB. While K values give us an indication of the extent of a reaction, they
give us no information whatsoever about the rate of reaction. So while we
get an indication of the proportion of reactants that may be converted into
products, there is no indication as to how fast the conversion (reaction)
proceeds.

If the calculated CF (concentration fraction) for a particular system is
not equal to the quoted K value, it is relatively easy to predict which
reaction - forward or reverse - will predominate as the system moves to
establish (or re-establish) equilibrium:

* If CF K, the forward reaction will predominate as the value of the CF
must increase. The rate of the forward reaction initially increases
relative to the rate of the reverse reaction.

* If CF K, the reverse reaction will predominate as the value of the CF
must decrease. The rate of the reverse reaction initially increases
relative to the rate of the forward reaction.

* If CF = K, then the system is at equilibrium. This is of course
characterised by the rates of the forward and reverse reactions being
equal.

These deductions are based on the fact that the value of the equilibrium
constant, K, for a particular equilibrium as defined by the stated equation
is just what it says, i.e. constant meaning non-changing (as long as the
temperature does not change). So for a system at constant temperature to
reach equilibrium, the reaction must proceed in the direction that will
enable the concentration fraction to assume the same value as the
equilibrium constant.

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