Chemical equilibrium is the state in which the forward reaction rate
and the reverse reaction rate are equal.
LEARNING OBJECTIVE [ edit ]
Recall the relationship between the forward and reverse reaction rates when a reaction is at
equilibrium
KEY POINTS [ edit ]
Equilibrium results when the forward reaction proceeds at the same rate as the reverse reaction.
The equilibrium state is one in which there is no net change in the concentrations of
the reactants and the products.
A catalyst speeds up the rate of a chemical reaction, but has no effect upon the equilibrium
position for that reaction.
TERMS [ edit ]
the equilibrium position
The point in a chemical reaction at which the concentrations of reactants and products are no
longer changing.
chemical equilibrium
In a chemical reaction, the state in which both reactants and products are present at
concentrations that have no further tendency to change with time.
Give us feedback on this content: FULL TEXT [edit ]
In theory, allchemical reactions are in fact double reactions: for every forward reaction, there
is a subsequent reverse reaction. The idea can be illustrated as follows:
reactants
⇌
prod ucts
For plenty of reactions, however, the
forward reaction is so favored, and the
reverse reaction is so negligible, that
reactions are written simply in terms of
the solid forward arrow, A
→
B
. However,
we will now consider forward/reverse
reaction pairs that exist in chemical
equilibrium with one another.
The Concept of
Chemical Equilibrium
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In a chemical reaction, chemical
equilibrium is the state in which the forward reaction rate and the reverse reaction rate are
equal. The result of this equilibrium is that the concentrationsof the reactants and
the products do not change. However, just because concentrations aren't changing
does not mean that all chemical reaction has ceased. Just the opposite is true; chemical
equilibrium is a dynamic state in which reactants are being converted into products at all
times, but at the exact rate that products are being converted back into reactants. The result
of such a situation is analogous to a bridge between two cities, where the rate of cars going
over the bridge in each direction is exactly equal. The result is that the net number of cars on
either side of the bridge does not change.
Cars entering and leaving a city over a bridge
Chemical equilibrium is akin to two cities connected a bridge. If the rate at which cars leave the city is
exactly equal to the rate at which cars enter the city, the result is that the net number of cars on both
sides of the bridge does not change.
Reaching Equilibrium and the Equilibrium Position
When a system consists of competing forward and reverse reaction rates, the reaction will
proceed until chemical equilibrium is reached. Consider the following general reaction
between A and B:
A
⇌
B
Let's say that A and B are introduced into a reaction vessel, and a reaction begins to proceed.
The concentrations of A and B will change over time, until they reach equilibrium.
Concentration vs. time for a reversible reaction
Note that equilibrium is reached when both the curves plateau, and the concentrations of both reactants
and products do not change afterward.
Note that over time, the curves level out, or plateau, and the concentrations of A and B are no
longer changing. This is the point at which the system has reached chemical equilibrium.
While there are various factors that can increase or decrease the amount of time it takes for a
given system to reach equilibrium, the equilibrium position itself is unaffected by these
factors. For instance, if a catalyst is added to the system, the reaction will proceed more
quickly, and equilibrium will be reached faster, but the concentrations of both A and B will be
the same at equilibrium for both the catalyzed and the uncatalyzed reaction.
Conclusion
The equilibrium state is one in which there is no net change in the concentrations of reactants
and products. Despite the fact that there is no apparent change at equilibrium, this
does notmean that all chemical reaction has ceased. Nothing could be further from the truth;
at equilibrium, the forward and reverse reactions continue, but at identical rates, thereby
leaving the net concentrations of reactants and products undisturbed.
Equilibrium lesson
This lesson introduces equilibrium as a condition where the forward and reverse reaction rates are
equal. We also explain how the concentrations remain constant at equilibrium.