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A chemical reaction reaches a state of chemical equilibrium when the rates of the forward and reverse reactions become equal. At this point, the concentrations of the reactants and products stabilize and become constant, although not always equal. The system is in a dynamic state where the reactants are continuously converting to products and vice versa at the same rate, but there is no net change in concentration observed.

Chemists can recognize a system that has reached chemical equilibrium by observing the following signs: 1. The reaction appears to have stopped, but there are still reactants and products present in the system. 2. The concentrations of the reactants and products remain stable and do not change significantly over time. 3. Changes in temperature, pressure, or concentration can disturb the equilibrium and cause the system to respond in a way that favors either the forward or reverse reaction until a new equilibrium is established.

When writing chemical equations, chemists indicate that a reaction comes to a state of chemical equilibrium by using a double arrow (⇌) instead of a single arrow (→). The double arrow shows that both the forward and reverse reactions are occurring simultaneously in the system. For example, the chemical equation for the equilibrium between nitrogen gas (N2), hydrogen gas (H2), and ammonia (NH3) would be written as: \[ N_2(g) + 3H_2(g) ⇌ 2NH_3(g) \] The double arrow indicates that the forward reaction (formation of ammonia from nitrogen and hydrogen) and the reverse reaction (breakdown of ammonia into nitrogen and hydrogen) are both occurring at the same time, and the system is at a state of chemical equilibrium.