Dynamic equilibrium · ⁨动态平衡⁩

Reactions that never finish

• A reversible reaction 可逆反应 can go both ways ($\rightleftharpoons$).
• In a closed system 封闭系统 it reaches a dynamic equilibrium 动态平衡.
• At equilibrium nothing appears to change — but both reactions are still going.

What equilibrium means

• At dynamic equilibrium:
  • the forward rate equals the reverse rate, and
  • the concentrations of reactants and products stay constant.
• It is dynamic because both reactions are still happening — they just cancel out.

• A closed system is needed, or products would escape and equilibrium could never be reached.

The features of equilibrium

• Equilibrium is only reached in a closed system.
• The macroscopic 宏观 properties (concentration, colour) stay constant once equilibrium is reached.

Cobalt chloride changes colour as its equilibrium shifts

A common misconception

• At equilibrium the amounts of reactants and products are constant, but not necessarily equal.
• The position of equilibrium can lie far to one side.

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