Dynamic equilibrium

What is a reversible reaction, and what symbol is used to represent one?

A reversible reaction is one that can proceed in both the forward direction (reactants → products) and the reverse direction (products → reactants). It is represented by the equilibrium sign ⇌ (a double half-arrow), rather than the single arrow → used for reactions that go to completion

What is dynamic equilibrium, and what two conditions must be met for it to exist?

Dynamic equilibrium is the state reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. Two conditions must be met: (1) the reaction must be reversible, and (2) it must take place in a closed system. At dynamic equilibrium, the concentrations of all reactants and products remain constant over time, but both the forward and reverse reactions are still occurring simultaneously — hence "dynamic."

Why do the concentrations of reactants and products stay constant at equilibrium, even though reactions are still occurring?

Because the forward reaction is converting reactants to products at exactly the same rate as the reverse reaction is converting products back to reactants. The two rates are equal, so there is no net change in the concentration of any species — the system appears static but is in fact constantly reacting in both directions.

What does the position of equilibrium mean?

The position of equilibrium describes the relative amounts of reactants and products present in the equilibrium mixture. If the position lies to the right, the equilibrium favours the products — there are more products than reactants at equilibrium. If it lies to the left, the equilibrium favours the reactants. The position of equilibrium can be shifted by changing concentration, pressure, or temperature.