Final answer:
At equilibrium, the concentrations of reactants and products remain constant. A system with essentially only products has high concentrations of products, while a system with essentially only reactants has high concentrations of reactants. A system with appreciable amounts of both products and reactants has concentrations that are relatively close to each other.
Step-by-step explanation:
When the system a + b ⟹ c + d is at equilibrium, it means that the forward and reverse reactions are occurring at the same rate. In other words, the concentrations of all the reactants and products remain constant over time.
In a system that contains essentially only products, the concentrations of the products are much higher than the concentrations of the reactants at equilibrium. This means that the forward reaction is favored and the reverse reaction is not significant.
In a system that contains essentially only reactants, the concentrations of the reactants are much higher than the concentrations of the products at equilibrium. This means that the reverse reaction is favored and the forward reaction is not significant.
In a system that contains appreciable amounts of both products and reactants, the concentrations of the reactants and products are relatively close to each other at equilibrium. This means that both the forward and reverse reactions are significant and are occurring at similar rates.