Final answer:
Equilibrium in a reaction where gas is produced can only be achieved in a closed system. Equilibrium can either be homogeneous or heterogeneous, depending on whether the reactants and products are in the same phase or different phases. Changes in temperature, concentration, volume, and pressure can disturb the equilibrium, but the presence of a catalyst or pure solids or liquids does not affect the equilibrium concentrations.
Step-by-step explanation:
In a reaction where a gas is produced, equilibrium can only be reached in a closed system. This is because a reversible reaction requires that no additional reactants are added or products removed for equilibrium to be established. For example, if in a reaction producing hydrogen iodide (HI), the HI gas were continuously removed, the reaction would not reach equilibrium as the removal of product would continuously drive the reaction forward to produce more HI.
It's important to note the distinction between homogeneous equilibrium and heterogeneous equilibrium. A homogeneous equilibrium involves reactants and products that are in the same phase, which can be a gas or liquid, whereas a heterogeneous equilibrium exists when the reactants and products are in different phases, such as when gas reacts with a solid or liquid.
When discussing systems at equilibrium, it's also important to understand that while catalysts and pure solids or liquids don't affect the equilibrium concentrations of reactants and products, changes in temperature, concentration, volume, and pressure can disturb the equilibrium. However, the effect of volume and pressure changes is significant only if the number of moles of gas changes on the reactant and product sides of the reaction. This is due to Le Chatelier's principle, which predicts how a system at equilibrium will respond to a disturbance.