Which of the following does not represent a heterogenous equilibrium?  

I) CH₄(g) + 2 O₂(g) ⇌ CO₂(g) + 2 H₂O(g)                  
II) CO₂(g) + C(s) ⇌ 2 CO(g)
III) 2 H₂O(l) ⇌ 2 H₂(g) + O₂(g)
IV) CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO^–(aq) + H₃O⁺(aq)

The following diagrams represent a hypothetical reaction A ¡ B, with A represented by red spheres and B represented by blue spheres. The sequence from left to right represents the system as time passes. Does the system reach equilibrium? If so, in which diagram(s) is the system in equilibrium? [Sections 15.1 and 15.2]

Ethene (C₂H₄) reacts with halogens (X₂) by the following reaction:

C₂H₄(𝑔) + X₂(𝑔) ⇌ C₂H₄X₂(𝑔)

The following figures represent the concentrations at equilibrium at the same temperature when X₂ is Cl₂ (green), Br₂ (brown), and I₂ (purple). List the equilibria from smallest to largest equilibrium constant. [Section 15.3]