Final answer:
The equilibrium constant expression for the given reaction is K = [FeCl3(g)]² / ([Fe(g)]² * [Cl2(g)]³). 'Heat' in the equation indicates that the reaction is exothermic and is a thermochemical equation.
Step-by-step explanation:
To write the equilibrium constant expression for the reaction 2Fe(g) + 3Cl2(g) ↔ 2FeCl3(g) + heat, we must first understand that the equilibrium constant (K) expression involves the concentrations of the gaseous products raised to the power of their coefficients in the balanced equation, divided by the concentrations of the gaseous reactants also raised to the power of their coefficients.
Since this is a reaction involving gases, the equilibrium constant will be expressed as Kp (using partial pressures) or Kc (using molarity) depending on the given data. For this particular reaction, the expression for the equilibrium constant K (which could be Kp or Kc) would be:
K = [FeCl3(g)]² / ([Fe(g)]² * [Cl2(g)]³)
Note that solids and pure liquids are not included in the equilibrium expression, and since heat is involved, we are discussing a thermochemical equation. Also, the inclusion of 'heat' implies that the reaction is exothermic. In thermochemical equations, if heat is produced, it indicates that the reaction is exothermic (q < 0).