Identify which of the following reactions represent homogeneous and heterogeneous equilibria:
$(a)$ $H_{2(g)} + I_{2(g)} \rightleftharpoons 2HI_{(g)}$
$(b)$ $Ca(OH)_{2(s)} \rightleftharpoons Ca^{2+}_{(aq)} + 2OH^{-}_{(aq)}$
$(c)$ $CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}$
$(d)$ $Ag_{2}O_{(s)} + 2H^{+}_{(aq)} \rightleftharpoons H_{2}O_{(l)} + 2Ag^{+}_{(aq)}$
$(e)$ $NH_{3(g)} + H_{2}O_{(l)} \rightleftharpoons N{H_{4}}^{+}_{(aq)} + OH^{-}_{(aq)}$
$(f)$ $CH_{3}COOC_{2}H_{5(l)} + H_{2}O_{(l)} \rightleftharpoons CH_{3}COOH_{(l)} + C_{2}H_{5}OH_{(l)}$

(N/A) homogeneous equilibrium is one in which all reactants and products are in the same phase.
$(a)$ All species are in the gas phase (homogeneous).
$(e)$ Reactants are gas and liquid,products are aqueous (heterogeneous).
$(f)$ All species are in the liquid phase (homogeneous).
$A$ heterogeneous equilibrium is one in which reactants and products are in different phases.
$(b)$ Solid reactant and aqueous products (heterogeneous).
$(c)$ Solid reactants and products with a gaseous product (heterogeneous).
$(d)$ Solid reactant and aqueous/liquid products (heterogeneous).
Summary:
Homogeneous: $(a), (f)$
Heterogeneous: $(b), (c), (d), (e)$