Which of the following reaction quotients are written correctly (pure solids and liquids omitted, activity of species in solution taken as concentration in $mol L^{- 1}$ , gases by partial pressure)?

A $CaCO X_{3} (s) CaO (s) + CO X_{2} (g)$ : $Q_{p} = p_{CO X_{2}}$
B $Fe X_{3} O X_{4} (s) + 4 H X_{2} (g) 3 Fe (s) + 4 H X_{2} O (g)$ : $Q_{p} = p_{H X_{2} O}^{4} / p_{H X_{2}}^{4}$
C $NH X_{4} Cl (s) NH X_{3} (g) + HCl (g)$ : $Q_{p} = p_{NH X_{3}} p_{HCl} / p_{NH X_{4} Cl}$
D $Zn (s) + Cu X^{2 +} (aq) Zn X^{2 +} (aq) + Cu (s)$ : $Q_{c} = [Zn X^{2 +}] / [Cu X^{2 +}]$

Answer: A, B, D

Pure solids have unit activity and are left out. This gives

Q_{p} = p_{CO X_{2}}

for the first reaction and

Q_{p} = p_{H X_{2} O}^{4} / p_{H X_{2}}^{4}

for the second. For

NH X_{4} Cl

the solid must not appear, so the correct form is

Q_{p} = p_{NH X_{3}} p_{HCl}

. The displacement reaction has

Q_{c} = [Zn X^{2 +}] / [Cu X^{2 +}]