For the ammonia synthesis equilibrium $N X_{2} (g) + 3 H X_{2} (g) 2 NH X_{3} (g)$ ( $Δ H < 0$ ), consider: (i) raising the temperature at constant pressure; (ii) adding helium at constant temperature and volume; (iii) adding helium at constant temperature and pressure; (iv) adding $N X_{2}$ at constant temperature and volume. How many of these changes decrease the equilibrium amount of $NH X_{3}$ relative to the original equilibrium?

Answer: D

(i) shifts backward for an exothermic forward reaction: decreases

NH X_{3}

. (ii) no shift. (iii) volume expands, favouring the side with more gas moles (reactants,

Δ n = - 2

): decreases

NH X_{3}

. (iv) extra

N X_{2}

drives the equilibrium forward: increases

NH X_{3}

. So two changes decrease it.