Question

100 g of silica and 100 g of graphite are placed in a rigid vessel of volume 20 L, which is evacuated at room temperature and then heated to 1500 °C, at which temperature the quartz and graphite reaction to form Sic. Calculate

Answer 1

(a)

2 SiO₂(s) + 4 C(s) $\leftrightharpoons$ SiC(s) + 3CO(g) + SiO(g)

∆G° = -266.9 + (-246.1)+(-57.0) -(0) - (-595.9) = 25.9kJ/mol

∆G = ∆G° + RT ln Q

At equilibrium ∆G =0 and Q = K_p

-25.9×1000 = 8.314×1773.15× ln K_p

K_p = e^-1.757 = 0.17258

K_p = p_CO×p_SiO

p_CO = 3 p_SiO

p_SiO = (0.17258/27)^1/4 = 0.282753 = 0.2828bar

p_CO = 3 × 0.282753 = 0.8483bar

(b)

PV = nRT

n (SiO) = (0.282753/1.01325)atm× 20L/(0.0821atm-L/K.mol × 1773.15K) = 0.03834mol

Moles of SiC formed = moles of SiO formed = 0.03834mol

Mass of SiC formed = 40.01g/mol × 0.03834mol = 1.5339g = 1.534 g. (Answer)

(c)

Moles of graphite consumed = 4 × moles of SiO formed

= 4×0.03834 = 0.153353mol

Mass of graphite consumed = 0.153353mol×12g/mol = 1.840g (Answer)