ScholarshipThermochemistryFree

Methanol as a liquid carrier for hydrogen

ΔH by two methods, entropy and temperature, Kc and Q, fuels and intermolecular forces

Methanol can be made from synthesis gas:

\ce{CO(g) + 2H2(g) <=> CH3OH(g)}

, using a Cu/ZnO catalyst at about 250 °C and 50–100 atm.

\Delta_fH

(kJ mol

^{-1}

): CO(g) −111;

\ce{CH3OH(g)}

−201. Bond enthalpies (kJ mol

^{-1}

): C≡O (in CO) 1077; H–H 436; C–H 414; C–O 358; O–H 463.

\Delta_cH

(kJ mol

^{-1}

\ce{CH3OH(l)}

−726;

\ce{H2(g)}

−286. M (g mol

^{-1}

\ce{CH3OH}

32.0;

\ce{H2}

2.0.

Facts you may need

ΔfH and bond enthalpy methods
entropy of gases
Kc and Q
intermolecular forces and state

(a)

Calculate

\Delta_rH

using (i)

\Delta_fH

data and (ii) bond enthalpies. Explain why the values differ.

(b)

Predict the sign of

\Delta S

for the system and explain why the reaction becomes less favourable at high temperature.

(c)

1.00 mol CO and 2.00 mol

\ce{H2}

are sealed in a 2.00 L vessel at the operating temperature. At equilibrium, 0.300 mol of

\ce{CH3OH}

is present. Calculate

K_c

(d)

Justify the industrial choice of 250 °C, 50–100 atm and a Cu/ZnO catalyst. First, for a mixture at the same temperature with

[\ce{CO}] = 0.20

[\ce{H2}] = 0.50

and

[\ce{CH3OH}] = 0.30

mol L

^{-1}

, determine the direction of net reaction.

(e)

Compare the energy released per kilogram by burning methanol and hydrogen. Despite its lower value, methanol is proposed as a way to store and transport hydrogen. Explain, with reference to particles and forces, why methanol is easier to store.