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Methanol: Structural Formula/Enthalpy Change Calculations, Etc.

Methanol (CH3OH) has been offered as a contender for alternative motor vehicle fuel for many years. In fact the technology of burning methanol in car engines, especially racing cars, has been use since the 1960s. Methanol (also known as wood alcohol) can be produced by the fermentation of biomass (naturally occurring organic materials such as trees and plants) but is more often produced industrially on a large scale from natural gas (methane), coal or naphtha (a fraction of crude oil). The process in which methanol is synthesized is via the hydrogenation of carbon monoxide from a mixture of gases known as synthesis gas. A schematic of the production of methanol from organic sources via different synthesis gas and some of its uses, is shown in Figure 6.
Figure 6 Industrial production of methanol via synthesis gas and some of its uses. (see attached file).

(a) The industrial production of methanol via synthesis gas occurs by the following reaction:
CO(g) + 2H2(g) â?? CH3OH(g) (Reaction 1)
(i) Draw out the full structural formula of a molecule of methanol. From your structural formula, identify and name any functional group(s) present. Highlight or circle the functional group(s) on your drawing.

(ii) Use the molar bond enthalpies in Table 2 to calculate the overall enthalpy change for the reaction between carbon monoxide and hydrogen (synthesis gas), to produce methanol (Reaction 1). You may assume that there is one triple bond in the carbon monoxide molecule and the reaction occurs solely in the gas phase. Show all the steps clearly in your calculation.

(iii) In terms of chemical equilibrium, from your answer in part (a) (ii), and from Reaction 1, suggest ways in which the yield of methanol could be increased. You should include in your answer brief reasoning as to why such measures would increase the yield of methanol.

(iv) Industrially, Reaction 1 is carried out at temperatures of about 500 K, but at this temperature the rate of reaction is slow. How could you ensure the rate of reaction is increased at this temperature? (A single sentence)

Table 2 Molar bond enthalpies for the production of methanol from synthesis gas.
Bond Molar bond
enthalpy/kJ molâ?'1
HH 436
CH 413
CO 358
C~O 1074
OH 464

(b)(i) The heat released by a fuel is commonly referred to as the enthalpy of combustion and for any carbon-based fuel, these values vary with number of carbons in the molecular formula. The enthalpies of combustion (Î"Hc) for several alkanes are given in Table 3. (Note that conventionally, even though combustion is an exothermic reaction, Î"Hc is quoted as a positive value this is simply for convenience.)
From the data in Table 3, draw a fully labelled graph of Î"Hc (on the vertical axis) and the number of carbons in the chain of the alkane (on the horizontal axis). From your graph, by extrapolation or using the best fit line, determine the enthalpy of combustion Î"Hc of octane (C8H18), the main constituent of petrol, which is missing from the table. Using your value of Î"Hc for octane, comment briefly (a couple of sentences) on how this value compares with the literature value of 5471 kJ mol1. Your graph may be drawn by hand or by using an appropriate computer software program.
Table 3 Enthalpy of combustion for several straight chain alkanes.
Hydrocarbon fuel name and molecular formula Number of carbons in chain Î"Hc /kJ mol1
methane CH4 1 840
ethane C2H6 2 1590
propane C3H8 3 2100
butane C4H10 4 2968
pentane C5H12 5 3537
hexane C6H14 6 4075
heptane C7H16 7 4818
octane C8H18 8

(ii) Normally we purchase fuel for motor cars by the litre rather than by mass or moles. The densities of methanol and octane (petrol) are similar, at
0.75 kg l1, and the enthalpies of combustion for methanol and octane are 726 and 5471 kJ mol1, respectively. Use this information to calculate the energy released per litre for each of these fuels. Give your final answers to two significant figures. (Use the relative atomic masses of C, H and O as 12.0, 1.0 and 16.0, respectively).

(iii) How do these two values compare if you were to use each of these fuels to power a car over the same distance and hence what would be the implication on the size of the car's fuel tank? (A few sentences)


Solution Preview

Attached is the full solution as a Word document.

A number of factors can influence the position of equilibrium.
(i) The concentration of a particular component (look up Le Chataliers Principle). So increasing the concentration of H2 or CO will drive the equilibrium towards product in order to counteract the change.
(ii) Change in temperature: For endothermic processes, heat can be thought of as a reactant and higher temperatures will drive the process towards products.
(iii) Removing product as ...

Solution Summary

The solution determines the structural formula/enthalpy change calculations for methanol.