Many power plants produce energy by burning carbon-based fuels, which also produ

Question

Many power plants produce energy by burning carbon-based fuels, which also produces carbon dioxide. Carbon dioxide is a greenhouse gas, so over-production can have negative effects on the environment. Use enthalpy of formation data to calculate the number of moles of CO2(g) produced per megajoule of heat released from the combustion of each fuel under standard conditions (1 atm and 25 °C).

(a) coal, C(s, graphite);

(b) natural gas, CH4(g);

(c) propane, C3H8(g);

(d) octane, C8H18(l) (Hf° = –250.1 kJ ·mol–1)

UNITS: mol * MJ^-1

Explanation / Answer

Standard enthalpy of formation:

1. C : 0 kJ/mol

2. CH4 : -74.81 kJ/mol

3. C3H8 : -103.8 kJ/mol

4. C8H18 : -250.1 kJ/mol

5. CO2 : -393.509 kJ/mol

6. H2O : -241.818 kJ/mol

7. O2 = 0 kJ/mol

A. C + O2 = CO2

dH of reaction = sum of formation enthalpies of products - sum of formation enthalpies of reactants

dH = -393.509 - 0 = -393.509 kJ

Ans : No of mole of CO2 produced per mega-joule= 1 megajoule/393.509 kJ= 106 / 393509 = 2.5 mol.MJ-1

B. CH4 + 2O2 = CO2 + 2H2O

dH = [(-393.509) + 2(-241.818)] -[(-74.81) + 0]

=-802.335 kJ

No of mole of CO2 produced per mega-joule = 106/802335 = 1.2 mol.MJ-1

C. C3H8 + 5O2 = 3CO2 + 4H2O

dH = [3(-393.509) + 4(-241.818)] -[(-103.8) + 0]

= -2044 kJ

No of mole of CO2 produced per mega-joule = 106 * 3 /2044000 = 1.46 mol.MJ-1 ...[ Multiplied by 3 because 3 mol of CO2 produced in a reaction]

D. 2C8H18 + 25O2 = 16CO2 + 18H2O

dH = [16(-393.509) + 18(-241.818)] -[2(-250.1) + 0]

= -10149 kJ

No of mole of CO2 produced per mega-joule = 106 * 16/10149000 = 1.57 mol.MJ-1

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