SHEET 2

1. Two forms of carbon are graphite, the soft, black, slippery material used

in “lead” pencils and as a lubricant for locks, and diamond , the brilliant,

hard, gemstone. Using the enthalpies of combustion for graphite (-394

kJ/mol) and diamond (-396 kJ/mole), calculate ΔH for the conversion of

graphite to diamond:

Cgraphite (s) → Cdiamond(s)

2. Methanol (CH3OH) is often used as a fuel in high performance engines in

race cars. Compare the standard enthalpy of combustion per gram of

methanol with that per gram of gasoline. Gasoline is actually a mixture of

compounds, but assume for this problem that gasoline is pure liquid octane

(C8H18). (Standard enthalpies of formation of CH3OH, C8H18, CO2(g) and

H2O(L) are -239, -269, -394 and -286 KJ/mol respectively).

3. Compare the energy available from the combustion of a given volume of

methane and the same volume of hydrogen at the same temperature and

pressure. (Standard enthalpy of formation of methane is -75 kJ/mol)

4. Assuming that the combustion of hydrogen gas provide three times as

much energy per gram as gasoline, calculate the volume of liquid H2

(density=0.0710 g/mL) required to furnish the energy contained in 80.0L

of gasoline (density = 0.740 g/mL). Calculate also the volume that this

hydrogen would occupy as a gas at 1.00 atm and 25°C.

5. When 1 mole of methane (CH4) is burned at a constant pressure, 890 kJ

of energy is released as heat. Calculate ΔH for a process in which a 5.8 g

sample of methane is burned at constant pressure.

6. Consider the following reaction:

CH4(g) + 2O2(g) →CO2(g) +2 H2O(L) ΔH=-891 kJ

Calculate the enthalpy change for each of the following cases:

a- 1.00 g methane is burned in excess oxygen.

b- 1.00x103 L methane gas at 740. torr and 25°C is burned in excess

oxygen.

7. The enthalpy of combustion of solid carbon to form carbon dioxide is

-393.7 kJ/mol carbon, and the enthalpy of combustion of carbon

monoxide to form carbon dioxide is -283.3 kJ/mol CO. Use these data to

calculate ΔH for the reaction:

2C(s) + O2(g) → 2CO(g)

8. Ethanol (C2H5OH) has been proposed as an alternative fuel. Calculate

the standard of enthalpy of combustion per gram of liquid ethanol.

Knowing that the standard enthalpy of formation of ethanol (liquid) = -278

kJ/mol.

9. Some automobiles and buses have been equipped to burn propane

(C3H8). Compare the amounts of energy that can be obtained per gram of

C3H8(g) and per gram of gasoline, assuming that gasoline is pure octane,

C8H18(L). Look up the boiling point of propane. What disadvantages are

there to using propane instead of gasoline as fuel. (Standard enthalpies of

formation of C3H8(g) and C8H18(L) are -104 and -269 kJ/mol respectively).

10. Assume that 4.19x106 kJ of energy is needed to heat a home. If this

energy is derived from the combustion of methane (CH4), what volume of

methane, measured at STP must be burned? (ΔH°combustion for CH4=-891

kJ/mol)

11. At 298 K, the standard enthalpies of formation for C2H2(g) and C6H6(L)

are 227 kJ/mol and 49 kJ/mol, respectively.

a- Calculate ΔH° for

C6H6(L) → 3 C2H2(g)

b- Both acetylene (C2H2) and benzene (C6H6) can be used as fuels. Which

compound will liberate more energy per gram when combusted in air?

12. Combustion of table sugar produces CO2(g) and H2O(L).

a- Assuming that table sugar is pure sucrose, C12H22O11(s), write the

balanced equation for the combustion reaction.

b- Calculate ΔH in kJ/mol C12H22O11 for the combustion reaction of

sucrose. (Standard enthalpy of formation of C12H22O11(s) is 2244

kJ/mol.

13. A sample of methane gas having a volume of 2.8 L at 25°C and 1.65 atm

was mixed with a sample of oxygen gas having a volume of 35.0 L at 31°C

and 1.25 atm. The mixture was then ignited to form carbon dioxide and

water. Calculate

a- the volume of CO2 formed at a pressure of 2.5 atm and a

temperature of 125°C.

b- the % excess of O2.

14. A compound containing only C, H, and N yields the following data.

i- Complete combustion of 35.0 mg of the compound produced 33.5 mg

of CO2 and 41.1 mg of H2O.

ii- A 62.5 mg sample of the compound was analyzed for nitrogen by the

Dumas method, giving 35.6 mL of N2 at 740 torr and 25°C.

iii- The effusion rate of the compound as a gas was measured and found

to be 24.6 mL/min. The effusion rate of argon gas under identical

conditions, is 26.4 mL/min.

What is the molecular formula of the compound?

15. An organic compound contains C, H, N, and O. Combustion of 0.1023 g

of the compound in excess oxygen yielded 0.2766 g of CO2 and 0.0991 g

of H2O. A sample of 0.4831 g of the compound was analyzed for nitrogen

by the Dumas method. At STP 27.6 mL of dry N2 was obtained. In a third

experiment, the density of the compound as a gas was found to be 4.02

g/L at 127°C and 256 torr. What are the empirical and molecular formulas

of the compound.

16. Consider a sample of a hydrocarbon (a compound consisting of only

carbon and hydrogen) at 0.959 atm and 298 K. Upon combusting the

entire sample in oxygen, you collect a mixture of gaseous carbon dioxide

and water vapor at 1.51 atm and 375 K. This mixture has a density of

1.391 g/L and occupies a volume four times as large as that of the pure

hydrocarbon. Determine the molecular formula of the hydrocarbon.

17. A plane uses C8H18 as a fuel. Assume complete combustion, calculate the

% excess air needed such that the water vapor in the combustion

products is about to condense at 50°C and total pressure of 736 mmHg.

Calculate the density of the combustion products and the partial pressure

of the CO2. (Saturated vapor pressure of water at 50°C is 92.4 mm Hg).

18. According to a law that limits the % CO2 in combustion product on dry

basis to 10%, calculate the % excess of air needed for the complete

combustion of C5H12.

19. Calculate the quantity of air needed for the combustion of 220 gm of

propane in a furnace that uses 20% excess air. Calculate the volume of air

at a pressure of 750 mmHg and a temperature of 32°C. If the combustion

products were cooled to 120°C and a pressure of 730 mmHg, calculate the

volume and density of the combustion products and the partial pressure of

CO2.

20. Methane (CH4) gas flows into a combustion chamber at a rate of 200

L/min at 1.50 atm and ambient temperature. Air is added to the chamber

at 1.00 atm and the same temperature, and the gases are ignited.

To ensure complete combustion of CH4 to CO2(g) and H2O(g), three times

as much oxygen as is necessary is reacted. Assuming air is 21 mole

percent O2 and 79 mole percent N2 calculate the flow rate of air

necessary to deliver the require

d amount of oxygen.

b- Under the conditions in part a., combustion of methane was not

complete as a mixture of CO2(g) and CO(g) was produced. It was

determined that 95.0% of the carbon in the exhaust gas was present

in CO2. The remainder was present as carbon in CO. Calculate the

composition of the exhaust gas in terms of mole fraction of CO, CO2,

O2, N2 and H2O.Assume CH4 is completely reacted and N2 is

unreacted.