9. (Points:1) Alkanes are hydrocarbons that are used as motor fuels orindustrial

Question

9.

(Points:1) Alkanes are hydrocarbons that are used as motor fuels orindustrial solvents. The unbalanced equation forthe combustion of octane is as follows:

C8H18(g) + O2(g) CO2(g) + H2O(g)

How much heat, in kJ, is liberatedwhen 4.93 kg of C8H18(g) is burned?

Species

ÄH°f(kJ/mol)

H2O(g)

-241.8

CO2(g)

-393.5

C8H18(g)

-208.4

10.

(Points:1) When a welder uses an acetylene torch, the combustion ofacetylene liberates the intense heat needed for welding metalstogether. The unbalanced equation for thiscombustion reaction is as follows:

C2H2(g) + O2(g) CO2(g) + H2O(g)

How much heat, in kJ, is liberatedwhen 4.93 kg of C2H2 is burned?

Species

ÄH°f(kJ/mol)

H2O(g)

-241.8

CO2(g)

-393.5

C2H2(g)

226.7

Express the answer to three significant figures.
Answer
Units Save Answer

Species

ÄH°f(kJ/mol)

H2O(g)

-241.8

CO2(g)

-393.5

C8H18(g)

-208.4

Explanation / Answer

Unbalanced equation:                                      C8H18(g) + O2(g) CO2(g) + H2O(g) Balanced chemical equation:                                           2C8H18(g) +50 O2(g) 16CO2(g) + 18 H2O(g)                             Hrxn = H products - H reactants                                         = (16 moles * -393.5 kJ / mol   + 18 * -241.8 kJ / mol) -( 2 moles * -208.4 kJ / mol )                                         = -629 kJ   - 4352.4 kJ + 416.8 kJ                                         = -4564.6 kJ            Molarmass of C8H18(g)   = 114.224 g/ mol Number of moles of C8H18(g)   = 4.93 * 1000g / 114.224 g / mol                                                    = 43.16 moles 43.16 moles ofC8H18(g)    releases 4564.6 kJof heat Then one mole releases = -4564.6 kJ / 43.16 moles                                      =-105.75 kJ / mol Unbalanced equation:                                        C2H2(g) + O2(g) CO2(g) + H2O(g) Balanced chemical equation:                                                              2 C2H2(g) + 5 O2(g) 4 CO2(g) + 2 H2O(g)
                                    Hrxn = H products - Hreactants                                                 = 4 moles * -393.5 kJ / mol + 2 * -241.8 kJ / mol - ( 2 * 226.7 kJ/ mol )                                                  =- 1574 kJ - 483.6 kJ -453.4 kJ                                                 = - 2511 kJ               Molar mass ofC2H2  =    26.036 g/ mol       Number of moles ofC2H2  =    4.93* 1000 g / 26.036 g / mol                                                 = 189.35 moles 189.35 moles releases 2511 kJ of heat Then one mole releases 2511 kJ / 189. 35 moles                                   =-13.26 kJ / mol           

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