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Chapter 5: Q5.15CYL (page 227)

Calculate the heat of combustion of 1 mole of ethanol, \({{\bf{C}}_{\bf{2}}}{{\bf{H}}_{\bf{5}}}{\bf{OH}}\)(l), when \({{\bf{H}}_{\bf{2}}}{\bf{O}}\)(l) and \({\bf{C}}{{\bf{O}}_{\bf{2}}}\)(g) are formed.

Use the following enthalpies of formation: \({{\bf{C}}_{\bf{2}}}{{\bf{H}}_{\bf{5}}}{\bf{OH}}\)(l), 鈭�278 kJ/mol; \({{\bf{H}}_{\bf{2}}}{\bf{O}}\)(l), 鈭�286 kJ/mol; and \({\bf{C}}{{\bf{O}}_{\bf{2}}}\)(g), 鈭�394 kJ/mol.

Short Answer

Expert verified

The heat of combustion of 1 mole of ethanol = -1924 kJ/mol.

Step by step solution

01

Enthalpies of formation

By using the following special form of the Hess鈥� law, we can calculate the heat of combustion of 1 mole of ethanol.

\({\bf{\Delta H}}_{{\bf{reaction}}}^{\bf{o}}{\bf{ = \Sigma n \times \Delta Hf}}_{{\bf{products}}}^{\bf{o}}{\bf{ - \Sigma n \times \Delta Hf}}_{{\bf{reactants}}}^{\bf{o}}.\)

Given: Enthalpies of formation:

  • \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{OH}}\)(l), 鈭�278kJ/mol
  • \({{\rm{H}}_{\rm{2}}}{\rm{O}}\)(l), 鈭�286kJ/mol
  • \({\rm{C}}{{\rm{O}}_{\rm{2}}}\)(g), 鈭�394 kJ/mol

The reaction of the combustion of ethanol is:

\({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{OH(l) + 3}}{{\rm{O}}_{\rm{2}}}\left( {\rm{g}} \right){\rm{\;}} \to {\rm{3}}{{\rm{H}}_{\rm{2}}}{\rm{O(l)\; + \;2C}}{{\rm{O}}_{\rm{2}}}{\rm{(g)}}{\rm{.}}\)

02

Change in enthalpy

From the combustion of ethanol, the products formed are water and carbon dioxide gas. The known values are substituted as shown below.

\(\begin{array}{c}{\rm{\Delta H}}_{{\rm{reaction}}}^{\rm{o}}{\rm{ = \Sigma n \times \Delta Hf}}_{{\rm{products}}}^{\rm{o}}{\rm{ - \Sigma n \times \Delta Hf}}_{{\rm{reactants}}}^{\rm{o}}\\{\rm{ = }}\left( {{\rm{3 \times \Delta Hf}}_{{{\rm{H}}_{\rm{2}}}{\rm{O(l)}}}^{\rm{o}}{\rm{\; + 2 \times \;\Delta Hf}}_{{\rm{C}}{{\rm{O}}_{\rm{2}}}{\rm{(g)}}}^{\rm{o}}} \right){\rm{ - \Delta Hf}}_{{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{OH(l)}}}^{\rm{o}}\\{\rm{ = 3}} \times \left( {{\rm{ - 286}}} \right){\rm{ + 2}} \times \left( {{\rm{ - 394}}} \right){\rm{ - }}\left( {{\rm{ - 278}}} \right)\\{\rm{ = - 1646 - 278 }}\\{\rm{ = \; - 1924 kJ/mol}}{\rm{.}}\end{array}\)

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Most popular questions from this chapter

The enthalpy of combustion of hard coal averages -35 kJ/g, that of gasoline, \({\bf{1}}{\bf{.28 \times 1}}{{\bf{0}}^{\bf{5}}}\)kJ/gal. How many kilograms of hard coal provide the same amount of heat as is available from 1.0 gallons of gasoline? Assume that the density of gasoline is 0.692 g/mL (the same as the density of isooctane).

When 50.0g of 0.200M NaCl(aq) at 24.1藲C is added to 100.0g of 0.100M AgNO3(aq) at 24.1藲C in a calorimeter, the temperature rises to 25.2藲C as AgCl(s) forms. Assuming the specific heat of the solution and products is 4.20J/g藲C, calculate the approximate amount of heat in joules produced.

The decomposition of hydrogen peroxide, \({{\bf{H}}_{\bf{2}}}{{\bf{O}}_{\bf{2}}}\), has been used to provide thrust in the control jets of various space vehicles. Using the data in Appendix G, determine how much heat is produced by the decomposition of exactly 1 mole of \({{\bf{H}}_{\bf{2}}}{{\bf{O}}_{\bf{2}}}\)under standard conditions.

\({\bf{2}}{{\bf{H}}_{\bf{2}}}{{\bf{O}}_{\bf{2}}}\left( {\bf{l}} \right) \to {\bf{2}}{{\bf{H}}_{\bf{2}}}{\bf{O}}\left( {\bf{g}} \right){\bf{ + }}{{\bf{O}}_{\bf{2}}}\left( {\bf{g}} \right)\)

Propane, \({{\bf{C}}_{\bf{3}}}{{\bf{H}}_{\bf{8}}}\), is a hydrocarbon that is commonly used as a fuel.

(a) Write a balanced equation for the complete combustion of propane gas.

(b) Calculate the volume of air at 25 掳C and 1.00 atmosphere that is needed to completely combust 25.0 grams of propane. Assume that air is 21.0 percent O2 by volume. (Hint: we will see how to do this calculation in a later

chapter on gases鈥攆or now use the information that 1.00 L of air at 25 掳C and 1.00 atm contains 0.275 g of O2 per liter.)

(c) The heat of combustion of propane is 鈭�2,219.2 kJ/mol. Calculate the heat of formation,螖Hf 掳of propane given that螖Hf 掳of H2O(l) = 鈭�285.8 kJ/mol and螖Hf 掳of CO2(g) = 鈭�393.5 kJ/mol.

(d) Assuming that all of the heat released in burning 25.0 grams of propane is transferred to 4.00 kilograms of water, calculate the increase in temperature of the water.

A piece of unknown metal weighs 217 g. When the metal piece absorbs 1.43 kJ of heat, its temperature increases from 24.5 掳C to 39.1 掳C. Determine the specific heat of this metal and predict its identity.
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