Standard Enthalpy of Combustion - UCalgary Chemistry Textbook

Standard enthalpy of combustion ($ΔH_C°$) is the enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions; it is sometimes called “heat of combustion.” For example, the enthalpy of combustion of ethanol, −1366.8 kJ/mol, is the amount of heat produced when one mole of ethanol undergoes complete combustion at 25 °C and 1 atmosphere pressure, yielding products also at 25 °C and 1 atm. $$C_2H_5OH(l)+3O_2(g)⟶2CO_2+3H_2O(l)\qquad ΔH°=−1366.8 kJ/mol$$

Enthalpies of combustion for many substances have been measured; a few of these are listed in the table below. Many readily available substances with large enthalpies of combustion are used as fuels, including hydrogen, carbon (as coal or charcoal), and hydrocarbons (compounds containing only hydrogen and carbon), such as methane, propane, and the major components of gasoline.

Substance	Combustion Reaction	Enthalpy of Combustion, ΔHc° (kJ/mol at 25°C)
carbon	$C(s)+O_2(g)⟶CO_2(g)$	−393.5
hydrogen	$H_2(g)+\frac{1}{2}O_2(g)⟶H_2O(l)$	−285.8
magnesium	$Mg(s)+\frac{1}{2}O_2(g)⟶MgO(s)$	−601.6
sulfur	$S(s)+O_2(g)⟶SO_2(g)$	−296.8
carbon monoxide	$CO(g)+\frac{1}{2}O_2(g)⟶CO_2(g)$	−283.0
methane	$CH_4(g)+2O_2(g)⟶CO_2(g)+2H_2O(l)$	−890.8
acetylene	$C_2H_2(g)+\frac{5}{2}O_2(g)⟶2CO_2(g)+H_2O(l)$	−1301.1
ethanol	$C_2H_5OH(l)+3O_2(g)⟶2CO_2(g)+3H_2O(l)$	−1366.8
methanol	$CH_3OH(l)+\frac{3}{2}O_2(g)⟶CO_2(g)+2H_2O(l)$	−726.1
isooctane	$C_8H_{18}(l)+\frac{25}{2}O_2(g)⟶8CO_2(g)+9H_2O(l)$	−5461

Using Enthalpy of Combustion

As the reaction above suggests, the combustion of gasoline is a highly exothermic process. Let us determine the approximate amount of heat produced by burning 1.00 L of gasoline, assuming the enthalpy of combustion of gasoline is the same as that of isooctane, a common component of gasoline. The density of isooctane is 0.692 g/mL.

The combustion of gasoline is very exothermic. (credit: modification of work by “AlexEagle”/Flickr)

Solution
Starting with a known amount (1.00 L of isooctane), we can perform conversions between units until we arrive at the desired amount of heat or energy. The enthalpy of combustion of isooctane provides one of the necessary conversions. The table above gives this value as −5460 kJ per 1 mole of isooctane (C₈H₁₈).

Using these data,

$$1.00\;\require{enclose}\enclose{horizontalstrike}{L\;C_8H_{18}}\times \frac{1000\;\enclose{horizontalstrike}{mL\;C_8H_{18}}}{1\;\enclose{horizontalstrike}{L\;C_8H_{18}}}\times \frac{0.692\;\enclose{horizontalstrike}{g\;C_8H_{18}}}{1\;\enclose{horizontalstrike}{mL\;C_8H_{18}}}\times \frac{1\;\enclose{horizontalstrike}{mol\;C_8H_{18}}}{114\;\enclose{horizontalstrike}{g\;C_8H_{18}}}\times \frac{−5460\;kJ}{1\;\enclose{horizontalstrike}{mol\;C_8H_{18}}}=−3.31×10^4\;kJ$$

The combustion of 1.00 L of isooctane produces 33,100 kJ of heat. (This amount of energy is enough to melt 99.2 kg, or about 218 lbs, of ice.)

Note: If you do this calculation one step at a time, you would find: $$1.00\;L\;C_8H_{18}⟶1.00×10^3\;mL\; C_8H_{18}$$ $$1.00×10^3\;mL\;C_8H_{18} ⟶692\;g\;C_8H_{18}$$ $$692\;g\;C_8H_{18}⟶6.07\;mol\;C_8H_{18}$$ $$692\;g\;C_8H_{18}⟶−3.31×10^4\;kJ$$

Check Your Learning
How much heat is produced by the combustion of 125 g of acetylene?

Answer:

$6.25×10^3\;kJ$

Emerging Algae-Based Energy Technologies (Biofuels)

As reserves of fossil fuels diminish and become more costly to extract, the search is ongoing for replacement fuel sources for the future. Among the most promising biofuels are those derived from algae. The species of algae used are nontoxic, biodegradable, and among the world’s fastest growing organisms. About 50% of algal weight is oil, which can be readily converted into fuel such as biodiesel. Algae can yield 26,000 gallons of biofuel per hectare—much more energy per acre than other crops. Some strains of algae can flourish in brackish water that is not usable for growing other crops. Algae can produce biodiesel, biogasoline, ethanol, butanol, methane, and even jet fuel.

(a) Tiny algal organisms can be (b) grown in large quantities and eventually (c) turned into a useful fuel such as biodiesel. (credit a: modification of work by Micah Sittig; credit b: modification of work by Robert Kerton; credit c: modification of work by John F. Williams)

According to the US Department of Energy, only 39,000 square kilometers (about 0.4% of the land mass of the US or less than $\frac{1}{7}$ of the area used to grow corn) can produce enough algal fuel to replace all the petroleum-based fuel used in the US. The cost of algal fuels is becoming more competitive—for instance, the US Air Force is producing jet fuel from algae at a total cost of under $5 per gallon.¹ The process used to produce algal fuel is as follows: grow the algae (which use sunlight as their energy source and CO₂ as a raw material); harvest the algae; extract the fuel compounds (or precursor compounds); process as necessary (e.g., perform a transesterification reaction to make biodiesel); purify; and distribute.

Algae convert sunlight and carbon dioxide into oil that is harvested, extracted, purified, and transformed into a variety of renewable fuels.

Click here to learn more about the process of creating algae biofuel.

Footnotes

1 For more on algal fuel, see http://www.theguardian.com/environment/2010/feb/13/algae-solve-pentagon-fuel-problem.