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THINK ABOUT IT

Combustion reactions are used to power vehicles, heat water and homes, and create electricity. But not all combustion reactions provide the same amount of energy. Some fuels heat water or power furnaces better than others.

How do different fuels compare?

To answer this question, you will explore

EXPLORING THE TOPIC

Burning Fuels

There are a number of different choices of fuels to power race cars. Ideally, you want to choose a fuel that transfers the most energy per gram. That way, the fuel does not unnecessarily add to the weight of the car. You can compare the energy from combustion of a variety of fuels using calorimetry to see which one would be best for a race car.

ETHANOL VERSUS METHANOL

Both ethanol, C₂H₆O, and methanol, CH₄O, have been used to power race cars. Suppose that you put the two fuels into alcohol burners and use them to heat two identical 50 mL samples of water by 30 °C.

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The chemical equations for the combustion of each type of fuel are shown here.

Combustion of ethanol: C₂H₆O(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

Combustion of methanol: 2CH₄O(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(l)

The experimental results show that the combustion of ethanol delivers more energy per gram of fuel consumed, compared with the combustion of methanol. So you would want to use ethanol, not methanol, in a race car if weight were a concern.

Heat of Reaction

The amount of energy transferred during a chemical reaction is called the heat of reaction. It is often designated with the symbol ΔH. The heat of reaction for a combustion reaction is usually referred to as the heat of combustion. Heat of reaction, ΔH, is expressed as a negative number when the reaction is exothermic. This is because energy is transferred from the system. When a reaction is endothermic, the system gains energy. Heat of reaction, ΔH, is expressed as a positive number when the reaction is endothermic.

Scientists have done carefully controlled experiments to measure the heat of combustion for many different fuels. Accurate measurements for the combustion of methanol result in a heat of reaction that is much larger in magnitude than the experimental value reported earlier.

Combustion of methanol: 2CH₄O + 3O₂ → 2CO₂ + 4H₂O
ΔH = –5400 cal/g

Combustion of ethanol: C₂H₆O(l) + 3O₂(g) → 2CO₂(g)+3H₂O(l)
ΔH = –7100 cal/g

UNITS

The metric unit that scientists use to express energy is the joule, J. A joule is a unit of energy that is 4.184 times larger than a calorie. The heat of reaction can be expressed in kilocalories per gram, kilocalories per mole, kilojoules per gram, or kilojoules per mole.

1 joule = 0.239 calorie

1 calorie = 4.184 joules

Comparing Fuels

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One way to compare fuels is to calculate the number of kilojoules of energy per mole (kJ/mol) of fuel. Simply multiply the kilojoules per gram (kJ/g) by the molar mass (g/mol) of each fuel.

This table provides the heats of combustion for several fuels in both kilojoules per gram and kilojoules per mole.

Notice that when the number of carbon atoms increases, the energy per mole increases. Also, the energy transferred per mole is smaller for the alcohols compared with the alkanes. This makes sense because combustion is a reaction in which a substance combines with oxygen, and the alcohols already contain an oxygen atom. Also notice that elemental hydrogen releases the highest energy per gram. This is why hydrogen is used as the fuel to launch a space shuttle. You get a lot of energy without adding much weight.

LESSON SUMMARY

How do different fuels compare?

One way to compare different fuels is to measure the amount of energy they transfer as a result of combustion. Calorimetry is the most common method of measuring this energy. The energy transferred can be expressed in kcal/mol, kcal/g, kJ/mol, or kJ/g of substance. The amount of energy transferred per gram or per mole during a chemical change is commonly called the heat of reaction, ΔH. For combustion reactions, the heat of reaction is negative and is also referred to as the heat of combustion.