Mason Delacroix
Energy density is a measure of how much energy is contained per amount, typically expressed as caloric heat per mass. Liquid hydrocarbons, such as gasoline, diesel, and kerosene, are currently the densest way to economically store and transport energy on a large scale. Alcohols, such as ethanol and methanol, have been used as fuels throughout history and continue to be used in automotive applications, particularly in racing cars. However, they are not as energy-dense as hydrocarbons due to several factors. Firstly, alcohols are already partially oxidized, and the C-O and O-H bonds in alcohols are stronger than the C-C and C-H bonds found in hydrocarbons. As a result, it takes less energy to break the C-H and C-C bonds in hydrocarbons, leading to a more exothermic combustion process. Additionally, the length of the hydrocarbon chain in alcohols affects their energy density, as shorter chains have lower energy density. Furthermore, the hydroxyl group (-OH) in alcohols increases their polarity, resulting in a significant attraction between molecules, which further impacts their energy density.
| Characteristics | Values |
|---|---|
| Energy density | Liquid hydrocarbons (fuels such as gasoline, diesel, and kerosene) are the densest way to store and transport energy at a large scale. |
| Energy per unit volume | The energy density of a fuel relates the stored energy to the volume of the storage equipment, e.g. the fuel tank. |
| Energy per unit mass | The energy of a fuel per unit mass is called its specific energy. |
| Alcohol-based fuels | Alcohol-based fuels have been used in automotive applications for a long time, particularly as high-octane fuels for racing cars. |
| Energy extraction | The energy is usually "extracted" by combustion. |
| Hydroxyl group | The hydroxyl group in alcohol contributes to its lower energy density. |
| Oxidation | Alcohol is already partially oxidized, which reduces its energy density. |
| Bond strength | C-O and O-H bonds in alcohol are stronger than C-C and C-H bonds, requiring more energy to break. |
| Bond dissociation energy | C-O bond dissociation energy is 1072 kJ•mol-1, C-C is 345kJ•mol-1, and C-H is 413 kJ•mol-1. |
| Enthalpy of combustion | Ethanol has a lower enthalpy of combustion than octane, resulting in less energy released. |
| Volatility | Alcohols are less volatile than comparable hydrocarbons due to the presence of the hydroxyl group. |
| Melting point | Alcohols have higher melting points than comparable hydrocarbons. |
| Water solubility | Alcohols have greater water solubility than comparable hydrocarbons due to hydrogen bonding. |
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What You'll Learn
Energy density of fuels
Energy density is a measure of the amount of energy that can be stored or transported for a given volume. The higher the energy density of a fuel, the more energy can be stored or transported. The energy density of a fuel is typically measured as the energy per unit mass, or the specific energy of the fuel.
Liquid hydrocarbons, such as gasoline, diesel, and kerosene, are currently the densest way to store and transport energy on a large scale. However, the search for alternative fuels with comparable energy densities is ongoing.
Alcohol-based fuels, such as ethanol and methanol, have been used in automotive applications, particularly in racing cars, due to their high octane rating. They burn more completely and produce lower emissions. However, alcohol-based fuels have a lower energy density than hydrocarbon fuels. This is because the alcohol is already partially oxidised, and the C-O and O-H bonds are stronger than the C-C and C-H bonds found in hydrocarbons. As a result, it takes less energy to break the C-H and C-C bonds in hydrocarbons, leading to a more exothermic combustion. Additionally, the longer hydrocarbon chains in fuels like octane mean there are more C-C and C-H bonds to break per molecule, further increasing the energy released during combustion.
While biobutanol, a four-carbon chain alcohol, has a higher energy density than ethanol and methanol, it is currently more challenging to produce. Butanol can be prepared from agricultural plant waste wood bases, and it possesses higher energy density and lower volatility compared to ethanol.
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Hydroxyl group
Alcohols are organic compounds characterised by one or more hydroxyl (-OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain). They are considered organic derivatives of water (H2O) where one of the hydrogen atoms has been replaced by an alkyl group.
The hydroxyl group is responsible for the high polarity of alcohols, which, when substituted on a hydrocarbon chain, gives the molecule a polar character. This polarity results in a significant attraction between molecules, particularly in the solid and liquid states. This attraction requires additional energy to break, which is why alcohols have higher boiling points than comparable hydrocarbons. The hydroxyl group also contributes to the water solubility of alcohols, as seen in methanol, where it accounts for almost half of the molecule's weight, making it completely soluble in water.
The presence of the hydroxyl group in ethanol, for example, lowers its energy density compared to hydrocarbons like gasoline. This is because the alcohol is already partially oxidised, and the C-O and O-H bonds are stronger than the C-C and C-H bonds found in hydrocarbons. As a result, it takes less energy to break the C-H and C-C bonds in hydrocarbons, leading to a more exothermic combustion.
Additionally, the length of the hydrocarbon chain in alcohols affects their solubility. Smaller alcohols, like methanol, are highly soluble in water due to hydrogen bonding, while the solubility decreases as the length of the hydrocarbon chain increases.
While alcohols have lower energy density than hydrocarbons, they have been used as alternative fuels due to their high octane rating, which can increase fuel efficiency. Alcohols like ethanol and methanol have been utilised in automotive applications, particularly in racing cars, as they burn more completely and produce lower emissions.
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Bond dissociation energy
The bond dissociation energy (BDE, D0, or DH°) is a measure of the strength of a chemical bond A−B. It is defined as the standard enthalpy change when the bond A−B is cleaved by homolysis to give fragments A and B, which are usually radical species. The enthalpy change is temperature-dependent, and the bond-dissociation energy is often defined as the enthalpy change of homolysis at 0 K (absolute zero). However, the enthalpy change at 298 K (standard conditions) is also frequently used. The BDE can be calculated using various experimental techniques, including spectrometric determination of energy levels, generation of radicals by pyrolysis or photolysis, and various calorimetric and electrochemical methods.
The BDE is an important concept in understanding the energy density of hydrocarbons and alcohols. Hydrocarbons are molecules consisting of carbon and hydrogen atoms, with strong C−H bonds. The strength of these bonds contributes to the high energy density of hydrocarbons, making them valuable as fuels. On the other hand, alcohols are organic compounds characterized by one or more hydroxyl (−OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain). The presence of the hydroxyl group in alcohols affects the BDE of the C−H bonds in the hydrocarbon chain.
The BDE of a bond can be calculated by measuring the energy required to break the bond. For example, the BDE for one of the C−H bonds in ethane (C2H6) is defined as the standard enthalpy change when one of the C−H bonds is broken, resulting in a CH3CH2−H fragment. The BDE for this bond is approximately 101.1 kcal/mol or 423.0 kJ/mol. The BDE values for different bonds can vary, and they are influenced by factors such as temperature and the presence of certain functional groups.
In the context of energy density, hydrocarbons have higher BDEs compared to alcohols due to the presence of multiple strong C−H bonds. The energy density of a substance is related to the amount of energy stored within it. Higher BDEs generally indicate a higher energy density, as more energy is required to break the bonds during combustion. This is one of the reasons why hydrocarbons, such as gasoline and diesel, have higher energy densities than alcohol-based fuels like methanol and ethanol.
While alcohols have lower BDEs and energy densities compared to hydrocarbons, they still have important applications as alternative fuels. Alcohols can be produced from various feedstocks, including renewable sources such as biomass and agricultural waste. They offer advantages such as lower production costs, reduced emissions, and higher octane ratings, which can improve fuel efficiency. Additionally, advancements in engine technologies and fuel blends can help optimize the performance of alcohol-based fuels, making them a viable alternative to traditional hydrocarbon fuels.
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Octane rating
The octane number is the average of two different octane rating methods: motor octane rating (MOR) and research octane rating (RON). MOR and RON differ primarily in the specifics of the operating conditions. The higher the octane number, the more stable the fuel. Retail gasoline stations in the United States sell three main grades of gasoline based on octane level: regular, mid-grade, and premium. Regular grade has an octane rating of 87, mid-grade averages 89 or 90, and premium-grade ratings range from 91 to 94.
Some companies have different names for these grades of gasoline, such as unleaded, super, or super premium, but they all refer to the octane rating. For example, Shell offers V-Power, advertised as "over 99 octane", instead of 98. In 2018, Shell launched a new variant, "Regular", rated at 90 RON, but this was discontinued in 2022.
Alcohol-based fuels, such as ethanol and methanol, are also high-octane fuels. They burn more completely and produce lower emissions, although they are still hydrocarbon fuels. One advantage shared by alcohol fuels is their high octane rating, which increases fuel efficiency and largely offsets the lower energy density of alcohol fuels compared to petrol/gasoline and diesel fuels. Biobutanol, for example, has an energy density closer to gasoline than simpler alcohols while still retaining an over 25% higher octane rating.
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Volatility
Ethanol, a type of alcohol, is less volatile than some other substances, such as water and methoxymethane. This is because ethanol molecules can form hydrogen bonds, which are stronger than the van der Waals dispersion forces that hold together non-hydrogen-bonding molecules. The hydroxyl group (-OH) in ethanol increases the potential for hydrogen bonding between neighbouring ethanol molecules, resulting in an overall stronger attractive force between them.
The volatility of ethanol is significant in various applications. For example, beverage makers can increase the concentration of ethanol in alcoholic drinks by heating the initial alcohol mixture to a temperature where most of the ethanol vaporises, collecting the ethanol vapour, and then condensing it in a separate container. This process results in a more concentrated product. Additionally, ethanol's volatility is relevant in the creation of perfumes as it determines how long the fragrance will last once applied.
Compared to hydrocarbons, alcohols are substantially less volatile, have higher melting points, and greater water solubility. This is due to the high polarity of the hydroxyl group (-OH) in alcohols, which confers a measure of polar character to the molecule. As a result, there is a significant attraction between alcohol molecules, particularly in the solid and liquid states.
The lower volatility of alcohols compared to hydrocarbons is also related to their energy density. Ethanol, for example, is less energy-dense than gasoline (a hydrocarbon) because it is already partially oxidised. The C-O and O-H bonds in ethanol are stronger than the C-C and C-H bonds in hydrocarbons, so it takes less energy to break the latter bonds, leading to a more exothermic combustion in hydrocarbon chains.
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Frequently asked questions
Alcohols are already partially oxidised, and the C-O and O-H bonds are stronger than C-C and C-H bonds. Therefore, it takes less energy to break the latter two bonds, leading to more exothermic combustion in alkane chains without the OH group.
Energy density is a measure of how much energy is obtained per amount, usually in the form of caloric heat per mass. The energy is typically extracted through combustion.
Additional energy is required to break the hydrogen bonds in a substance, which causes the boiling point to rise. Therefore, substances with higher boiling points will have higher energy densities.
The energy density of gasoline is higher than that of alcohol. This is because alcohol has a lower energy density per unit mass, resulting in a lower amount of energy extracted through combustion.
