Combustion of alcohols
for Post 16 chemistry.
Alcohols like methanol and ethanol are commonly used as fuels in combustion reactions due to their ability to burn and release energy. Here’s a description of how they are utilised as fuels:
Combustion Process:
Combustion is a chemical reaction where a fuel reacts with oxygen to produce heat, light, and various combustion products. The chemical equations for the complete combustion of methanol (CH3OH) and ethanol (C2H5OH) are as follows:
Methanol: CH3OH + 1.5O2 → CO2 + 2H2O ΔcH = – 726 kJmol -1
Ethanol: C2H5OH + 3O2 → 2CO2 + 3H2O ΔcH = – 1367 kJmol -1
Energy Release:
- During combustion, the chemical bonds within methanol and ethanol molecules break, and new bonds form in the resulting products.
- Much more energy is released into the surroundings during bond formation than is taken from the surrounding during bond breaking.
- Combustion releases a significant amount of energy in the
form of heat and light.
Applications:
- Methanol and ethanol can be used as fuels in various applications, including:
- Internal Combustion Engines: They are often used in engines such as those found in cars, boats, and small machinery. They are mixed with gasoline to create biofuels, like E85
(a blend of 85% ethanol and 15% gasoline). - Stoves and Heaters: Methanol and ethanol can be used as clean-burning fuels for stoves, portable heaters, and even as fuel for alcohol lamps.
- Internal Combustion Engines: They are often used in engines such as those found in cars, boats, and small machinery. They are mixed with gasoline to create biofuels, like E85
- Methanol and ethanol can be used as fuels in various applications, including:
Advantages:
- Alcohols like methanol and ethanol have some advantages as fuels:
- They are renewable and can be produced from various sources, including plants (for ethanol) and natural gas or synthesis gas (for methanol).
- They have relatively low greenhouse gas emissions compared to fossil fuels.
- They can be used in engines with minor modifications, making them a feasible alternative to traditional fuels.
- Alcohols like methanol and ethanol have some advantages as fuels:
Challenges:
- While alcohols have advantages, they also have some drawbacks:
- Lower energy density compared to gasoline and diesel, which means vehicles may need larger fuel tanks.
- Corrosive properties that can affect certain engine components.
- Energy-intensive production processes for ethanol and methanol.
- Land used to grow crops for making ethanol from sugar may result in less food-crops being grown.
- While alcohols have advantages, they also have some drawbacks:
Used above for ‘chafing’ (keeping food warm).
Burning for over 3 hours.
Ethanol burning in a spirit burner.
The colour of a flame is determined by the specific wavelengths of light emitted during the combustion process, which, in turn, is influenced by the chemical composition of the fuel and the conditions under which it burns. In the case of methanol and ethanol, the difference in the colour of their flames can be attributed to various factors, including the presence of different chemical compounds and the temperature of the flames.
QUESTION. DETERMINING THE ΔcH OF METHANOL.
A student carried out an experiment to determine the enthalpy change when a sample of methanol (CH3OH) was burned.
The student found that the temperature of 140 g of water increased by 7.50 oC when 0.0110 mol of methanol was burned in air and the heat produced was used to warm the water.
Find the enthalpy change of combustion in kJ mol –1.
ANSWER.
Q = m c ΔT =140 x 4.18 x 7.50
= 4389 J = 4.389 kJ
Moles = 0.0110
ΔH = – 399 kJmol -1
The actual value is – 726 kJmol -1.
The discrepancy is down to significant heat-losses to the surroundings giving a ΔT value much lower than it should be.
Some incomplete combustion might also contribute to this.
Biofuels.
OTHER BIOFUELS. You will also learn about biodiesel when you study the transesterification of vegetable oils.
⇓ Video below: Shows the combustion of some hydrocarbons and alcohols. ⇓
