Alcohols are organic compounds, which contain at least one hydroxyl group (-OH) attached to the saturated carbon atom. They have the general formula of CnH2n+1OH. Methanol (CH3OH) and ethanol (C2H5OH) are the first two simplest members of the alcohol family.
The term alcohol is referred to the four types of alcohols, namely, ethyl alcohol, denatured alcohol, isopropyl alcohol, and rubbing alcohol.
Out of these four alcohols, ethyl alcohol describes the term alcohol efficiently. It is also known as ethanol (common name) and grain alcohol as it is obtained from the fermentation of fruits, grains, or sugar. Ethanol is used in the synthesis of drugs and it is also present in various alcoholic drinks.
Let us study the important characteristics of alcohol especially ethanol, which is its flammability. In this article, we will use the common term alcohol for ethyl alcohol or ethanol for simplification.
So, is alcohol flammable? Yes, alcohol is flammable because it is easily burned in presence of air by the supply of an ignition source. The flashpoint of alcohol is 12.8 °C, which makes it flammable material. The low flashpoint suggests the burning of alcohol in the air by an ignition source. It is a volatile substance and hence, its vapors will catch the fire easily.
Let us work on this explanation in detail.
We will start with the general characteristics of the alcohol i.e., ethyl alcohol.
Ethyl alcohol (C2H5OH) is the second member of the alcohol, an organic compound with a hydroxyl functional group.
It is referred to as alcohol because of its presence in various alcoholic drinks such as wine, whiskey, and beer.
Ethyl alcohol is a colorless liquid with a characteristics odor. Its low boiling point (78.23 °C) and high vapor pressure (5.95 kPa) make it volatile material.
Ethyl alcohol is naturally produced by the fermentation of fruits or sugar by yeasts. It can also be synthesized in the laboratory by the reaction of ethane with water i.e., the hydration of the ethene (C2H4) molecule, as shown in the following reaction:
C2H4 + H2O + catalyst —–> C2H5OH
Flammability is the property of combustible materials, by which they can catch fire easily without any extra effort in the air by the supply of heat via ignition source.
The ignition source may be hot surfaces, sparks, static electricity, and open flames. Whether a combustible material is flammable or not?
It depends on the flashpoint of that material.
The flashpoint of the material is the minimum temperature, which is required for catching fire in the air by vapors of that particular material with an ignition source.
The flashpoint of flammable materials lies between 0 to 60 °C.
Why Alcohol is Flammable?
The flashpoint of the alcohol i.e., the ethyl alcohol is 12 °C which is in the permissible range of flammable material. Hence, alcohol is a flammable material.
Hence, the vapors of the alcohol will catch fire at its flashpoint i.e, 12 °C in the air by an ignition source.
Combustion of Alcohol
The burning of alcohol in the air is known as its combustion. In combustion, ethyl alcohol combines with oxygen molecules present in the air and produces carbon dioxide and water. It can be represented as follows:
C2H5OH + 3O2 → 2CO2 + 3H2O
The activation energy for the combustion of ethanol is not very high and hence, oxidation of ethanol takes place easily.
The activation energy is the minimum amount of energy required by reactants to carry out a reaction.
It is well understood by the energy profile of the particular reaction.
Energy Profile of Combustion of Ethyl Alcohol
If we see the reaction carefully, we can observe that one molecule of ethyl alcohol reacts with three molecules of oxygen and produces two molecules of carbon dioxide and three water molecules.
Hence, the total number of molecules is greater on the product side than the reactant side. It indicates that the combustion of ethanol produces more entropy and hence, the change in entropy is positive. The increase in entropy favors the formation of the products.
The combustion of ethanol is an exothermic reaction as it requires a small amount of activation energy, which can be supplied by an ignition source. In turn, this reaction generates a large amount of energy in the form of heat and light.
Hence, the change in enthalpy for the reaction will be negative.
The energy supplied by an ignition source easily overcomes the activation energy of the combustion reaction.
The exothermic reaction indicates the energy of reactants is higher than the products and hence, the reaction is favored in the forward direction.
Now, we know that the change in Gibbs free energy (∆G) is related to the change in enthalpy (∆H) and change in entropy (∆S) by the following equation:
∆G = ∆H − T∆S
For combustion reaction, ∆H > T∆S, ∆H is negative, and ∆S is positive. Hence, ∆G will be negative, which explains the spontaneity of the combustion reaction from the thermodynamic point of view.
We can understand the flammability of ethanol by looking over the mechanism of the combustion of ethyl alcohol.
Mechanism of combustion of ethyl alcohol
Combustion of ethyl alcohol is a free radical reaction, which means that combustion involves the movement of one electron at a time rather than in a pair.
The free radical reaction involves three steps: Initiation, propagation, and termination of radical.
Let us understand these three steps in detail.
Step 1: The initiation step includes the formation of oxygen radicals by an ignition source.
O2 + Ignition source —–> 2O
Step 2: The propagation step is basically the generation of hydrogen radical by breakage of the C-H bond.
Afterward, the C-OH bond break to accept the hydrogen radical formed earlier.
The hydroxyl group is a good acceptor of hydrogen radical as compared to the hydrocarbons and hence, increases the rate of the reaction. It explains the flammability of ethyl alcohol.
Step 3: In the termination step, two oxygen radicals combine with the four released hydrogen radicals, and one hydroxyl radical combines with one released hydrogen radical.
Hence, three molecules of water are formed in the reaction.
However, the ethane part of ethyl alcohol is converted into two molecules of carbon dioxides by combining one carbon radical with two oxygen radicals.
Let us understand the flammability of alcohols with respect to hydrocarbons in terms of the energy of electrons present in the bonds of reactants and products.
Alcohols versus Hydrocarbons
Alcohols (CnH2n+1OH) are a slightly oxidized form of hydrocarbons (CnH2n+2) because complete oxidation of hydrocarbons will produce carbon dioxide.
As we know that, the combustion of hydrocarbons also produces carbon dioxide and water. Hydrocarbons are also flammable because the energy of electrons is higher in the C-H and C-C bonds of the reactant than the C-O and O-H bonds of the products.
Similarly, alcohols are flammable because they also contain high-energy electrons in C-H and C C bonds. Hence, these bonds can easily combine with the oxygen molecules by a free radical mechanism.
Uses of Ethyl alcohol
Ethyl alcohol is used as an antiseptic and disinfectant. It is also used for the production of useful organic compounds, where it acts as a solvent.
It is also used as a solvent in the preparation of liquid medicines for cold, cough, and mouth wash.
Ethyl alcohol has been used as an antidote to various poisons like ethylene glycol and methanol. It is miscible with water still its high concentration is used to dissolve various water-insoluble compounds.
The important characteristics of ethyl alcohol i.e., its flammability will be discussed here in detail. For that, we should be aware of some basic terms such as flammability, and flashpoint.
In this article, we have focused on the flammability of ethyl alcohol, which is simply named alcohol as it is an active ingredient in alcoholic drinks.
Alcohol is a flammable material owing to its low flashpoint, which makes it burn easily in the air by an ignition source. The flashpoint is the minimum temperature at which vapors of volatile material catch fire easily by an ignition source.
We hope that you understand the flammability of ethanol.
If you have any doubt regarding this topic, you are most welcome.