4.3.1. Combustion

Carbon, in all its allotropic forms, burns in oxygen to give carbon dioxide along with the release of heat and light. Most carbon compounds also release a large amount of heat and light on burning.

Combustion is a chemical reaction where carbon or carbon-containing substances react with oxygen. During this process, the carbon atoms combine with oxygen atoms to form carbon dioxide (CO₂). This reaction also emits energy in the form of heat and light, making it an exothermic reaction. This principle applies not just to pure carbon, but to all carbon-based compounds, which release similar amounts of energy upon combustion.

Think of a campfire. When you burn wood, it contains carbon. As it burns, it reacts with oxygen in the air, producing carbon dioxide and releasing heat and light, which keeps you warm and allows you to see at night.

These are the oxidation reactions that you learnt about in the first Chapter: (i) C + O → CO + heat and light (ii) CH₄ + O₂ → CO₂ + H₂O + heat and light (iii) C₂H₅OH + O₂ → CO₂ + H₂O + heat and light.

The equations represent different examples of combustion reactions. In the first reaction, elemental carbon (C) reacts with oxygen (O₂) to form carbon monoxide (CO). In the second reaction, methane (CH₄), the primary component of natural gas, fully combusts in oxygen to produce carbon dioxide (CO₂) and water (H₂O). The third example shows ethanol (C₂H₅OH), which also burns in oxygen to yield carbon dioxide and water. The energy released in these reactions is harnessed for various purposes, such as heating or powering engines.

When you light a gas stove, methane gas in the pipe burns with oxygen from the air, producing a flame. The heat from this flame not only cooks your food but also illustrates the combustion reaction transforming energy from the gas into heat and light.

Saturated hydrocarbons will generally give a clean flame while unsaturated carbon compounds will give a yellow flame with lots of black smoke. This results in a sooty deposit on the metal plate.

The type of flame produced during combustion depends on whether the carbon compound is saturated or unsaturated. Saturated hydrocarbons, like octane in gasoline, burn completely with sufficient oxygen, producing a clean blue flame. In contrast, unsaturated compounds, like those containing double bonds, may not combust completely, leading to incomplete reactions that produce carbon soot, creating a yellow flame and leaving black deposits around. This indicates inefficiency in burning, meaning not all carbon is converted to carbon dioxide.

When using a gas-based heater, you often see a blue flame. This indicates complete combustion. However, if you accidentally use an oily product to light a fire, you might see a bright yellow flame that smokes a lot – that’s the incomplete combustion producing soot!

The gas/kerosene stove used at home has inlets for air so that a sufficiently oxygen-rich mixture is burnt to give a clean blue flame.

Efficient combustion is crucial in everyday appliances like stoves. To ensure that flames burn cleanly with minimal smoke and soot, gas stoves are designed with air inlets. This allows more oxygen to mix with the gas before ignition, promoting complete combustion instead of incomplete (which produces soot). Clean burning reduces flue gases and minimizes air pollution inside and outside the home.

Imagine blowing air into a campfire to help it burn brighter and cleaner. The air (oxygen) makes the fire hotter and allows for more complete combustion. This is similar to how gas stoves work for efficient cooking without producing excess smoke.

Fuels such as coal and petroleum have some amount of nitrogen and sulphur in them. Their combustion results in the formation of oxides of sulphur and nitrogen which are major pollutants in the environment.

When carbon-based fuels like coal and petroleum combust, they do not only produce carbon dioxide; they also release nitrogen oxides (NOx) and sulfur oxides (SOx) due to the presence of nitrogen and sulfur impurities. These gases contribute to environmental pollution, leading to smog, acid rain, and various health problems in humans. Reducing the combustion of these fuels or improving combustion efficiency can help limit these pollutants put into the atmosphere.

Think about a busy city with smog. This haze is partly caused by cars and factories burning fossil fuels. When they burn, not only do they release CO₂, but also harmful nitrogen oxides that contribute to poor air quality, similar to what happens when coal is burned in power plants.

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