Let's turn up the heat on redox reaction chemistry: Combustion

April 18, 2021

Let's turn up the heat on redox reaction chemistry :Combustion

As the seasons change, from spring into summer, many people start to enjoy outdoor activities such as grilling on a barbeque or sitting around a firepit cooking food while enjoying the company of friends and family. Whether you use a gas-powered car to get from point a to point b, or a gas-powered stove or water heater to heat your food and home, they all have one thing in common, combustion.  

Combustion is a highly exothermic (gives off heat) redox (adds oxygen) reaction that takes place between a fuel source (hydrocarbon, C-H), oxygen from either the air or an accelerant, to form products such as carbon dioxide (CO2) and water(H2O).(1) If you use wood as a fuel source, activated carbon(charcoal) is formed. Common fuel sources that are abundant as forms of hydrocarbons are Butane(C4H10) seen in lighter fuel, propane for grills and back-up generators and Octane (C8H16)which is seen in many types of fuel ranging from cars to planes.

In order to form fire and to keep it going, one must take into account the three items that make up the “fire triangle”, which are Fuel, Oxygen, and Heat (to overcome the energy of activation) to start the fire.(2) The energy of activation that is needed to overcome the chemicals (fuels) inert state and to start the reaction(set on fire) is generated from a heat source, like the sun (concentrated beam), a spark, or another open flame source such as a lighter.

                                            Example of Octane (Fuel) combustion reaction:

Octane (liquid) and Oxygen (gas) yields Carbon Dioxide (gas) and Water (gas)

To calculate what has been oxidized wetake a look at the methane molecules (CH4) and its oxidation.

You would look at the total # of carbons and hydrogens and set them equal to 0. (C + 4H = 0) you would then look at the normal charge associated with hydrogen which is +1 and solve for the carbon oxidation number. (X + 4(+1) = 0) therefore (X= -4), (X=-4 for Carbon). For CO2you would set carbon to X and solve likewise with oxygen atom being -2 charge.(X + 2(-2) = 0) therefore (X = +4). Looking at the carbon on the left side has an oxidation number of negative 4 and on the right has an oxidation number of positive 4, with a total change of 8 electrons being lost, therefore the carbon was the element that was oxidized with oxygen as the element reduced.

Combustion at a glance

Let's break down combustion:

·        It is both a reduction and oxidation (Redox) chemistry reaction between the fuel and air.

·        The common source of fuel is Hydrocarbons (C-H chains) that are typical of butane and octane.

·        Common combustion products are carbon dioxide, water, and heat

·        The spark required to start afire is known to overcome the Energy of Activation for the combustion reaction to get it started.

·        To keep a fire going one must have an adequate amount of fuel and oxygen as seen in the fire triangle.

·        Common items that use combustion are cars, lighters, barbeques/stoves, water heaters

Recreate the Fire Triangle in the activity below to check your understanding of combustion:

1)     https://www.chem.fsu.edu/chemlab/chm1020c/Lecture%207/01.php

2)     http://www.auburn.edu/academic/forestry_wildlife/fire/combustion.htm

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