1 https://chem.libretexts.org/@go/page/53090
Combustion Reactions
Identify a general chemical equation for combustion reactions Describe some of the characteristics of combustion reactions
Combustion reactions are common and very important. Combustion means burning, usually in oxygen but sometimes with other oxidants such as fluorine. A combustion reaction happens quickly, producing heat, and usually light and fire. Combustion describes how the reaction happens, not the reactants and products. Chemists as early as Lavoisier suggested that people get their energy from combustion-like reactions, but even though the products and reactants are the same when you burn food in a fire and in your body, the way it happens is different. In a combustion reaction, the thing that burns (the reactant that isn't O or F ) is called the fuel. Combustion reactions are a type of redox reaction.
A simplified diagram of a combustion reaction where oxygen is the oxidant.
The classic chemistry class combustion reaction involves a compound of C and H reacting with O to form CO and H O.
Sometimes the reactant has some other elements, like O, S or N in it. The O will form water, the S will form usually SO and the N will often produce N , but sometimes a little bit of a nitrogen oxide. For class purposes, you can usually write equations in which carbon dioxide is produced. In real life, often some or a lot of CO is produced, depending on how much oxygen is present and other factors. In general, most elements in a compound that is combusted will form oxides, but you won't be able to say for sure how much of each oxide will be produced (CO or CO , SO or SO , etc). Here are some example equations. When you balance combustion reactions, usually start with the C, and the fuel, and do the oxygen last.
Reaction 4.2 is sugar burning, which may also represent an animal or plant using stored energy.
Contributors and Attributions
Emily V Eames (City College of San Francisco)
Combustion Reactions is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.
Skills to Develop
2 2
2 2 2
2 2
2 2 3
(g) +5 (g) → 3C (g) +4 O(g)
C3H8 O2 O2 H2 (1)
(s) +12 (g) → 6C (g) +6 O(g)
C6H12O6 O2 O2 H2 (2)
1 https://chem.libretexts.org/@go/page/53089
Decomposition Reactions
Describe decomposition reactions with chemical equations
Decomposition reactions are very common, and this word is used so much that many chemists just say "decomp". When people use this word in the lab, they might just mean that something didn't work, or that a chemical reacted in an unwanted way, especially while it was sitting in a bottle for a long time. The official meaning of decomposition is a little bit more specific, and means a reaction in which one chemical splits into two or more chemicals, like this:
Decomposition reactions are often undesirable, but not always. For instance, many explosions are decompositions, and explosives are very important for many purposes other than weapons. Decomposition reactions might be hard to predict at first.
Some Simple Decomposition Patterns
The decomposition reactions in intro chemistry classes often result from heating a substance. For instance, when heated or struck, a salt of a complex anion (chlorate, carbonate, azide) may lose a gas (oxygen, carbon dioxide or nitrogen) leaving behind a simpler salt or metal. This could happen explosively, depending on the compound. Or, when heated, a metal hydroxide loses water to form the metal oxide (the reverse of the basic anhydride combination reaction). Here are some examples:
Students often get confused into thinking that combination reactions, the opposite of decomposition reactions, are called composition reactions. Actually, composition is not a type of reaction, rather, it has a different meaning. Composition means the ratio of elements in a compound, such as 75% C and 25% H.
Contributors and Attributions
Emily V Eames (City College of San Francisco)
Decomposition Reactions is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.
Skills to Develop
A → B +C (1)
CaC (s) → CaO(s) +C (g)O3 O2 (2)
2Na (s) → 2Na(s) +3 g)N3 N2 (3)
2KCl (s) → 2KCl(s) +3 (g)O3 O2 (4)
Cu(OH (s) → CuO(s) +)2 H2O(g or l) (5) Note