{"id":145,"date":"2016-01-11T19:59:32","date_gmt":"2016-01-11T19:59:32","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/composition-decomposition-and-combustion-reactions-2\/"},"modified":"2020-05-06T18:11:56","modified_gmt":"2020-05-06T18:11:56","slug":"composition-decomposition-and-combustion-reactions","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/composition-decomposition-and-combustion-reactions\/","title":{"raw":"Composition, Decomposition, and Combustion Reactions","rendered":"Composition, Decomposition, and Combustion Reactions"},"content":{"raw":"
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Learning Objectives<\/p>\r\n\r\n<\/header>\r\n

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  1. Recognize composition, decomposition, and combustion reactions.<\/li>\r\n \t
  2. Predict the products of a combustion reaction.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nThree classifications of chemical reactions will be reviewed in this section. Predicting the products in some of them may be difficult, but the reactions are still easy to recognize.\r\n\r\nA [pb_glossary id=\"1309\"]composition reaction[\/pb_glossary]\u00a0(sometimes also called a combination reaction<\/em> or a synthesis reaction<\/em>) produces a single substance from multiple reactants. A single substance as a product is the key characteristic of the composition reaction. There may be a coefficient other than one for the substance, but if the reaction has only a single substance as a product, it can be called a composition reaction. In the reaction\r\n

    2H2<\/sub>(g) +\u00a0O2<\/sub>(g) \u2192\u00a02H2<\/sub>O(\u2113)<\/p>\r\nwater is produced from hydrogen and oxygen. Although there are two molecules of water being produced, there is only one substance\u2014water\u2014as a product. So this is a composition reaction.\r\n\r\nA [pb_glossary id=\"1310\"]decomposition reaction[\/pb_glossary]\u00a0starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),\r\n

    2NaHCO3<\/sub>(s) \u2192\u00a0Na2<\/sub>CO3<\/sub>(s) +\u00a0CO2<\/sub>(g) +\u00a0H2<\/sub>O(\u2113)<\/p>\r\nsodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.\r\n\r\nComposition and decomposition reactions are difficult to predict; however, they should be easy to recognize.\r\n

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    Example 4.9<\/p>\r\n\r\n<\/header>\r\n

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    Problems<\/h1>\r\nIdentify each equation as a composition reaction, a decomposition reaction, or neither.\r\n
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    1. Fe2<\/sub>O3<\/sub> + 3SO3<\/sub> \u2192 Fe2<\/sub>(SO4<\/sub>)3<\/sub><\/li>\r\n \t
    2. NaCl + AgNO3<\/sub> \u2192 AgCl + NaNO3<\/sub><\/li>\r\n \t
    3. (NH4<\/sub>)2<\/sub>Cr2<\/sub>O7<\/sub> \u2192 Cr2<\/sub>O3<\/sub> + 4H2<\/sub>O + N2<\/sub><\/li>\r\n<\/ol>\r\n

      Solutions<\/h2>\r\n
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      1. In this equation, two substances combine to make a single substance. This is a composition reaction.<\/li>\r\n \t
      2. Two different substances react to make two new substances. This does not fit the definition of either a composition reaction or a decomposition reaction, so it is neither. In fact, you may recognize this as a double-replacement reaction.<\/li>\r\n \t
      3. A single substance reacts to make multiple substances. This is a decomposition reaction.<\/li>\r\n<\/ol>\r\n

        Test Yourself<\/h1>\r\nIdentify the equation as a composition reaction, a decomposition reaction, or neither.\r\n

        C3<\/sub>H8<\/sub> \u2192 C3<\/sub>H4<\/sub> + 2H2<\/sub><\/p>\r\n \r\n

        Answer<\/h2>\r\nDecomposition.\r\n\r\n<\/div>\r\n<\/div>\r\nA combustion reaction\u00a0occurs when a reactant combines with oxygen, many times from the atmosphere, to produce oxides of all other elements as products; any nitrogen in the reactant is converted to elemental nitrogen, N2<\/sub>. Many reactants, called fuels<\/em>, contain mostly carbon and hydrogen atoms, reacting with oxygen to produce CO2<\/sub> and H2<\/sub>O. For example, the balanced chemical equation for the combustion of methane, CH4<\/sub>, is as follows:\r\n

        CH4<\/sub> +\u00a02O2<\/sub> \u2192\u00a0CO2<\/sub> +\u00a02H2<\/sub>O<\/p>\r\nKerosene can be approximated with the formula C12<\/sub>H26<\/sub>, and its combustion equation is\r\n

        2C12<\/sub>H26<\/sub> +\u00a037O2<\/sub> \u2192 24CO2<\/sub> +\u00a026H2<\/sub>O<\/p>\r\nSometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C2<\/sub>H5<\/sub>OH, whose combustion equation is\r\n

        C2<\/sub>H5<\/sub>OH +\u00a03O2<\/sub> \u2192\u00a02CO2<\/sub> + 3H2<\/sub>O<\/p>\r\nIf nitrogen is present in the original fuel, it is converted to N2<\/sub>, not to a nitrogen-oxygen compound. Thus, for the combustion of the fuel dinitroethylene, whose formula is C2<\/sub>H2<\/sub>N2<\/sub>O4<\/sub>, we have\r\n

        2C2<\/sub>H2<\/sub>N2<\/sub>O4<\/sub> +\u00a0O2<\/sub> \u2192\u00a04CO2<\/sub> +\u00a02H2<\/sub>O +\u00a02N2<\/sub><\/p>\r\n\r\n

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        Example 4.10<\/p>\r\n\r\n<\/header>\r\n

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        Problems<\/h1>\r\nComplete and balance each combustion equation.\r\n
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        1. the combustion of propane, C3<\/sub>H8<\/sub><\/li>\r\n \t
        2. the combustion of ammonia, NH3<\/sub><\/li>\r\n<\/ol>\r\n

          Solutions<\/h2>\r\n
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          1. The products of the reaction are CO2<\/sub> and H2<\/sub>O, so our unbalanced equation is C3<\/sub>H8<\/sub> + O2<\/sub> \u2192 CO2<\/sub> + H2<\/sub>O. Balancing (and you may have to go back and forth a few times to balance this), we get\r\nC3<\/sub>H8<\/sub> + 5O2<\/sub> \u2192 3CO2<\/sub> + 4H2<\/sub>O.<\/li>\r\n \t
          2. The nitrogen atoms in ammonia will react to make N2<\/sub>, while the hydrogen atoms will react with O2<\/sub> to make H2<\/sub>O, thus NH3<\/sub> + O2<\/sub> \u2192 N2<\/sub> + H2<\/sub>O. To balance this equation without fractions (which is the convention), we get 4NH3<\/sub> + 3O2<\/sub> \u2192 2N2<\/sub> + 6H2<\/sub>O.<\/li>\r\n<\/ol>\r\n

            Test Yourself<\/h1>\r\nComplete and balance the combustion equation for cyclopropanol, C3<\/sub>H6<\/sub>O.\r\n

            Answer<\/h2>\r\n

            C3<\/sub>H6<\/sub>O + 4O2<\/sub> \u2192 3CO2<\/sub> + 3H2<\/sub>O<\/p>\r\n \r\n\r\n[caption id=\"attachment_144\" align=\"aligncenter\" width=\"294\"]\"Purple<\/a> Propane is a fuel used to provide heat for some homes. Propane is stored in large tanks like that shown here.[\/caption]\r\n\r\n<\/div>\r\n<\/div>\r\n

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            Key Takeaways<\/p>\r\n\r\n<\/header>\r\n

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