Chapter 8 Chemical Reactions and Thermochemistry

Chapter 8: Chemical Reactions and Thermochemistry - - - - One type of reaction is combination, or it can also be called synthesis. This means the formation of a compound from the union of its elements. - 𝑍𝑛(𝑠) + 𝑆(𝑠) → 𝑍𝑛𝑆(𝑠) - 2𝐻2 (𝑔) + 𝑂2 (𝑔) → 2𝐻2 𝑂(𝑔) - 𝐶(𝑠) + 𝑂2 (𝑔) → 𝐶𝑂2 (𝑔) The second type of reaction, decomposition, can also be referred to as analysis. This means the breakdown of a compound to release its components as individual elements or other compounds. - 2𝐻2 𝑂(𝑙) → 2𝐻2 (𝑔) + 𝑂2 (𝑔) - 𝐶12 𝐻22 𝑂11 (𝑠) → 12𝐶(𝑠) + 11𝐻2 𝑂(𝑙) - 2𝐻𝑔𝑂(𝑠) → 2𝐻𝑔(𝑠) + 𝑂2 (𝑔) The 3rd type of reaction is called single replacement or single displacement. This can best be shown in which one substance is displacing another. - 𝐹𝑒(𝑠) + 𝐶𝑢𝑆𝑂4 (𝑎𝑞) → 𝐹𝑒𝑆𝑂4 (𝑎𝑞) + 𝐶𝑢(𝑠) - 𝑍𝑛(𝑠) + 𝐻2 𝑆𝑂4 (𝑎𝑞) → 𝑍𝑛𝑆𝑂4 (𝑎𝑞) + 𝐻2 (𝑔) - 𝐶𝑢(𝑠) + 2𝐴𝑔𝑁𝑂3 (𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 (𝑎𝑞) + 2𝐴𝑔(𝑠) The last type is calle double replacement or double displacement b/c there is an actual exchange of “partners” to form new compounds. - 𝐴𝑔𝑁𝑂3 (𝑎𝑞) + 𝑁𝑎𝐶𝑙(𝑎𝑞) → 𝐴𝑔𝐶𝑙(𝑠) + 𝑁𝑎𝑁𝑂3 (𝑎𝑞) - 𝐻2 𝑆𝑂4 (𝑎𝑞) + 2𝑁𝑎𝑂𝐻(𝑎𝑞) → 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞) + 2𝐻2 𝑂(𝑙) PREDICTING REACTIONS 1. Combination - A heat of formation table gives the number of kj evolved or absorbed when a mole of the compound in question is formed by the direct union of its elements. - A positive number indicates that heat is absorbed, a negative number when heat is evolved. - If the heat of formation is a large number preceded by a minus sign, the combination is likely to occur spontaneously and the reaction is exothermic. - If the number is small and negative or is positive, heat will be needed to get the reaction to proceed. - The symbol 𝛥𝐻is used to indicate heat of formation. Ex: 𝑍𝑛(𝑠) + 𝑆(𝑠) → 𝑍𝑛𝑆(𝑠) + 202.7 𝑘𝐽 𝛥𝐻 = −202.7 𝑘𝐽 This means that 1 mole of zinc (65 grams) reacts w/ 1 mole of sulfur (32 grams) to form 1 mole of zinc sulfide (97 grams) and releases 202.7 kJ of heat. 2. Decomposition - The prediction of decomposition uses the same source of information, the heat of the formation table. - If the heat of formation is a high exothermic (𝛥𝐻is negative), the compound will be difficult to decompose since this same quantity of energy must be returned to the compound. - A low heat of formation indicates decomposition would not be difficult 3. Single Replacement - A prediction can be based on a comparison of the heat of formation of the original compound and that of the compound to be formed Ex: 𝑍𝑛(𝑠) + 2𝐻𝐶𝑙(𝑎𝑞) → 𝑍𝑛𝐶𝑙2 (𝑎𝑞) + 𝐻2 (𝑔) 𝛥𝐻 = 2 × −92.3 𝑘𝐽 2 × −92.3 𝑘𝐽 −184.6 𝑘𝐽 ≠ −415.5 𝑘𝐽 This comparison leaves an excess of 230.9 kJ of heat given off, so the reaction would occur. - Nother simple way to predict single replacement reactions is to check the active positions of the 2 elements in the activity series. - If the element that is to replace the other in the compound is higher on the chart, the reaction will occur. If it is below, there will be no reaction. Ex: 𝑍𝑛(𝑠) + 2𝐻𝐶𝑙(𝑎𝑞) → 𝑍𝑛𝐶𝑙2 (𝑎𝑞) + 𝐻2 (𝑔) - In predicting the replacement of hydrogen by zinc in hydrochloric acid, zinc will replace hydrogen and this reaction will occur. - Most metals in the activity series would replace hydrogen. If a metal such as copper were chosen, no reaction would occur. 4. Double Replacement - 2 compound seacat where either the cations or anions witch but not both to form 2 new compounds 5. Hydrolysis Reactions - The water and salt react to form an acid and a base Entropy - Another driving force to reactions is related to their state of disorder or of randomness called entropy. - A reaction is also driven by a need for a greater degree of disorder. THERMOCHEMISTRY - The sum of the internal energy (heat content) of a system is called enthalpy and is symbolized by 𝛥𝐻. - The heat released by the complete combustion of 1 mole of a substance is called the heat of combustion. The First Law of Thermodynamics states that the total energy of the universe is constant and cannot be created or destroyed.