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Oxides And Oxy-acids Of Chlorine : Thermochemistry |
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That branch of chemistry which is concerned with heats of reaction is called thermochemistry. The fundamental law is that of Hess, and by means of this all heats of reaction may be calculated from the heats of formation of the compounds concerned. The heat of formation of a compound is the quantity of heat evolved during the formation of 1 mol (molecular weight in grams) of the compound from its elements. The substances must be in specified states, and the products must be brought to the same temperature as the initial substances. The heat of formation may refer to a reaction at constant volume or to a reaction at constant pressure. Reactions in which heat is evolved are called exothermic reactions, those in which heat is absorbed, endothermic reactions. If a compound is formed from its elements with evolution of heat it is called an exothermic compound; if formed from its elements with absorption of heat it is called an endothermic compound. Heats of Formation
The values are in k. cal. for room temperature and 1 atm. Round brackets () denote that the substances are gases, square brackets [] that they are solids; liquids are written without brackets. Sulphur is the rhombic (a) form, carbon is taken as graphite. If we suppose all the compounds on the left of an equation to be decomposed into their elements, an amount of heat is absorbed equal to the algebraic sum of the heats of formation of these compounds. If we now suppose the elements to be combined to form the compounds on the right of the equation, an amount of heat is evolved equal to the algebraic sum of the heats of formation of these compounds. It follows from Hess's law that: Heat of reaction-sum of heats of formation of final compounds - sum of heats of formation of initial compounds. The energies of the compounds are all referred to those of the elements as zero. The amounts of energy associated with the different elements are not, of course, zero, nor are they equal, but it is only the difference between the amounts of energy associated with the elements when in combination and when free that is required. Thus, the equation [Cu] + (Cl2) = [CuCl2] + 51.6 k. cal. may be written in the form: o+o = [CuCl2] + 51.6 k. cal., or [CuCl2] = -51.6 k. cal., indicating that [CuCl2] contains 51.6 k. cal. less energy than [Cu] + (Cl2). The symbols of compounds thus represent quantities of energy, which may be added or subtracted. We may therefore, in the thermochemical equation, write the negative values of the heats of formation instead of the chemical symbols, and solve for the unknown heat of reaction. Example 1. - Find the heat of the reaction: [CaCl2] + 2[Na] = [Ca] + 2[NaCl] + x. The heats of formation of CaCl2 and NaCl are 191 k. cal. and 97.7 k. cal., respectively, hence: -191 = -2 x 97.7 + x, or x = 4.4 k. cal. If substances are produced in aqueous solution, we have to take account of the heats of solution. These vary with the amount of water, but become constant when this is very large; we usually suppose so much water taken that the heat of solution is constant. This amount of water is denoted by Aq. Thus: (NH3) + Aq = NH3,Aq + 8400 g. cal. means that when 17 grams of ammonia gas dissolve in a large quantity of water, 8400 g. cal. are evolved. If still more water is added, no heat change occurs, hence Aq does, not need to be specially stated. The heat of solution of perchloric acid is very large: HClO4 + Aq = HClO4,Aq + 20,100 g. cal. Example 2. - Find the heat of formation of gaseous hydrogen iodide from hydrogen and solid iodine from the following heats of reaction:
We write (4), (5) and (6) in the reverse order underneath (i), (2) and (3) and add all the equations, when we find: ½(H2) + [I] = (HI) - 5.93 k. cal. |
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