QUESTIONS. 279. The following quantities of water are mixed together : Calculate the temperature of the mixture. ! 280. If one kilogram of mercury at 20° C. be mixed with one kilogram of water at o°, the temperature of the mixture will be 0'634° C. Calculate the specific heat of mercury. ! 281. Determine the specific heat of mercury from the observation that when the same vessel is filled successively with water and mercury, and heated to the same temperature, the water and mercury cool through the same number of degrees in 10 minutes and 270 seconds respectively. The specific gravity of mercury being considered constantly at 13.6. 282. Calculate the specific heat of mercury from the following numbers obtained by Kopp, according to his method : Temperature of mercury bath Weight of water in calorimeter 51°1 C. 283. Calculate the specific heat of phosphorus from the following determination by Kopp : Temperature of mercury bath Initial temperature in calorimeter 38°.8 C. 13° 20 grams. 26.95 †284. The specific heat of water is 4 times, and its density 770 times, that of air. Supposing a cubic mile of water to yield up one degree of its heat to a cold atmosphere, what quantity would 1000 cubic miles of the atmosphere be heated? 285. A bar of platinum weighing 150 grams is heated in a furnace until its temperature becomes constant, when it is thrown into a kilogram of water, the temperature of which it raises from 15° to 20°. Required the temperature of the furnace on the assumption that the specific heat of platinum is o°03308+00000042t between o° and t°. (Pouillet.) 286. A determination of the specific heat of iron made in the calorimeter of Lavoisier and Laplace yielded the following data. Calculate the specific heat of iron : Weight of iron taken 100 grams. Initial temperature of iron 100° C. 79°25 287. The mechanical equivalent of heat is 425 metrekilograms. What is this in foot-pounds ? 288. From what height must a block of ice at o° C. fall that the heat generated by its collision with the earth shall be just competent to melt it? From what height must it fall that the heat generated may be sufficient to convert it into steam? 289. If Ww = the number of scale divisions on a Bunsen's calorimeter equivalent to one gramme-degree unit of heat, T = the observed movement of the thread in scale divisions, G the weight of substance taken = and its temperature, then sp. ht. = = W.G.t 99.82° C., Ww the sp. ht. of indium if G = 1'1514, t : 14657 and T = 100*2. Time correction for T - 3'45.) = 290. From the data given, find the specific heats of (A) cast silver, (B) cast zinc, (C) cast antimony, (D) cast cadmium, (E) roll sulphur, as determined by Bunsen's method. Wo = 14.657. 291. One kilogram of steam at 100° C. is condensed in forty-nine kilos of water at 16° C. ; what is the temperature of the mixture? 292. It is required to distil 2 kilos. of ethyl alcohol (BP. 78.3°) per hour. What must be the supply of water at 16° C. in order that the temperature of the water round the worm may not average higher than 25° C. ? (Specific heat of ethyl alcohol 0.615). = 293. 75 grams of water are placed in a calorimeter of which the water equivalent is 5 grams; when at 15° C., 5 grams of steam are passed into and condensed by the water with the result that the temperature of the calorimeter and its contents is raised to 51.6° C.; from these data calculate the latent heat of vaporization of steam. 294. Using the atomic weights given in Appendix I., calculate the atomic heats of Pb, Ag, Cu, Fe, S, and P. 295. Taking the mean value of the atomic heats of the elements as 64, and the specific heats given in the table, find the atomic weights of Ag, Zn, Bi, Sn, and Fe. The stochiometrical quantities of these elements found by analysis as equivalent to 35'37 parts of Cl are as follows:Ag 107.66, Zn 32'44, Bi 69'16, Sn 29'339, and Fe 18626; from these numbers deduce the exact atomic weights of the elements in question. 296. It is found that in many compounds the sum of the atomic heats of the atoms of which the molecule is built up equals the product of the molecular weight into the specific heat of the compound (the molecular heat); assuming this to be always the case, deduce the atomic heats in the solid state of Cl and O, from the data that the (Specific heats,-Ga, o'079; In, o'057.) 297. Deduce the atomic heat of oxygen from the molecular heat 28.3 of potassium permanganate. (Specific heats,-—K, o'166; Mn, o'122.) 298. Find the approximate atomic heat of hydrogen if the molecular heat of NH4Cl be taken as 20, and the atomic heats of N and Cl as 5'6 and 6'4 respectively. 299. The atomic heat of lead is deduced from Regnault's value for the specific heat to be about 63, the specific heat of cerussite = o'080, of calc-spar strontianite = O'145, and of witherite atomic heats of Ca, Sr, and Ba. 0'206, of O'109; find the = = HEAT OF SOLUTION; HEAT OF COMBINATION; CALORIFIC POWER; CALORIFIC INTENSITY. CHEMICAL change is usually accompanied by changes in the distribution of energy in the system considered. By far the larger part of the energy lost to the changing systems during chemical reactions is given out in the form of heat; reactions in which heat is evolved are said to be exothermic. There exists also a class of reactions requiring heat to be imparted from without the system to the reacting bodies to enable the change to occur; such reactions are termed endothermic. It is convenient to consider the thermal effects of the solution of substances together with the changes of energydistribution due to strictly chemical reactions. The thermal unit in general use in connection with problems of this character is defined to be the amount of heat required to raise the temperature of one kilogram of water one degree Centigrade (the Calorie). Generally the heat of combination is only one of a number of factors in the total thermal effect, heat being absorbed in the liquefaction or vaporization and evolved on the solidification of the reacting substances and products. In the phenomena of ordinary combustion the terms calorific power and calorific intensity are used; by the former we understand the amount of heat produced by the combustion of one unit of weight of the burning substance, whereas the latter indicates the temperature to which the products of combustion can be raised by the heat evolved. The calorific intensity 1 = H m11+m252 + M3S3 + where H represents the calorific power, S1 S2 S3 &c., the specific heats, and m1 m, m, &c., the masses of the products of the combustion of one unit weight of substance. The following table expresses the calorific powers of a number of substances burnt in oxygen : The calorific power of any substance is a constant, being the same whether the body be burnt in oxygen or in air and whether it be burnt rapidly or slowly; the calorific intensity is modified by the circumstances under which the combustion takes place, any mixture of inert material-e.g. nitrogen in air, ash in coal-lessening I since heat must be used to raise the temperature of the foreign matters. Again, if radiation be allowed to take place freely the temperature reached during slow combustion will not be nearly so high as that attained by a more rapid burning. A special form of notation is used in thermo-chemistrye.g. [H2, C12] = 44,000+. in words means 'two kilograms of hydrogen combine with seventy-one kilograms of chlorine with the evolution (+) of 44,000 Calories.' EXAMPLES. I. The calorific power of carbon is 8080, what is its calorific intensity when burnt in oxygen? |