. Practical physical chemistry . ids, in order toprevent air-currents and evaporation of the liquid in the calori-meter. The lids are suitably cut to allow of the passage of thethermometer and stirrer. Units.—Various imits are employed in which to expressthe amount of heat. One of the oldest units is the amount ofheat required to raise the temperature of i gram of water from15°-! 6° C. This quantity of heat is called the calorie (or gram-calorie), and is represented by cal. Since this unit is rathersmall, and since the expression of heats of reaction in this unitwould lead to the use of very l
. Practical physical chemistry . ids, in order toprevent air-currents and evaporation of the liquid in the calori-meter. The lids are suitably cut to allow of the passage of thethermometer and stirrer. Units.—Various imits are employed in which to expressthe amount of heat. One of the oldest units is the amount ofheat required to raise the temperature of i gram of water from15°-! 6° C. This quantity of heat is called the calorie (or gram-calorie), and is represented by cal. Since this unit is rathersmall, and since the expression of heats of reaction in this unitwould lead to the use of very large numbers, the centuple calorie,represented by K, is frequently employed. This is the amountof heat required to raise the temperature of i gram of waterfrom 0° to 100°. It is practically equal to 100 cal. A stilllarger unit is now frequently used, viz. the large calorie (abbrevi-ated Cal.) which is equal to 1000 small or gram calories. In many cases, especially where calculations of transformation THERMO-CHEMISTRY n 277. Fig. 92, 278 PRACTICAL PHYSICAL CHEMISTRY of energy are involved, it is better to employ absolute this system, the unit of energy is called an erg, and the valueof 1 cal. is 4i83 X loergs. Since i joule is equal to i X loergs, it follows that i cal. is equal to 4i83 joules; or i Cal. =4i83 kilojoules {kj), o\ i kj — 0-2391 Cal. A.—Heat of Neutralization of Acids and BasesIN Dilute Solution According to the theory of electrolytic dissociation, themolecules of acids, bases, and salts in aqueous solution, areregarded as undergoing ionization to a greater or less this ionization were complete, then the process of neutraliza-tion, say of hydrochloric acid by caustic soda, could be repre-sented by the equation, H -f- CI -|- Na- 4- OH = Na -|- CI +HjO. That is to say, the process would consist merely in thecombination of hydrion and hydroxidion to form (practically)unionized water. This may be regarded as sufficiently near tothe truth i
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