Gas-forming reactions

Measurement of relative humidity depends on the system used. Systems employing vacuum are usually evacuated prior to introduction of water vapor [29]. For cases in which there is not a gas-forming reaction occurring, measurement of total pressure in the system can be used as a measure of water vapor pressure. Systems in which air is not evacuated require specific measurement of water vapor pressure. (For the latter type of system, caution should be taken to assure that the relative humidity source is in close proximity to the solid, since the diffusion of water vapor through air to the solid is required to maintain a constant relative humidity in the immediate vicinity of the solid.) A wide variety of pressure measuring instrumentation is commercially available with varying accuracy, precision, and cost. 

Thermal gravitational analysis (TGA) Chemical testing technique that precisely measures weight loss (due to gas forming reactions) as a function of temperature and time. 

It is also possible that the analyte must be calculated from a gaseous product of another reaction. In this case, one would want the gas to react with the titrant as soon as it is formed, since it could escape into the air because of a high vapor pressure. Thus an excess of the titrant would be present in a solution through which the gas is bubbled. After the gas-forming reaction has stopped, the excess titrant in this bubble flask could be titrated with the back titrant and the results calculated. This latter experiment is one form of the Kjeldahl titration. 

The currently preferred method for the study of gas forming reactions as function of temp is DSC. Here the specimen and the ref sample are heated at programmed heating rates (in controlled atms, if desired) while the differential energy input to the specimens is recorded. Hence the pen movement is directly proportional to the heat capacity while the area under the curve represents the enthalpy change. New equipment is now on the market which can operate up to 1200° (Ref 79) and which is therefore adequate for expl and propint studies. Limitations on the use of DSC for kinetics studies of expls will be discussed under the entry Thermochemistry in this Vol... 

Some gas-forming reactions proceed explosively. The decomposition of nitroglycerin is a detonation in which all the products are gases ... 

Explain how you could prepare barium sulfate by (a) an acid-base reaction, (b) a precipitation reaction, and (c) a gas-forming reaction. The materials you have to start with are BaCOj, Ba(OH)2, Na2S04, and H2SO4. 

Classify each of these exchange reactions as an acid-base reaction, a precipitation reaction, or a gas-forming reaction. Preict the products of the reaction, and then balance the completed equation. 

Gas-Forming Exchange Reactions. The last type of exchange reaction involves the formation of a gas. Common gas-forming reactions involve the reaction between an acid and a metal carbonate. The common products are a salt and carbonic acid, H3CO3, which decomposes into CO (g) and water H O (1). Gas-forming reactions can also occur with sulfates, in which case SO is produced. 

In many examples of interest, a single component from a gas stream is selectively dissolved into a hquid solvent. This situation was seen previously in Chapter 6, in the case of gas-forming reactions. In that situation, a species that could form a gas was produced by the reaction, and the product would leave the liquid solution to form a separate gas phase. In a similar way, a gas can be dissolved into a liquid and be present as a reactant. 

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