The Stoichiometry of Reacting Gases

The ammonia synthesis reaction is represented by Eq. 11.6, which establishes the stoichiometry of 2 mol (volumes) of ammonia produced for each 4 mol (volumes) of reacting gases. 

The gas laws can be applied to calculate the stoichiometry of reactions in which gases are reactants or products. Recall that the coefficients in chemical equations represent molar amounts of substances taking part in the reaction. For example, hydrogen gas can react with oxygen gas to produce water vapor. 

Third, we use the stoichiometry of the reaction to determine the changes in concentration that occur as the reaction proceeds to equilibrium. The H2 and I2 concentrations will decrease as equilibrium is established and that of HI will increase. Let s represent the change in concentration of H2 by x. The balanced chemical equation tells us the relationship between the changes in the concentrations of the three gases. For each x mol of H2 that reacts, X mol of I2 are consumed and 2x mol of HI are produced ... 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

This information will help you with a certain type of gas stoichiometry problem. When a gas reacts to produce another gas, you can use Gay-Lussac s law of combining volumes to find the volumes of the gases. The following Sample Problem shows you how. 

To be useful, reactions must occur at a reasonable rate. To produce the 20 million tons of ammonia needed each year for fertilizer, we cannot simply mix nitrogen and hydrogen gases at 25°C and wait for them to react. It is not enough to understand the stoichiometry and thermodynamics of a reaction we must also understand the factors that govern the rate of the reaction. The area of chemistry that concerns reaction rates is called chemical kinetics. 

The case where two gases A and dissolve in a liquid phase and react together is a case for which a large number of commercial examples exist (33). A general stoichiometry can be written... 

Metal oxide surfaces react with gases or solutions they can be used as active phases or as supports for catalysts. The behavior of metal oxide surfaces is controlled by (i) coordination—sites of low coordination are in general more reactive than sites of high coordination (ii) acid/base properties—clean and anhydrous metal oxide surfaces present two different types of active sites, cations and anions (acid/base pairs) which determine reactivity towards gas-phase adsorbates (iii) the redox mechanism—when the oxide deviates from the stoichiometry due to the presence of defects such as vacancies or adatoms, the oxidation state of surface atoms varies [52],... 

Our study of stoichiometry has shown that substances react in definite mole and mass proportions. Using previously discussed gas laws, we can show that gases also react in simple, definite proportions by volume. For example, owe volume of hydrogen always combines (reacts) with one volume of chlorine to form two volumes of hydrogen chloride, if all volumes are measured at the same temperature and pressure... 

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