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Chemical Reaction Equilibrium SVNA 13

Chemical Reaction Equilibrium SVNA 13. If sufficient data exist, we can describe the equilibrium state of a reacting system. If the system is able to lower its Gibbs energy through a change in its composition, this reaction is favourable.

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Chemical Reaction Equilibrium SVNA 13

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  1. Chemical Reaction Equilibrium SVNA 13 • If sufficient data exist, we can describe the equilibrium state of a reacting system. • If the system is able to lower its Gibbs energy through a change in its composition, this reaction is favourable. • However, whether or not a reaction will occur in a finite period of time is a question of reaction kinetics. • There are several industrially important reactions that are both rapid and “equilibrium limited”. • Synthesis gas reaction • production of methyl-t-butyl ether (MBTE) • In these processes, it is important to know the thermodynamic limit of the reaction extent under different operating conditions.

  2. Reaction Extent • Given a feed composition for a reactive system, we are most interested in the degree of conversion of reactants into products. • A convenient measure is the reaction extent, e. • Consider the following reaction: • In terms of stoichiometric coefficients: • where, nCH4 = -1, nH20 = -1, nCO = 1, nH2 = 3 • For any change in composition due to this reaction, • 13.2 • where de is called the differential extent of reaction.

  3. Reaction Extent • Note that: • (i=1,2,…,N) 13.3 • The extent of reaction is zero before the reaction starts. • We can integrate 13.3 from the start from the start of the reaction to find the number of moles of any species in terms of  • so that • 13.4 • How does this help us?

  4. Reaction Extent and Mole Fractions • Relating the reaction extent to mole fractions is accomplished by calculating the total number of moles in the system at the given state. • Where, • Mole fractions for all species are derived from: • 13.5 • What happens if there is an inert component in the reaction mixture?

  5. Multiple Reactions and the Reaction Extent • The reaction extent approach can be generalized to accommodate two or more simultaneous reactions. • For j reactions of N components: • (i=1,2,…,N) • and the number of moles of each component for given reaction extents is: • 13.6 • and the total number of moles in the system becomes: • where we can write:

  6. Chemical Reaction Equilibrium Criteria • To determine the state of a • reactive system at equilibrium, • we need to relate the reaction • extent to the total Gibbs • energy, GT. • We have seen that GT of a • closed system at T,P • reaches a minimum at • an equilibrium state: • Eq. 14.64 [14.68]

  7. Reaction Extent and Gibbs Energy • Consider a single-phase system in which chemical reactions are possible. • Changes in Gibbs energy resulting from shifts in temperature, pressure and composition are described by the fundamental equation: • At constant temperature and pressure, this reduces to: • and the only means the system has to lower the Gibbs • energy is to alter the number of moles of individual • components. • Let’s relate the changes in moles to the reaction extent.

  8. Criterion for Chemical Equilibrium • For a single chemical reaction, we can apply equation 13.3 which relates the reaction extent to the changes in the number of moles: • 13.3 • Substituting for dni in the fundamental equation yields: • At equilibrium at constant T and P, we know that d(nG)/d, = 0. Therefore, • 13.8

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