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Chemical Reaction Equilibrium SVNA 13PowerPoint Presentation

Chemical Reaction Equilibrium SVNA 13

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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.

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.

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?

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?

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:

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]

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.

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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