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Open And Closed Systems

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Optimising the Conditions for. a Reaction ... Optimising Reaction Conditions. CONCENTRATION. remove the product or increase the reactants, ... – PowerPoint PPT presentation

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Title: Open And Closed Systems


1
Open And Closed Systems
  • Open system - exchanges material with
    surroundings.
  • Closed systems - no exchange can reach
    equilibrium
  • molecules/s returning to liquid escaping
    to vapour

2
Equilibria with Pure Liquid or Pure Solid
  • concentrations (or partial pressures) are
    actually ratios of the actual concentration to
    the concentration in the standard state.
  • ACTIVITIES

3
Activity
  • BY CONVENTION
  • for solution standard state 1 M activity
    concentration.
  • for gas, standard state 1 atmos, activity
    pressure
  • for solids liquids standard state pure solid
    or liquid, whose concentration does not change
    activity 1 always

4
  • Kc O2 Kp pO2
  • independent of amount of solid or liquid present
  • Example evaporation of water

Vapour pressure in sealed vessel in presence of
liquid is always the same (at given temperature)
irrespective of how much liquid water is present
cf textbook fig 17.4
5
Example
  • Aluminium production by electrolysis
  • molten cryolite, Na3AlF6, is used as solvent for
    aluminium ore bauxite (Al2O3).
  • Cryolite undergoes slight decomposition with heat
    to produce tiny amount F2 (may escape into air).
  • Kc 2 ? 10-104 M3 at 1300 K
  • What is the concentration of F2 at this
    temperature?
  • Kc F23
  • hence F2 (2 ? 10-104 )1/3 3 ? 1035 M

6
Temperature Variation of KC
  • size of Kc determined by thermodynamics
  • time to reach equilibrium determined by Ea
  • Endothermic reaction lies more to right at higher
    T

7
Optimising the Conditions for a Reaction
  • Kc favours NH3 at low T (exothermic), but rate
    too slow
  • A large equilibrium constant indicates that the
    reaction proceeds toward the right almost
    completely but indicates nothing about how fast
    the reaction occurs
  • Run at high T, high pressure with V2O5 catalyst
  • Gives compromise between rate and yield (ie, how
    fast reaction goes to equilibrium vs optimal
    concentrations at equilibrium)

8
Optimising Reaction Conditions
  • CONCENTRATION
  • remove the product or increase the reactants, ie
    make Qc smaller and push the reaction toward
    products
  • eg liquefy the ammonia and recycle the N2 and H2
  • PRESSURE
  • increasing the total pressure (for this
    reaction!) makes Qp smaller since there are 4
    moles of gaseous reactants to 2 moles gaseous
    product reaction is shifted toward products
  • CATALYSTS
  • No effect on value of Kc or position of
    equilibrium but increases rate at which
    equilibrium is attained.

9
Le Châteliers Principle
  • When a chemical system in a state of equilibrium
    is disturbed, it reattains equilibrium by
    undergoing a net reaction that reduces the effect
    of the disturbance
  • Understand in terms of the Reaction Quotient, Q,
    and the temperature variation of Kc

10
1 Write balanced equation 2. Write reaction
quotient expression, Q 3. Convert all amounts to
M or atm
Preliminary Setting Up
4. If reaction direction is unknown compare
Q with K 5. Construct a reaction table
Working on the Reaction Table
check sign of x
6. Substitute amounts into Q 7. To simplify the
maths assume x is negligible (Ainit - x
Ainit) 8. Solve for x 9. Solve for the
equilibrium amounts
Solving for x and Equilibrium Concentrations
check assumption justified (lt5 error)
check reproduce K
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