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