Title: Open And Closed Systems
1Open And Closed Systems
- Open system - exchanges material with
surroundings. - Closed systems - no exchange can reach
equilibrium - molecules/s returning to liquid escaping
to vapour
2Equilibria 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
3Activity
- 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
5Example
- 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
6Temperature 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
7Optimising 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)
8Optimising 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.
9Le 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
101 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