Équilibre chimique

Question 1

Define reversible rxn

Answer 1

A reversible rxn is at dynamic chemical eqm when:
1. Rate of forward rxn = rate of backward rxn
2. Substances still react tgt though conc. of rxt & pdt remain constant (system does not undergo any observable change)

Question 2

What is the expression for Kc and Kp for a reversible reaction aA+bB ⇌ cC+dD

Answer 2

Kc:
[C]^c [D]^d
_____________
[A]^a [B]^b

Kp:
(Pc)^c (Pd)^d
_____________
(P a)^a (Pb)^b

Question 3

What is the formula for partial pa?

Answer 3

Partial pressure of A = nA/nT x P T

Where P T is total pressure, nA is amt of A, nT is total amt of gas

Question 4

What do Kc values >1, = 1 and <1 mean?

Answer 4

Kc>1:
- higher prop n of pdt than rxt at eqm
- rxn proceed left to right to greater extent & form n of pdt favoured
- eqm pos n lies to the right

Kc=1:
- At eqm, conc. of both rxt & pdt abt the same

Kc<1:
- Lower prop n of pdt than rxt at eqm
- Rxn proceed right to left to greater extent & form n of rxt favoured
- eqm pos n lies to the left

Question 5

What are properties of eqm constants?

Answer 5

• temperature dependent.
• A change in temp hv diff impact on exo & endo rxn.

Question 6

What is the difference btw homogenous and heterogenous equilibria?

Answer 6

Homogenous:
All rxt, pdt in same physical state
eg all gas/all liquid
All rxt, pdt included in eqm constant expression (includes conc of water)

Heterogenous:
Not all rxt, pdt in same physical state
eg NH3(g) + H2O (l) ⇌ NH4+ (aq) + OH-(aq)
Expression no include:
- conc of water bcos it is the medium (its conc effectively constant)
- solids (bcos uniform conc)

Question 7

What method to use to solve eqm problems?

Answer 7

• ICE table
  (Initial, Change, Eqm amt)

Question 8

State Le Chatelier’s Principle

Answer 8

When system in eqm subjected to change/stress which disturb eqm, system react to counteract effect of change/stress to re-establish eqm

Question 9

How does eqm pos n change?

Answer 9

Shifts right, left or remain unchanged (meaning forward or backward rxn is favoured)

Question 10

How is eqm pos n affected by conc change?

Answer 10

• Increase in [rxt]/decrease in [pdt] shift eqm pos n to right
• Decrease in [rxt]/increase in [pdt] shift eqm pos n to left

Question 11

How is eqm pos n affected by Pa change?

Answer 11

• Increase in Pa shift eqm pos n to decrease amt of gas
• Decrease in Pa shift eqm pos n to increase amt of gas

Question 12

What happens with change of amt of gas in a rxn?

Answer 12

• change amt of gas hv same effect as change partial Pa of that gas
  (from pV=nRT, p is proportional to conc and to amt)

Question 13

What happens with volume change of rxn vessel?

Answer 13

• Compress n causes decrease in volume & corresponding increase in total Pa (& increase in partial Pa of each component gases in rxn)
• Expans n of rxn vessel causes increase in volume & corresponding decrease in total Pa (& decrease in partial Pa of each component gases in rxn)

Question 14

How does temp affect eqm pos n?

Answer 14

• Increase temp
  eqm pos n shift towards endo rxn to absorb heat
• Decrease temp
  eqm pos n shift towards exo rxn to release heat

Question 15

What are 2 effects of catalyst?

Answer 15

1. Increase both rate of forward & backward rxn to same extent, so eqm pos n is unchanged
   - thus, [rxt],[pdt] constant b4 & after add catalyst
   - Kc, Kp stay same
2. Increase both rate of forward & reverse rxn to same extent so that dynamic eqm is established more quickly within shorter time

Question 16

State and explain conditions used for Haber Process

Answer 16

1. High Pa of 250atm
   too high Pa too expensive as thicker pipes needed to withstand high Pa, so 250atm is enough
2. Moderate temp 450 C
   too low temp decrease rxn rate (not industrially viable), so moderate temp used
3. Iron catalyst
   to increase rxn rate (does NOT affect NH3 yield)
4. Molar ratio of N2: H2 is 1:3

(Thus, there is a compromise btw conflict demand of satisfy high yield, high rate & low cost of rxn)

Question 17

What is Gibbs Free Energy Change, dG? Explain

Answer 17

• measure of spontaneity of rxn

dG<0: rxn spontaneous in forward rxn

dG>0: rxn not spontaneous in forward rxn, spontaneous in backward rxn

dG=0: system in eqm

Question 18

What is the eqn for standard Gibbs Free Energy Change, dGe? Explain

Answer 18

At eqm where dG=0 & Q=K,

dGe=-RT ln K

where R = 8.31, T is temp in Kelvin, K is Kc/Kp

If dGe«0, K&raquo_space;1; eqm pos n lies far to right, so rxn effectively complete. More pdt present

If dGe»0, K«1; eqm pos n lies far to left, so more rxt present