chemical equilibrium Flashcards
1
Q
chemical equilibrium
A
a state of dynamic balance where rate of forward reaction equals rate of the reverse reaction
2
Q
Le Chatelier’s Principle
A
- if a stress is applied to a system at equilibrium
- system readjusts to oppose stress applied
3
Q
Le Chatelier’s Principle and gases
A
Le Châtelier’s Principle predicts that in an all-gaseous reaction, an increase in pressure will favour the reaction which takes place with a reduction in volume
4
Q
Equilibrium constant
A
formula in hardback
5
Q
Kc
A
[ ] means concentration in moles per litre
- if greater than 1, products are favoured. The larger the value of Kc, the greater is the extent to which equilibrium is pushed towards products
- if less than 1, reactants are favoured. Low value of Kc - only small fraction of reactants converted to products
6
Q
Le Chatelier and industry
A
ammonia and haber process
sulfuric acid and contact process
7
Q
ammonia and haber process
A
predict max yield at high pressure / low temperature
reality = 200 atm and 500°C
8
Q
sulfuric acid and contact process
A
predict max yield at high pressure / low temperature
reality = one atm and 450°C
9
Q
large Kc
A
equilibrium far to right
lots of product produced
10
Q
Small Kc
A
equilibrium far to left
v little product formed
11
Q
2Mg + O₂ –> 2MgO
A
- 2 Mg atoms react with one O₂ molecule to form 2 molecules of magnesium oxide
- v hard to turn MgO back to magnesium and oxygen
- reaction “goes to completion”
12
Q
N₂ + 3H₂ –> 2NH₃
A
- not all reactions go to completion
- some reactions are ‘reversible’
- some of ammonia formed decomposes and becomes nitrogen and hydrogen again
- reaction takes place both in forward and backward reactions
13
Q
chemical equilibrium
A
a balance where the rate of the forward reaction is equal to the rate of the backwards reaction
14
Q
dynamic equilibrium
A
equilibrium is constantly ‘moving’
15
Q
stress
A
stress is a change in reaction conditions
- concentration of reactants/products
- temperature
- pressure
- catalyst
if we change conditions of reaction at equilibrium, reaction readjusts itself to relieve stress
16
Q
pressure
A
- if there are same no. of molecules on both sides of reaction, change in pressure will have no effect on equilibrium
- pressure only has effect on gaseous reactions
17
Q
temperature
A
- forward reaction is exothermic ~(produced heat)
- when temp of reaction is changed, reaction shifts itself to relieve stress
18
Q
concentration
A
- changing conc of one of the reactants shifts reaction to right (forwards) to relieve stress
- changing conc of product shifts reaction to left (backwards) to relieve stress
19
Q
stresses that can be applied
A
pressure increased/decreased
temperature increased/decreased
concentration of ‘x’ increased/decreased
20
Q
system’s response - pressure increased
A
equilibrium shifts to the side with the lower total number of molecules
21
Q
system’s response - pressure decreased
A
equilibrium shifts to the side with the higher total number of molecules
22
Q
system’s response - temperature increased
A
endothermic reaction is favoured
23
Q
system’s response - temperature decreased
A
exothermic reaction is favoured
24
Q
system’s response - concentration of ‘X’ is increased
A
reaction that removes ‘X’ is favoured
25
system's response - concentration of 'X' is decreased
reaction that forms 'X' is favoured
26
haber process
N₂ + 3H₂ 2NH₃
- iron catalyst
- forward reaction is exothermic
- made at IFI plant in Cobh, Cork
27
haber process - pressure
- 4 molecules on left, 2 on right
- increase pressure will favour production of ammonia
- keeping pressure high is expensive
- 200 atmospheres - most cost effective + safe
28
haber process - temperature
- forward reaction in exothermic
- lowering temp will favour production of ammonia
- too slow at v low temps
- 500°C temperature
29
N₂ + 3H₂ 2NH₃
| chemical equilibrium
- at beginning, conc of ammonia is 0
- as reaction proceeds, conc of N₂ + H₂ decreases, conc of NH₃ increases
- when rate of production of NH₃ equals its rate of decomposition, reaction has reached equilibrium
30
stoichiometry - empirical formula
gives the simplest whole number ratio of the numbers of the different atoms present in the molecule
31
stoichiometry - molecular formula
simple multiple of the empirical formula
32
effect of increasing pressure on equilibrium mixture
higher pressure favours fewer moles
33
effect of increasing temperature on equilibrium mixture
favours endothermic reaction according to Le Chatelier's Principle
34
effect of using (charcoal) catalyst on equilibrium constant
no effect, catalyst only effects time to reach equilibrium, does not affect equilibrium concentrations
35
Le Chatelier's principle predicts best yields in a certain equilibrium process at low temps + high pressures
Why might these conditions not be used industrially?
- temperature too low gives low rate (slow reaction) / temp low adds to costs
- pressure too high is unsafe (costly)
36
Kc equation
aA + bB ⇌ cC + dD
Kc = [C]ᶜ x [D]ᵈ / [A]ᵃ x [B]ᵇ
37
Kc equation example
N₂ + 3H₂ ⇌ 2NH₃
Kc = [NH₃]² / [N₂][H₂]³
38
Why is chemical equilibrium described as a dynamic state?
both reactions continues / reaction doesn't stop
39
Would there be a change in the value of Kc at T if a different initial concentration of X was used? why?
No change
- Kc constant at given temp even if concentrations change
- only change in temp will result in change in Kc
40
how to know which side is endothermic/exothermic
indicated by ΔH
-ΔH indicates that the reaction gives out heat in the side it is placed at]
eg. 2NO₂ ⇌ N₂O₄ ΔH = -58.1 kJ mol⁻¹
means that reaction gives out heat in forward direction
41
example of equilibrium mixture and which side is exothermic and endothermic
exothermic
2NO₂ ⇌ N₂O₄
endothermic
42
effect of a change in pressure on an all gaseous reaction in which there is the same no. of molecules on both sides
a change in pressure will have no effect on the position of equilibrium
43
effect of a catalyst on a reaction that has reached a state of dynamic equilibrium
- catalyst increases both the rate of the forward reaction and the reverse reaction by the same amount
- does not affect position of equilibrium in reaction or composition of the equilibrium mixture
- does not alter yeild of product
- speeds up rate at which equilibrium is reached but does not change position of equil.
44
What will happen to an equilibrium mixture if NaOH is added?
solution will go to colour of side which has H+ removed
- will remove the [H+]concentration,
- move it to right/left
45
What will happen to an equilibrium mixture if HCl is added?
solution will go to colour of side which has more H+
- Hcl will increase the [H+] concentration
- equilibrium will move to left/right
46
effect increase in pressure would have on value of Kc
no effect
47
what will effect Kc?
only temperature
48
use le chatelier's principle to predict + explain the conditions of temp/pressure that would max yield
low temp:
- favours exothermic reaction (which is probably forward reaction/product in the given question),
- high temp would favour endothermic reaction
High pressure:
- favours fewer moles
- low pressure would favour more moles
49
effect of presence of catalyst on equilibrium yield
no effect, alters rate of forward and reverse reactions equally
50
how temp would affect pressure of equilibrium mixture
increased pressure
```
increased velocity (energy) of gaseous molecules.particles / more collisions with walls of container
(/more gaseous product /more forward reaction(?))
```
51
how is sulfuric acid manufactured
manufactured by a method known as Contact Process
52
catalyst used in manufacture of sulfuric acid (contact process)
vanadium pentoxide (V₂O₅)
53
use le chat principle to preduct levels of temp + pressure to max yield of X when equilibrium is established
temp: raising temp will favour endothermic reaction, lead to decrease in yield of
54
Explain how an acid-base indicator, which is itself a weak acid and is represented by HX, functions
HX ⇌ H+ + X-
blue red
- in acid equilibrium lies on elft giving colour of HX (blue)
- in base equilibrium lies on right (shifts forwards) giving colour of ion X- (red)
55
effect of temp on Kc
increased temp: reaction shifts in endothermic direction to oppose added head
decreased temp: reaction shifts in exothermic direction to relieve stress applied/oppose decreased temp
56
name given to industrial process of formation of ammonia, important process in fertilizer industry
Haber Process
57
catalyst for formation of ammonia from its elements
Iron (Fe)
58
conditions that are actually used in manufacture of ammonia and why
high pressure (200 atmosphere), favours products
High temperature 673K, at low temps rate is too low
59
why changing pressure has no effect in equilibrium mixture of solution (aqueous)
equilibrium in solution/liquid phase, liquid solution not compressible / no gases involved
60
state + explain effect on equilibrium conc of triiodide ions of adding a substance that reacts with iodine, eg. starch
reaction shifts backward to restor iodine concentration
61
if the solution becomes a colour on one side of the reaction, has the reaction ceased? (equilibrium reaction)
no, forward and reverse reactions continue at same rate
62
other than heating, mention one way of reversing the change caused by cooling the reaction mixture
change concentration of one side
63
what is a reversible reaction
products can react to give back the reactants
64
how could you use a selection of chemicals to establish a chemical equilibrium?
make solution of hydrated molecule in water
65
Eg answer of effect of a stress
"Higher yield of products bc higher temp favours endothermic reaction according to Le Chatelier's principle"
"Decomposition of N2O4 is exothermic as higher temp favours endothermic reaction (ie it favours N2O4)"
66
"calculate the number of gaseous moles at equilibrium"
total up the amount of moles at the end of he question
67
graph: kc increasing with t, "deduce whether the dissociation of gas A is exothermic or endothermic"
endothermic
kc increases with temp. At higher temp more products (favours endothermic reaction)
68
Explain how an increase in the storage temp would affect the pressure of the equil mixture
2015 q b
increase
more gaseous products, more forward reaction, more collisions
69
effect of increasing conc of one of the reactants (A) to the conc of the other reactant (b)
the conc of the other reactant (b) decreases as reaction shifts forward/right to oppose stress (to dec the conc of A)
70
"why does changing the pressure have no effect on this equilibrium mixture?"
equilibrium in solution / no gases involved
71
when writing Kc expression and reaction has ions
include ions in the expression
72
Kc unit
M^-1
73
"is the forward reaction exothermic or endothermic?" eg of an answer
endothermic
cooling shifts in the backward/left direction therefore backward reaction is exothermic
74
"explain the lilac colour in the test tube" eg answer
| reactants are pink, products are blue, turned lilac after dropping a few drops of the reactants
"shifted to right/forward direction to relieve stress (inc in HCl conc) according to Le chateliers principle"
75
"state and explain colour observed when methyl orange added to acid/base"
eg. answer:
acid: red, equilibrium shifted to (left) bc... (high H+ conc)
base: yellow, equilibrium shifted to (right) bc ... (low H+ conc)
76
carbon monoxide and chlorine reaction
Cl2 + CO --> COCl2
77
why pressure has no effect on kc
only temp affects kc, value of Kc is constant at constant temp
78
effect of inc in temp to equilibrium
write: "reaction shifts in endothermic reaction to oppose heat added /temp increase" or "reaction shifts in endothermic direction to relieve stress applied"
79
writing Kc for ions
write the ions in the kc formula, dont write the full molecule
eg [Fe3+] rather than [FeCl3]
if given in ions in the equilibrium equation
80
when finding equil concentration of smth
make sure to divide by the volume at the end to get it in terms of 1 litre!! bc M = mole per litre!!!
ie. when you find the change (x), dont just put the concentration answer as x, get the answer as x/volume!!!
81
when colour is (in between colour) has the reaction stopped?
no, chemical equilibrium is a dynamic state, both forward and reverse reactions still occur / conc of reactants and products unchanged bc rates equal /
82
h2 + i2 2HI , effect of inc pressure on yield of product?
none, equal no of molecules on both sides
83
"state the colour change"
write becomes x
| eg. becomes pink
