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LC Chemistry > chemical equilibrium > Flashcards

chemical equilibrium Flashcards

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1
Q

chemical equilibrium

A

a state of dynamic balance where rate of forward reaction equals rate of the reverse reaction

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2
Q

Le Chatelier’s Principle

A
  • if a stress is applied to a system at equilibrium

- system readjusts to oppose stress applied

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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

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4
Q

Equilibrium constant

A

formula in hardback

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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
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6
Q

Le Chatelier and industry

A

ammonia and haber process

sulfuric acid and contact process

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7
Q

ammonia and haber process

A

predict max yield at high pressure / low temperature

reality = 200 atm and 500°C

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8
Q

sulfuric acid and contact process

A

predict max yield at high pressure / low temperature

reality = one atm and 450°C

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9
Q

large Kc

A

equilibrium far to right

lots of product produced

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10
Q

Small Kc

A

equilibrium far to left

v little product formed

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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”
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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
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13
Q

chemical equilibrium

A

a balance where the rate of the forward reaction is equal to the rate of the backwards reaction

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14
Q

dynamic equilibrium

A

equilibrium is constantly ‘moving’

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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

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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
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17
Q

temperature

A
  • forward reaction is exothermic ~(produced heat)

- when temp of reaction is changed, reaction shifts itself to relieve stress

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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
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19
Q

stresses that can be applied

A

pressure increased/decreased
temperature increased/decreased
concentration of ‘x’ increased/decreased

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20
Q

system’s response - pressure increased

A

equilibrium shifts to the side with the lower total number of molecules

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21
Q

system’s response - pressure decreased

A

equilibrium shifts to the side with the higher total number of molecules

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22
Q

system’s response - temperature increased

A

endothermic reaction is favoured

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23
Q

system’s response - temperature decreased

A

exothermic reaction is favoured

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24
Q

system’s response - concentration of ‘X’ is increased

A

reaction that removes ‘X’ is favoured

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25
Q

system’s response - concentration of ‘X’ is decreased

A

reaction that forms ‘X’ is favoured

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26
Q

haber process

A

N₂ + 3H₂ 2NH₃

  • iron catalyst
  • forward reaction is exothermic
  • made at IFI plant in Cobh, Cork
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27
Q

haber process - pressure

A
  • 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
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28
Q

haber process - temperature

A
  • forward reaction in exothermic
  • lowering temp will favour production of ammonia
  • too slow at v low temps
  • 500°C temperature
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29
Q

N₂ + 3H₂ 2NH₃

chemical equilibrium

A
  • 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
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30
Q

stoichiometry - empirical formula

A

gives the simplest whole number ratio of the numbers of the different atoms present in the molecule

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31
Q

stoichiometry - molecular formula

A

simple multiple of the empirical formula

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32
Q

effect of increasing pressure on equilibrium mixture

A

higher pressure favours fewer moles

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33
Q

effect of increasing temperature on equilibrium mixture

A

favours endothermic reaction according to Le Chatelier’s Principle

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34
Q

effect of using (charcoal) catalyst on equilibrium constant

A

no effect, catalyst only effects time to reach equilibrium, does not affect equilibrium concentrations

35
Q

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?

A
  • temperature too low gives low rate (slow reaction) / temp low adds to costs
  • pressure too high is unsafe (costly)
36
Q

Kc equation

A

aA + bB ⇌ cC + dD

Kc = [C]ᶜ x [D]ᵈ / [A]ᵃ x [B]ᵇ

37
Q

Kc equation example

A

N₂ + 3H₂ ⇌ 2NH₃

Kc = [NH₃]² / [N₂][H₂]³

38
Q

Why is chemical equilibrium described as a dynamic state?

A

both reactions continues / reaction doesn’t stop

39
Q

Would there be a change in the value of Kc at T if a different initial concentration of X was used? why?

A

No change

  • Kc constant at given temp even if concentrations change
  • only change in temp will result in change in Kc
40
Q

how to know which side is endothermic/exothermic

A

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
Q

example of equilibrium mixture and which side is exothermic and endothermic

A

exothermic
2NO₂ ⇌ N₂O₄
endothermic

42
Q

effect of a change in pressure on an all gaseous reaction in which there is the same no. of molecules on both sides

A

a change in pressure will have no effect on the position of equilibrium

43
Q

effect of a catalyst on a reaction that has reached a state of dynamic equilibrium

A
  • 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
Q

What will happen to an equilibrium mixture if NaOH is added?

A

solution will go to colour of side which has H+ removed

  • will remove the [H+]concentration,
  • move it to right/left
45
Q

What will happen to an equilibrium mixture if HCl is added?

A

solution will go to colour of side which has more H+

  • Hcl will increase the [H+] concentration
  • equilibrium will move to left/right
46
Q

effect increase in pressure would have on value of Kc

A

no effect

47
Q

what will effect Kc?

A

only temperature

48
Q

use le chatelier’s principle to predict + explain the conditions of temp/pressure that would max yield

A

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
Q

effect of presence of catalyst on equilibrium yield

A

no effect, alters rate of forward and reverse reactions equally

50
Q

how temp would affect pressure of equilibrium mixture

A

increased pressure

increased velocity (energy) of gaseous molecules.particles / more collisions with walls of container 
(/more gaseous product /more forward reaction(?))
51
Q

how is sulfuric acid manufactured

A

manufactured by a method known as Contact Process

52
Q

catalyst used in manufacture of sulfuric acid (contact process)

A

vanadium pentoxide (V₂O₅)

53
Q

use le chat principle to preduct levels of temp + pressure to max yield of X when equilibrium is established

A

temp: raising temp will favour endothermic reaction, lead to decrease in yield of

54
Q

Explain how an acid-base indicator, which is itself a weak acid and is represented by HX, functions

HX ⇌ H+ + X-
blue red

A
  • 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
Q

effect of temp on Kc

A

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
Q

name given to industrial process of formation of ammonia, important process in fertilizer industry

A

Haber Process

57
Q

catalyst for formation of ammonia from its elements

A

Iron (Fe)

58
Q

conditions that are actually used in manufacture of ammonia and why

A

high pressure (200 atmosphere), favours products

High temperature 673K, at low temps rate is too low

59
Q

why changing pressure has no effect in equilibrium mixture of solution (aqueous)

A

equilibrium in solution/liquid phase, liquid solution not compressible / no gases involved

60
Q

state + explain effect on equilibrium conc of triiodide ions of adding a substance that reacts with iodine, eg. starch

A

reaction shifts backward to restor iodine concentration

61
Q

if the solution becomes a colour on one side of the reaction, has the reaction ceased? (equilibrium reaction)

A

no, forward and reverse reactions continue at same rate

62
Q

other than heating, mention one way of reversing the change caused by cooling the reaction mixture

A

change concentration of one side

63
Q

what is a reversible reaction

A

products can react to give back the reactants

64
Q

how could you use a selection of chemicals to establish a chemical equilibrium?

A

make solution of hydrated molecule in water

65
Q

Eg answer of effect of a stress

A

“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
Q

“calculate the number of gaseous moles at equilibrium”

A

total up the amount of moles at the end of he question

67
Q

graph: kc increasing with t, “deduce whether the dissociation of gas A is exothermic or endothermic”

A

endothermic

kc increases with temp. At higher temp more products (favours endothermic reaction)

68
Q

Explain how an increase in the storage temp would affect the pressure of the equil mixture
2015 q b

A

increase

more gaseous products, more forward reaction, more collisions

69
Q

effect of increasing conc of one of the reactants (A) to the conc of the other reactant (b)

A

the conc of the other reactant (b) decreases as reaction shifts forward/right to oppose stress (to dec the conc of A)

70
Q

“why does changing the pressure have no effect on this equilibrium mixture?”

A

equilibrium in solution / no gases involved

71
Q

when writing Kc expression and reaction has ions

A

include ions in the expression

72
Q

Kc unit

A

M^-1

73
Q

“is the forward reaction exothermic or endothermic?” eg of an answer

A

endothermic

cooling shifts in the backward/left direction therefore backward reaction is exothermic

74
Q

“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

A

“shifted to right/forward direction to relieve stress (inc in HCl conc) according to Le chateliers principle”

75
Q

“state and explain colour observed when methyl orange added to acid/base”

A

eg. answer:

acid: red, equilibrium shifted to (left) bc… (high H+ conc)
base: yellow, equilibrium shifted to (right) bc … (low H+ conc)

76
Q

carbon monoxide and chlorine reaction

A

Cl2 + CO –> COCl2

77
Q

why pressure has no effect on kc

A

only temp affects kc, value of Kc is constant at constant temp

78
Q

effect of inc in temp to equilibrium

A

write: “reaction shifts in endothermic reaction to oppose heat added /temp increase” or “reaction shifts in endothermic direction to relieve stress applied”

79
Q

writing Kc for ions

A

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
Q

when finding equil concentration of smth

A

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
Q

when colour is (in between colour) has the reaction stopped?

A

no, chemical equilibrium is a dynamic state, both forward and reverse reactions still occur / conc of reactants and products unchanged bc rates equal /

82
Q

h2 + i2 2HI , effect of inc pressure on yield of product?

A

none, equal no of molecules on both sides

83
Q

“state the colour change”

A

write becomes x

eg. becomes pink

LC Chemistry (26 decks)

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