Chemical Equilibria and Le Chatelier's principle Flashcards
What are irreversible reactions
Irreversible reactions are reactions where reactants convert to products
- and where products cannot convert back to reactants.
> For example, you cannot turn a baked cake back into its raw ingredients
For the reaction: CaCO3(s) → CaO(s) + CO2(g)
- This equation means 1 mole of CaCO3 is broken down
- on heating to form 1 mole of CaO and 1 mole of CO2.
-
- Theres mole of a substance on left side of equation
- and 2 moles of substances on right.
> Equations shd be understood in moles
How do reversible reactions work
Many chemical reactions are reversible.
- The reversible arrow shows a reaction is reversible
- A reversible reaction is where reactants are converted into products
- but the products are then converted back into for the reactants
..
general reversible reaction: A(g) + B(g) = AB(g)
- In general, a particle of A reacts with a particle of B and forms a particle of AB
- a particle of AB breaks down to form a particle of A and a particle of B.
-
- Imagine particles of A and B moving around randomly.
- When they collide, may pair up if collide with sufficient energy to form pair AB.
- However a pair AB can also break up to release one particle of A and one of B.
Whats the dynamic equilibrium and a steady state
The reversible reaction starts with A and B only; as reaction starts some AB pairs appear.
» The reaction is described as having reached equilibrium
> when number of particles of A, B and AB remains constant.
>
> However the A, B and AB particles present are not always the same ones
> even though the number of each remains same; are constantly being formed and broken up.
This is called a dynamic equilibrium where you have a steady state
>because rates of the forward reaction and the reverse reaction are the same
..
The definition of a dynamic equilibrium is a system where:
• the concentrations of reactants and products remain constant, and
• where forward and reverse reactions are proceeding at equal rates.
What are Homogenous and heterogeneous reactions?
A homogeneous reaction
is where all reactants and products are same physical state.
- eg. N2(g) + 3H2(g) = 2NH3(g)
- All reactants and products are in the same state, i.e. here are all gases.
-
- When explaining why a reaction is considered to be homogeneous,
- you shd explain homogeneous means all reactants/ products are in same state
- but also make it specific eg. explaining they are all gases.
Whats the position of equilibrium
The position of equilibrium is a notional measure
- of how far reaction is to left-hand side or to the right-hand side.
— In homogeneous A and B example:
A(g) + B(g) - AB(g)
If The conditions of the reaction,
- such as temperature, pressure and concentration, are changed,
–> likely to affect position of equilibrium and SO result in more reactants OR more products formed
** diagram 1**
Whats le chateliers principle?
Le Chatelier’s principle states that
- if a factor is changed wch affects a system in equilibrium,
- the position of equilibrium will move in a direction
- so as to oppose the change.
Factors wch may affect position of equilibrium are
changes in temperature, pressure
or conc of a particular reactant or product.
..
The principle can also be stated as:
When a system in equilibrium is disturbed, position of equilibrium will move in direction to reduce effect of disturbance.
How do changes in pressure impact the position of equilibrium
For the A and B homogeneous equilibrium: A(g) + B(g) <> AB(g)
- There are 2 moles of gas on left-hand side (1 mole of A(g) and 1 mole of B(g))
- whereas theres only 1 mole of gas on right-hand side (1 mole of AB(g)).
..
An increase in pressure at constant temperature
- shifts position of equilibrium to the side with a smaller gas volume/moles
-
- Increasing pressure on this equilibrium moves position from left to right, TO OPPOSE INCREASE
- results in an increase in conc of AB.
> A smaller number of moles of gas has a smaller gas volume.
A decrease in pressure, constant temp,
- shifts position to side with a larger gas volume/more moles
-
- The position of equilibrium will move from right to left TO OPPOSE DECREASE
- as are more moles on left-hand side
> More moles of gas has a greater volume.
How does NO change in pressure impact the position of equilibrium
If theres the same amount of moles of gas on each side of the equilibrium reaction
- then a change in pressure will have no effect on the position of that equilibrium.
For example:
2H1(g) <> H2(g) + 12 (g)
A change in pressure would have no effect on the position of this equilibrium
- as there are 2 moles of gas on each side of the equation.
How does change in temperature impact the position of equilibrium
The position of equilibrium can be altered by changing temp,
- depends on whether the reaction is exothermic or endothermic.
For equilibrium: N2(g) + 3H2(g) <> 2NH3(g)
ΔH = -92 kJ mol-1
- The ΔH (change in enthalpy) given is for the forward reaction; it is negative,
- so the forward reaction is exothermic.
- Means reverse reaction is endothermic.
..
•For a reaction in equilibrium where the forward reaction is exothermic,
- an increase in temp would shift position of equilibrium
- in direction of reverse endothermic reaction so position of equilibrium would move from right to left.
•For a reaction in equilibrium where forward reaction is exothermic,
- a decrease in temp shifts position of equilibrium to forward exothermic reaction
- and the position of equilibrium would move from left to right.
..
•For a reaction in equilibrium where
forward reaction is endothermic,
- an increase in temp shifts position of equilibrium
- in direction of forward endothermic reaction
- the position of equilibrium would move from left to right.
•For a reaction in equilibrium where
the forward reaction is endothermic,
- a decrease in temp shifts position of equilibrium
- in direction of reverse exothermic reaction
- the position of equilibrium would move from right to left.
How to answer a question on change in temp on position of equilibrium
Some mark schemes insist on the endothermic reactions absorbing heat and again for a full answer include:
• the forward OR reverse reaction?
• direction position of equilibrium moves in terms of endothermic or exothermic?
• which direction position of equilibrium moves (left to right or left to right)?
• and if required a statement about the yield
> eg. Decrease in temp leads to leads to decrease in yield of ammonia
> eg. compromise temp if low temp needed but to slow
How do changes in concentration impact position of equilibrium
If more of a reactant or product is
- added to/removed from equilibrium system at constant temp and pressure,
- this alters position of equilibrium.
-
- The equilibrium will adjust to replace any substance that has been removed
- or remove any substance that has been added.
..
N2(g) + 3H2(g) = 2NH3(g)
Adding more nitrogen will push position of equilibrium from left to right
>as are more nitrogen molecules to react; will increase yield of ammonia.
>
>Removing ammonia (cooling it to condense it) will result in moving to right-hand side to form more ammonia,
>will lower conc of nitrogen and hydrogen present at equilibrium.
>
consider the AB example.
- if all AB were removed, the only reaction wch could occur is forward reaction,
- so more AB would be formed.
What impact do catalysts have on position if equilibrium
A catalyst has no effect on the position of equilibrium
- but simply allows the reaction to get to equilibrium faster.
A catalyst increases the rate of the forward and reverse reactions equally
- so equilibrium is attained much more quickly in catalyst’s presence
- but it does not affect the position of equilibrium.
What are industrial examples (ethene hydration) of change in position of equilibrium (1 = catalyst)
Methanol and ethanol are important chemicals and fuels.
- The importance of both of them is growing with rise in use of biotuels.
-
- Methanol can be used to synthesize other chemicals.
- Ethanols produced by sugar fermentation and hydration (ethene and steam)
An equilibrium is established and the conditions used ensure highest yield of ethanol is produced.
hydration of ethene:
C2H4 (g) + H2O(g) = CH3CH2OH(g);
ΔH = -42 kJ mol-1
- A hydration reaction is where waters added to a compound.
- These 5 conditions are used to increase rate of reaction
and to maximise the yield of ethanol:
..
1) Conc phosphoric acid is the catalyst (conc sulfuric acid can also be used).
• The presence of a catalyst allows equilibrium to be attained more rapidly.
> The catalyst increases rate of reaction.
• A catalyst has no effect on position of equilibrium; doesnt affect yield of product • but will ensure that equilibrium is attained quicker, making product faster
What are industrial examples (ethene hydration) of change in position of equilibrium (2 = pressure)
2) High pressure between 5-10MPa’s used.
Using Le Chatelier’s principle,
- Its understandable a high pressure will increase equilibrium yield of ethanol
- as there are 2 moles of gas on left and 1 mole of gas on right
- so a high pressure will move position of equilibrium to the right.
• A higher yield of ethanol would be obtained at higher pressures
• but as discussed before high pressure is expensive to apply due to
•
• increased electrical pumping costs; requires expensive strong-walled vessels
• and expensive valves and other equipment to withstand the pressure.
What are industrial examples (ethene hydration) of change in position of equilibrium (3=temp)
3) temp used is between 300-600°C.
• The forward reaction is exothermic so an increase in temp shifts position
• in direction of reverse - endothermic.
• A tempbetween 300°C and 600°C is a compromise temp between rate and yield.
• It increases rate of reaction at expense of a loss in some yield at equilibrium.
What are industrial examples (ethene hydration) of change in position of equilibrium (4=excess)
4) Excess ethene / steam can be used
• Excess ethene (or steam) will move the position of equilibrium to right
• to remove the excess reactant so increasing the yield of ethanol.
• Any unreacted ethene is recycled back into the reaction mixture.
What are industrial examples (ethene hydration) of change in position of equilibrium (5=concentration)
5) The ethanol can also be condensed out of the reaction mixture to lower conc
•
• If a product is removed from an equilibrium reaction,
• the position of equilibrium will shift to replace it.
• In this example the position of equilibrium will move to the right
• to replace the ethanol which has been removed
Whats the law of chemical equilibrium
the law of chemical equilibrium
> states the direction taken by a reaction is dependent
- not only on mass of the various components of the reaction,
- but also upon the concentration.
-
When analysing results of experiments, Guldberg and Waage noticed
- when they arranged equilibrium concs into a specific form of ratio,
- the resulting value was same no matter what combos of initial concs were mixed at a fixed temperature.
..
This value they called the equilibrium constant.
• Kc represents the equilibrium constant.
>The subscript letter after K shows what type of equilibrium is being expressed.
•
• Kc is equilibrium constant calculated from reactants and products concs (moldm-3).
• Kc can be calculated for reactions in solution
• or homogeneous gaseous reactions, as the conc of a solution
•or a gas can be calculated as number of moles in a certain volume (in dm3).
• All equilibrium constants are only constant at constant temperature.
• If the temp remains constant, the equilibrium constant wont change.
> If any other factor is varied, eg pressure or concof reactants,
> the value of the equilibrium constant remains constant.
..
The temperature should be quoted when value of any equilibrium constant is given
-but often its stated that its at a given temperature.
How to work out expressions for equilibrium constant, Kc ⭐️
Used for expressions:
• For the reaction: aA + bB → cC + dD
Kc = [C]c[D]d / [A]a[B]b
> where [C] represents concentration of C in mol dm-3 in equilibrium mixture
and c is balancing number for C in the equation for the reaction.
The same applies to A, B and D.
- The concs of all products at equilibrium are on the top line of the expression
- raised to the power of their balancing numbers,
- The concs of all reactants at equilibrium are on the bottom line of the expression,
- again raised to the power of their balancing numbers.
• Concentration is calculated as
the moles of a reactant or products /
the volume (most often in dm3).
..
The position of equilibrium may vary when external factors are changed
»_space;but only changes in temperature will affect tvalue of the equilibrium constant.
How to find units of Kc ⭐️
Units of Kc = (mol dm-3)^(c+d) /
(mol dm-3)^(a+b)
• The units are in terms of conc in mol dm-3
• but overall power depends on balancing numbers in equation for reaction
Remember that when the same term is multiplied, their powers are added
and if the same term is divided then the powers are subtracted.
Why does Kc sometimes have no units
If Kc has no units then it is because there are
- an equal number of moles on both sides of the equation
- they cancel each other out in the Kc expression.
Calculating Kc ⭐️ (calculating equilibrium moles 1)
Calculating a value for K,
may involve calculating and using equilibrium amounts,
>in moles or equilibrium concentrations.
>
Kc is only constant at constant temperature
- wch may be quoted in °C or K
- or simply stated as a constant temperature or given temperature or particular temperature.
•Calculating equilibrium moles
The general format in diagram2 is followed
-to calculate the amount, in moles, of each substance present at equilibrium (equilibrium moles).
..
If the equilibrium moves from right to left then reacting moles
-will be lost from the products (and be negative on the right)
-and gained by the reactants (and be positive on the left).
»_space;This would be unusual.
Calculating equilibrium concentrations
The equlibrium moles questions can be taken one stage further.
- If a volume is given in example,
- the equilibrium moles are divided by the volume in dms
- to give the equilibrium concentration in mol dm-3.
- (divide by V if volume is not given).
> The equilibrium concentration may then be substituted into the K,
- expression to calculate a value for Kc
-
- For a Kc expression which has no units, the volumes will cancel out
- as there are equal number of moles on each side of the equilibrium equation.
diagram 3
Calculating Kc from concentrations ⭐️
Equilibrium concs of reactants and products may be given
and the equilibrium constant, Kc, is calculated from these values.
- You will often be asked to deduce units as well in this question type.
-
- The equilibrium constant for reverse reaction is reciprocal of value for forward.
- Simply use 1/Kc to calculate value of the equilibrium constant for the reverse.
>The units of equilibrium constant for the reverse reaction are again
>the reciprocal of the units of the equilibrium constant for forward reaction.
-Eg. if units of equilibrium constant for forward reaction are mol-1 dm3,
-the units of value of the equilibrium constant of the reverse reaction are
(mol-1 dm3)-1 = mol dm-3.
diagram 4
