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 R4. REVERSIBLE REACTIONS & EQUILIBRIUM SYSTEMS 

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 (a) Haber process - synthetic nitrogen fixation

 (b) Contact process

 (c) Catalytic hydration of ethene 

 (d) Steam reforming of methane & water-shift gas reaction

 R4.1   INTRODUCTION TO REVERSIBLE CHANGE 

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 (a) Reversible physical change 

 (a) Reversible physical change 

 (b) System and surroundings 

Ideas on chemical equilibrium developed in the c.19th along with its terminology, some of which may seem a bit strange.  It is necessary, therefore, to define some important terms which textbooks often take for granted.  These will become an integral part of your scientific vocabulary.

In chemical equilibrium - and, later, energetics - it is necessary to define the region or specify the quantity of matter of interest.  This is called the SYSTEM.  Two examples are

(b) System and surroundings 
R4.1   INTRODUCTION TO REVERSIBLE CHANGE

(i) one in which no reaction is taking place, e.g., 20 g of NaBr under different physical conditions:

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(ii) 

The SURROUNDINGS are everything other than the system.  In certain circumstances it is important to specify whether the container, e.g., a crucible in NaBr case, or the flask and syringe for Mg + HCℓ, is to be considered part of the system or part of the surroundings.

 (c) Open, closed & isolated systems

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 (c) Open, closed & isolated systems 

 (d) Reversible chemical change

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1.

2.

(d) Reversible chemical change 
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axis label

URL link (accessed Feb 2022) www.youtube.com/watch?v=-0a_zi0vhaE

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 R4.2   TYPES OF EQUILIBRIUM SYSTEM 

There are many situations in which opposing changes occur at the same time, leading to a state of equilibrium. It can be useful to classify different types of equilibrium based upon whether a chemical reaction is occurring, i.e., chemical processes and physical processes.

 R4.2   TYPES OF EQUILIBRIUM SYSTEM 

 (a) Physical processes

(a) Physical processes
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 (b) Chemical processes 

5.    Chemical equilibria: reactants & products in the same phase - HOMOGENEOUS
6.    Chemical equilibria: reactants & products in different phases - HETEROGENEOUS

 

In chemistry, mixtures are far commonly dealt with than (pure) compounds or elements.  The technical term component is used by chemists to identify the different substances present in a mixture.  In chemistry,
the term substance used correctly refers to a single material of definite chemical composition.

 

(b) Chemical processes 

 R4.3   PHASES 

 (a) Real chemical examples

R4.3   PHASES 

A phase is defined as any uniform, i.e., homogeneous, part of a system which is different from the rest of the system and separated from it by a distinct boundary seen in these examples 1. & 2., e.g.,

(a) Real chemical examples
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 (b) Distinction between homogeneous and heterogeneous

Homogeneous – sampling provides a mixture of uniform composition,
e.g., concentration, colour.  Consisting of only one phase, it has no visible boundaries of separation.  True solutions must be homogeneous.

 

Heterogeneous – sampling provides mixtures of variable composition, e.g., concentration depends on the region of the mixture sampled.  Consisting of two or more phases, a visible boundary of separation can be identified.

(b) Distinction between homogeneous and heterogeneous
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 R4.4   CHARACTERISTIC PROPERTIES OF EQUILIBRIUM STATES 

It should be emphasised that the principles which apply to any one equilibrium system
apply to all.  All equilibrium systems share the same general characteristics.

R4.4   CHARACTERISTIC PROPERTIES OF EQUILIBRIUM STATES 

 (a) Closed systems: a pre-requisite for equilibrium 

For equilibrium systems to be established, either a closed system is required or one which very nearly approximates to that.  This prevents exchange of matter with the surroundings so that reactants and products continually interact in the same proportions.

 (a) Closed systems: a pre-requisite for equilibrium 
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Time

Initially the forward reaction occurs rapidly, but as the concentrations of the reactants fall its rate decreases.

In a closed system, for any physical or chemical process, a state of equilibrium will eventually be reached in which both forward and reverse reactions are occurring simultaneously, but at equal rates.

Assuming that only reactants were present at the start, the reverse reaction initially cannot occur at all, but as soon as products start to form its rate increases.  Eventually the rate of the two reactions becomes equal and equilibrium is established.

 (b) Equilibrium processes are dynamic 

It is important to note that, once equilibrium is attained, the reactions occurring do not cease, rather the froward reaction and reverse reaction both continue, but at equal and opposite rates.  At equilibrium, the macroscopic properties of a system are not observed to alter, e.g., colour, but the nano-scopic processes are continuing, i.e., those occurring on the molecular or/& ionic levels.

(b) Equilibrium processes are dynamic 

The equilibrium is a dynamic process rather than static.  That the nano-scopic processes do continue is supported by experimental evidence.  Examining that, however, lies just a little way beyond the scope of the present discussion.  Nevertheless, a pair of analogies might help visualize these competing changes.

3.

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 (c) Meaning of position of equilibrium 

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At equilibrium, if the concentration of reactants is vanishingly small compared with the products, the reaction is said to have ‘gone to completion’, whereas if the situation is the opposite, the reaction ‘does not occur’.

In the plot above, the equilibrium position lies in favour of the products,
i.e., the concentration of the products exceeds that of the reactants at equilibrium.

(c) Meaning of position of equilibrium 

The relative proportions of product(s) and reactant(s) in the equilibrium mixture is referred to qualitatively as the equilibrium position.

N.B.

Even although the concentrations of reactant and product are constant at equilibrium,
do not infer from this that they are equal: it is possible, but not in most cases encountered. The fact of the matter is that some reactions tend to favour the formation of products, and others the formation of reactants.

Attainment of equilibrium
possible from either direction

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SUMMARY

FEATURES OF THE EQUILIBRIUM STATE

1.

2.

Dynamic nanoscopic properties

3.

Attainment of equilibrium
possible from either direction

4.

Closed systems

Constant macroscopic properties

INTERPRETATION

Concentrations, and other observable properties like colour & density that depend on it, remain constant as products and reactants are being formed and removed at equal rates

Molecular or/& ionic processes are continuing

Same equilibrium mixture results irrespective of whether the reaction started with only reactants, only products, or with a non-equilibrium mixture of both

Prevents exchange of matter with the surroundings so that reactants and products continually interact in the same proportions

 R4.5   FACTORS AFFECTING THE POSITION OF EQUILIBRIUM 

A system in chemical equilibrium can be defined by an equation.  However, one system can have many different states, and change between one state and another can be brought about in various ways.

R4.5   FACTORS AFFECTING THE POSITION OF EQUILIBRIUM 

 (a) Disturbing the position of equilibrium

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(a) Disturbing the position of equilibrium
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It is likely that you are already familiar with the factors which affect the rates of chemical reactions from your study of chemical reaction kinetics, viz

1. Concentration

2. Temperature

3. Pressure

4. Catalysts

A system at equilibrium appears to consist of two reactions: a forward reaction and its reverse. It seems reasonable to assume therefore, that the factors which affect rates of reaction might also affect equilibrium systems.

 (b) Effect on equilibrium of changing concentrations

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The following simple procedure demonstrates how a system in equilibrium responds to changes in concentration of components in the mixture.  The results of this experiment can then be applied to other systems.

The equilibrium system just considered has been used to study the effect of concentration changes within homogeneous systems.  The situation is usually simpler in heterogeneous systems, as we now see.

(b) Effect on equilibrium of changing concentrations
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 (c) Pure solids & pure liquids in equilibrium systems 

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(c) Pure solids & pure liquids in equilibrium systems 

So, by adding more sugar to the system above, the amount of solid present is changed but not its concentration.  Since an equilibrium state, or position, is specified by the concentrations of substances present, it follows that adding more solid sugar does not affect the equilibrium position.

A very similar argument can be applied to pure liquids in equilibrium systems, as in the following exercise.

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 (d) Effect on equilibrium of changing temperature 

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(d) Effect on equilibrium of changing temperature 
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Having discussed how equilibrium systems respond to changes in temperature, it is now appropriate to consider changes in pressure.

 (e) Effect on equilibrium of changing pressure 

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give us the fact that, at low pressure, to a very good approximation, p N, where N represents the number of gas-phase molecules.

The reaction which results in fewer gas molecules being formed will thus lower the pressure of the system.

So, for gaseous equilibria, an increase in pressure, at constant temperature, will shift an equilibrium in the direction where fewer gas molecules will result.

Conversely, a decrease in pressure, at constant temperature, will shift the equilibrium in the direction where a greater number of gas molecules will result.

These generalizations were proposed originally by Jacobus van’t Hoff (1832-1911) and can usefully be applied when one, and only one, variable has been changed.

(e) Effect on equilibrium of changing pressure 
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Now attempt the following exercises which deal initially with homogeneous gaseous equilibria (Q19), then both homogeneous and heterogeneous gaseous equilibria (Q20).

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 (f) Effect on equilibrium of catalysts

(f) Effect on equilibrium of catalysts

A catalyst is a substance that usually increases the rate of a chemical reaction without undergoing any overall consumption or permanent chemical change.  Catalysts that speed up a reaction do so by providing an alternative reaction pathway possessing a lower activation energy by which the net reaction (reactants → products) can take place.

In industry, chemists and chemical engineers must ensure a high yield of product and will therefore consider all the factors which shift equilibrium position to the right, in favour of the desired products.  In addition, the rate at which equilibrium is achieved and the underlying cost benefits in shifting the equilibrium position must also be considered.

Answers to the exercise above should make one realise that the operating conditions which provide for the highest yield are not always those used in practice.  The reasons why are considered in the next section: INDUSTRIAL CASE STUDIES.

 R4.6   INDUSTRIAL CASE STUDIES 

R4.6 INDUSTRIAL CASE STUDIES

Many industrial processes involve equilibria.  The economics of a process demand the desired product is generated as efficiently as possible, i.e., rapidly, but with the minimum amount of waste and the minimum input of energy.  This requires a consideration of both the reaction kinetics and the corresponding equilibria.

Key industrial reactions, some mentioned above, are:

Completing the table below should help to consolidate the ideas behind the optimal conditions under which a particular process might be run.

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HABER PROCESS

CONTACT PROCESS

CATALYTIC HYDRATION OF ETHENE

STEAM REFORMATION OF METHANE

WATER-GAS SHIFT REACTION

low

high

ambient

atmospheric

active

450 °C

no effect

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