mass
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
R4. REVERSIBLE REACTIONS & EQUILIBRIUM SYSTEMS
(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
(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
(i) one in which no reaction is taking place, e.g., 20 g of NaBr under different physical conditions:
(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
(d) Reversible chemical change
1.
2.
URL links (accessed Feb 2022)
www.youtube.com/watch?v=eX3JiKmenuU
www.youtube.com/watch?v=58EEcCit7ls (from 34 s in, until 2.5 min)
www.youtube.com/watch?v=UjmcHP7WFTA
axis label
URL link (accessed Feb 2022) www.youtube.com/watch?v=-0a_zi0vhaE
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.
(a) Physical processes
(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.
R4.3 PHASES
(a) Real chemical examples
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.,
(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.
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.
(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.
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.
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.
(c) Meaning of position of equilibrium
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.
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
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.
(a) Disturbing the position of equilibrium
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
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.
(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.
(d) Effect on equilibrium of changing temperature
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
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.
Now attempt the following exercises which deal initially with homogeneous gaseous equilibria (Q19), then both homogeneous and heterogeneous gaseous equilibria (Q20).
(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
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.
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