Reversible and irreversible reactions, Chemistry tutorial


It was assumed that all the chemical reactions were irreversible till the year 1803, when French chemist Claude Louis Berthollet introduced the concept of reversible reactions. Primarily he observed that the sodium carbonate and calcium chloride react to yield calcium carbonate and sodium chloride; though, after observing sodium carbonate formation around the edges of salt lakes, he comprehended that huge amount of salts in the evaporating water reacted by calcium carbonate to form sodium carbonate, pointing out that the reverse reaction was taking place.

The chemical reactions are symbolized by chemical equations. These equations generally encompass a unidirectional arrow (→) to symbolize irreversible reactions. The other chemical equations might encompass a bidirectional harpoons (↔) that symbolize reversible reactions (not to be confused by the double arrows ↔ employed to point of the resonance structures).

Irreversible Reactions:

Define: The chemical reactions that carry on to completion in one direction only are termed as irreversible reactions. In irreversible reactions reactants are fully transformed into products in a certain interval of time. In such reactions products don't form reactants again.

A basic concept of chemistry is that chemical reactions occurred whenever reactants reacted by each other to form products. Such unidirectional reactions are termed as irreversible reactions, reactions in which the reactants transform to products and where the products can't transform back to the reactants. Such reactions are necessarily like baking. The ingredients, acting as the reactants, are mixed and baked altogether to form a cake that acts as the product. This cake can't be transformed back to the reactants (like the eggs, flour and so on), just as the products in an irreversible reaction can't transform back to the reactants.

An illustration of an irreversible reaction is the combustion. The combustion comprises burning an organic compound like a hydrocarbon and oxygen to produce carbon-dioxide and water. As water and carbon-dioxide are stable, they don't react with one other to form the reactants. The combustion reactions take the given form:

CxHy + O2 → CO2 + H2O

Reversible reactions:

Define: The chemical reactions that carry on in both the directions forward and backward concurrently are termed as reversible reactions. Such reactions never go to completion however for all time continue in both the directions.

In reversible reactions, the reactants and products are never fully consumed; they are each continually reacting and being generated. The reversible reaction can take the given summarized form:

218_Reversible reactions.jpg

Fig: Reversible reactions

These two reactions are taking place concurrently that signifies that the reactants are reacting to yield the products, as the products are reacting to generate the reactants. Collisions of the reacting molecules cause chemical reactions in the closed system. After products are made up, the bonds between such products are broken whenever the molecules collide with one other, producing adequate energy required to break the bonds of the product and reactant molecules.

Dissimilar to irreversible reactions, reversible reactions lead to the equilibrium: In reversible reactions, the reaction carries in both the directions whereas in irreversible reactions the reaction carries in only one direction.

If the reactants are made up of at the same rate as the products, a dynamic equilibrium exists. For illustration, if a water tank is being filled by water at similar rate as water is leaving the tank (via a hypothetical hole), then the amount of water remaining in the tank remains steady.

The Carnot Cycle:

In the early 19th century, the steam engines came to play an increasingly significant role in industry and transportation. Though, a systematic set of theories of the conversion of thermal energy to motive power via steam engines had not yet been developed. Nicolas Leonard Sadi Carnot (1796 - 1832), a French military engineer, introduced 'Reflections on the Motive Power of Fire' in the year 1824. The book stated a generalized theory of heat engines, and also an idealized model of the thermodynamic system for a heat engine which is now termed as the Carnot cycle. Carnot made the basis of the second law of thermodynamics, and is often illustrated as the 'Father of thermodynamics'.

The Carnot cycle has the maximum efficiency possible of an engine (however other cycles encompass the similar efficiency) based on the supposition of the absence of incidental wasteful processes like friction, and the supposition of no conduction of heat between various parts of the engine at various temperatures.       

The heat engine having Carnot cycle, as well termed as Carnot heat engine, can be simplified via the given model:

The reversible heat engine absorbs heat QH from the high-temperature reservoir at TH und discharges heat QL to the low-temperature reservoir at TL. The temperatures TH and TL remain unaffected. And we are familiar from the 1st law of thermodynamics, work is done by the heat engine, W = QH + QL. Here QH > 0 and QL < 0.

The Carnot cycle in this heat engine comprises of two isentropic and two isothermal methods.

The Carnot cycle comprises of the given four processes:

A) Reversible isothermal gas expansion process: In this method, the ideal gas in the system absorbs qin amount heat from the heat source at a high temperature Th, expands and does work on the surroundings.

B) Reversible adiabatic gas expansion process: In this method, the system is thermally insulated. The gas carries to expand and do work on surroundings that causes the system to cool to a lower temperature, Tl.

C) Reversible isothermal gas compression process: In this method, surroundings do work to the gas at Tl, and causes a loss of heat, qout.

D) Reversible adiabatic gas compression process: In this method, the system is thermally insulated. The surroundings carry on to do work to the gas that causes the temperature to mount back to Th.

2099_carnot cycle.jpg

Fig: Carnot cycle

Efficiency of a Carnot engine:

The Carnot cycle forms an engine. The PV diagram below illustrates the operation of a Carnot engine, where the 'working fluid' which expands and contracts in the cylinder is the ideal gas.

1291_Efficiency of a Carnot engine.jpg

Fig: Efficiency of a Carnot engine

Here the high temperature TH and the low temperature TL are temperatures as measured on the ideal gas thermometer, that is:

T = pV/NR

One cycle of the Carnot engine acts as follows:

=> Leg 1: Isothermal expansion at high temperature.

As the gas expands, it lifts a huge pile of sand, that is, it does work |W1|. We have observed that expansion generally sends the temperature down. To keep the similar temperature, the gas should absorb heat |Q1| from its surroundings.

=> Leg 2: Adiabatic expansion.

As the gas expands, it lifts a huge pile of sand, that is, it does work |W2|.

The expansion sends the temperature down. As it is adiabatic, no heat is absorbed or removed.

=> Leg 3: Isothermal compression at low temperature.

As the gas contracts, it lets down a small pile of sand, that is, work |W3| is done on the gas. We have seen that compression generally sends the temperature up. To keep the similar temperature, the gas should eject heat |Q2| into its surroundings.

=> Leg 4: Adiabatic compression.

As the gas contracts, it lets down a small pile of sand, that is, work |W4| is done on the gas. The compression sends the temperature up. As it is adiabatic, no heat is absorbed or removed.

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