Overview of Le Chatelier’s Principle

Le Chatelier’s Principle

Le Chatelier’s Principle is used to predict changes in an equilibrium system with respect to the shift of the equilibrium position. It states that when a dynamic equilibrium system is disturbed or placed under a stress, the system will move to counteract the change in order to re-establish a new equilibrium. However, the new equilibrium restored will not return to the original since concentrations will be changed.

Three main variables that affect the equilibrium shift include:

1. Changes in concentration
2. Changes in pressure and volume
3. Changes in temperature

Commercial synthesis to produce organic and inorganic compounds advances multiple aspects of our life. For instance, the organic synthesis of esters and alcohols as well as the inorganic synthesis of sulfuric acid and lime from limestone.

Industrial synthesis of esters

Chemistry of the reaction

Esters are derivatives of carboxylic acids where the hydrogen attached to the oxygen is replaced by an alkyl group. Esters are polar molecules due to the dipole-dipole interaction in the carbonyl group. However, they are less polar, less soluble in water, and have lower boiling point as compared to the carboxylic acids and alcohols with similar molecular weight due to the absence of the hydroxyl group which is able to form hydrogen bond with themselves and water (Foundation, 2013). Esters with lower molecular weight are extremely volatile. Hence, enable us to smell them.

Figure 1. The reaction involved to form an ester (Formation of esters, n.d.).

Esters are organic compounds formed by reacting carboxylic acids with alcohols, water is produced as a by-product (Figure 1). The industrial synthesis of esters is achieved through the direct esterification of carboxylic acid and alcohol. This process is referred to as the Fischer Esterification, which is a reversible reaction catalysed by an acid, usually concentrated sulfuric acid. The mechanism for Fischer Esterification relates to a nucleophile substitution reaction where the alcohol nucleophile (as the oxygen atom has two lone electron pairs) is added to the carboxylic acid, followed by a series of eliminations.

Figure 2. The process of condensation (Sheng, 2013).

Esterification involves the process of condensation, where the water molecule is split out from the carboxylic acid and alcohol (Figure 2). This enables a larger molecule to be formed as there is a linkage between the reactants.

In an industrial scale, the esterification process is achieved through an esterification plant with the general setup as indicated (Figure 3). A batch esterification process indicates that the feed mixture is added to the reactor all at once. The raw materials for the synthesis are placed in the reactor and are mixed at a certain temperature below 220˚C. The vapour produced moves upwards towards the condenser where it cools and condenses. The reflux drum collects the condensate where some gets diverted to the reactor while the rest is diverted to the outlet collector. Cooling water is added into the reactor to separate the ester from the reaction mixture (Figure 4).

Figure 3. Industrial batch esterification setup (Psenterprise, n.d.).

Availability of reagents

Carboxylic acid and alcohol are cheap reagents that are readily available. Carboxylic acid in particular, is abundant in nature as a result of the oxidation of alcohols and aldehydes during cell metabolism (Kunjapur and Prather, 2015). Primary alcohols can be oxidised to aldehydes which is then oxidised to carboxylic acids using strong oxidising agents. Alcohol is also readily available. It is naturally contained in fruits and ethanol can be easily obtained via fermentation (Macejkovic, 2015).

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Reaction conditions

Due to the reversible nature of the reaction, the desired esters are obtained by using excess amount of alcohols or removing water under azeotropic condition. The concentrated sulfuric acid catalyst acts as a dehydrating agent which eliminates the water produced. Hence, shifting the equilibrium position to the right. This shift results in an increase in the conversion of raw materials. Moreover, the water by-product is removed continuously from the reaction mixture so that the forward reaction is favoured to counteract the change.

The pressure in the reactor is kept between 0.013 to 1.07 bar, and the temperature is maintained below 220˚C (Psenterprise, n.d.). The reflux process allows vapour to be trapped and condense back into the mixture during a period of heating without any loss of reactants and products. Therefore, increasing the yield.

Yield and purity

The purity of the product is 97%. A high product yield is achieved by supressing water as the by-product. A 100% yield can be achieved when an excess of alcohol is added.

Industrial uses of the product

Esters are industrially synthesised to mimic the natural fragrance and flavour. Volatile esters with distinctive fragrances are used in perfumes, cosmetics, synthetic flavouring in food and beverages. Esters are also used as solvents for varnishes, and as an activator in paints. Less volatile esters are used as softening agents in the manufacturing of plastics (Rogers, n.d.). Saponification, which refers to the alkaline ester hydrolysis of fat, is used to make soap.

Environmental issue – High concentration of esters can reduce biogas production, esters initiates antimicrobial activity against microbes, the accumulation of fatty acids decreases the pH which inhibits cell metabolism (Yanti et al., 2014), reagents like ethanol is an air pollutant.

Social issue – Fatty acid ethyl esters are toxic mediators for ethanol ingestion

Economic issue – The batch esterification process can take up to 8 hours to complete. Therefore, a significant consumption of energy and high-pressure steam increases the cost. Furthermore, the concentrated acid used can cause wear and corrosion damage to the reactor.

The Contact Process

Chemistry of the reaction

The contact process is the method used to produce concentrated sulfuric acid. Sulfuric acid is an inorganic compound due to the absence of the carbon atom. It consists of the sulfur atom double-bonded to two oxygen atoms and single-bonded to two hydroxyl groups. The two polar hydroxyl groups forms hydrogen bonds with water, hence, sulfuric acid is soluble in water and has a high boiling point.

Figure 4. Outline of the Contact Process represented in a flowchart (Larapedia, n.d.).

The Contact Process consists of three major steps (Figure 4):

1. Sulfur melted and combusted to produce sulfur dioxide: S _(l) + O_{2 (g)}   SO_{2 (g)}
2. Sulfur dioxide oxidises to sulfur trioxide: 2SO_{2 (g)} + O_{2 (g)}  ⇌ 2SO_{3 (g)}
3. Sulfur trioxide dissolves in sulfuric acid to produce oleum: SO_{3 (g)} + H₂SO_{4 (l)}   H₂S₂O_{7 (l)}

Oleum is diluted to produce sulfuric acid: H₂S₂O_{7 (l)}  + H₂O _(l)   2H₂SO_{4 (l)}

Availability of reagents

Oxygen is readily available in the air and it is a cheap way to increase the yield of sulfur trioxide in step 2.

Sulfur is common and accessible as it:

•  be extracted from mineral deposits through the Frasch process
• can be obtained from hydrogen sulfide, which is found in petroleum and natural gas
• can be obtained in the form of sulfur dioxide during the smelting of sulfide ore

Reaction conditions

The Contact Process proceeds under a temperature of 400˚C to 450˚C, and pressure of 1 to 2 atm. Since the production of sulfur trioxide (step 2) is a reversible reaction, reaction conditions are considered to produce a higher yield:

Temperature:

In order to increase the yield, the forward reaction is favoured. Since the forward reaction is exothermic, by Le Chatelier’s Principle, it will be favoured if there is a decrease in temperature as the equilibrium will attempt to counteract the change by producing more heat. However, according to collision theory, a lower temperature results in an insufficient kinetic energy of particles, thus, only a small proportion of reactant particles will have kinetic energy greater than the activation energy. This decreases the likelihood of successful collisions and the rate of reaction will be slower. Hence, a compromise temperature of 400˚C to 450˚C is used to produce a fairly high portion of SO₃.

Pressure:

There are three moles of reactants and two moles of products. When the pressure increases, by Le Chatelier’s Principle, the equilibrium will attempt to counteract the change by favouring the reaction that produces less gaseous molecules. Moreover, the increase in pressure also speeds up the reaction rate as particles collide more frequently and results in more successful collisions. Thus, a high pressure is preferred to effectively maximise the yield of SO₃. Nonetheless, the actual reaction take place under low pressures close to atmospheric pressure yet still maintains a 99.5% conversion rate and is economically viable.

Concentration:

When the concentration of reactants is increased, by Le Chatelier’s Principle, the equilibrium will counteract the change by lowering the concentration of the products. Hence, favouring the forward reaction. A small excess of oxygen is used during the reaction to increase the yield of sulfur trioxide. This increase in concentration also increases the reaction rate due to the increased collision frequency as well as the likelihood for successful collisions.
Catalyst:

Vanadium oxide is added as a catalyst during the reaction. The addition of a catalyst does not affect the equilibrium position. It lowers the activation energy while increasing the rate of both forward and reverse reaction. Hence, equilibrium is achieved faster while concentration remains the same (Chemguide, n.d.).

Yield and purity

99.5% of the sulfur dioxide is converted to sulfur trioxide, resulting in a 98% yield of sulfuric and the other 2% is water remained in the absorbers. The purity obtained is 97%. Impurities within the reagent gases decreases the yield of sulfur dioxide.

Industrial uses of the product

Sulfuric acid is mainly used in fertiliser manufacturing as well as metal processing which includes the removal of impurities, and rust. Concentrated sulfuric acid is used as a dehydrating agent in organic compounds synthesis due to its strong affinity for water.

Environmental issues – the reagent sulfur can oxidise to sulfur dioxide, which is an air pollutant that contributes to acid rain. It can also undergo reduction and form hydrogen sulfide, which impacts aquatic organisms once in enters the water.

Social issues – when sulfur is oxidised, the sulfur dioxide triggers lung irritation when under exposure, associated with fine soot that results in respiratory diseases. Hydrogen sulfide also poses health concerns as it is toxic and can cause fatal damages when sulfur is reduced.

Economic issues – since the conversion from sulfur dioxide to sulfur trioxide is an exothermic reaction, heat is produced thus the large amount of energy obtained is utilised for the operation of the plant. This enhances the overall efficiency of the process while ensuring it is cost effective. The conversion process only requires a low pressure which is also cost effective.

Bibliography

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