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Contact Process

   

   

Industrial manufacture of Sulphuric (VI)acid - contact process

Contact process. Contact process, modern industrial method of producing sulfuric acid; it has largely replaced the chamber, or lead-chamber, process. Sulfur dioxide and oxygen, passed over a hot catalyst, unite to form sulfur trioxide, which in turn combines with water to make sulfuric acid.

The process can be divided into five stages:

Combining of sulfur and oxygen (O2) to form sulfur dioxide
Purifying the sulfur dioxide in a purification unit
Adding an excess of oxygen to sulfur dioxide in the presence of the catalyst vanadium pentoxide at 450 C and 1-2 atm
The sulfur trioxide formed is added to sulfuric acid which gives rise to oleum (disulfuric acid)
The oleum is then added to water to form sulfuric acid which is very concentrated.

Purification of the air and sulfur dioxide (SO2) is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried with sulfuric acid.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulfur dioxide and dioxygen then react as follows:

2 SO2(g) + O2(g) ⇌ 2 SO3(g) : change in H = -197 kJmol−1

According to the Le Chatelier's principle, a lower temperature should be used to shift the chemical equilibrium towards the right, hence increasing the percentage yield. However too low of a temperature will lower the formation rate to an uneconomical level. Hence to increase the reaction rate, high temperatures (450 C), medium pressures (1-2 atm), and vanadium(V) oxide (V2O5) are used to ensure an adequate (95%) conversion. The catalyst only serves to increase the rate of reaction as it does not change the position of the thermodynamic equilibrium. The mechanism for the action of the catalyst comprises two steps:

Oxidation of SO2 into SO3 by V5+:

2SO2 + 4V5+ + 2O2− → 2SO3 + 4V4+

Oxidation of V4+ back into V5+ by dioxygen (catalyst regeneration):

4V4+ + O2 → 4V5+ + 2O2−

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H2SO4 in the absorption tower to form oleum:

H2SO4 (l) + SO3 (g) → H2S2O7 (l)

Note that directly dissolving SO3 in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H2SO4.

H2S2O7 (l) + H2O (l) → 2 H2SO4 (l)


Standard conditions for measuring Enthalpy changes
Energy Changes in Chemical and Physical Processes

Our bodies need energy in order to perform various tasks such as get out of bed, make breakfast, pedal a bicycle, read a chemistry book and so on. The energy needed to carry out these activities comes from the chemical changes that our bodies induce in the food we eats.

What is energy, and what different forms does it take? Why do some chemical changes release energy while others absorb it? This topic will attempt to answer such questions and then apply our understanding of energy to some of the important environmental issues that people face today.

Energy is the ability to do work. This energy is stored in food, fossil fuels such as coal and oil as chemical energy. There are many forms of energy which include, chemical, heat, electrical and mechanical energy. Some substances have energy as a result of their particles moving. Such energy is known as kinetic energy (K.E.). Other substances have energy by virtue of their position e.g. coiled spring.

This type of energy is known as potential energy (P.E). The two forms of energy are interconvertible; potential energy can be transformed to kinetic energy. For example, electrical energy can be transformed into light energy and heat energy by passing electricity through a bulb filament. Energy cannot be created or destroyed. Energy changes bring about physical and chemical changes in substances. For example, when other forms of energy are converted to heat energy, there is a change in temperature.

Energy is the ability to do work. This energy is stored in food, fossil fuels such as coal and oil as chemical energy. There are many forms of energy which include, chemical, heat, electrical and mechanical energy. Some substances have energy as a result of their particles moving. Such energy is known as kinetic energy (K.E.). Other substances have energy by virtue of their position e.g. coiled spring.

This type of energy is known as potential energy (P.E). The two forms of energy are interconvertible; potential energy can be transformed to kinetic energy. For example, electrical energy can be transformed into light energy and heat energy by passing electricity through a bulb filament. Energy cannot be created or destroyed. Energy changes bring about physical and chemical changes in substances. For example, when other forms of energy are converted to heat energy, there is a change in temperature.

Molar heat of vaporization is the amount of heat energy required to convert one mole of a liquid substance into a gas at its boiling point.

Molar heats of fusion and molar heat of vapourization are used to estimate the strengths of bonds holding the particular together in solids and liquids.
Melting point of substances is high when forces holding particles in solid structure are strong, while the melting point is low when these bonds are weak.

In water for example, its melting point is higher (00C) than that of ethanol (-1170C), while boiling point of water (1000C) is higher than that of ethanol (780C). The inter molecular forces holding water molecules together are stronger than those holding ethanol molecules together . Latent heats of fusion and vaporization of water are thus higher than those for ethanol.


Oxides of nitrogen: Nitrogen(I)oxide, nitrogen(II)oxide and nitrogen (IV)Oxides

A natural source of nitrogen oxides occurs from a lightning stroke. The very high temperature in the vicinity of a lightning bolt causes the gases oxygen and nitrogen in the air to react to form nitric oxide. The nitric oxide very quickly reacts with more oxygen to form nitrogen dioxide
 

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