the various processes of chemical weathering

chemical weathering is decomposition and disintegration of rocks due to chemical processes and reactions wherein the minerals of the rocks weather away.The rates of weathering due to chemical reactions are not well understood generally in thermodynamic aspects.then there is the effect of temperature on the yeilds,which can be predicted by using the Le chatelier's principles.the effect of a temperature increase on rection rates is to increase them.An approximte rule is that for every increase of 10(degree)Centigrade the reaction rate is doubled.The quantitative relation between temperature and wethering rate is governed by Arrhenius equation.

The various reactions leading to weathering are as follows:-

1)Hydration:-It is an exothermic reaction.It does not involve reaction with water but on association between ions and water mollecules.Vo bnds are broken in this process.Indiavidual polar water moleculaes orient themselves to charged moneral surface in a way that one end of a molecule bonds to the mineral surface and the other end projects away.Their rigidity increases with the temperature fall resulting in development of repulsion forces and is aided by expansion of water volume as temperature falls below 4(degree)celcius,causing weathering.the process prepares mineral surface for further alteration by carbonation and oxidation.

2)Hydrolysis:-In this process one O-h bond of the water is broken.It may occur under the neutral,acidic and basic conditions.It is reaction between chemical compound and water(H+ ions and OH- ions).In this process the cations give rise to weak bases or the anions give rise to weak acids,or both the cation and anion gives rise to both weak acid and base(weak).

                                                                         

3)Carbonation:-It is concerned with weathering effects of carbon-di-oxide in aqueous solution and interaction particularly with calcium carbonate and may give rise to solution landscapes(karst).Dissolution of carbondioxide in water form weak carbonic acid.Atmosphereic water in equilibrium has a ph pf 5.6 due to prescence of dissloved carbon di oxide.In ground water concentration of carbon di oxide can be 20 - 30 times higher due to carbon di oxide addition by biological activity.Example:-green plants respire approximately 40% of their carbon di oxide through their root system.Base addition will lower calcum carbonate solubility by reversal reactions.underground water comes into equilibrium with carbon di oxide at a pressure higher than that in the atmosphere.When the concentration of hyrochloric acid is lower and equilibrium is acheived calcium carbonate precipitates forming stalactites.When there is inequilibrium the drop may fall forming stalagmaites.

4)Redox reaction:-It occurs commonly in aqueous solution.It requires something to be reduced and something else to be oxidised.Oxidisation is electron loss while reduction is electron gain.

a)oxidation:-

This occurs with oxygen as the oxidising agent which sis normally dissolved in water described by Henry's law.The most commonly oxidised material is iron which is converted into ferreous(Fe2+) to ferric iron.Yellowish brown  to red colour of the tropical soil is due to oxidation of iron due to weathering .There are 2 oxidation mechanisms:-

1)Oxidation of ferrous iron in the moneral and subsequent changes;

2)Release of ferrous and it's rapid oxidation to ferric.

It results in rusting of iron containing rocks and their subsequent disintegration.

b)Reduction:-

It is lesssignificant than the oxidation and occurs in anaerobic environment such as that found in waterlogged soils.the colour of such soilsb in gereen grey.Organic matter commonly functions as reducing agent inweathering .the organic compounds are themselves oxidised to form carbon-di-oxide and water or tot form new organic compounds.Sulphate ions may be reduced by bacteria who use the oxygen in sulphate to oxidise organic material.the process also leads to metal formation.

5)Cation excahange:-

Fine soil produced by weathering provide reservoir of exchangeble cations and anions.the colloidal nature particles are either clay or humus.Exchangeable cations are  more comon than exchangeable anions.In clay,replacement of Al3+ for Si4+ at the center of a tetrahedron of oxygen's cause an excessive negative charge for the one .In humus prescence of acidic organic group imparts negative charge.these cations may be exchangeable if water containing other cations is passed through the soil/weathering material.Diferent soils have different cation exchange capacities which have different effect on soil solution whoch may modify weathering process in regolith.

6)Solution:-

The rpocess is first step in chemical decomposition.It depends on ionic potential of a compound which determines the ease with an ion is removable from compound .It also depends on the rock types nature and on the solvent slolute raito and also on the solubitlity of the solid rocks(ie.,solubility product).Highly mobile low ionic potential ease weathering because it makes dissolution in water faster.the temperature effetc on solubility occurs in a manner cocnsistent with The solubility product is also afected by 2 other factors:-

1)Common ion effect;

2)Solution's ionic strength is repersented by the formula:

7)Chelation and complex:

It is a complex organic process by which metallic cations are incorporated into hydrocarbon molecules.

 A complex is formed when coordination number of any ion is greater than oxidation number of the complex involving etylenedyamine is an example of chelate complex.Chelate complexes are more stable than complexes involving monodentate ligands.

       Ferric iron and Al(Al3+) mobilization and release during chelation results in complex formation with fulvic acid (found in humus).These molecules form chelate complexes which are soluble and move down the soil profile before being these complexes.Chelation explains why certain metals are more mobile in podzolisation.