SOLUTION OF EXAMPLE

In the casting of metals the top of the mould can be constructed of substance which evolve heat when they come into contact with a source of heat such as molten metal this evalution of heat by the exothermic materials as they are often called keeps the metal att he top of the casting molten allowing it to feed into the casting and fill the contraction cavity which forms when the metal solidifies one of the reaction tacking place in the exthermic material is the combustion of aluminum powder,the heat change accompanying this reaction is large and negative so that alarge quantity of heat is evolved and the reaction is known as an exothermic reaction hence the name of the materials used in the top of the mould to produce heat.carbon is used to reduce zinc oxide in the commerical extraction of zinc and the reaction can be reperseented, because this reaction absorbs alarge quantity of heat energy the energy is supplied by external heating of the retorts in which the reaction is carrird out with
It has been demonstrated that the condition of chemical equilibrium can often be perdicated by thermodynamic considerations and therfore thetendency of areaction to proceed can be measured.The thermodynamic approach dose not however, give any indication of the rate at which a reaction will proceed,and two examples of methallurigically phenomena show the importance of the study of the mechanism and rate of reaction.If aplain carbon steel in the austenitic stracture say at 850c is quenched in watera metastable martensitic structure is produced.this structure will tend to transform to a mixture of ferrite and cementite of lower free energy but the rate of transformation at room tempreature is so slow that the martensite remains apparently unchanged .Tempering at 650 allows the transformation to be completed within an hour.The reaction between carbon and oxygen in liguid steel to form carbon monoxide is accompanid by a substantial negative free energy change but the the carbon and oxygen contents can be raised well above the equilibrium values without any apperciable reaction. If an inrt gas such as argon is bubbled through the liguid steel ,introdusing suitable nuclei for reaction avigorous boil caused by carbon monoxide gas evaluation can be produced ,