KINETICS - THE RATE OF CHEMICAL REACTION BASIC INFORMATION AND TUTORIALS

What is chemical kinetics?

Whereas thermodynamics describes the energy requirements of a reaction, the speed at which it progresses is termed kinetics. It is important to be able to control the rate of chemical reactions for commercial and safety reasons.

If a reaction takes too long to progress the rate at which a product is manufactured would not be viable. Alternatively, if reactions progress too fast and ‘runaway’ out of control there could be dangers such as explosions.

The rate at which reactions take place can be affected by the concentration of reactants, pressure, temperature, wavelength and intensity of light, size of particles of solid reactants, or the presence of catalysts (i.e. substances which alter the speed of reactions without being consumed during the reaction) or impurities.

Catalysts tend to be specific to a particular reaction or family of reactions. Thus nickel is used to facilitate hydrogenation reactions (e.g. add hydrogen to C==C double bonds) whereas platinum is used to catalyse certain oxidation reactions. Sometimes care is needed with the purity of reactants since impurities can act as unwanted catalysts; alternatively, catalysts can be inactivated by ‘poisoning’.


For reactions which progress slowly at room temperature it may be necessary to heat the mixture or add a catalyst for the reaction to occur at an economically-viable rate. For very fast reactions the mixture may need to be cooled or solvent added to dilute the reactants and hence reduce the speed of reaction to manageable proportions.

In general the speed of reaction:

• doubles for every 10°C rise in temperature;
• is proportional to the concentration of reactants in solution;
• increases with decreased particle size for reactions involving a solid;
• increases with pressure for gas phase reactions.

WATER TREATMENT CHEMISTRY BASIC INFORMATION AND TUTORIALS

The chemistry behind water treatment.

The treatment of water can be considered under two major categories: (1) treatment before use, and (2) treatment of contaminated water after it has passed through a municipal water system or industrial process.

In both cases, consideration must be given to potential contamination by pollutants and their removal from water to acceptable levels.

Several operations may be employed to treat water prior to use. Aeration is used to drive off odorous gases, such as H2S, and to oxidize soluble Fe2+and Mn2+ions to insoluble forms.

Lime is added to remove dissolved calcium (water hardness). Al2(SO4)3 forms a sticky precipitate of
Al(OH)3, which causes very fine particles to settle. Various filtration and settling processes areemployed to treat water.

Chlorine, Cl2, is added to kill bacteria. Formation of undesirable byproductsof water chlorination may be avoided by disinfection with chlorine dioxide, ClO2, orozone, O3.

Municipal wastewater may be subjected to primary, secondary, or advanced water treatment. Primary water treatment consists of settling and skimming operations that remove grit, grease, and physical objects from water.

Secondarywater treatment is designed to take out biochemical oxygen demand (BOD). This is normally accomplished by introducing air and microorganisms such that waste biomass in the water, {CH2O}, is removed by aerobic respiration of microorganisms acting on degradable biomass:

{CH2O} + O2 → CO2+ H2O (aerobic respiration).