Thursday, 29 September 2011

Chemistry note: Kinetics III - catalyst and gas volume calculations

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Catalyst effect on rate of reaction
Catalyst is defined as the substance that alters the rate of reaction. Positive catalyst increases while negative catalyst decreases the rate of reaction. In general A+BC is completed with catalyst by A+catalystA-catalyst and react with B to form C. The activation of the reaction path is lower so that higher portion of molecules has enough energy for reaction to occur, hence faster rate of reaction. Feature of catalyst:
-          Not used up in reaction (always regenerated)
-          Only need a small amount in the reaction mixture
-          Does not affect yield but it only affect rate only
-          Often specified for a certain reaction only
-          Increasing surface area for solid catalyst raises the catalytic effect
-          Maybe “poisoned” by small amount of other impurities with decreasing catalytic effect.
Heterogeneous catalyst: catalyst is in different phase with the reactants (i.e. not miscible in simple meaning, though not exactly), e.g. iron for Haber process and V2O5 for contact process.
Principle of heterogeneous catalyst: Catalyst adsorb reactants and weaken their bond and producing more reactive fragment; and the concentration of reactant around the catalyst increase, which increase the effective collision frequency and increase the rate.
Homogeneous catalyst: the catalyst is in the same phase with the reactant, like hydrogen ion in esterfication. The principle is that the catalyst takes part in the reaction mechanism which changes the rate significantly. For example the redox reaction 2I- + S2O82- I2 + 2SO42-, the rate of reaction is slow due to electrostatic repulsion, but under the catalyst iron(II) or (III), the reaction speeds up as iron ion breaks the reaction into two in which positively and negatively ion attracts by the following reaction:
1)       S2O82- + 2Fe2+ 2SO42- + 2Fe3+
2)       2I- + 2Fe3+ I2 + 2Fe2+ (Note: iron(II) is regenerated)
Catalytic convertor is frequently used to reduce pollutants like rhodium and platinum in car catalytic convertor, changing NO, CO and unconsumed hydrocarbon into CO2 and H2O by 2NO+2CON2+2CO2.
Enzyme is a type of biological catalyst and one of the typical enzyme is the yeast decomposing maltose and glucose to give ethanol and carbon dioxide by:
1)       (maltose)C12H22O11 + H2O 2C6H12O6 (glucose)
2)       C6H12O6 2EtOH (ethanol) + CO2
It’s more environmental-friendly like lower temperature required (they usually function in environment slightly hotter than r.t.) and less metal consumed. Some of them are stereo-specific as well in body mechanisms.
Gas volume calculations
Avogadro’s law states that all gases, under same temperature and pressure, contain equal moles of gases (and equal number of particles).
Molar volume is the volume occupied by 1 mole of gas. We can determine by the following experiment: React Mg and HCl of known mass in a closed flask connected to a syringe. Measure volume and number of moles of H2 released, the syringe conserves the pressure inside and outside the flask so the volume-mole ratio is recorded under atm.
In 298K and 1atm, the molar volume of [ideal] gas is 24 dm3. (or 24 litre)
Assuming gases are ideal, we have the ideal gas law PV=nRT.
1)       In constant pressure, mole ratio = volume ratio.
2)       In constant volume, mole ratio = pressure ratio.

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