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The combined energy of the colliding particles is called the collision energy. Only when the collision energy is equal to or greater than the collision energy, reaction takes place. In reactions that are not instantaneous, one of the ways by which the rate of the reaction can be increased is through decreasing the activation energy. This can be done by adding a catalyst. As shown in the enthalpy diagram (Fig. 1) the uncatalysed reaction has a high activation energy. In the presence of a catalyst, an alternative reaction pathway with a lower activation energy is followed (Baron, 2010). The details of this pathway are: when a catalyst is added, it first combines with the reactants to form an intermediate complex (also called the transition state) having a lower activation energy. In the next step, the complex breaks down releasing the product(s), and the unchanged catalyst which is now free to interact with fresh reactant molecules to continue the reaction. The catalysed reaction proceeds faster as the activation energy is reduced.
2. List the gases emitted by a car engine. For each one specify any health or pollution problems associated with it. Explain, with balanced symbol equations, how each of these gases can be converted into less polluting gases.
The major gases emitted by a car engine are (a) nitrogen, (b) oxides of nitrogen NOx), (c) carbon dioxide, (d) carbon monoxide (CO), (e) volatile organic compounds, VOCs (mainly unburnt hydrocarbons, HC), and (f) water vapour.
Of the above gaseous products of combustion, nitrogen is inert and non-polluting. Besides, it is a major constituent (78%) of air. Therefore, it does not require any treatment. Similarly, carbon dioxide and water vapour are also generally harmless except that carbon dioxide contributes to global warming. Carbon monoxide, VOCs i.e., unburnt hydrocarbons and oxides of nitrogen require to be made less polluting through suitable conversion reactions (AECC, Association for Emissions Control by Catalyst, 2011). Carbon monoxide and hydrocarbons can be made less polluting through oxidation to carbon dioxide (CO2) and CO2 and water, respectively, as follows: [O] 2CO + O2 ------------------> 2 CO2 Carbon monoxide Pt-Palladium Carbon dioxide (highly hazardous) catalyst (less harmful) [O] CxH2x+2 + [(3x+1)/2]O2 -------------> xCO2 + (x+1)H2O Hydrocarbon Pt-Palladium Carbon water (hazardous) catalyst dioxide Oxides of nitrogen are converted into nitrogen and oxygen by a reduction reaction: (Reduction) 2NOx ------------> N2 + xO2 Nitrogen oxide Pt-rhodium catalyst 3. The majority of catalysts in cars are made from the precious metals platinum and palladium, which are examples of heterogeneous catalysts. Explain this term, and describe (with the help of diagrams) the adsorption and desorption mechanism by which heterogeneous catalysts work. Heterogeneous catalysts are those that catalyse a reaction in which the reactants are present in a different phase. For example, the platinum and palladium, and platinum and rhodium used in cars are catalysts in the solid phase that act upon gaseous reactants such as CO, unburnt HC, and nitrogen oxides. The basic principle of heterogeneous catalysis is that when the reactant molecules are concentrated on the catalyst surface, their collision frequency is much enhanced than when they are in the gas phase (Delpierre and Sewell, 2011) www.physchem.co.za). The mechanism of heterogeneous catalysis with gaseous reactants and a solid catalyst involves a number of steps as shown in Fig. 2 below. Step 1. Diffusion of the reactant gases on to the
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