CHEMICAL REACTIONS CAN BE EITHER EXOTHERMIC OR ENDOTHERMIC

Before it reaches the catalytic converter, the exhaust contains such pollutants as NO, CO, and hydrocarbons. 

A catalytic converter reduces the pollution caused by automobile exhaust by converting such harmful combustion products as NO, CO, and hydrocarbons to harmless N2, 02, and COz. The catalyst is typically platinum, Pt, palladium, Pd, or rhodium, Rd. 

Living organisms rely on special types of catalysts known as enzymes, which allow exceedingly complex biochemical reactions to occur with ease. The nature and behavior of enzymes are discussed in Chapter 13. 

How does a catalyst lower the energy barrier of a chemical reaction  

Ss the preceding two sections have discussed, reactants must have a certain amount of energy in order to overcome the energy barrier so that a chemical reaction can proceed. Once a reaction is complete, however, there may be either a net release or a net absorption of energy. Reactions in which there is a 

The exhaust from automobiles today is much cleaner than before the advent of the catalytic converter, but there are many more cars on the road. In I960, there were 70 million rt stered motor vehicles in the United State.s. In 2000, there were more chan 200 million. 

For the chemical reactions taking place in burning wcmkI, there is a net release of energy. For those taking place in a photosynthetic plant, there is a net absorption of eneigy. 

TABLE 9.1 SELECTED BOND ENERGIES Bond Energy Bond Energy 

Do all covalent single bonds have the same bond energy  

---

All sorbent desiccants release heat (exothermic) when water is captured. Similarly, desorption requires addition of heat energy (endothermic). Reactant desiccants can be either exothermic or endothermic, depending upon the nature of the chemical reaction. This is also known as chemisorption. Other pertinent details are noted in Endnote C and Footnote 40 of Chapter 4. 

Chemical blowing agents generate a gas, typically nitrogen, water, carbon monoxide or carbon dioxide, by a chemical reaction that can be either exothermic (i.e., heat is generated chemically and given out by the system, resulting in a rise in temperature) or endothermic (heat is absorbed... 

The background for Equation 3.26 can be illustrated by the following mental exercise let us assume that an exothermic chemical reaction proceeds in a reactor for an infinitesimally short period of time, df. Subsequently, an amoimt of heat R(—AHr)Vn dt (in /) is released. This heat partially escapes from the reactor (dQ = Qdt) and is partially consumed to increase the temperature of the reaction mixture by the amount dT mCpdT. Consequently, balance Equation 3.26 is obtained, which can be proved valid even in the case of an endothermic reaction. The heat flux Q can be either positive or negative, depending on the temperature of the reactor (T) and its surroundings (Tc). Q is typically given as... 

Finally, chemical reactors can be classified according to the mode of heat removal. We can have either isothermal or non-isothermal reactors, a sub-category of which is adiabatic reactors. Often it is desirable to use the heat released by an exothermic reaction for an endothermic reaction, in order to achieve higher heat integration. 

C) Chemical reactions are either endothermic or exothermic (they can t be both). 

Equation 16-7 not only shows the simple way that K, depends on temperature, it also shows a simple way to determine the enthalpy change for a reaction. By determining the value of e at several different temperatures, and then plotting log Ke versus 1 IT, we should get a straight line whose slope is -AE/2.3.R. If the reaction is exothermic LH is negative), the slope will be positive if the reaction is endothermic (A/f is positive), the slope will be negative (Figure 16-1). Equation 16-7 applies to all chemical equilibria and is independent of the concentration units used either Kp or < can be use(j equally... 

---