Heat energy exothermic reactions

In a heat-producing (exothermic) reaction the molecular energy of the products is lower than that of the reactants. In the cninbustion of methane, the energy stored in the bonds of the molecules COj plus two molecules of H,0 is less than that in CH4 plus two molecules of O,. If the molecular energy of the products is greater than that of the products (an endothermic reaction), energy (often in the form of heat) must be added for the reaction to occur. 

Non-toxic Opportunity for replacing VOCs Naturally occurring Inexpensive Non-flammable High specific heat capacity -exothermic reactions can be more safely controlled Distillation is energy intensive Contaminated waste streams may be difficult to treat High specific heat capacity - difficult to heat or cool rapidly... 

Hydrogen and carbon monoxide produced by the gasification reaction react with each other and with carbon. The hydrogenation of carbon to produce methane, reaction (24-15), is exothermic and contributes heat energy. Similarly, methanation of CO, reaction (24-19), can also contribute heat energy. These reactions are affected by the water-gas-shift reaction (24-18), the equilibrium of which controls the extent of reactions (24-16) and (24-17). 

As you have seen, reactions can release or absorb energy of several kinds, including electricity, light, sound, and heat. When heat energy is gained or lost in reactions, special terms are used. Endothermic (en dob THUR mihk) reactions absorb heat energy. Exothermic (ek soh THUR mihk) reactions release heat energy. You may notice that the root word therm refers to heat, as it does in thermos bottles and thermometers. 

The solution behaves as a "trap" or "sink" for energy released in the exothermic process. The temperature increase indicates a gain in heat energy. Endothermic reactions, on the other hand, take heat energy away from the solution, lowering its temperature. 

As in all chemical engineering, a major trend has been towards the complete utilization of all raw materials and energy and, for example, all heat from exothermic reactions or Joule heating is used elsewhere in the plant. Moreover the plants have had to change in order to comply with the legal requirement to monitor and control emissions of possible pollutants in the chlor-alkali industry the major concerns have been mercury and chlorine itself. Normally both the atmosphere and the effluent will be monitored for Hg and CI2 and the hazards have been much reduced. 

There may be an output of energy greater than the activation energy of the reaction, so that the energy level of the products is lower than that of the starting materials. This is an exothermic reaction — it proceeds with the output of heat. An exothermic reaction will proceed spontaneously once the initial activation energy has been provided. 

A reactor system is shown in Figure 2 to which the HAZOP procedure can be appHed. This reaction is exothermic, and a cooling system is provided to remove the excess energy of reaction. If the cooling flow is intermpted, the reactor temperature increases, leading to an increase in the reaction rate and the heat generation rate. The result could be a mnaway reaction with a subsequent increase in the vessel pressure possibly leading to a mpture of the vessel. 

Uijferential Scanning Calorimetry (DSC) Sample and inert reference materials are heated in such a way that the temperatures are always equal. If an exothermic reaction occurs in the sample, the sample heater requires less energy than the reference heater to maintain equal temperatures. If an endothermic reaction occurs, the sample heater requires more energy input than the reference heater. 

Thus, the heat of a reaction is obtained by taking the difference between the heat of formation (AHi) of the products and reactants. If the heat of reaction is negative (exothermic), as is the case of most combustion reactions, tlien energy... 

This situation has some similarities to the chemical change in a spontaneous, exothermic reaction. The reactants of high heat content react spontaneously to form products of lower heat content. As each molecular reaction occurs, the excess heat content becomes kinetic energy. The product molecules separate from each other with high kinetic energy. As they collide with... 

Consider two reactions for which ° shows that products are favored, one an exothermic reaction, and the other an endothermic reaction. For the exothermic reaction, when the reactants are mixed they are driven toward equilibrium in accord with the tendency toward minimum energy. Now contrast the endothermic reaction for which ° shows that equilibrium favors products. When these reactants are mixed, they approach equilibrium against the tendency toward minimum energy (since heat is absorbed). This reaction is driven by the tendency toward maximum randomness. 

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