Exothermic reactions Releasing heat

In an exothermic reaction, heat is given off (released) when you go from reactants to products. The reaction between oxygen and methane eis you light a gas stove (from the earlier section Reactants and Products Reading Chemical Equations ) is a good exeimple of an exothermic reaction. 

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The first law of thermodynamics also tells you that if no work is done on or by the sample, that is, pressure and volume are held constant, any heat flow is counterbalanced by a change in internal energy. An exothermic reaction releasing heat to the surroundings, therefore, is accompanied by a decrease in internal energy, whereas an endothermic reaction has a concomitant increase in internal energy. 

If the AH listed for a reaction is negative, then that reaction releases heat as it proceeds — the reaction is exothermic (exo- = out). If the A//listed for the reaction is positive, then that reaction absorbs heat as it proceeds — the reaction is endothermic (endo- = in). In other words, exothermic reactions release heat as a product, and endothermic reactions consume heat as a reactant. 

Entropy increases markedly as an exothermic reaction releases heat. How can this entropy increase be minimized ... 

If the sum of the enthalpies of the products is less than the sum of the enthalpies of the reactants, then A Hrxn is negative and the reaction is exothermic. Exothermic reactions release heat into their surroundings. 

As you learned in Chapter 1, chemical reactions can be exothermic or endothermic. Recall that an exothermic reaction releases heat, and an endothermic reaction absorbs heat. Figure 20.1 pictures an exothermic process as a reaction that s going downhill energetically and an endothermic process as a reaction that s going uphill. 

Another stress that we can impose on a chemical equilibrium system is an increase or decrease in temperature. All chemical equilibrium systems are affected by temperature changes. An increase in temperature causes a shift in one direction or the other, either to left or to the right. Also, a decrease in temperature causes a shift in one direction or the other, either to the left or to the right. Reactions that shift to the right when the temperature is increased are described as endothermic reactions. Reactions that shift to the left when the temperature is increased are described as exothermic reactions. Endothermic reactions require heat to make them proceed, while exothermic reactions release heat as the reaction proceeds. 

Air Enthalpy change T he heat of reaction, or difference in strength between the bonds broken in a reaction and tire bonds formed. When All is negative, the reaction releases heat and is exothermic. When A IT is positive, the reaction absorbs heat and is endothermic. 

Think for a moment about the connection between bond strengths and chemical reactivity. In an exothermic reaction, more heat is released than is absorbed. But since making product bonds releases heat and breaking reactant bonds absorbs heat, the bonds in the products must be stronger than the bonds in the reactants. In other words, exothermic reactions are favored by stable products with strong bonds and by reactants with weak, easily broken bonds. 

Cylinders have the advantage that they are cheap to manufacture. In addition to varying the shape, the distribution of the active material within the pellets can be varied, as illustrated in Figure 6.7. For packed-bed reactors, the size and shape of the pellets and the distribution of active material within the pellets can be varied through the length of the reactor to control the rate of heat release (for exothermic reactions) or heat input (for endothermic reactions). This involves creating different zones in the reactor, each with its own catalyst designs. 

An endothermic process absorbs heat energy from its surroundings an exothermic process releases heat energy to its surroundings. If a reaction is endothermic in one direction, it is exothermic in the opposite direction. For example, the melting of 1 mole of ice water is an endothermic process requiring 6.02 kJ of heat ... 

However, the large reactors encountered in industry are usually so large that it is impossible to thermostat the reactor at a fixed temperature that would be independent of the reactions and conversions. Further, it is frequently desired to use the heat release from exothermic reactions to heat the reactor above the feed temperature to attain a higher rate. Qn the other hand, one of the greatest problems in exothemic reactions is that of overheating if the heat generated is too large. 

Enthalpy, H, is the heat content of the reacting system. It reflects the number and kinds of chemical bonds in the reactants and products. When a chemical reaction releases heat, it is said to be exothermic the heat content of the products is less than that of the reactants and AH has, by convention, a negative value. Reacting systems that take up heat from their surroundings are endothermic and have positive values of AH. 

Energy is conserved. It may be converted from one form to another, say from potential to kinetic energy, but the total amount of energy in the universe is constant. The energy that an exothermic reaction releases always goes somewhere in the environment, usually in the form of thermal energy (heat). 

It is interesting that the temperature maximum on the Todes line is shifted somewhat to the left (see Fig. 1 or 5) with respect to the tangent point B so that between the maximum and the point B there is a paradoxical region in which exothermic reaction and heat release are accompanied by a decrease in the temperature as a result of the simultaneous expansion of the material. Heat release in this region is accompanied by a growth in entropy. Tentative calculations show that the maximum temperature exceeds the temperature at the point B at the end of the reaction by about 50-100°. 

Any material that will burn in air in solid form may explode if it is dispersed in aerosol form. Explosions of foods, metals, pharmaceuticals, grain products, polymers, and other organic materials all have occurred in the past. Since oxidation is an exothermic reaction, the heat released in burning will rapidly raise the temperature of small particles nearby, and the large surface area presented by these particles encourages more reaction to take place and hence more heat to be produced. A runaway reaction can be the result. 

Reactions that release more energy than they take in are called exothermic reactions. In this type of reaction, the potential energy of the products is lower than the potential energy of the reactants, with the extra energy being released, usually in the form of heat. For this reason, exothermic reactions will heat the area around them. An example of an equation for an exothermic reaction is shown here ... 

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. 

Combustion reactions of fossil fuels are familiar examples of exothermic reactions. Hydrocarbons—including methane, the main component of natural gas, and octane, a minor component of gasoline—undergo combustion with an excess of O2 to yield CO2 and H2O. These reactions release heat energy. The amounts of heat energy released at constant pressure are shown for the reactions of one mole of methane and of two moles of octane. 

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