Endothermic reaction Energy

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. 

The positive sign indicates that there is a net absorption of energy, meaning the reaction is endothermic. For any endothermic reaction, energy can be considered a reactant and is thus sometimes included before the arrow of the chemical equation ... 

In this reaction, water from the solid (s) gypsum is released in gas (g) form. It is an endothermic reaction (energy is absorbed). The reverse reaction, in which liquid (1) water is added to the plaster of Paris, is exothermic (energy is released) ... 

Endothermic reaction (Section 6.4) A reaction in which the energy of the products is higher than the energy of the reactants. In an endothermic reaction, energy is absorbed and the hH° is a positive value. 

In an endothermic reaction, energy is absorbed by the chemicals that are reacting. If you have ever baked a cake or a loaf of bread, you have seen an example of such a reaction. Recipes for both products require either baking soda or baking powder. Both baking powder and baking soda contain a chemical that causes dough to rise when heated in an oven. 

To do this we need to classify reactions according to whether they produce heat or absorb heat. A reaction that produces heat (heat is a "product") is said to be exothermic. A reaction that absorbs heat is called endothermic. Because heat is needed for an endothermic reaction, energy (heat) can be regarded as a "reactant" in this case. 

The key components of a chemical energy transmission system or a chemical heat pipe are a primary energy source to provide heat, an input catalyst reactor where in an endothermal reaction energy is stored in newly created products, and an output catalyst converter, where the reverse exothermal chemical reaction helps to extract the stored heat for further consumption. 

In endothermic reactions, energy must be added to start and continue to be added to sustain the reaction. 

In an endothermic reaction, energy must be added to the reactants to make the reaction happen. Heat is a reactant. The decomposition of ammonia into its elements is an example ... 

For an endothermic reaction, energy is absorbed from the surroundings by the chemicals in the reaction. So... 

Cracking reactions are endothermic the energy balance is obtained by the production of coke that deposits on the catalyst and that is burned in the regenerator. 

The direct splitting of H2S, analogous to the splitting of water, is not economically feasible because of the high energy iaput requirement for the endothermic reaction. 

Thermal decomposition of spent acids, eg, sulfuric acid, is required as an intermediate step at temperatures sufficientiy high to completely consume the organic contaminants by combustion temperatures above 1000°C are required. Concentrated acid can be made from the sulfur oxides. Spent acid is sprayed into a vertical combustion chamber, where the energy required to heat and vaporize the feed and support these endothermic reactions is suppHed by complete combustion of fuel oil plus added sulfur, if further acid production is desired. High feed rates of up to 30 t/d of uniform spent acid droplets are attained with a single rotary atomizer and decomposition rates of ca 400 t/d are possible (98). 

Additional energy to sustain the endothermic reaction is provided chemically by the addition of siUcon carbide grain or electrically by use of electrothermal fluidized beds (33—34), induction heating, or resistance heating. Chlorine efficiencies are typically 98% or better. 

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. 

Most chemical reactions are exothermic. In the few endothermic reactions that are known, heat is absorbed into the reaction product or products, which are known as endothermic or energy-rich compounds. Such compounds are thermodynamically unstable because heat woiild be released on decomposition of their elements. The majority of endothermic compounds possess a tendency toward insta-bihty and possibly explosive decomposition under various circumstances of initiation. 

When a gas reacts with a solid, heat will be transfened from the solid to the gas when the reaction is exothermic, and from gas to solid during an endothermic reaction. The energy which is generated will be distributed between the gas and solid phases according to the temperature difference between the two phases, and their respective thermal conductivities. If the surface temperature of the solid is T2 at any given instant, and that of the bulk of the gas phase is Ti, the rate of convective heat transfer from the solid to the gas may be represented by the equation... 

In general, for basic petrochemicals that are not much more expensive than fuel (energy) itself, the energy recovery or use is important. Therefore, exothermic reactions should be executed at the highest temperature and endothermic reaction at the lowest, within the range that the reaction permits. In addition, reactors should not be optimized only for their own performance, but also for the optimum economy of the full synthesis loop or the full technology. 

Figure 1-2. Potentiai energy curve for an endothermic reaction.

In addition to molecular geometry, the most important quantity to come out of molecular modeling is the energy. Energy can be used to reveal which of several isomers is most stable, to determine whether a particular chemical reaction will have a thermodynamic driving force (an exothermic reaction) or be thermodynamically uphill (an endothermic reaction), and to ascertain how fast a reaction is likely to proceed. Other molecular properties, such as the dipole moment, are also important, but the energy plays a special role. 

The irradiation of 3-carbomethoxyisoxazole (47) gave the corresponding oxazole (48) in very low yields (5-8%) without the isolation of the corresponding azirine (Scheme 22) [71JCS(C)1196]. Also in this case calculations show that the energy of the triplet state allows the formation of the biradical intermediate and then of the azirine. However, the low yields of the conversion can be explained considering that the transformation of the biradical intermediate into the azirine is an endothermic reaction (Fig. 10) [99H(50)1115]. 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

Sketch a potential energy diagram which might represent an endothermic reaction. (Label parts of curve representing activated complex, activation energy, net energy absorbed.)... 

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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