Exothermic reaction enthalpies

AH is positive when heat is supplied to a system which is free to change its volume and negative when the system releases heat (as in an exothermic reaction). Enthalpy is related to the internal energy of a system by the relationship... 

In this formula, E is the power provided by the drive unit, Q is the heat flow through the wall into the extruder. Hr is the (exothermal) reaction enthalpy per unit mass, Q AP is the energy needed to give a volume flow Q a pressure rise AP, AT is the temperature rise of the material, and is the conversion enthalpy (heat required for melting 1 kg of material), p and Cp are the density and the specific heat, respectively. With this simple balance it is possible to evaluate some overall effects without the need to solve the complete flow field and the energy balances. The temperature rise can be written as... 

ANstreams = enthalpy change between feed and product streams AI/react = reaction enthalpy (negative in the case of exothermic reactions)... 

Figure 5-7 Enthalpy as a Eunction of Temperature for the Exothermic Reaction A B.

By allowing compounds to react in a calorime ter It IS possible to measure the heat evolved in an exothermic reaction or the heat absorbed in an en dothermic reaction Thousands of reactions have been studied to produce a rich library of thermo chemical data These data take the form of heats of reaction and correspond to the value of the enthalpy change AH° for a particular reaction of a particular substance... 

Figure 8.4a (p. 204) shows the enthalpy relationship between reactants and products for an exothermic reaction such as... 

In an exothermic reaction (a), the products have a lower enthalpy than the reactants thus AW is negative, and heat is given off to the surroundings. In an endothermic reaction (b), the products have a higher enthalpy than the reactants, so AW is positive and heat is absorbed from the surroundings. 

It is more common to find that AH° and AS° have the same sign (Table 17.2, III and IV). When this happens, the enthalpy and entropy factors oppose each other. AG° changes sign as temperature increases, and the direction of spontaneity reverses. At low temperatures, AH° predominates, and the exothermic reaction, which may be either the forward or the reverse reaction, occurs. As the temperature rises, the quantity TAS° increases in magnitude and eventually exceeds AH°. At high temperatures, the reaction that leads to an increase in entropy occurs. In most cases, 25°C is a low temperature, at least at a pressure of 1 atm. This explains why exothermic reactions are usually spontaneous at room temperature and atmospheric pressure. 

With a reaction enthalpy of A RH = -170 kJ/g mol the sulfonation with S03 is strongly exothermic. As the color of the acid is dependent not only on the residence time but also to a considerable extent on the reaction temperature, it is necessary to have an effective thermal dissipation. This applies to all of the reactors listed in Table 13. The falling film reactors, of which there are various designs, have the advantage that a very short residence time can be realized [152]. 

When we transfer energy to a constant-pressure system as hear, the enthalpy of the system increases by that amount. When energy leaves a constant-pressure system as heat, the enthalpy of the system decreases by that amount. For example, the formation of zinc iodide from its elements is an exothermic reaction that (at constant pressure) releases 208 kj of heat to the surroundings for each mole of Znl2 formed ... 

We can therefore report that AH = —208 kj because the enthalpy of the reaction mixture decreases by 208 kj in this reaction (Fig. 6.18). An endothermic process absorbs heat, and so when ammonium nitrate dissolves in water the enthalpy of the system increases (Fig. 6.19). Note that AH < 0 for exothermic reactions, whereas AH > 0 for endothermic reactions. 

STRATEGY We expect a strongly negative value because all combustions are exothermic and this oxidation is like an incomplete combustion. First, add up the individual standard enthalpies of formation of the products, multiplying each value by the appropriate number of moles from the balanced equation. Remember that the standard enthalpy of formation of an element in its most stable form is zero. Then, calculate the total standard enthalpy of formation of the reactants in the same way and use Eq. 20 to calculate the standard reaction enthalpy. 

The enthalpies of both reactants and products increase with temperature. If the total enthalpy of the reactants increases more than that of the products when the temperature is raised, then the reaction enthalpy of an exothermic reaction becomes more negative (Fig. 6.35). On the other hand, if the enthalpy of the products increases more than that of the reactants, then the reaction enthalpy... 

FIGURE 6J5 If the heat capacity ot the reactants is larger than that of the products, the enthalpy of the reactants will increase more sharply with increasing temperature. If the reaction is exothermic, the reaction enthalpy will become more negative, as shown here. If the reaction is endothermic, the reaction enthalpy will become less positive and may even become negative. 

Use the estimates of molar constant-volume heat capacities given in the text (as multiples of R) to estimate the change in reaction enthalpy of N2(g) + 3 H,(g) —> 2 NH.(g) when the temperature is increased from 300. K to 500. K. Ignore the vibrational contributions to heat capacity. Is the reaction more or less exothermic at the higher temperature ... 

Step 2 N202 + H2 — N,0 + H,0 Step 3 N20 + H2 — N, + H,0 (a) Which step in the mechanism is likely to be rate determining Explain your answer, (b) Sketch a reaction profile for the overall reaction, which is known to be exothermic. Label the activation energies of each step and the overall reaction enthalpy. 

The addition of an ion to butadiene is clearly an exothermic process in the gas phase due to the formation of aa-bond substituting a rc-bond. The agreement of the reaction enthalpies of the reactions (11) and (12) with equal R (except R = H) is surprising (Table 11). 

The units are correct and the numerical value is typical of reaction energies and enthalpies. Moreover, it makes sense for the chemical industry to develop exothermic reactions for large-scale industrial production. 

One investigation referred to using a Crignard reaction for industrial purposes [134]. It served as model reaction meeting the relevant criteria highly exothermic, temperature sensitive, fast and difficult to handle in a stirred vessel. The reaction chosen had a reaction enthalpy of 300 kj mol and occurred in a time frame of about 10 s, as did most of the side reactions did. The avoidance of side reactions, i.e. an increase in selectivity, was the main motivation for the development of a micro-channel process. 

First H2 is physically adsorbed on the surface. The forces are weak and the adsorption enthalpy is only slightly negative. Subsequently, chemical bonding between Ni atoms and the hydrogen atoms of H2 molecule takes place while simultaneous dissociation of the H2 molecule takes place. The chemisorption enthalpy is strongly negative (exothermic reaction). 

Two types of situation may generally arise in respect of this equation. In the first, the enthalpy of the products exceeds that of the reactants (AH is positive), while in the second the converse happens (AH is negative). A reaction that conforms to the former situation is called an exothermic reaction and a reaction that corresponds to the latter situation is called an endothermic reaction. An exothermic reaction is accompanied by evolution of heat. An endothermic reaction, in contrast, occurs with absorption of heat. Enthalpy changes are... 

Note A negative sign is necessary in equation 3.24 as Qr is positive when heat is evolved by the reaction, whereas the standard enthalpy change will be negative for exothermic reactions. Qp will be negative when cooling is required (see Section 3.4). 

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