Exothermic reactions thermochemical equations

There are three different ways to represent the enthalpy change of an exothermic reaction. The simplest way is to use a thermochemical equation a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents. For example, consider the exothermic reaction of one mole of hydrogen gas with half a mole of oxygen gas to produce liquid water. For each mole of hydrogen gas that reacts, 285.8 kj of heat is produced. Notice that the heat term is included with the products because heat is produced. 

Consider bond breaking and bond making to explain why the combustion of acetylene is exothermic. Then write a thermochemical equation for the reaction, using the following balanced equation ... 

How can a balanced thermochemical equation tell you whether a chemical reaction is exothermic or endothermic ... 

The nature of the reaction or process described by a thermochemical equation is often written as a subscript of Mi. For example, the highly exothermic combustion (comb) of glucose (C6Hi20g) occurs in the body as food is metabolized to produce energy for activities such as the one shown in Figure 16-9. The thermochemical equation for the combustion of glucose is... 

A final assumption of thermochemical equations is that when you reverse the direction of a chemical reaction, you only change the sign of the enthalpy. For example, the synthesis of ammonia is an exothermic process that releases 91.8 k.T of heat. If you reverse the direction to show the decomposition of ammonia, the reaction becomes endothermic ... 

The negative sign for AH tells us that this reaction is exothermic. Notice that AH is reported at the end of the balanced equation, without explicitly specifying the amounts of chemicals involved. In such cases the coefficients in the balanced equation represent the number of moles of reactants and products producing the associated enthalpy change. Balanced chemical equations that show the associated enthalpy change in this way are called thermochemical equations. 

This equation tells us two important things. First, we can immediately see whether the reaction is exothermic or endothermic, because Ml carries a sign. We know that the combustion of methane must be exothermic, and the negative sign on AH confirms this. Second, the thermochemical equation includes the numerical value of the value of AH, so we can tell exactly how much heat will be released. It is important to realize that the heat of reaction shown is for the equation exactly as written if one mole of methane reacts with two moles of oxygen, 890.4 kJ will be released. But if more fuel is burned, more heat will be released. So if the stoichiometric coefficients are multiplied by some factor, the heat of reaction must also be multiplied by that factor. Thus another thermochemical equation for the combustion of methane is... 

For an exothermic reaction, heat flows out of the system. We might say that this heat is a product of the reaction. We can note this explicitly in a form of a thermochemical equation ... 

When we reverse a chentical equation, we change reactants to products and products to reactants. The magnitude of AH for the equation remains the same, but its sign changes. Thus, an endothermic reaction becomes exothermic and an exothermic reaction becomes endothermic, as shown by reversing the thermochemical equations for the melting of ice and the combustion of methane ... 

Write complete units, including substance formulas.Then remember to scale the heat change to the amount of reaction described in the balanced thermochemical equation. Heat is released, so this Is an exothermic reaction and has a negative value of AH . 

We reverse equation (1) to give (-1) when the equation is reversed, the sign of AH is changed because the reverse of an exothermic reaction is endothermic. Then we add the thermochemical equations. 

List some rocket fuels and corresponding oxidizers. Give thermochemical equations for the exothermic reactions of these fuels with the oxidizers. 

An equation that includes a change in energy is a thermochemical equation. There are two kinds of thermochemical equations. One simply writes the AH of the reaction to the right of the conventional equation. For example, if you burn two moles of ethane, CjHg, 2855 kJ of heat is given off. This is an exothermic reaction, so AH is negative AH = —2855 kJ. The thermochemical equation is... 

The second form of thermochemical equation includes energy as if it were a reactant or product. In an exothermic reaction, heat is produced, so it appears as a positive quantity on the product side of the equation ... 

Thermochemical equations are usually written hy designating the value of AH, rather than by writing the energy as a reactant or product. For an exothermic reaction, AH is always negative because the system loses energy. So, the thermochemical equation for the formation of 2 mol of gaseous water would be written as... 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

For thermoneutral identity reactions, there is no thermochemical driving force. In the case of non-identity nucleophilic substitution reactions - when the nucleophile and nucleofuge are different - reaction exothermicity may be taken quantitatively into account. This can be quite elegantly considered by applying the simple Marcus equation [104-109]. For cationic reactions, where interactions with the neutral nucleophile and nucleofuge are quite weak,... 

When this last addition is made, we get the overall equation for the reaction and the AH, calculated by Hess law, is +66.6 — 159.9 = —93.3, which is close to the observed value of —93.7 kcal. This thermochemical analysis makes clear that the exothermicity of the overall reaction comes from the formation of crystalline KBr. The energy released in this last reaction, called the CRYSTAL lattice energy, arises because the ions in the solid are closely arranged so that there is a strong net electrostatic attraction between ions of opposite charge. 

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