Methane enthalpy changes from

Enthalpy Change from Standard Enthalpies of Formation Use standard enthalpies of formation to calculate AH° for the combustion of methane. 

Equations (1) and (2) are the heats of formation of carbon dioxide and water respectively Equation (3) is the reverse of the combustion of methane and so the heat of reaction is equal to the heat of combustion but opposite in sign The molar heat of formation of a substance is the enthalpy change for formation of one mole of the substance from the elements For methane AH = —75 kJ/mol... 

Note several points about this definition. First, the "reaction" to form a substance from its constituent elements can be (and often is) hypothetical. We can t combine carbon and hydrogen in the laboratory to make methane, for instance, yet the heat of formation for methane is A H°f = —74.8kJ/mol, which corresponds to the standard enthalpy change for the hypothetical reaction... 

An estimate of the standard enthalpy change for the formation of the methyl anion and a hydrogen cation from methane may be obtained by a calculation based on Hess s Law, as shown in Table 1. 

Bond enthalpy terms, on the other hand, are quantities assigned to each bond in a molecule such that the sum over all bonds is equal to the enthalpy change associated with the conversion of the molecule into separate atoms. Bond enthalpy terms are assumed to be constant, and therefore transferable from molecule to molecule. So for methane ... 

Methane expands in an isothermal process at 200 K from 800 to 100 kPa. Calculate the enthalpy change per kmol methane using the Soave... 

This enthalpy change is used to calculate the remaining unknown from the experimental values of Af//298 for methane and ethane... 

As an example let us consider the enthalpy change when methane is formed from its elements, both reactants and product being in their standard states. This is called the standard enthalpy of formation and written AH°. 

In a formation equation, 1 mole of a compound forms from its elements. The standard heat of formation (AH ) is the enthalpy change for the formation equation when all the substances are in their standard states. For instance, the formation equation for methane (CH4) is... 

In the mid-19 century, some thought that the sign of the enthalpy change (AH), the heat added or removed at constant pressure (qp), was the criterion for spontaneity. They thought that exothermic processes (AH < 0) were spontaneous and endothermic ones (AH > 0) were nonspontaneous. This hypothesis had a lot of support from observation after all, many spontaneous processes are exothermic. All combustion reactions, such as methane burning, are spontaneous and exothermic ... 

It does not matter that we cannot make this reaction occur cleanly and therefore cannot directly measure its enthalpy change. As seen above, indirect methods can be used. In such ways it is found that for this reaction is -17.9 kcal. Since we have arbitrarily taken the enthalpies of the reactants to be zero, it follows that on this basis the enthalpy of CH4 (g) is —17,9 kcal mol" h This is known as the enthalpy of formation, AHf, of methane. It.is important to use the term enthalpy of formation only for the formation of the compound from elements in their standard states. 

The heat of combusion of a substance is defined as the enthalpy change that occurs when 1 mole of a substance reacts with elemental oxygen to form liquid water and gaseous COz. Determine the heat available from the combustion of methane, at 25 C and a constant pressure. Express the... 

Consider the reaction of methane, CH4 (the principal constituent of natural gas), burning in oxygen at constant pressure. How much heat could you obtain from 10.0 g of methane, assuming you have an excess of oxygen You can answer this question if you know the enthalpy change for the reaction of 1 mol of methane. The thermochemical equation is... 

Enthalpy changes of formation are often difficult to measure in practice due to competing side reactions and slow rates of reaction. For example, methane and potassium manganate(vii) cannot be prepared from their elements via the following thermochemical equations ... 

The standard heat of formation (AHp, is the enthalpy change for formation of one mole of a compound directly from its elements, and Is one type of heat of reaction. In cases such as the formation of CO2 or FI2O from the combustion of carbon or hydrogen, respectively, the heat of formation of a substance can be measured directly. In most other cases, heats of formation are not measured experimentally but are calculated from the measured heats of other reactions. Consider, for example, the heat of formation of methane. The reaction that defines the formation of methane from its elements. 

Recall from Section 5.3 that the enthalpy change for the combustion of a mole of methane depends on whether the product water is liquid or gas. More heat is given off by the reaction that produces liquid water. We can use this example to illustrate Hess s law by envisioning the first of these reactions happening in two steps. In step 1, methane and oxygen are converted to carbon dioxide and liquid water, releasing heat. 

In Section 3.7, we discussed the chlorination of methane. Now we ll return to that reaction and consider it in a more general way and from a different perspective. We will consider the enthalpy changes that accompany the various propagation steps in the reaction. We will examine the enthalpy change for each step, and see how it contributes to the overall enthalpy change. 

Calculate the standard enthalpy change of formation of methane from the following standard enthalpy changes of combustion carbon = -394kJmoH... 

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