Combustion, enthalpy change

Heat of formation (AH ) the enthalpy change for formation of a compound directly from the ele ments is one type of heat of reaction In cases such as the formation of CO2 or H2O from the combustion of carbon or hydrogen respectively the heat of forma tion of a substance can be measured directly In most... 

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... 

The stoichiometric flame temperature ( Tg ) is used to characterize the burning gas surrounding the droplets because combustion naturally predominates at a distance where the fastest burning mixture is produced. This mixture approximates to the stoichiometric composition. The selection of the droplet surface temperature BP is discussed below. The enthalpy change for vaporization AH is given by... 

The gross healing value or higher healing value (HHV) represents tlie enthalpy change or heat released when a compound is stoicliionietrically combusted at 60°F with the final (flue) products at 60°F and any otlicr water... 

It is difficult to measure the enthalpy change of this reaction. However, standard enthalpies of combustion reactions are easy to measure. Calculate the standard enthalpy of this reaction from the following experimental data ... 

The heat of combustion of a substance is the enthalpy change that occurs when one mole of the substance (an element or a compound) is completely burnt in oxygen. 

Figure 15.20 The enthalpy change from initial to final state in a combustion process is independent of path.

The terms enthalpy of fusion, enthalpy of vaporization, enthalpy of combustion, and many more cause some students to believe that there are many different kinds of enthalpies. There are not. These names merely identify the processes with which the enthalpy term is associated. Thus, there are processes called fusion (melting), vaporization, sublimation, combustion, and so forth. The corresponding enthalpy changes are called by names that include these descriptions. 

There is considerable variation in the heat of reaction data employed in different articles in the literature that deals with this reaction. Cited values differ by more than an order of magnitude. If we utilize heat of combustion data for naphthalene and phthalic anhydride and correct for the fact that water will be a gas instead of a liquid at the conditions of interest, we find that for the first reaction (equation 13.2.3) the standard enthalpy change will be approximately — 429 kcal/g mole for the second reaction it will be approximately — 760 kcal/g mole. These values will be used as appropriate for the temperature range of interest. Any variation of these parameters with temperature may be neglected. 

Heat of combustion the enthalpy change for the complete oxidation of a compound => for a hydrocarbon means converting it to C02 and water. 

Standard enthalpy of combustion, AH%, is the enthalpy change when one mole of a substance is completely burned in oxygen, under standard conditions. 

Example Calculate the standard enthalpy change for the oxidation of ethanol (C2H5OH) to ethanal (CH3CHO), given the following standard enthalpies of combustion ... 

Most authors abbreviate combustion to just c, and symbolize the enthalpy change as AH . Others write... 

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry... 

Chemists use different subscripts to represent enthalpy changes for specific kinds of reactions. For example, AWcomb represents the enthalpy change of a combustion reaction. 

Magnesium ribbon burns in air in a highly exothermic combustion reaction. (See equation (1).) A very bright flame accompanies the production of magnesium oxide, as shown in the photograph below. It is impractical and dangerous to use a coffee-cup calorimeter to determine the enthalpy change for this reaction. 

Instead, you will determine the enthalpy changes for two other reactions (equations (2) and (3) below). You will use these enthalpy changes, along with the known enthalpy change for another reaction (equation (4) below), to determine the enthalpy change for the combustion of magnesium. 

A 10.0 g sample of pure acetic acid, CH3CO2H, is completely burned. The heat released warms 2.00 L of water from 22.3°C to 39.6°C. Assuming that no heat was lost to the calorimeter, what is the enthalpy change of the complete combustion of acetic acid Express your answer in units of kj/g and kj/mol. 

Design an investigation to determine the enthalpy change of the combustion of ethanol using a wick-type burner, similar to that in a kerosene lamp. 

This reaction, having equal number of mols of gas reactants and products, has a negligible change in entropy and thus a negligible heat effect if carried out reversibly at constant temperature. The maximum work available from a fuel cell under these circumstances would then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the... 

In some cases, the value given in the table depends on that calculated previously for some other bond. For example, to obtain ec-c, we combine the enthalpy of combustion of ethane, —1,588kJ mol , with the proper multiples of the AHm s in Equations (4.38)-(4.41) to obtain the enthalpy change for the reaction... 

This reaction equation describes the combustion of methane, a reaction you might expect to release heat. The enthalpy change listed for the reaction confirms this expectation For each mole of methane that combusts, 802 kJ of heat is released. The reaction is highly exothermic. Based on the stoichiometry of the equation, you can also say that 802 kJ of heat is released for every 2 mol of water produced. (Flip to Chapter 9 for the scoop on stoichiometry.)... 

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