Ethanol standard enthalpy

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

The choice of a given database as source of auxiliary values may not be straightforward, even for a thermochemist. Consistency is a very important criterion, but factors such as the publication year, the assignment of an uncertainty to each value, and even the scientific reputation of the authors or the origin of the database matter. For instance, it would not be sensible to use the old NBS Circular 500 [22] when the NBS Tables of Chemical Thermodynamic Properties [17], published in 1982, is available. If we need a value for the standard enthalpy of formation of an organic compound, such as ethanol, we will probably prefer Pedley s Thermodynamic Data and Structures of Organic Compounds [15], published in 1994, which reports the error bars. Finally, if we are looking for the standard enthalpy of formation of any particular substance, we should first check whether it is included in CODATA Key Values for Thermodynamics [16] or in the very recent Active Thermochemical Tables [23,24],... 

Why did we prefer to use the cycle in figure 2.1 instead of the easier method after equation 2.21 Simply because we had considered that the best data for the standard enthalpies of formation of pure ethanol and acetic acid are those recommended in Pedley s tables [15]. The values (-277.6 kJ mol-1 and -484.3 kJ mol-1, respectively) are both about 1 kJ mol-1 less negative than those in the NBS Tables, and their difference nearly cancels when the reaction enthalpy is calculated. But of course, we are seldom so lucky. Using data from different databases may lead to much larger discrepancies. 

In the foregoing calculation of Asin//(1) and Asin//(3), we have used the tabulated values for the standard enthalpies of formation of ethanol and acetic acid aqueous solutions. This looks sensible (after the definitions given in section 2.3), because the standard states of ethanol and acetic acid solutions in water correspond to 1 mol of C2H5OH or CH3COOH in about... 

The standard enthalpy of formation, AHf°, of a substance is the standard reaction enthalpy for the formation of a substance from its elements in their most stable form. (Phosphorus is an exception white phosphorus is used because it is much easier to obtain pure than the other, more stable allotropes.) Standard enthalpies of formation are expressed in kilojoules per mole of the substance (kj-mol-1). We obtain AHf for ethanol, for instance, from the thermochemical equation for its formation from graphite (the most stable form of carbon) and gaseous hydrogen and oxygen ... 

Use the information in Tables 6.2, 6.6, and 6.7 to estimate the enthalpy of formation of the following compounds in the liquid state, given that the standard enthalpy of sublimation of carbon is +717 kJ-mol 1 (a) NH3 (b) ethanol, CH3CH2OH (c) acetone, CH3COCH3. 

If we want to determine the heat of reaction, where do we even begin The easiest place is to look at a measurement known as the standard enthalpy of formation, A H°f This is based on two different units, the enthalpy of formation, AHfi which represents the enthalpy change that occurs when a compound is formed from its constituent elements, and the standard enthalpy of reaction, AH0, which is the enthalpy for a reaction when all reactants and products are in their standard state (the state they exist in at 25°C and 1 atm). The standard enthalpy of formation is 1 mole of a compound from its constituent elements in their standard states. Enthalpies of formation can be found in many different reference books. Let s take a look at how we can use enthalpies of formation to determine the enthalpy of reaction for the combustion of ethanol. 

Ethanol (C2H5OH) has been proposed as an alternative fuel. Calculate the standard enthalpy of combustion per gram of liquid ethanol. 

Determine the standard enthalpy of formation of ethanol (C2H5OH) from its standard enthalpy of combustion (—1367.4 kJ/mol). 

J Standard enthalpy of formation, AH, is the enthalpy change, under standard conditions, when one mole of a compound is formed from its elements in their standard states, e.g. A//, for ethanol is the enthalpy change for the reaction ... 

We usually report AHj values at 298 K. If an element exists in more than one form under standard conditions, the most stable form of the element is usually used for the formation reaction. For example, the standard enthalpy of formation for ethanol, C2H5OH, is the enthalpy change for the reaction... 

Ethanol (C2H5OH) is currently blended with gasoline as an automobile fuel, (a) Write a balanced equation for the combustion of liquid ethanol in air. (b) Calculate the standard enthalpy change for the reaction, assuming H20(g) as a product (c) Calculate the heat produced per liter of ethanol by combustion of ethanol under constant pressure. Ethanol has a density of 0.789 g/mL. (d) Calculate the mass of CO2 produced per k) of heat emitted. 

Fig. 7.6. Standard enthalpy of solution zl//s°(II) (in kcal/mole) of methane as a function of mole fraction of ethanol at two temperatures.

Using the standard enthalpies of formation listed in Table 5.3, calculate the enthalpy change for the combustion of 1 mol of ethanol ... 

For example, the standard enthalpies of combustion of hydrogen, methane and ethanol are... 

The solubilities of D-xylose and D-mannose in aqueous ethanol (0-100%) at 25 C have been measured by use of refractometry and h.p.l.c., and some physical properties (m.p., solubilities) of crystalline anhydrous a-lactose, , e-lactose, and 8-lactose have been reported. The excess Gibbs free energies of aqueous solutions of carbohydrates and other polyols at 25°C have been determined and correlated with the sugar configurations. The standard enthalpy of formation (1263.4 1.2 kJmol"" ) of o-D-mannose at 298.15 K has been calculated from the corresponding standard energy of combustion (-15.6123 0.0065 kJg ) which was... 

Self-test 1.4 J Estimate the enthalpy change for the reaction between 1 mol C2H5OH as liquid ethanol, a fuel made by fermenting corn, and 02(g) to yield C02(g) and H20(l) under standard conditions by using the bond enthalpies, mean bond enthalpies, and the appropriate standard enthalpies of vaporization. 

Self-test 1.5 ) CalculatethestandardenthalpyofthefermentationCfiHi206(s) —> 2 C2H50H(1) -I- 2 C02(g) from the standard enthalpies of combustion of glucose and ethanol (Table 1.5). 

Astudent calculated the standard enthalpy change of combustion of ethanol [C HjOH] by calorimetry as... 

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