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Topic: Standard enthalpy

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Enthalpy

Standard enthalpy change of formation

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	Enthalpy - Wikipedia, the free encyclopedia
		Enthalpy is a quantifiable state function, and the total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead.
		Enthalpy is a thermodynamic potential, and is useful particularly for nearly-constant pressure process, where any energy input to the system must go into internal energy or the mechanical work of expanding the system.
		Enthalpy is most useful when pressure is held constant through exposure to the surroundings, to analyse reactions that increase the volume of the system, causing it to do mechanical work on the surroundings and lose energy.
	en.wikipedia.org /wiki/Enthalpy (516 words)

	Chapter 2 First Law: The Concepts
		Hess’s Law: The standard enthalpy of an overall reaction is the sum of the standard enthalpies of the individual reactions into which a reaction may be divided.
		The standard enthalpies of formation for the elements in their reference state are 0 at all temperatures.
		Estimate of standard enthalpy of formation at 298 K of 2,2-dimethylpropane in a) the gas phase, b) the liquid phase.
	www.cst.cmich.edu /users/Teckl1mm/PChemI/Chm351Ch2-Ch3F01.htm (630 words)

	Enthalpy Changes by Calorimetry (Site not responding. Last check: 2007-10-14)
		Enthalpy is a state function, and so if one wants to define uniquely the enthalpy change in a physical or chemical process, one needs to define only the initial and final states of the system when the process occurs.
		The standard enthaly of formation of a chemical compound, DHf, is the heat evolved or absorbed when the compound is formed in its standard state from its constituent elements in their standard states.
		The standard state of a substance is defined as the stable form of that substance at a pressure of 1 bar and a specified temperature.
	wwwchem.uwimona.edu.jm /lab_manuals/c10p4.html (1222 words)

	University Chemistry: Reaction Enthalpy Changes Under Standard Conditions
		The standard enthalpy of formation of any pure compound (its enthalpy under standard conditions) is the enthalpy change (heat) of the reaction that forms the compound from its constituent chemical elements under standard conditions.
		The standard molar heat of formation of a compound, then, is the enthalpy change of the reaction which forms that compound from its constituent elements.
		The standard molar enthalpy of formation of a compound is the enthalpy change of the reaction which forms one mole of that compound from its constituent elements.
	www.ualberta.ca /~jplambec/che/p101/p01081.htm (3066 words)

	Chapter 5, Section 7 (Site not responding. Last check: 2007-10-14)
		The standard state of a substance is its pure form at atmospheric pressure (1 atm; see Section 10.2) and the temperature of interest, which we usually choose to be 298 K (25°C).
		The standard enthalpy of a reaction is defined as the enthalpy change when all reactants and products are in their standard state.
		°, is the change in enthalpy for the reaction that forms 1 mol of the compound from its elements, with all substances in their standard states.
	cwx.prenhall.com /bookbind/pubbooks/blb/chapter5/medialib/blb0507.html (1180 words)

	The standard molar enthalpy of formation of 2,3-dichlorodibenzo-p-dioxin (Site not responding. Last check: 2007-10-14)
		ABSTRACT.The standard molar enthalpy of combustion of crystalline 2,3-dichlorodibenzo-p-dioxin was determined by rotating-bomb calorimetry to be equal to (5406.4 +- 6.6) kJ/mol.
		Using this result and the standard enthalpy of sublimation, (108.6 +- 1.0) kJ/mol, the values of the standard molar enthalpies of formation of 2,3-dichlorodibenzo-p-dioxin in the crystalline and gaseous states were found to be -(220.5 +- 6.8) kJ/mol and -(111.9 +- 6.9) kJ/mol, respectively.
		The standard molar enthalpy of formation of 2,3,7,8- tetrachlorodibenzo-p-dioxin was estimated as equal to -164.6 kJ/mol.
	www.chem.msu.su:8081 /eng/publ/r98-5.html (110 words)

	[No title] (Site not responding. Last check: 2007-10-14)
		This is the standard molar enthalpy change (heat given out or taken in per mole of reactant) when a species loses an electron to form a positive ion and yield an electron, in the gaseous phase.
		In situation whereby the second electron is in an inner shell, the size of ionisation energy is influenced by both the size of the charge and distance of the second electron to the nucleus.
		This is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions (298 K or 25°C, 1 atm or 100 kPa).
	www-users.york.ac.uk /~ndm500/siti.org.uk/generated/enthalpy_change_1.doc.htm (317 words)

	Standard-state enthalpies
		For liquids and solids, the standard state is the thermodynamically stable state at a pressure of 1 atm and a specified temperature.
		For gases, the standard state is the gaseous state at a pressure of 1 atm and a specified temperature, based on the assumption of ideal gas behavior.
		For dissolved species, the standard state is a 1 M solution at a pressure of 1 atm and specified temperature, based on the assumption of ideal solution behavior.
	nyu.edu /classes/tuckerman/honors.chem/lectures/lecture_15/node9.html (674 words)

	Organometallic Thermochemistry Database
		Standard enthalpies of formation are the building blocks of any thermochemical database.
		Additional enthalpies of solution may be available in the literature (including the reference(s) indicated in a given record), enabling the calculation of reaction enthalpies with the species in different physical states.
		It must be stressed that parameters like enthalpies of reaction or combustion are the true experimental quantities, whereas enthalpies of formation are computed from those parameters and rely on enthalpies of formation of other compounds.
	webbook.nist.gov /chemistry/om (1906 words)

	Quiz 1A Answers
		The enthalpy of sublimation of mercury is 61.6 kJ/mol and its enthalpy of vaporization is 59.3 kJ/mol.
		Because enthalpy is a state function, the enthalpy of sublimation is equal to the enthalpy of fusion plus the enthalpy of vapourization.
		The standard enthalpy of formation of a substance is the enthalpy change for the formation of 1 mole of substance from its elements in their most stable forms at 1 atm and a specified temperature, usually 25
	www.chembio.uoguelph.ca /balahura/quiz_answers.html (485 words)

	Thermochemistry Notes
		The equation associated with the standard enthalpy of formation and any thermochemical equation must specify the state (s, l, g, aq) of each compound in the equation.
		the standard reaction enthalpy for the combustion of one mole of a substance at 1 atm.
		the change in enthalpy when a bond is broken in the gaseous state to form the corresponding gaseous atoms.
	www.chembio.uoguelph.ca /educmat/chm19105/thermochemistry_notes.htm (249 words)

	Enthalpy (Chapter 5)
		Because enthalpy is a state function (doesn’t depend upon the path of the reaction only the initial and final states) it doesn’t matter whether the reaction occurs in two steps as shown above or all at once.
		This implies that the enthalpy of 2 moles of water in the liquid state is 88 kJ lower than 2 moles of water in the gaseous state.
		Note the standard enthalpy of oxygen is zero by definition, since it is in its standard elemental form.
	www.chemistry.ohio-state.edu /~woodward/ch121/ch5_enthalpy.htm (1197 words)

	General Chemistry Online: Glossary: Energy and chemical change
		Enthalpies of hydration for ions are always negative because strong ion-water attractions are formed when the gas-phase ion is surrounded by water.
		The standard state for a gas is the the state of the pure substance in the gaseous phase at the standard pressure
		The standard state for liquids and solids is the state of the most stable form of the substance at the standard pressure.
	antoine.frostburg.edu /chem/senese/101/thermo/glossary.shtml (1938 words)

	Chemical Energetics Definitions
		Because the enthalpy change is affected by changes in temperature and pressure, we use standard conditions.
		The bond dissociation enthalpy is the enthalpy change when 1 mole of bonds is broken.
		The bond enthalpy term is an average value of the bond dissociation enthalpies of a particular bond.
	www.teachmetuition.co.uk /Chemistry/Energetics/chemical_energetics1.htm (516 words)

	Thermochemistry o
		However, standard enthalpies are often given for data collected at 298 K. The most stable state at the standard condition is the standard state.
		The enthalpy of an element at its standard state is assigned 0 for reference.
		The enthalpy of formation of CO from graphite is -393.5 kJ.
	www.science.uwaterloo.ca /~cchieh/cact/c120/thermosum.html (1786 words)

	[No title] (Site not responding. Last check: 2007-10-14)
		Ans: = 2.00 10+4 atm 32.8 The standard Gibb's free energy change for the vaporization of carbon disulfide: CS2 (l) -----> CS2 (g) at 38.1 C is 177.4 cal. Calculate the equilibrium vapor pressure of carbon disulfide at 38.1 C. Neglect the variation of the Gibbs’ free energy of the liquid with pressure.
		Assume that the standard absolute entropies are independent of temperature and that the density of water which is 0.9584 g/ml at 62.6 C is independent of pressure.
		Standard Standard Ans 14.4 / mole Standard Standard Standard Standard lculate the change in the Gibbs’s vaporized at 62.6 C and 110.1 ensity of water which is 0.9584 Ans 8.45 10 oooooo o o o ooooo 17.0!ÿÿ ÿÿ ÿÿx# }"òØÙóôõiÀ{>?ab°abrstthe equilibrium vapor pressure of ice at -10.0 C is 1.950 torr.
	www.mtech.edu /chem/syllabi/CHEM_336/Problem/Problem_Set_33.doc (1605 words)

	Chemical Energy
		Like the internal energy, enthalpy is also a thermodynamic state function, depending only on the initial and final states of the system, but not on the rate of reaction.
		A general rule is to consider standard enthalpy of formation of all elements at the standard condition to be zero.
		Standard enthalpies of formation and standard entropies are important thermodynamic data, and this link gives an extensive table of values for some key compounds.
	www.science.uwaterloo.ca /~cchieh/cact/c120/chemener.html (1777 words)

	Standard Enthalpy of Formation
		The standard enthalpy of formation for an element in its standard state is zero.
		Thus, the standard state for carbon is solid, for water is liquid and for hydrogen is gas.
		That is because graphite is the standard state for carbon, not diamond.
	dbhs.wvusd.k12.ca.us /webdocs/Thermochem/StandardEnthalpyFormation.html (902 words)

	The standard molar enthalpy of formation of 2,3-dichlorodibenzo-p-dioxin (Site not responding. Last check: 2007-10-14)
		The standard molar enthalpy of combustion of crystalline 2,3-dichlorodibenzo-p-dioxin, -5406.4 +- 6.6 kJ/mol, was determined by rotating bomb calorimetry.
		Using this result and the standard molar enthalpy of sublimation, 108.6 +- 1.0 kJ/mol, the values of the standard molar enthalpies of formation of 2,3-dichlorodibenzo-p-dioxin in the crystalline and gaseous states were found to be -220.5 +- 6.8 kJ/mol and -111.9 +- 6.9 kJ/mol, respectively.
		The standard molar enthalpy of formation of gaseous 2,3.7,8- tetrachlorodibenzo-p-dioxin, the most toxic pollutant, was estimated to be -164.6 kJ/mol.
	www.chem.msu.su:8081 /eng/publ/r99-065.html (110 words)

	Chemical Sciences: Reaction Enthalpies Under Standard Conditions
		The enthalpy change which occurs in a chemical reaction is called the reaction enthalpy and is given the symbol DH.
		The reaction enthalpy, which is the enthalpy change that occurs in the reaction, is always calculated as the sum of the enthalpies of the products less the sum of the enthalpies of the reactants, because when this is done the sign of DH has a real physical meaning.
		Reaction enthalpy, then, as a state function depends only upon the initial and final conditions of the reactants and products and not upon the course of the reaction or the conditions under which the reaction actually occurs--if in fact it occurs at all.
	www.psigate.ac.uk /newsite/reference/plambeck/chem2/p01076.htm (977 words)

	Handout (Site not responding. Last check: 2007-10-14)
		O (g) is condensed isothermally and reversibly to liquid water at 100°C. The standard enthalpy of vaporization of water at 100°C is 40.656 kJmol
		the temperature increased by 7.793 K. Calculate (a) the standard molar internal energy of combustion, (b) the standard enthalpy of combustion, and (c) the standard enthalpy of formation of glucose.
		The enthalpies of formation, in kJ/mol, of NAD, Ethanol, NADH and Acetaldehyde are –10.26, -290.76, -41.38, and –213.97, respectively.
	opbs.okstate.edu /~soulages/4224_2005/handouts/handout_2.htm (543 words)

	Kids.net.au - Encyclopedia Enthalpy -
		Enthalpy is a useful function for two reasons.
		Second, if a process begins and ends at the same pressure, the amount of energy exchanged with the environment as heat is given by the change in the system's enthalpy.
		An enthalpy change (ΔH) in chemistry describes the heat energy exchange with the surroundings at constant pressure.
	www.kids.net.au /encyclopedia-wiki/en/Enthalpy (156 words)

	Standard Enthalpies of Formation ?Hfo
		Even though it is the same chemical reaction, the enthalpy changes are different: In the first reaction water is produced in liquid form, whereas the second reaction produced water vapor.
		The standard state of an element or a compound is the most stable physical state at given temperature and pressure.
		The standard enthalpy of formation for elements in its most stable form is generally zero.
	wc.pima.edu /~skolchens/C152OL/Ch6/StdEntha.htm (343 words)

	Erik's Chemistry: First Law of Thermodynamics Terms
		They lie at a higher enthalpy than their component elements and have a positive standard enthalpy of formations.
		The standard enthalpy of a reaction is the sum of the standard enthalpies of the reactions into which the overall reaction may be divided.
		The standard enthalpy (per mole of substance) of a for its formation from its elements in their reference states.
	eppe.tripod.com /pc2term.htm (658 words)

	ORBSEARCH.COM \| encyclopedia of knowledge
		In chemistry, the standard enthalpy change of formation is the enthalpy change (i.e.
		Due to conservation of energy, enthalpies of formation can be used to calculate the heat absorbed or released in any chemical reaction.
		This replacement is unfortunate, since the thermodynamicists had good reasons to avoid a zero in the notation: the standard state does not refer to zero temperatures, pressures and concentrations but to an arbitrarily chosen set of finite ones.
	www.orbsearch.com /st/Standard_enthalpy_change_of_formation.php (195 words)

	unit03-sect03-les01-lessona (Site not responding. Last check: 2007-10-14)
		The standard molar enthalpy of formation (often shortened to molar heat) for a compound is the enthalpy change that occurs when the compound is formed from its elements in their standard states.
		The standard state of an element is its state of matter at 25°C and 100 kPa (SATP conditions).
		A way to overcome this problem is to use standard molar enthalpy of formation data for the species involved in the reaction.
	www.cdli.ca /courses/chem3202/unit03/section03/lesson03/3-lesson-a.htm (767 words)

	Standard States of the Elements and Compounds (Site not responding. Last check: 2007-10-14)
		, is defined as the enthalpy change accompanying the formation of 1 mole of the substance in its standard state from its elements which are also in their standard staes at a definite temperature and constant pressure.
		According to this scale, the standard enthalpies of all elements in their standard state are zeros.
		Then the standard enthalpy of a compound is its standard heat of formation.
	www.accd.edu /sac/chemistr/madappat/lesson5.htm (223 words)

	Practice Questions: Thermodynamics
		From the indicated standard enthalpies of formation given in kJ/mol, please calculate the standard enthalpy of reaction for:
		The standard enthalpy of reaction is -284 kJ and the standard entropy of reaction is -87 J/K. Estimate the temperature at which the equilibrium begins to favor the decomposition of CO Assume that the enthalpy and the entropy of reaction are not affected by temperature.
		From the indicated standard entropies given in J/K, please calculate the standard entropy of reaction for:
	www.towson.edu /users/debye/chem111/practice/111thermo.html (317 words)

	pthermo (Site not responding. Last check: 2007-10-14)
		Use the the enthalpy change for the combustion of benzene, which was determined experimentally, to calculate the standard molar enthalpy of formation of benzene.
		Calculate the enthalpy change under standard conditions (in joules) for this reaction.
		Standard enthalpy change is the enthalpy change when molar quantities of reactants (as defined by the coefficeints of the balanced equation) are converted to molar quantities of products.
	colossus.chem.umass.edu /chandler/ch111/pthermo.htm (1220 words)

	ChemLab - Chemistry 3/5 - The Enthalpy of Formation of MgO - Introduction
		H is a characteristic and important property of the reaction; convenient sets of units for the molar enthalpy are kJ mol
		These units emphasize that enthalpy is an extensive property, which depends upon the amount of reactants consumed.
		Enthalpies of reaction (often called "heats of reaction") are usually tabulated for reactions occurring isothermally (at constant temperature) at, for example, 25 °C. Since enthalpy is a state function, it is not necessary for the reaction mixture to stay at constant temperature during the entire reaction.
	www.dartmouth.edu /~chemlab/chem3-5/calor1/full_text/intro.html (264 words)

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