define standard heat of formation with example
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The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states.
For example, although oxygen can exist as ozone (O3), atomic oxygen (O), and molecular oxygen (O2), O2 is the most stable form at 1 atm pressure and 25°C. Similarly, hydrogen is H2(g), not atomic hydrogen (H).
Answer:
The Hope diamond is a very expensive piece of jewelry, currently worth about $350 million. A pencil can be purchased for less than a dollar. Both items contain carbon, but there is a big difference in how that carbon is organized. The diamond was formed under very different reaction conditions than the graphite, so it has a diffStandard Heat of Formation
A relatively straightforward chemical reaction is one in which elements are combined to form a compound. Sodium and chlorine react to form sodium chloride (see video below). Hydrogen and oxygen combine to form water. Like other reactions, these are accompanied by either the absorption or release of heat. The standard heat of formation (Delta H{_f}^circ) is the enthalpy change associated with the formation of one mole of a compound from its elements in their standard states. The standard conditions for thermochemistry are 25°C and 101.3 kPa. Therefore, the standard state of an element is its state at 25°C and 101.3 kPa. For example, iron is a solid, bromine is a liquid, and oxygen is a gas under those conditions. The standard heat of formation of an element in its standard state is by definition equal to zero. The Delta H{_f}^circ=0 for the diatomic elements, H 2 (g), N 2 (g), O 2 (g), F 2 (g), Cl 2 (g), Br 2 (l), and I 2 (g). The graphite form of solid carbon is its standard state with Delta H{_f}^circ = 0 , while diamond is not its standard state. Some standard heats of formation are listed in the Table below.erent hSubstance ΔHfº (kJ/mol) Substance ΔHfº (kJ/mol)
Al2O3(s) -1669.8 H2O2(l) -187.6
BaCl2(s) -860.1 KCl(s) -435.87
Br2(g) 30.91 NH3(g) -46.3
C (s, graphite) 0 NO(g) 90.4
C (s, diamond) 1.90 NO2(g) 33.85
CH4(g) -74.85 NaCl -411.0
C2H5OH(l) -276.98 O3(g) 142.2
CO(g) -110.5 P(s, white) 0
CO2(g) -393.5 P(s, red) -18.4
CaO(s) -635.6 PbO(s) -217.86
CaCO3(s) -1206.9 S(rhombic) 0
HCl(g) -92.3 S(monoclinic) 0.30
CuO(s) -155.2 SO2(g) -296.1
CuSO4(s) -769.86 SO3(g) -395.2
Fe2O3(s) -822.2 H2S(g) -20.15
H2O(g) -241.8 SiO2 -859.3
H2O(l) -285
\Delta_f H^\ominus = standard enthalpy of formation for a chemical reaction at standard temperature and pressure (STP)
v = coefficients of each respective reactant or product in the balanced chemical reaction
\Delta_f H^\ominus \text{(products)} = sum of enthalpy of each individual product in the balanced chemical reaction
\Delta_f H^\ominus \text{(reactants)} = sum of enthalpy of each individual reactant in the balanced chemical reaction