Which of the following have standard enthalpies of formation values that are not
ID: 718130 • Letter: W
Question
Which of the following have standard enthalpies of formation values that are not zero: Ar(g), Br2(l), H(g), W(s), Hg(s), Li2(g)? Explain.
When ammonia is formed according to the following equation:
®2 NH3(g)
the standard enthalpy of reaction (as written) is -92.6 kJ/mol. If 25.4 kg of ammonia (NH3) is formed under standard conditions, what is the enthalpy change? Is this reaction exothermic or endothermic? Explain.
Alcoholic fermentation is the process by which carbohydrates decompose into ethanol and carbon dioxide. The reaction is multi-step and complex, but the overall reaction is
®2 C2H5OH(l) + 2 CO2(g)
Given that the standard enthalpies of formation are C6H12O6(s) = -1274.5 kJ/mol, C2H5OH(l) = -277.0 kJ/mol, and CO2(g) = -393.5 kJ/mol, calculate the standard enthalpy for the fermentation reaction.
Explanation / Answer
Ans 1
The enthalpy of formation for a substance in natural state = 0
Because a naturally occured substance does not form from any other substance so they neither absorb nor release the energy.
Argon is a natural gas
standard enthalpy of formation of Ar(g) = 0
Bromine is a natural liquid
standard enthalpy of formation of Br2(l) = 0
H is a not the natural elemental state of hydrogen gas. It is formed by breaking down the H2 gas molecule, so it needs energy to form H(g).
standard enthalpy of formation of H(g) is not 0.
Tungsten (W) is a natural solid.
standard enthalpy of formation of W(s) = 0
Hg(s) is a not the natural elemental state of mercury. Natural state of mercury is liquid. Hg(s) is formed from Hg(l), so it releases energy to form Hg(s)
standard enthalpy of formation of Hg(s) is not 0.
Li(g) is a not the natural elemental state of lithium . Natural state of lithium is solud. Li(g) is formed from Li(s) so it absorbs energy to form Li2(g).
standard enthalpy of formation of Li2(g) is not 0.
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