Concept explainers
Interpretation:
Standard enthalpy of formation of a given compound is to be described.
Concept introduction:
Hess’s law : According to this law, total enthalpy change of a reaction is the sum of all the reactions regardless of the steps or stages of reaction.
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Answer to Problem 89A
The standard enthalpy (heat) of formation ( ΔH°f= - 396kJ ) is defined as the change in enthalpy that accompanies the formation of one mole of the compound in its standard state from its elements in their standard states.
Explanation of Solution
Hess’s law allows to calculate the unknown ΔH values using known reactions and their experimentally determined ΔH values. The ΔH value for such a reaction is called the standard enthalpy (heat) of formation of the compound. The standard enthalpy (heat) of formation ( ΔH°f ) is defined as the change in enthalpy that accompanies the formation of one mole of the compound in its standard state from its elements in their standard states. A typical standard heat of formation reaction is the formation of one mole of SO3 from its elements
S(s)+32O2(g)→SO3(g). ΔH°f= - 396kJ
The product of this equation is SO3, a suffocating gas that produces acid rain when mixed with moisture in the atmosphere.
Standard enthalpies of formation are stated based on the following assumption: Elements in their standard state have a ΔH°f of 0.0 kJ.
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Chemistry: Matter and Change
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