The standard heat of formation, H f, is defined as the enthalpy change for the f

Question

The standard heat of formation, Hf, is defined as the enthalpy change for the formation of one mole of substance from its constituent elements in their standard states. Thus, elements in their standard states have Hf=0. Heat of formation values can be used to calculate the enthalpy change of any reaction.

Consider, for example, the reaction

2NO(g)+O2(g)2NO2(g)

with heat of formation values given by the following table:

Then the standard heat of reaction for the overall reaction is

Hrxn===Hf(products)2(33.2)114 kJ

For which of the following reactions is Hrxn equal to Hf of the product(s)?

You do not need to look up any values to answer this question.

Check all that apply.

Hints

Check all that apply.

Explanation / Answer

Answer:

Hrxn = Hf [product(s)]

This will be true only when all the reactants used in given chemical reaction are in their native state.

This is the case with reactions

Na(s)+12Cl2(g)NaCl(s)

2Na(s)+Cl2(g)2NaCl(s)

S(s)+O2(g)SO2(g)

Where, Na (s), Cl2 (g) S(s) & O2(g) both are in the so-called elemental state.

Note: Cl2(l) or Cl2 (s) state are not elemental states.

Hence Answe options : (1), (4), (6).

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