Question

Read Examples 5.11 and 5.12, then determine which of the following reactions have the same ΔHrxn...

Read Examples 5.11 and 5.12, then determine which of the following reactions have the same ΔHrxn ° and ΔHf ° .

a) 2 CO(g) + O2(g)  2 CO2(g)

b) 3/2 H2(g) + 1/2 N2(g)  NH3(g)

c) H2S (g) + 2 O2(g)  H2SO4(l)

(ans: b)

***Explain why the other two reactions above do not represent heat of formation equations

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Answer #1

Ans : b) 3/2 H2 (g) + 1/2 N2 (g) = NH3 (g)

The formation enthalpy is the amount of heat that is required to prepare one mole of the substance in its standard state using the reactants in the standard states as well.

Here one mole of NH3 is formed in (b) reaction ,using the reactants N2 and H2 in their standard states, so its reaction enthalpy and formation enthalpy is same.

Now in the other two reactions , either the product formed is not one mole (a) or the reactants used are not in their pure standard states (c) . So these reactions do not have the same reaction and formation enthalpies.

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