Question
Asked Nov 23, 2019
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Select which of the following standard enthalpy of formation values is not zero at 25°C:

  
Na(s)
  
CH4(g)
  
H(g)
  
Hg(l)
  
Ne(g)
  
S8(rhombic)
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Expert Answer

Step 1

Those whose standard enthalpy of formation values is not zero at 25°C are to be selected:

a.

Na(s)

b.

CH4(g)

c.

H(g)

d.

Hg(l)

e.

Ne(g)

f.

S8(rhombic)

Step 2

The standard enthalpy of formation is the change in enthalpy when 1 mole of substance in the standard state is formed from its pure elements under the same conditions.

  • The standard enthalpy of formation of a pure element in its reference form is zero. For example, the standard of formation of Carbon in graphite form is zero.
Step 3
  1. Na (s)-The standard form of sodium is in solid state. Therefore, ΔHfo(Na,s) = 0
  2. CH4 (g) - It is not a pure element. Therefore, ΔHfo (CH4,g) is not 0.
  3. H(g) – The standard form ...

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