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Question:

For which of the following reactions, ΔH is equal to ΔU?

N2(g)+3H2(g)→2NH3(g)

2NO2(g)→N2O4(g)

2SO2(g)+O2(g)→2SO3(g)

2HI(g)→H2(g)+I2(g)

Solution:

For the reaction, 2HI(g)→H2(g)+I2(g), ΔH is equal to ΔU. The relationship between ΔH and ΔU is given by:

ΔH = ΔU + ΔnRT

where:

  • ΔH is the change in enthalpy
  • ΔU is the change in internal energy
  • Δn is the change in the number of moles of gas (moles of gaseous products - moles of gaseous reactants)
  • R is the ideal gas constant
  • T is the temperature in Kelvin

For the reaction 2HI(g) → H2(g) + I2(g), the change in the number of moles of gas (Δn) is:

Δn = (moles of gaseous products) - (moles of gaseous reactants) = (1 + 1) - 2 = 0

Since Δn = 0, the equation becomes:

ΔH = ΔU + 0

Therefore, ΔH = ΔU for this reaction.