Answered


For the reaction, NH3(g) + HCl(g) NH4Cl(s) run at 25°C, ΔG = −91368 J and ΔS = −284 J/K. What is the enthalpy change for this reaction?


184,632 J

−91,084 J

−98,468 J

−176,000 J


TRUE or FALSE. A negative change in free energy, or −ΔG, represents a spontaneous reaction.

Answer :

For question 1:

25°C = 298 K
ΔG = ΔH - T ΔS
ΔH= ΔG + TΔS = −91368 + (298)(−284)=−91368-84632=-175730
Therefore ΔH is approx - 176,000 J

For question 2:

The statement "A negative change in free energy, or −ΔG, represents a spontaneous reaction." is false 

Answer:

For 1: The enthalpy change for the given reaction is -176000 J.

For 2: Yes, the negative sign of the gibbs free energy represents a spontaneous reaction.

Explanation:

  • For 1:

For the given chemical reaction:

[tex]NH_3(g)+HCl(g)\rightarrow NH_4Cl(s)[/tex]

We are given:

[tex]\Delta G[/tex] = Gibbs free energy = -91368 J

T = Temperature = [tex]25^oC=[273+25]K=298K[/tex]

[tex]\Delta S[/tex] = entropy of the reaction = -284 J/K

To calculate the enthalpy of the reaction, we use the equation for gibbs free energy, which is:

[tex]\Delta G=\Delta H-T\Delta S[/tex]

Putting values in above equation, we get:

[tex]-91368J=\Delta H-(298K)(-284J/K)\\\\\Delta H=-176000J[/tex]

Hence, the enthalpy change for the given reaction is -176000 J.

  • For 2:

Gibbs free energy is defined as the energy which is available to do some work. It is represented as [tex]\Delta G[/tex]

Sign convention for [tex]\Delta G[/tex]

  • If [tex]\Delta G[/tex] value comes out to be positive, it is considered as non-spontaneous reaction.
  • If [tex]Delta G[/tex] value comes out to be negative, it is considered as spontaneous reaction.

So, yes the negative sign of the gibbs free energy represents a spontaneous reaction.

Other Questions