Question

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 1

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.

Step-by-step explanation:

• For 1:

For the given chemical reaction:

`NH_3(g)+HCl(g)\rightarrow NH_4Cl(s)`

We are given:

`\Delta G` = Gibbs free energy = -91368 J

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

`\Delta S` = entropy of the reaction = -284 J/K

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

`\Delta G=\Delta H-T\Delta S`

Putting values in above equation, we get:

`-91368J=\Delta H-(298K)(-284J/K)\\\\\Delta H=-176000J`

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
`\Delta G`

Sign convention for
`\Delta G`

• If
  `\Delta G` value comes out to be positive, it is considered as non-spontaneous reaction.
• If
  `Delta G` 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.

Answer 2

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