The normal freezing point of ammonia is -78 degrees celsius. Predict the signs of Delta H, Delta S, and Delta G for ammonia when it freezes at -80 degrees celsius and 1 atm: NH3 (l) —> NH3 (s)
The correct answer and explanation is:
Answer:
For the phase transition: NH3(l)→NH3(s)\text{NH}_3 (l) \rightarrow \text{NH}_3 (s)
at -80°C and 1 atm, the signs of thermodynamic parameters are:
- ΔH (Enthalpy Change): Negative (-)
Freezing is an exothermic process because heat is released when ammonia molecules transition from a higher-energy liquid state to a lower-energy solid state. - ΔS (Entropy Change): Negative (-)
Entropy decreases because a liquid has more molecular disorder than a solid. When ammonia freezes, its molecules become more ordered, leading to a reduction in entropy. - ΔG (Gibbs Free Energy Change): Negative (-)
Since ammonia normally freezes at -78°C, freezing at -80°C means the process occurs spontaneously. At temperatures lower than the normal freezing point, the solid phase is favored, making ΔG negative.
Explanation
The freezing of ammonia at -80°C follows the principles of thermodynamics.
- Enthalpy Change (ΔH):
When a substance transitions from liquid to solid, intermolecular forces strengthen as molecules arrange into a fixed structure. This releases heat into the surroundings, making the process exothermic. Hence, ΔH is negative. - Entropy Change (ΔS):
The second law of thermodynamics states that entropy, a measure of disorder, generally decreases when a substance solidifies. Liquids have more molecular motion and randomness than solids. As ammonia transitions from liquid to solid, its molecules become more ordered, reducing entropy, so ΔS is negative. - Gibbs Free Energy Change (ΔG):
The Gibbs free energy equation is: ΔG=ΔH−TΔSΔG = ΔH – TΔS At temperatures below the freezing point (-78°C), solid ammonia is the thermodynamically favored state. Since the system releases heat (ΔH < 0) and the decrease in entropy is more significant at low temperatures, the ΔG value becomes negative, indicating a spontaneous process.
Thus, at -80°C, ammonia spontaneously freezes because ΔG < 0, confirming that the process occurs naturally without external energy input.
Now, I will generate an image illustrating this phase transition.
Here is the scientific illustration showing the phase transition of ammonia from liquid to solid at -80°C, including thermodynamic properties like enthalpy (ΔH < 0), entropy (ΔS < 0), and Gibbs free energy (ΔG < 0). Let me know if you need any modifications!
