Zero entropy

[victorias]

I know that when the number of moles of gases increases, entropy increases and when we go from solid to liquid to gas, entropy increases. But I am wondering under what circumstances is entropy zero? The only example I can think of is that if the number of moles of a given phase does not change, then entropy would be zero. eg. 2A (g) + 3B(g) -> 4C(g) + 1D(g) Entropy would be zero here since the total number of moles of gas (5 moles) remains the same. Are there any other situations where entropy remains zero?

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[aldol16]

You should specify that you're talking specifically about change in entropy and not absolute entropy. Absolute entropy is zero if and only if there is only one microstate the system can occupy, which is generally taken to occur at absolute zero for a perfect crystal. Perfection is a must because if you have impurities, the system can organize in multiple ways, which gives you a distribution of available microstates.

With regard to your question, you can take change in entropy to be roughly zero if the stoichiometric amounts of gas do not change during the reaction. I say roughly because the number of available microstates for a system depends on that system's identity in the real world (since we're not dealing with ideal gases usually).