[Gautham Siddharth]

for the reaction 2Cl_(g)  Cl_2(g) the signs of ΔH and ΔS respectively are
a) +,-
b) +,+
c) -,-
d) -, +

what is the best answer ?

i think its - and + because it is a spontaneous process and in all spontaneous process, entropy is positive and hence enthalpy will be negative

but there is also another way !
in this process the reaction is accompanied by decrease in no of molecules of reactant. therefore entropy is negative and hence enthalpy will be positive .

but both cannot be the answer ..what do u think is the answer ?

[mjc123]

it is a spontaneous process

How do you know?

in all spontaneous process, entropy is positive

Not true. Entropy of the UNIVERSE increases, but entropy of the system doesn't necessarily increase. (Note: "Entropy" is always positive. What we are interested in is the sign of the CHANGE in entropy.)

entropy is positive and hence enthalpy will be negative

Not necessarily. ΔH may be of the same or opposite sign to ΔS. What actually determines whether a process is spontaneous?

in this process the reaction is accompanied by decrease in no of molecules of reactant.

This is true of all reactions - reactant is consumed and product is produced. That's what a reaction is. What I think you mean is that there is a decrease in the total number of molecules of reagents - reactants plus products.

therefore entropy is negative

Entropy CHANGE is negative if the number of moles of reagents in the gas phase decreases, which is true here. (If there is no change in the amount of gas, ΔS is more subtle and not so easy to predict.)

entropy is negative and hence enthalpy will be positive

Wrong, as explained above. Why not look at the process and ask yourself whether the enthalpy CHANGE is likely to be positive or negative, rather than making unjustified inferences from the entropy?

[Gautham Siddharth]

Cl₂ molecule is more stable than Cl atom ..so this process is spontaneous and hence entropy change is positive ....but the sign of enthalpy change is ...?

[mjc123]

this process is spontaneous and hence entropy change is positive

Once again - not true and non sequitur. I ask again, what is the condition for a process to be spontaneous? Under what conditions can a process be spontaneous despite having a negative entropy change?

[Gautham Siddharth]

This question takes place in plus two thermodynamics chapter. the correct answer for this question is still doubtful. according to plus two thermodynamics, a process is said to be spontaneous when entropy change is positive. here chlorine molecule is more stable than chlorine atom which occurs spontaneously. so entropy change is positive . i do not have clear idea about entropy and enthalpy. sorry. please explain me. replies are thankfully accepted.

[Corribus]

according to plus two thermodynamics, a process is said to be spontaneous when entropy change is positive.

Well this is just incorrect, no way around it. The Gibbs energy change for a process, a measure of spontaneity, is equal to ΔH-TΔS. If a reaction is endothermic enough, it can be non-spontaneous even if the entropy change is positive - that is, if ΔH is more positive than TΔS. You have to consider BOTH ΔH and ΔS (and T) to determine spontaneity.

here chlorine molecule is more stable than chlorine atom which occurs spontaneously. so entropy change is positive . i do not have clear idea about entropy and enthalpy. sorry. please explain me. replies are thankfully accepted.

What do you mean by the bolded phrase?

In any case, spontaneity has nothing to do with determining the answer to the question. You need to determine what the enthalpy change is and what the entropy change is. mjc123 has already given you a number of hints, but I'll ask directly.

What is the enthalpy change related to? What is the entropy change related?

Those are the questions you need to understand in order to be able to get the right answer to this problem.