Rasaa22
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Hi guys, I’ve been asked to write expressions for Kc for a few reactions, I’m not sure if I’m doing it properly.
For instance, for Sodium and Chlorine to produce Sodium Chloride. How do I write this as an Kc expression ?
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David Tan
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Kc represents equilibrium constant and is applicable only for reversible reactions. An example of a reversible reaction is alcohol + carboxylic acid to generate ester + water. I don't think the reaction between sodium and chlorine is a reversible one.

Perhaps, you can provide us with the examples that you have in your school notes so that it will be easier for us to explain the concept to you.

Cheers.
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Ðeggs
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Doesn’t exist as the reaction isn’t reversible...
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Rasaa22
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Ah okay, why isn’t it a reversible reaction? And if it isn’t a reversible reaction, can I still write it as Kc expression?
So my answer is:
Kc = [NaCl]^2 / [Na]^2 + [Cl2]
(Original post by David Tan)
Kc represents equilibrium constant and is applicable only for reversible reactions. An example of a reversible reaction is alcohol + carboxylic acid to generate ester + water. I don't think the reaction between sodium and chlorine is a reversible one.

Perhaps, you can provide us with the examples that you have in your school notes so that it will be easier for us to explain the concept to you.

Cheers.
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syrup!
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ignore that, it is a reversible reaction

the expression is

[NaCl]/[Na+][Cl-]
(Original post by Rasaa22)
Ah okay, why isn’t it a reversible reaction? And if it isn’t a reversible reaction, can I still write it as Kc expression?
So my answer is:
Kc = [NaCl]^2 / [Na]^2 + [Cl2]
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David Tan
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Imagine burning a piece of paper to get ashes. Can u get back the same piece of paper from the ashes? If you can, then it is a reversible reaction. Can you get back sodium metal and chlorine gas by performing some "magic" on the table salt, NaCl? You can't and so it is not a reversible reaction. It is an irreversible reaction.

Hope it helps. Cheers.
(Original post by Rasaa22)
Ah okay, why isn’t it a reversible reaction? And if it isn’t a reversible reaction, can I still write it as Kc expression?
So my answer is:
Kc = [NaCl]^2 / [Na]^2 + [Cl2]
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Rasaa22
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Kc = [NaCl]^2 / [Na]^2 + [Cl2] Isn’t this the answer?
And what makes it an irreversible reaction?
Also, surely at normal temperature and pressure it is irreversible?
(Original post by syrup.02)
ignore that, it is a reversible reaction

the expression is

[NaCl]/[Na+][Cl-]
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syrup!
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that could be as well, it depends on the exact info ur given
and you cant know for sure tbh, you have to use the info given. if they are asking for Kc it must be reversible
(Original post by Rasaa22)
Kc = [NaCl]^2 / [Na]^2 + [Cl2] Isn’t this the answer?
And what makes it an irreversible reaction?
Also, surely at normal temperature and pressure it is irreversible?
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Rasaa22
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Just been told to write Kc expression for ‘sodium and chlorine react to form sodium chloride) with a side note; ‘some of these examples are not reversible reactions but apply the rules as you have been taught them to work out an answer’
(Original post by syrup.02)
that could be as well, it depends on the exact info ur given
and you cant know for sure tbh, you have to use the info given. if they are asking for Kc it must be reversible
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anonymoussse
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i fought nacl was a soluble salt so the reaction is reversible as long as the nacl is still in solution
(Original post by David Tan)
Imagine burning a piece of paper to get ashes. Can u get back the same piece of paper from the ashes? If you can, then it is a reversible reaction. Can you get back sodium metal and chlorine gas by performing some "magic" on the table salt, NaCl? You can't and so it is not a reversible reaction. It is an irreversible reaction.

Hope it helps. Cheers.
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David Tan
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His example was Na(s) + Cl₂(g) to generate NaCl(s). There is no water in his example. The example you are referring to is the enthalpy change of solution of NaCl and its equation is NaCl(s) + aq → NaCl(aq). However, this is a bit off topic from his original post.

If the question wants you to assume that the reaction [Na(s) + 0.5 Cl₂(g) ⇄ NaCl(s)] is reversible, then you have to express it as Kp = 1/ (partial pressure of chlorine gas) raised to the power of 0.5. If you are asked to use Kc, then it shall be Kc = 1/[Cl₂] raised to the power of 0.5. Kc uses square bracket to represent concentration. Kp uses partial pressure. Both Na and NaCl are solid and you do not incorporate solid into your equilibrium constant expression, be it Kc or Kp. The reason is because solids have fixed concentration and density.

Hope it helps.
(Original post by ggxsywes)
i fought nacl was a soluble salt so the reaction is reversible as long as the nacl is still in solution
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Pigster
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Delta H is negative, delta S is also negative, so delta G can be negative.

All an irreversible reaction is is one in which Kc is so large than the proportion of reactant is soooo low that we can effectively ignore it.
(Original post by David Tan)
Imagine burning a piece of paper to get ashes. Can u get back the same piece of paper from the ashes? If you can, then it is a reversible reaction. Can you get back sodium metal and chlorine gas by performing some "magic" on the table salt, NaCl? You can't and so it is not a reversible reaction. It is an irreversible reaction.

Hope it helps. Cheers.
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David Tan
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You are using the Gibbs free energy equation to explain why a reaction is irreversible/reversible. There is nothing wrong with that. I would prefer my students to focus on understanding first and then use equation as a complementary or supplementary tool to assist their understanding. If the students can "understand" why a reaction is irreversible by referring to examples that we commonly witness in our everyday life, then we can introduce the equation to enhance their "understanding".

History has taught us that past science greats always came up with the core idea first before coming up with equations to represent their ideas. Hope it helps. Cheers.
(Original post by Pigster)
Delta H is negative, delta S is also negative, so delta G can be negative.

All an irreversible reaction is is one in which Kc is so large than the proportion of reactant is soooo low that we can effectively ignore it.
Last edited by David Tan; 1 year ago
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