I do not know much about Chemistry and whatever I knew has become rusty. I have a very simple query. I take a beaker containing pure $\ce{H2SO4}$ solution and immerse a rod of pure $\rm Zn$ into it. Now, the $\rm Zn$ will react with $\ce{H2SO4}$ to form $\rm ZnSO_4$ and hydrogen gas. $$\ce{H2SO4(aq) + Zn(s) -> ZnSO4(aq) + H2(g)}$$ Why does not the reaction spontaneously reverse once again forming pure $\rm Zn$ and $\ce{H2SO4}$ molecules?

$$\ce{H2SO4(aq) + Zn(s) <- ZnSO4(aq) + H2(g) ?}$$

If your answer contains any technical jargon of Chemistry, please explain that in simple terms.

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    $\begingroup$ Not quite right, the reaction forms hydrogen gas which bubbles off making the reaction irreversible. $\endgroup$ – Waylander Mar 21 at 12:50
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    $\begingroup$ Imagine a crowded bus during rush hour. Some people board the bus, others get off the bus. Why don't they spontaneously reverse? Well, sometimes they do, but most would eventually get where they want to be. Same thing here. $\endgroup$ – Ivan Neretin Mar 21 at 12:51
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    $\begingroup$ Dear @Waylander, I am afraid that logic "the reaction forms hydrogen gas which bubbles off making the reaction irreversible" is not correct. Assume we conduct this reaction at 10,000 psi; at this pressure we will not see any bubbles of H2, can we prevent zinc from dissolving in sulfuric acid or reverse this reaction? Most likely not. The formation of hydrogen is highly favored thermodynamically and its equilibrium constant (if it exists) must be extremely large. $\endgroup$ – M. Farooq Mar 21 at 14:15
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    $\begingroup$ @M.Farooq The OP asked for a simple explanation without jargon. Feel free to provide a full answer to the question $\endgroup$ – Waylander Mar 21 at 15:44
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    $\begingroup$ M.Farooq i'll have to agree. Zn2+ will not spontaneously return to metallic state. Hydrogen, even if contained in solution, will not change anything. @Waylander your statement is certainly not a simplification! $\endgroup$ – William R. Ebenezer Mar 21 at 17:49

Before I talk about the chemistry, I will use an analogy from physics. If I drop a piece of chalk, it falls down. It does not at some point later "fall up" again. However, the atoms inside the piece of chalk do go up and down a bit because of thermal vibration. The chalk does not start jumping up because these vibrations are random rather than synchronized.

Why does not the reaction spontaneously reverse once again forming pure Zn and H2SO4 molecules?

If you are asking why at no time the mass of elemental Zn (metal) increases again, it is because there is a strong driving force toward turning the reactants into products for this specific reaction. If you are asking about individual particles, it turns out every now and then, the reverse reaction does occur (this is true for all reactions and is called micro-reversibility).

For some reactions, this back and forth results in the reaction not going to completion. Instead, these reactions may reach an equilibrium where reactant and products are present, and the forward and reverse reactions go on at the same speed so nothing seems to change (this state is called a dynamic equilibrium).

In the reaction you are asking about the driving force forward is too large, and one of the products continuously "escapes" from the reaction mix, so the reaction goes on until one of the reactants is used up (goes to completion rather than reach equilibrium).

In any reaction, reversible or not, once the reaction reaches equilibrium or goes to completion, it does not spontaneously go in the opposite direction unless something changes (add materials, change temperature, do work).


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