Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)
why cant MgCl2(aq) and H2(g) react to form Mg(s) and 2HCl(aq)?
why cant MgCl2(aq) and H2(g) react to form Mg(s) and 2HCl(aq)?
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Spontaneous == occurring with no outside intervention
Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g) ..... ΔG is negative, the reaction is spontaneous. It's equivalent to a bowling ball placed on a hill. It will roll down. Down to a lower potential energy.
MgCl2(aq) + H2(g) --> Mg(s) + 2HCl(aq) ..... ΔG is positive, the reaction is nonspontaneous. The bowling ball doesn't roll uphill when placed on a hill.
Things in nature spontaneously move to lower energy, not the other way around. Magnesium and HCl reacting is moving to a lower energy. MgCl2 and H2 reacting is the other way around, and it won't happen spontaneously.
================ Follow up ================
Cpnj127 wasn't doing too badly, until the end, "... endothermic reactions are less stable because it absorbs energy and puts it at a higher energy state." If things in nature move to lower energy (ie more stable) then why are there endothermic reactions, which according to Cpnj127 are "less stable."
Cpnj127 has only mentioned half the story. Whether a reaction occurs or not isn't simply a matter of exothermic vs endothermic. That is ΔH. The spontaneity of a reaction depends on both ΔH and ΔS, the entropy change. When there is an endothermic reaction taking place we know that the entropy change is what is making it spontaneous. ΔG = ΔH - TΔS The endothermic reaction is still at a lower energy despite having a positive ΔH.
Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g) ..... ΔG is negative, the reaction is spontaneous. It's equivalent to a bowling ball placed on a hill. It will roll down. Down to a lower potential energy.
MgCl2(aq) + H2(g) --> Mg(s) + 2HCl(aq) ..... ΔG is positive, the reaction is nonspontaneous. The bowling ball doesn't roll uphill when placed on a hill.
Things in nature spontaneously move to lower energy, not the other way around. Magnesium and HCl reacting is moving to a lower energy. MgCl2 and H2 reacting is the other way around, and it won't happen spontaneously.
================ Follow up ================
Cpnj127 wasn't doing too badly, until the end, "... endothermic reactions are less stable because it absorbs energy and puts it at a higher energy state." If things in nature move to lower energy (ie more stable) then why are there endothermic reactions, which according to Cpnj127 are "less stable."
Cpnj127 has only mentioned half the story. Whether a reaction occurs or not isn't simply a matter of exothermic vs endothermic. That is ΔH. The spontaneity of a reaction depends on both ΔH and ΔS, the entropy change. When there is an endothermic reaction taking place we know that the entropy change is what is making it spontaneous. ΔG = ΔH - TΔS The endothermic reaction is still at a lower energy despite having a positive ΔH.
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But in what conditions will MgCl2 react with H2? thanks
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it can, just not under the same conditions, and thus it will not be an equilibrium/reversible reaction at the given conditions. consider the enthalpy of formation for MgCl2.
Mg(s) + Cl2(g) ==> MgCl2(s) .....ΔHf° = -614.6 kJ molˉ¹
the reverse reaction would require 614.6 kJ molˉ¹ input in order to separate MgCl2.
generally speaking, atoms and molecules have a tendency to prefer more stable states that less stable states. forming Mg2+ from MgCl2 is less stable than forming MgCl2 from Mg2+. endothermic reactions are less stable because it absorbs energy and puts it at a higher energy state.
Mg(s) + Cl2(g) ==> MgCl2(s) .....ΔHf° = -614.6 kJ molˉ¹
the reverse reaction would require 614.6 kJ molˉ¹ input in order to separate MgCl2.
generally speaking, atoms and molecules have a tendency to prefer more stable states that less stable states. forming Mg2+ from MgCl2 is less stable than forming MgCl2 from Mg2+. endothermic reactions are less stable because it absorbs energy and puts it at a higher energy state.