# Hydration, Coordinate Bonding, and Reduction Potential in Groups 1 and 2 Elements [closed]

Whenever an alkali metal such as Lithium is hydrated/solvated, coordinate bonding occurs and it is the high hydration enthalpy of lithium which makes it the strongest reducing agent in aqueous solution. My question is, if lithium is hydrated and becomes surrounded by water molecules in coordinate bonding, then how does the overall reaction of adding lithium to water produce LiOH and H2(gas) ? Is the coordinated ion an intermediate product?

I also am wondering what happens to the "lost" electrons from the metal in coordinate bonding. Take Beryllium dissolving in water as an example. As it is hydrated it will lose the 2 valence electrons in its outer orbital and then it hybridizes its orbitals into 4 empty orbitals so that 4 molecules of water, supplying 1 lone pair each, can bond with the positive beryllium ion. Where did the original 2 electrons go? How do these electrons vanish in order to allow the hybridization of the 4 empty orbitals for water molecules to bond with?

And if Lithium is the strongest reducing agent in aqueous solution I assume it means it is reducing water itself, in order to create lithium hydroxide and hydrogen gas, but again, how does that come about from the coordinate bonding and the hydrated lithium? Coordinate bonds preserve the charge of the cation but I am not quite sure how this is so.

I have included an image below from a website that attempts to explain it, but I am confused about to where the original electrons go.

The website from which I took the image is: https://www.chemguide.co.uk/inorganic/group2/beryllium.html#top

$$\ce{2Li}(s) + \ce{2H2O}(l) \rightarrow \ce{H2}(g) + 2\ce{Li+}(aq) + 2\ce{OH-}(aq)$$

Whenever an alkali metal such as Lithium is hydrated/solvated, coordinate bonding occurs and it is the high hydration enthalpy of lithium which makes it the strongest reducing agent in aqueous solution. My question is, if lithium is hydrated and becomes surrounded by water molecules in coordinate bonding, then how does the overall reaction of adding lithium to water produce LiOH and H2(gas) ? Is the coordinated ion an intermediate product?

The reaction mechanism for this reaction involves some radical chemistry:

Lithium hydroxide is soluble in water, and will exist as separate ions as long as the solution is not saturated. As soon as the ions are generated, they will be solvated by nearby water molecules. The solvated ions are not intermediates - they might better be viewed as products.

The lithium metal is very reactive due to its free electron, and will react quickly. Besides, Li metal is neutral, and will not immediately form any strong interactions with the polar water molecules, I would think.

And if Lithium is the strongest reducing agent in aqueous solution I assume it means it is reducing water itself, in order to create lithium hydroxide and hydrogen gas, but again, how does that come about from the coordinate bonding and the hydrated lithium? Coordinate bonds preserve the charge of the cation but I am not quite sure how this is so.

Indeed, in the course of the reaction water is reduced and Li is oxidized. But I don't think the Li metal (neutral atomic Li) is solvated by water. The reaction with water takes place extremely quickly. And as mentioned above, it is not intuitive to me why water would want to solvate a Li atom in the first place, due to it being non-polar.

• If we recall though that in the alkali and alkaline earth metals the ratio of charge to ionic radius becomes very important in influencing the behavior, especially in the elements near the top of the respective groups, such as Lithium and Beryllium. The positive charge of lithium in such a small atom strongly attracts polar water and permits coordinate bonding, where 4 water molecules surround lithium, thus hydrating it. So, would the hydration occur only after it has already reduced other water molecules to produce lithium hydroxide and hydrogen gas? – MattGeo Jun 19 '19 at 23:18
• I was certain that the hydration of lithium had to occur first, because the hydration enthalpy is what actually permits, thermodynamically, for lithium to so easily give up its outer electron, being that it is so positive. The ionization energy decreases as you go down the alkali metal group, meaning it is easier to remove an electron, but lithium is a special case at the top which defies this trend, due to its hydration enthalpy and ratio of charge to size. This makes it the strongest reducing agent in water. So does hydration actually occur first? then the reaction you demonstrated above? – MattGeo Jun 19 '19 at 23:24
• Can you give a reference for the high hydration enthalpy of the lithium atom (not the lithium cation)? I'm trying to find something about this, but I cannot. The link you provided for Be talk about the hydration energy for the Be cation, not the Be atom. – Yoda Jun 20 '19 at 9:08
• You are correct in that it is the lithium cation and not the neutral lithium atom which has this very high enthalpy of hydration, but this is also where some of the confusion arises for me. Lithium is the strongest reducing agent in aqueous solution because of the high hydration enthalpy can compensate for its high ionization energy. Lithium will not give up its outer lone electron, it is too electronegative to do so despite being an alkali metal, but since the cation can be hydrated so thoroughly and efficiently, it will give up the electron very readily, having highest reduction potential – MattGeo Jun 21 '19 at 0:38
• I am wondering how we end up with LiOH and H2(gas) though in the very end. Does the hydration happen at the same time as the radical reaction mechanism which you have already shown? We have hydration of lithium cation occurring, but we also have lithium reducing water to form lithium cation, hydroxide ions, and hydrogen gas. How does this all unfold? I want to understand step by step what is occurring. – MattGeo Jun 21 '19 at 0:40