Water in Chemical Reactions

Meaning of Water in Chemical Reactions

In chemical reactions, water can act in many ways. Some of these include as a hydrogen bond donor or acceptor, as well as a proton (H⁺) donor or acceptor, which allows water to facilitate reactions in many ways.

Most importantly, water acts as a solvent in many chemical reactions. In many chemical contexts, the chemical reaction does not take place without water. Here, water allows these reactions to proceed.

Role of Water in Chemical Reactions

Here we will examine how water is able to facilitate multiple types of chemical reactions.

Most importantly, due to its solvent abilities, it allows the free movement of ions and compounds, which is what allows for many reactions to take place. Because the ions or compounds can move around freely in a dissolved state (such as when dissolved in water), it is easier for chemical reactions to occur, which further demonstrates water's ability to facilitate chemical reactions.

But what is water? And how can it help chemical reactions?

How does water act in chemical contexts?

So what is water? The chemical formula of pure water is: H₂O. This means it is a molecule made up of two hydrogen atoms, covalently bonded to an oxygen atom. This works in the context of atomic bonding due to the need of oxygen to gain two additional electrons (oxidation state of oxygen being -2) which it gains from two hydrogens, where each one shares its one electron (as the oxidation state of hydrogen is +1).

The bent molecular geometry of a water molecule

Looking at the molecular geometry of water, we can see it is bent. You can see this in the diagram on the side, but why might that be?

Taking a closer look at the central atom, in this case oxygen, we can use the VSEPR theory to determine the shape of the molecule. As oxygen has four electron domains (two bonded pairs and two lone pairs) they will arrange themselves in a tetrahedral geometry. Due to there only being two bonds, only these will be prominent and shown.

This shape is what gives water many of its interesting properties.

Due to the fact that water is a bent molecule and since there are differences in electronegativity between O and H, the water molecule is actually a dipole.

This also allows for water to form hydrogen bonds with each other. As you can see on the diagram, the water molecules interact with each other through these partial charges, which give water all its physical and chemical properties.

A diagram showing the molecular shape and angle of a water molecules with partial poles in A, and the hydrogen bonding of water in B

But why is it important for water to be a dipole? Since water is a dipole, i.e, a polar molecule, it is able to allow many chemical reactions to take place. But how?

Formula of Water in Chemical Reactions

Next, we shall cover how water acts in three different ways in chemical reactions. This includes dissolving ionic compounds, forming hydrogen bonds with other compounds, and also acting in acid/base reactions.

1. Dissolution of ionic compounds

Firstly, water is a great solvent due to being a molecule with a dipole. This means that water can easily dissolve any ionic compound, such as any salt. It splits apart the ionic bond to leave two ions - a positive and a negative one. Because water is a dipole, it can shield and protect the ions in both ways. It surrounds the cation (the positive ion) with the negatively charged oxygen atoms, and the anion (the negatively charged ion) with the hydrogens. This allows for appropriate stability of the ionic compound in water. As seen below, this is what occurs.

Ionic Salt_(s) + \( H_2O\) => Cation⁺_(aq) + Anion^-_(aq) + \(H_2O\)

2. Hydrogen bonding with compounds

As water is a polar molecule, it is also able to dissolve any polar substance. In a similar way to the previous example of ionic compounds, the water molecules arrange themselves in such as way as to protect specific parts needed of a polar molecule. This might not cause the splitting of bonds such as in the example with ionic salts, but rather might rely on both its polar properties and also its hydrogen bonding properties.

Water can interact with substances that are able to form hydrogen bonds to dissolve them, thus making them more mobile in water and allowing reactions to take place. It can act both as a hydrogen bond acceptor and donor. Here, the compound in question will stay the same and be unchanged, yet its dissolution will provide greater means for chemical reactions to take place.

Compound_(solid) + \(H_2O\) => Compound_(dissolved) + \( H_2O\)

3. Proton donor/acceptor role

The last key property of water is its ability to be a proton donor or acceptor. This relies on water's intrinsic ability to add or split a hydrogen from itself. The following reactions show this ability:

$$ H_2O\rightarrow H^+ + OH^- $$

$$ H_2O + H^+ \rightarrow H_3O^+ $$

When this occurs, either a hydroxide ion (OH^-) is formed, or a hydronium ion (H₃O⁺) is formed. Here the positive hydrogen ion (H⁺) is referred to as a proton, as it is merely just an atom with one proton and no neutrons or electrons.

Due to this property of water, it can help many reactions to take place. Most importantly acid-base reactions, but it is not limited to that. This property of water is also employed in many organic synthesis reactions and even catalysis of certain reactions.

Examples of Water in Chemical Reactions

First off, the most basic of reactions could be considered the dissolution of solid salts through water. Here is the example of common table salt (NaCl) dissolved, which creates aqueous ions.

$$ NaCl_{(s)} + H_2O \rightarrow Na^+_{(aq)} + H_2O $$

In a similar way, water can also be used to dissolve other covalently bonded molecules, usually larger than salts. Here, the compound does not split apart in water, yet it is still dissolved. An example of this would be glucose. This sugar will form hydrogen bonds with the water molecules and be able to be dissolved by the surrounding water. Note how in the example below, the composition of the compound (glucose in this example) does not change from solid to its aqueous form.

$$ C_6H_{12}O_{6(S)}+H_2O\rightarrow C_6H_{12}O_{6(aq)}+H_2O $$

Another common reaction that water is involved in is acid and base mediated reactions. As we've seen before, the ability of water to split and create hydroxide or hydronium ions is the reaction that can be used by acid or base reactions. Below, is an example of how hydrochloric acid is able to form a strong acid in water:

$$ HCl_{(aq)}+H_2O\rightarrow H_3O^++Cl^- $$

Chemical Reaction of Chlorine in Water

What happens when chlorine is added to water? If you perform a simple litmus test you will notice that the pH decreases. Take a look at the equation below which justifies this behaviour.

$$ Cl_2+H_2O\rightarrow HOCl+HCl $$

This reaction is sometimes called hydrolysis. Its products are HCl, which is a strong acid, hence the decrease of pH upon addition of chlorine. Another product is oxygen gas, so if you perform this experiment you will notice bubbles of this gas forming.