Question

Explain why, although it is not an ionic compound, we still assign oxygen an oxidation state of -2 in water, \(\mathrm{H}_{2} \mathrm{O}\). Give an example of a compound in which oxygen is not in the -2 oxidation state.

Short answer

Oxygen is assigned an oxidation state of -2 in water due to its greater electronegativity compared to hydrogen, which results in an uneven distribution of electrons. This assignment helps us understand the electron distribution within the compound and predict its behavior in chemical reactions. In hydrogen peroxide, \(\mathrm{H}_{2}\mathrm{O}_{2}\), oxygen has an oxidation state of -1, as each oxygen atom forms a single bond with both a hydrogen atom and another oxygen atom.

Step-by-step solution

Define oxidation state

Oxidation state is the charge that an atom appears to have when counting the electrons according to a set of rules. In the context of covalent bonds, it represents an imaginary ionic charge on an atom if all bonds were completely ionic. Although oxidation states do not always correspond to real charges, they help us to balance chemical equations and predict chemical reactions.

Explain why oxidation states are assigned

Oxidation states are assigned to the atoms in a compound to help us understand the electron distribution within that compound. In molecules, atoms share electrons in covalent bonds, but they do not share them equally. The electronegativity difference between the atoms influences the electron distribution: electrons are more attracted to the more electronegative atom. Assigning oxidation states is a way to approximate this electron distribution and predict the behavior of elements in chemical reactions.

Assign oxidation state of oxygen in water

In water, oxygen forms two covalent bonds with hydrogen atoms. Oxygen is more electronegative (3.44) than hydrogen (2.20), so it attracts the electrons shared in these bonds more strongly. According to the rules for assigning oxidation states, if a single bond is involved, both electrons are assigned to the more electronegative atom. As a result, both electrons in each bond are assigned to oxygen, making its oxidation state as -2. This is a useful simplification to describe the electron distribution and calculate chemical reactions involving water, even though its bonding is covalent and not ionic.

Provide an example of oxygen with a different oxidation state

In hydrogen peroxide, \(\mathrm{H}_{2}\mathrm{O}_{2}\), oxygen has an oxidation state of -1. This is because each oxygen atom forms a single bond with both a hydrogen atom and another oxygen atom. The shared electrons in the hydrogen-oxygen bond are again assigned to the oxygen atom, but since oxygen atoms have the same electronegativity, the shared electrons in the oxygen-oxygen bond are divided equally. As a result, both oxygen atoms in hydrogen peroxide have an oxidation state of -1. In summary, oxygen has an assigned oxidation state of -2 in water to approximate its electron distribution and help us describe the compound's chemical behavior. Other molecules, like hydrogen peroxide, can have oxygen in a different oxidation state.

Key concepts

Oxygen in Compounds

Oxygen is one of the most common elements and is present in a wide variety of compounds. In its compounds, oxygen often takes on an oxidation state of -2. This is because oxygen is very electronegative, meaning it attracts electrons strongly.
When combined with less electronegative elements, such as hydrogen in water (\(\mathrm{H}_2\mathrm{O}\)), oxygen pulls the shared electrons in its direction. So, even though water is a covalent compound, we often assign oxygen an oxidation state of -2 as it effectively "owns" the shared electrons.
However, oxygen doesn’t always have this oxidation state. In hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)), for example, each oxygen is assigned an oxidation state of -1. This difference arises from the fact that the oxygen atoms are bonded to each other, sharing electrons equally.

Electron Distribution

In chemical compounds, electron distribution refers to how electrons are shared among the atoms. This sharing is influenced by the atoms' electronegativities, or their ability to attract electrons.
Oxygen is more electronegative than hydrogen, which means it pulls electrons towards itself when bonding.

• In water, for example, electrons in the bonds are more drawn towards oxygen, leading to an oxidation state of -2.
• This concept applies to the electron distribution in any molecule, helping chemists predict how molecules will behave in reactions.

Understanding electron distribution is key to determining oxidation states and predicting the nature of chemical bonds in a compound.

Chemical Reactions

Chemical reactions involve the rearrangement of electrons between atoms. During these reactions, ultimate electron distributions determine how new compounds form. Knowing oxidation states helps chemists understand electron flow in reactions.
For instance, in oxidation-reduction reactions, elements undergo changes in oxidation states as electrons are transferred. Monitoring these changes allows us to follow the process and predict possible products.

• When oxygen reacts with other elements, it often takes on electrons, reducing its counterpart while being reduced itself.

Assigning oxidation states helps to systematically understand the transformation of substances, ensuring accurate predictions of reactants and products.

Oxidation State Rules

Oxidation states are a useful tool in chemistry for keeping track of electron transfer. Here are some basic rules:

• The oxidation state of a free element is always 0.
• The oxidation state for a pure ion is equal to its charge.
• In compounds, oxygen usually has an oxidation state of -2.
• Exceptions include peroxides like hydrogen peroxide where it's -1.
• More electronegative elements in a compound take electrons away from less electronegative ones.

These rules help in determining oxidation states across various compounds and balancing chemical equations.
Although these states are hypothetical, they provide a framework for understanding electron distribution and predicting chemical reaction outcomes.