Question

What are the oxidation states of phosphorus in the following compounds? \(\mathrm{H}_{3} \mathrm{PO}_{2}, \mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{Mg}_{2} \mathrm{P}_{2} \mathrm{O}_{7}, \mathrm{PH}_{3}, \mathrm{HPO}_{3}\) (a) \(+1,+3,+3,+3,+5\) (b) \(+3,+3,+5,+5,+5\) (c) \(+1,+2,+3,+5,+5\) (d) \(+1,+5,+5,-3,+5\)

Short answer

\(+1,+5,+5,-3,+5\)

Step-by-step solution

Oxidation state in H3PO2

Find the oxidation state of phosphorus in \(\mathrm{H}_3\mathrm{PO}_2\). Hydrogen is usually \(+1\) and oxygen is \(-2\). With three hydrogens, the total positive charge is \(+3\) and since there are two oxygens, the total negative charge is \(-4\). To balance the charges, phosphorus must have an oxidation state of \(+1\) to give a neutral compound.

Oxidation state in H3PO4

For \(\mathrm{H}_3\mathrm{PO}_4\), again hydrogen has a \(+1\) oxidation state contributing \(+3\) in total. With four oxygen atoms contributing \(-8\), phosphorus must have an oxidation state of \(+5\) to balance out the negatives and achieve a neutral compound.

Oxidation state in Mg2P2O7

In \(\mathrm{Mg}_2\mathrm{P}_2\mathrm{O}_7\), magnesium has a \(+2\) oxidation state. Since there are two magnesium atoms, their total contribution is \(+4\). Seven oxygens contribute \(-14\). To balance, the two phosphorus atoms must have a total oxidation state of \(+10\), so each phosphorus atom has an oxidation state of \(+5\).

Oxidation state in PH3

In \(\mathrm{PH}_3\), hydrogen is again \(+1\) and contributes \(+3\) in total. Phosphorus is the only other element, so it must have a corresponding negative oxidation state to balance hydrogen. The compound is neutral, so phosphorus has an oxidation state of \(-3\).

Oxidation state in HPO3

For \(\mathrm{HPO}_3\), the one hydrogen contributes \(+1\), and three oxygens contribute \(-6\). Phosphorus must compensate for this by having an oxidation state of \(+5\), again to achieve a neutral molecule.

Key concepts

Understanding Chemical Compounds

Chemical compounds are substances formed when two or more elements combine and bond together. In a compound, the elements are present in a fixed ratio and the compound has properties different from those of its constituent elements. Phosphorus, for example, can combine with hydrogen and oxygen to create various compounds, each with unique characteristics and properties.

It's essential to recognize these compounds are built upon the principle of chemical stability. Atoms in the compound will share, donate, or take on electrons in such a way that the electron configuration is as stable as possible, often resembling that of the nearest noble gas in the periodic table. Stable electron configurations typically mean that atoms will have full outer shells, leading to the formation of compounds through ionic or covalent bonds.

Balancing Charges in Compounds

Balancing charges in compounds is central to understanding chemical reactions and creating stable molecules. Atoms combine to form compounds where the total charge must equal zero, which requires an equal sum of positive and negative charges.

In the case of phosphorus compounds, you'll often be balancing the +1 charge typically found on hydrogen ions with the -2 charge typically carried by oxygen ions. When figuring out how these charges balance, you must account for all atoms present. For example, in the compound \(\mathrm{H}_3\mathrm{PO}_4\), three hydrogen atoms contribute a total of +3 in positive charges, while four oxygen atoms contribute -8. Therefore, phosphorus must carry a +5 charge to neutralize the molecule.

Understanding how to balance charges can also be seen in the exercise where \(\mathrm{Mg}_2\mathrm{P}_2\mathrm{O}_7\) is formed, and magnesium's +2 charge must be counterbalanced by both phosphorus and oxygen to result in a neutral compound.

Determining Oxidation Numbers

Oxidation numbers or states represent how electrons are distributed among atoms in a molecule. The concept is significant when analyzing oxidation-reduction (redox) reactions or when balancing chemical equations. Phosphorus, being a versatile element, can exhibit a variety of oxidation states in different compounds, as seen in our initial exercise.

To determine the oxidation state of phosphorus or any element in a compound, follow these guidelines: List known oxidation states for common elements like hydrogen (+1) and oxygen (-2), use algebra to balance the overall charge of the compound, accounting for all atoms involved, and remember that the sum of oxidation states must equal the total charge of the molecule (which is zero for neutral compounds).

For instance, in \(\mathrm{PH}_3\), hydrogen, usually +1, is balanced by phosphorus, resulting in phosphorus having an oxidation state of -3 to neutralize the molecule. Similarly, in \(\mathrm{HPO}_3\), the +1 charge from hydrogen and the total -6 from three oxygens require phosphorus to have an oxidation state of +5.