Question

Use oxidation states to identify the oxidizing agent and the reducing agent in the redox reaction. $$ 2 \mathrm{Na}(\mathrm{s})+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NaOH}(a q)+\mathrm{H}_{2}(g) $$

Short answer

The reducing agent is sodium (Na), and the oxidizing agent is water (H2O).

Step-by-step solution

Assign Oxidation Numbers

Determine the oxidation states of each element in the reactants and products. In Na(s), sodium has an oxidation state of 0 since it is a pure element. In H2O, hydrogen has an oxidation state of +1 and oxygen has an oxidation state of -2. In NaOH, sodium has an oxidation state of +1, oxygen of -2, and hydrogen of +1. In H2(g), hydrogen has an oxidation state of 0.

Identify the Changes in Oxidation Numbers

Compare the oxidation states of elements in the reactants and products to identify the changes. Sodium's oxidation state changes from 0 to +1, which means it is oxidized. Hydrogen's oxidation state in water changes from +1 to 0, which means it is reduced.

Identify the Oxidizing and Reducing Agents

The species that is oxidized (loses electrons) is the reducing agent, and the species that is reduced (gains electrons) is the oxidizing agent. Sodium (Na) is the reducing agent because it is oxidized, and water (H2O) acts as the oxidizing agent because it is the species that contains the element hydrogen, which is reduced.

Key concepts

Oxidation States

Oxidation states, also known as oxidation numbers, play a critical role in understanding chemical reactions, especially those involving electron transfer. An oxidation state is a theoretical charge that an atom would have if all bonds to atoms of different elements were completely ionic. Assigning oxidation states to elements in a compound helps us keep track of electron movement during chemical reactions.

For elements that exist in their pure form, the oxidation state is zero. This is the case with Na(s) in our exercise. In compounds, the oxidation states differ. For instance, oxygen typically has an oxidation state of -2, and hydrogen, when bonded with non-metals, is usually +1. Determining the oxidation state of each element in a reaction is the first step to identify which species gets oxidized and which gets reduced.

Redox Reaction

A redox (reduction-oxidation) reaction involves the transfer of electrons between two species. It encompasses two complementary processes: oxidation and reduction. Oxidation is the loss of electrons, while reduction is the gain of electrons. Every redox reaction consists of two halves - one half involves the oxidation process, and the other involves the reduction process.

Our example reaction between Na(s) and H2O(l) to form NaOH(aq) and H2(g) is a classic redox reaction. In this process, one species loses electrons (Na) and another gains them (H in H2O). Understanding redox reactions is vital for a multitude of scientific disciplines, including electrochemistry, biochemistry, and environmental science.

Assigning Oxidation Numbers

To assign oxidation numbers, one must follow a set of guidelines. Elements in their pure form, like Na(s) and H2(g), have an oxidation number of 0. For ions that consist of a single atom, the oxidation number is equal to the charge of the ion. When dealing with molecules, we assign oxidation numbers based on electronegativity - the more electronegative atom gets a negative oxidation number.

Here's how you would assign oxidation numbers in our example: Na in Na(s) is 0, H in H2O is +1, O in H2O is -2, Na in NaOH is +1, H in NaOH is +1, and H in H2(g) is 0. These numbers give us a clear overview of which atoms undergo oxidation or reduction.

Identifying Changes in Oxidation Numbers

Once oxidation numbers are assigned, we look for changes in these numbers from the reactants to the products to identify which atoms are oxidized and which are reduced. An increase in the oxidation state signifies oxidation, while a decrease indicates reduction.

In our reaction, sodium's oxidation number increases from 0 to +1, meaning it loses electrons and is oxidized. Concurrently, hydrogen in H2O goes from an oxidation state of +1 to 0 in H2(g), indicating it gains electrons and is reduced. These changes are the essence of a redox process, where electron transfer and changes in oxidation states manifest as chemical change.