The blood alcohol level of a person can be detected by reacting a sample of blood plasma with dichromate ion, \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\), which takes part in an electron-transfer reaction with ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), in the blood: \(2 \mathrm{Cr}_{2} \mathrm{O}_{7}{ }^{2-}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}+16 \mathrm{H}^{+} \rightarrow\) \(4 \mathrm{Cr}^{3+}+2 \mathrm{CO}_{2}+11 \mathrm{H}_{2} \mathrm{O}\) Assign an oxidation state to each atom in this reaction and indicate the oxidizing agent and the reducing agent.

Understanding oxidation states is crucial when studying chemical reactions, especially in oxidation-reduction (redox) processes. An oxidation state, also referred to as an oxidation number, is an indicator of the degree of oxidation or reduction of an atom in a chemical compound. It is a hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic.

Oxidation states help us track how electrons are transferred in a reaction. The key rule is that the sum of oxidation states for all atoms in a molecule or an ion must equal the overall charge. For example, in the dichromate ion \(\mathrm{Cr}_{2}\mathrm{O}_{7}^{2-}\), oxygen atoms are typically assigned an oxidation state of -2 due to their higher electronegativity. Since the overall charge of the ion is -2 and there are seven oxygen atoms, chromium must balance this with a +6 oxidation state for each chromium atom.

Using oxidation states, we can determine which elements are oxidized or reduced during a reaction. An increase in oxidation state signifies oxidation, while a decrease indicates reduction. In the provided exercise, ethanol \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) increases its carbon's oxidation state from -2 to +4, undergoing oxidation, while chromium in the dichromate ion decreases its oxidation state from +6 to +3, undergoing reduction.

An oxidizing agent, also known as an oxidant or electron acceptor, is a substance that gains electrons and becomes reduced in a redox reaction. It 'oxidizes' the other substance by taking electrons away from it. These agents are key to driving the oxidation process of other reactants in a chemical reaction.

In the blood alcohol detection reaction, the dichromate ion \(\mathrm{Cr}_{2}\mathrm{O}_{7}^{2-}\) serves as the oxidizing agent. It accepts electrons from ethanol during the reaction, as indicated by the decrease in chromium's oxidation state from +6 to +3. Oxidizing agents are typically characterized by having high electronegativity, a high oxidation state, or a vacant d-orbital, which allows them to receive electrons.

In contrast to oxidizing agents, reducing agents, or reductants, are substances that lose electrons and become oxidized in the process. They 'reduce' other substances by donating electrons. A strong reducing agent has a higher tendency to lose electrons, thus providing them to other reactants requiring electron gain.

For the exercise in question, ethanol \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) acts as the reducing agent. It donates electrons to the dichromate ion, which is evident from the increase in the oxidation state of carbon within the alcohol from -2 (in ethanol) to +4 (in carbon dioxide). Reducing agents often have low electronegativity and a low oxidation state, which enables them to transfer electrons to other substances.

Electron-transfer reactions, fundamental to redox reactions, involve the movement of electrons from one reactant to another. These reactions play a vital role in many biochemical, industrial, and environmental processes. The core characteristic of any electron-transfer reaction is that one substance transfers electrons to another substance.

The alcohol level in blood plasma is detected via an electron-transfer reaction between ethanol and the dichromate ion. During this process, the reactants undergo changes in their oxidation states. Ethanol loses electrons (as it is oxidized), and the dichromate ion gains electrons (as it is reduced). Such reactions are part of analytical methods used in alcohol detection and various other chemical analyses. Understanding these concepts allows us to comprehend how compounds interact at an atomic level and predict the products of chemical reactions.