Question

Potassism dichromate oxidation of ethanol to acetic acid is the basis for the original breath alcohol sereen ing test ased by law enforcememt agencies to determine a person's blood alcohod content. The test is based on the difference in color between the dichromate ion (redidish orange) in the reagent and the chromium $$ \mathrm{CH}_{3} \mathrm{CH} \mathrm{OH}+\mathrm{Cr}_{2} \mathrm{O}_{7}{ }^{2}-\frac{\mathrm{H}_{2} \mathrm{YO}_{4}}{\mathrm{H}_{2} \mathrm{O}} $$ Ethanol Dichroanate ion (redutish exange)

Short answer

Short Answer: The potassium dichromate oxidation of ethanol (CH3CH2OH) to acetic acid (CH3COOH) involves a redox reaction between ethanol, dichromate ions (Cr2O7^2-), and hydrogen ions (H+). The balanced chemical equation for the reaction is: 3 CH3CH2OH + Cr2O7^2- + 14 H+ → 3 CH3COOH + 2 Cr^3+ + 7 H2O

Step-by-step solution

Write the unbalanced equation

First, we need to write the unbalanced chemical equation for the redox reaction between ethanol, potassium dichromate, and acid: $$ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} + \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + \mathrm{H}^{+} \rightarrow \mathrm{CH}_{3} \mathrm{COOH} +\mathrm{Cr}^{3+} + \mathrm{H}_{2} \mathrm{O} $$

Assign oxidation states

Before we can balance the redox reaction, we need to assign oxidation numbers to each atom involved in the reaction: $$ \begin{array}{|c|c|c|c|} \hline \text{Atom} & \text{Ethanol} & \text{Dichromate} & \text{Acetic Acid,Water} \\ \hline \mathrm{C} & -2 & & +2 \\ \hline \mathrm{H} & +1 & & +1 \\ \hline \mathrm{O} & -1 & -2 & -1, -2 \\ \hline \mathrm{Cr} & & +6 & +3 \\ \hline \end{array} $$

Balance the oxidation and reduction half-reactions

Now, we divide that chemical equation into two half-reactions: one for reduction and one for oxidation. Reduction half-reaction (Cr goes from +6 to +3 oxidation state): $$ \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + 14 \mathrm{H}^{+} + 6 e^{-} \rightarrow 2 \mathrm{Cr}^{3+} + 7 \mathrm{H}_{2} \mathrm{O} $$ Oxidation half-reaction (C goes from -2 to +2 oxidation state): $$ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \rightarrow \mathrm{CH}_{3} \mathrm{COOH} + 2 \mathrm{H}^{+} + 2 e^{-} $$

Balance the redox equation

In order to balance the redox equation, we need to combine these half-reactions so that the number of electrons lost equals the number of electrons gained. Since the oxidation half-reaction produces 2 electrons and the reduction half-reaction consumes 6 electrons, multiply the oxidation half-reaction by 3 to balance the electrons: $$ 3 \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \rightarrow 3 \mathrm{CH}_{3} \mathrm{COOH} + 6 \mathrm{H}^{+} + 6 e^{-} $$ Now, add the balanced half-reactions together: $$ 3 \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} + \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + 14 \mathrm{H}^{+} \mathrm{ }\rightarrow\mathrm{ } 3\mathrm{CH}_{3} \mathrm{COOH} + 2 \mathrm{Cr}^{3+} + 7\mathrm{H}_{2}\mathrm{O} $$ This is the balanced equation for the potassium dichromate oxidation of ethanol to acetic acid.

Key concepts

Redox Reaction

Redox reactions are fundamental to understanding chemistry, especially when it involves the transfer of electrons between chemical species. This type of reaction is characterized by the reduction (gain of electrons) of one substance and the oxidation (loss of electrons) of another. In the context of the exercise, we see potassium dichromate serving as the oxidizing agent, accepting electrons from ethanol which is being oxidized. The color change from reddish orange to green is a classic indicator of the dichromate ion receiving electrons and being reduced to chromium ions.

In learning about redox reactions, it is crucial for students to recognize the movement of electrons and how they influence the oxidation states of different atoms in the reaction. By doing so, one can predict the outcome of the reaction and the substances formed — key skills in understanding not just inorganic chemistry, but also numerous biological and environmental processes.

Ethanol Oxidation

Ethanol oxidation plays a vital role in many biological and chemical processes, including the breath alcohol screening test mentioned in our exercise. When ethanol undergoes oxidation, its chemical structure is altered, resulting in acetic acid — a compound with different properties and applications. For students, it's important to see the bigger picture: oxidation reactions involving organic compounds like ethanol often form part of metabolic pathways in living organisms, including the human body's processing of alcoholic beverages.

The oxidation of ethanol to acetic acid can be understood as the removal of hydrogen atoms from ethanol, which changes the oxidation state of the carbon atom involved. Furthermore, being able to write and balance the corresponding chemical equation is a valuable skill that showcases a student's understanding of the fundamental principles of chemical reactions and stoichiometry.

Chemical Equation Balancing

Balancing chemical equations is an essential skill in chemistry. It ensures the law of conservation of mass is respected, meaning the number of atoms for each element remains the same before and after the reaction. When it comes to redox reactions, balancing becomes slightly more complex as it involves not only the mass balance but also the charge balance. The exercise showcases the step-by-step process of balancing a redox reaction where ethanol is oxidized by potassium dichromate.

Students should first separate the reaction into half-reactions, balancing the atoms other than O and H, then balance O atoms by adding H₂O, H atoms by adding H⁺, and finally the charges by adding electrons. After that, the half-reactions can be combined, making sure electrons are balanced. This systematic approach simplifies the process and enables students to tackle even the most complex redox reactions confidently.

Oxidation Number Assignment

Assigning oxidation numbers to the atoms within a molecule or ion is a foundational concept for understanding redox processes. It involves giving a charge value to an atom based on a set of rules, primarily considering electronegativity and bond type. Oxidation numbers help clarify the specific changes that occur to atoms during chemical reactions and are crucial when balancing redox equations, as done in our exercise.

For students, grasping how to assign oxidation numbers is the first step in identifying which atoms are oxidized and which are reduced in a reaction. For instance, in the oxidation of ethanol, the carbon atom's oxidation number increases from -2 to +2, signifying its oxidation. Conversely, the chromium atom in the dichromate ion decreases from +6 to +3, indicating its reduction. Clear understanding and correct assignment of oxidation numbers not only support the student’s ability to balance redox reactions but also enhance the comprehension of a wide array of chemical phenomena.