Problem of the Week

Chapter 20: Redox Reactions

Problem of the Week

Alcohol in the Blood
Ethanol, C₂H₅OH, is also known as ethyl alcohol. This clear colorless liquid is produced by the fermentation of fruits and grains. Except for alcoholic beverages, nearly all the ethanol used industrially is a mixture of 95% ethanol and 5% water. Ethanol is a primary alcohol and the oxidation of ethanol produces acetaldehyde, also known as ethanal. When ethanol is consumed, this reaction takes place in the liver and acetaldehyde builds up in the blood. Ethanol is one of few easily vaporized 'drug' molecules in the blood stream. Any ethanol that an individual drinks shows up in the breath because this alcohol can be absorbed from the mouth, throat, stomach, and intestines into the blood stream. Because ethanol is not digested before it is absorbed, it is not chemically changed in the bloodstream. As the blood moves through the individual's lungs, some of the ethanol moves across the alveoli into the air because ethanol is volatile. The concentration of ethanol in the alveolar air is related to the concentration of alcohol in the blood. Ethanol in the alveolar air when exhaled can be detected by breath alcohol testing devices.

A Breathalyzer is a device that is used to determine the amount of alcohol in an individual's blood stream. Individuals are asked to blow into a Breathalyzer. The exhaled alveolar air is used to determine the concentration of alcohol in the individual's blood stream. The ratio of breath-alcohol to blood-alcohol is 2,100:1. This means that 2,100 milliliters (ml) of alveolar air will contain the same amount of alcohol as 1 ml of blood. The device has a mouthpiece, a tube through which the individual exhales air, and a sample chamber where this air sample in analyzed. The breath sample is bubbled through a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The sulfuric acid removes the alcohol from the air sample and converts it into a liquid solution. Sulfuric acid also provides the acidic conditions necessary for the reaction to take place. Silver nitrate is a catalyst in this reaction and the dichromate ion is the reactant that undergoes a color change when it reacts with ethanol. The degree of color change is directly related to the amount of ethanol in the air sample. Dichromate reagents are a dark orange-red color and are chemically transformed into chromium III ions, which are green. Potassium permanganate is another reactant that is sometimes used. Permanganate ions are purple and turn colorless with the oxidation of ethanol. These color changes are the basis of the Breathalyzer.

Consider the reaction above. Notice the oxidation states of the carbon atoms in the ethanol molecule, the acetaldehyde molecule, and the acetic acid molecule. Determine if the ethanol is oxidized or reduced and explain your answer. Write half reactions for the reactions: a. ethanol → acetaldehyde, and b. acetaldehyde → acetic acid, and c. ethanol → acetic acid.

One way to determine the alcohol content in wine involves using a solution that contains dichromate ions. The equation for this reaction between ethanol and dichromate ions in an acidic environment is as follows:

CH₃CH₂OH(g) + Cr₂O₇^2-(aq) + H+(aq) → CH₃CO₂H(aq) + Cr₃+(aq) + H₂O(l)

a. Determine the oxidation states of each atom involved.
b. Write the half reaction for the oxidation reaction.
c. Write the half reaction for the reduction reaction.
d. Determine the oxidizing agent and the reducing agent
e. Balance the reaction in an acidic environment.