Video Transcript
Which of the following reactions is not an indication of the acidity of a carboxylic acid? (A) Its reaction with sodium. (B) Its reaction with sodium hydroxide. (C) Its reaction with ethanol. (D) Its reaction with calcium oxide. (E) Its reaction with sodium carbonate.
The question is asking us to determine which of the reactions listed in the answer choices is not an indication of the acidity of a carboxylic acid. One definition of acidity is the capacity of a substance to donate hydrogen ions to other substances. A carboxylic acid, shown here with its general formula RCOOH, reacts with water to form a carboxylate anion and the hydronium H3O+. In this reaction, the carboxylic acid is demonstrating acidity by donating a hydrogen ion to a water molecule.
It’s helpful to recall that carboxylic acids demonstrate acidity when they react with active metals and with bases. Because the question is asking us to find a reaction that does not indicate acidity, we will need to eliminate the answer choices in which carboxylic acid molecules are donating hydrogen ions during the reaction.
In choice (A), a carboxylic acid is reacting with sodium. Sodium, or Na, is an alkali metal, making it a very reactive metal. In fact, according to the reactivity series for metals, sodium is one of the most reactive metals. Metals that appear higher than hydrogen on the reactivity series tend to react with carboxylic acids.
Let’s go ahead and write a balanced equation for the reaction of a carboxylic acid with sodium metal. When two carboxylic acid molecules react with two sodium atoms, two hydrogen ions are released, forming diatomic hydrogen. Because the carboxylic acid molecules released hydrogen ions to form a new substance, this is a demonstration of acidity, so (A) is not a correct answer choice.
In choice (B), a carboxylic acid is reacting with sodium hydroxide. Sodium hydroxide is a metal hydroxide base. Let’s write a balanced chemical equation for the reaction of a carboxylic acid with sodium hydroxide. During the reaction, a hydrogen ion is released from the carboxylic acid joining together with a hydroxide ion to form a molecule of water. This is a demonstration of acidity, so choice (B) is not a correct answer choice.
In choice (C), a carboxylic acid is reacting with ethanol. Ethanol is an alcohol and has a condensed chemical formula of CH3CH2OH. When a carboxylic acid reacts with ethanol, an ester and water are produced. During the reaction, the carbon-oxygen single bond of the carboxylic acid and the oxygen-hydrogen single bond of the alcohol are broken. An ester and water are produced. The important point here is that the carboxylic acid molecule does not release a hydrogen ion during the reaction, so this reaction does not demonstrate acidity. (C) is our correct answer choice.
In choice (D), a carboxylic acid is reacting with calcium oxide or CaO. Calcium oxide is a metal oxide base. Let’s write a balanced chemical equation for this reaction. During the reaction, two hydrogen ions are released from the carboxylic acid molecules to combine with an oxide ion to form a molecule of water. This is a demonstration of acidity, so choice (D) is not a correct answer choice.
Finally, in choice (E), a carboxylic acid is reacting with sodium carbonate. Sodium carbonate or Na2CO3 is a metal carbonate base. Let’s write a balanced chemical equation for the reaction. During the reaction, two hydrogen ions are released from the carboxylic acid molecules to combine with the carbonate ion, forming a molecule of carbon dioxide and water. This is a demonstration of acidity, so choice (E) is an incorrect answer choice.
Which of the following reactions is not an indication of the acidity of a carboxylic acid? Choice (C), its reaction with ethanol.
