Video Transcript
An alcohol is ignited and a mixture of two different gases is collected. What conclusion could be drawn from this observation? (A) The reaction is endothermic. (B) The reaction is irreversible. (C) The reaction is reversible. (D) The reaction equation would use the symbol with a double arrow. (E) The organic alcohol contains water of crystallization.
The reaction in this equation begins with an alcohol. The alcohol is then ignited. In other words, we’ve given it enough energy to start a combustion reaction. In a combustion reaction, the thing that is combusting, in this case the alcohol, reacts with the oxygen from the air. The products of a combustion reaction are carbon dioxide and water, both in the gaseous form. These products match the two different gases that are collected at the end of the reaction, as described in the question.
An important thing to know about a combustion reaction is that it releases energy in the form of heat. We can think of a campfire as a continuous combustion reaction that releases heat to warm the surrounding area. Since this reaction releases energy, it’s an exothermic reaction as opposed to an endothermic reaction, which absorbs energy. From this piece of information, we can eliminate our first answer choice. Choice (A) the reaction is endothermic is incorrect because, as we’ve shown, the reaction is exothermic.
Another observation to make is that no alcohol is collected at the end of the reaction. We just collect the two gaseous products carbon dioxide and water. The absence of alcohol implies that this is an irreversible equation. In other words, the products can’t recombine to form the reactants. Unless we supplied a prohibitively large amount of energy, we could not take water and carbon dioxide to reform alcohol and oxygen. So, we can draw the conclusion from choice (B). This reaction, like all other combustion reactions, is irreversible. Since the formation of the products releases a lot of energy, it’s very difficult to recapture that energy to form the reactants once again. Because the reaction is irreversible, it cannot be reversible. So, we can eliminate choice (C) from consideration.
The double arrow symbol in choice (D) signifies a reversible reaction. Since this reaction is not reversible, it will not use this symbol. Lastly, we can eliminate choice (E) as well. If the alcohol contained water of crystallization, we would expect a solid product if it were ignited or evaporated. The leftover ionic compound in the crystal should show up. But since only gases are collected, that is not the case in this reaction.
So, looking at this reaction, what conclusion can be drawn? That’s the conclusion in choice (B). The reaction is irreversible.
