Video Transcript
Which of the following processes is
not an endothermic reaction? (A) Cooking food, (B)
photosynthesis, (C) evaporation, (D) freezing, or (E) thermal decomposition.
This question is about endothermic
reactions. Four of the answer choices will be
endothermic reactions, while the one that is not will be an exothermic reaction. An endothermic reaction is a
reaction where energy goes into the system. In other words, energy is
absorbed. In exothermic reactions, energy
goes out of the reaction. In other words, energy is
released.
Let’s take a look at the answer
choices to see which ones are endothermic and which one is exothermic. It’s worth noting that three of our
answer choices involve the substance being heated, often by a flame. For these reactions, the energy
often, but not always, takes the form of heat. For all three of these reactions,
the energy is going into the system, making them endothermic reactions. So we can eliminate choices (A),
(C), and (E) from consideration.
Choice (B), photosynthesis,
involves plants absorbing sunlight through their leaves to produce sugars. In this situation, the Sun’s energy
is added into the chemical reaction that takes place in the leaf. Since the plant absorbs energy,
this process is endothermic and not the correct answer to the question.
An example of the freezing process
would involve placing a water bottle in a freezer. While we might typically
conceptualize this process as the bottle getting colder, another way to think about
it from an energetics perspective is that the water is releasing its energy in the
form of heat to its surroundings. Since the water is releasing
energy, the process of freezing is exothermic.
Choice (D), freezing, is the
correct answer. It is the only one of these five
processes that involves releasing energy rather than absorbing it. So, which of the following
processes is not an endothermic reaction? That’s choice (D), freezing.
