Video Transcript
When reporting an experiment, a
student writes that “combustion is an exothermic reaction because energy is produced
in the form of heat.” Why is this description not
correct? (A) There are no energy changes
during an exothermic reaction. (B) Combustion is an endothermic
reaction. (C) Heat is not a form of
energy. (D) Energy cannot be produced. Or (E) exothermic reactions absorb
heat.
A key point to highlight is that
the question is asking why is this description not correct. Let’s analyze what the student
wrote to determine what exactly is incorrect about it. Generally, when discussing
combustion, it refers to a reaction where a substance reacts with oxygen. For example, burning pieces of wood
is a combustion reaction. We know that this releases energy
in the form of heat and light.
Combustion reactions, like the
combustion of methane, are also known to release energy, typically in the form of
heat. In this reaction, the bonds that
hold the two types of reactants, methane and oxygen, together are broken in order
for the carbon, oxygen, and hydrogen atoms to come together to form new bonds to
create the products carbon dioxide and water. The bonds in the products are
stronger than the bonds in the reactants. So, overall, we see a release of
energy or heat into the surroundings, which is why combustion reactions are good
examples of exothermic reactions.
A reaction is exothermic if more
energy is released to the surroundings than is taken in. The most common result of an
exothermic reaction is an increase in the temperature of the surroundings. Using this information, let’s start
eliminating answer choices.
As we just discussed, exothermic
reactions release energy or heat to the surroundings. It is endothermic reactions that
absorb heat. So, answer choice (E) is false and
is also eliminated as the correct answer for this question. As we have stated, heat is a form
of energy, which eliminates answer choice (C). Because combustion is an exothermic
reaction and not an endothermic reaction, answer choice (B) is false and can be
eliminated as a potential answer for this question.
Lastly, if heat is released or
transferred to the surroundings in an exothermic reaction, then an exothermic
reaction does in fact demonstrate an energy change, eliminating answer choice (A) as
well. This means the correct answer is
answer choice (D). Let’s briefly discuss why this is
the case.
While the first portion of the
student’s statement is correct because combustion really is an exothermic reaction,
the second part stating that this is because energy is produced in the form of heat
is not correct. This is because we know that energy
is never created or in other words produced or destroyed, only converted from one
form to another. As a result, a reaction cannot
produce energy. Therefore, why was the student’s
description incorrect? The answer is option (D), because
energy cannot be produced.
Which of the following is a more
accurate description of the combustion reaction? (A) Combustion is an endothermic
reaction because energy is produced in the form of heat. (B) Combustion is an exothermic
reaction because energy is not produced. (C) Combustion is an exothermic
reaction because chemical energy is converted to heat. (D) Combustion is an exothermic
reaction because heat is converted to chemical energy. Or (E) combustion is an endothermic
reaction because thermal energy is released.
This question is asking which of
the answer choices provided is the most accurate description of a combustion
reaction. We have established that exothermic
reactions involve an overall transfer of energy to the surroundings and that
combustion is an exothermic reaction. Based on this information, we can
eliminate answer choices (A) and (E) because they both say combustion is endothermic
when it is actually exothermic.
We have also established that
energy cannot be produced, only converted from one form to another. Using this information more
specifically to combustion reactions, when substances like methane react with oxygen
and combust, chemical potential energy or chemical energy, for short, is converted
into other forms of energy like heat. In fact, heat energy typically
makes up the majority of the energy converted from chemical energy. This is why we often simplify
descriptions of exothermic reactions as the release of heat.
Answer choice (B) states that
energy is not produced in an exothermic reaction. This is true, but it is also true
for all reactions. No reaction can produce energy. All reactions involve the
conversion of chemical energy to other forms of energy or vice versa. So, while answer choice (B) is true
in that energy is not produced in exothermic reactions, it is not the most accurate
description of a combustion reaction. Therefore, we can eliminate option
(B).
Answer choices (C) and (D) both
correctly describe combustion as exothermic and identify that energy is converted
from one form to another. However, only answer choice (C)
correctly identifies that in a combustion reaction, chemical energy is converted to
heat and not the other way around as indicated in answer choice (D). Therefore, which answer most
accurately describes the combustion reaction? The answer is option (C):
combustion is an exothermic reaction because chemical energy is converted to
heat.
