Some substances are able to reduce
the activation energy. How would this affect the energy
changes during a chemical reaction? (A) More energy would be required
for the reaction to proceed. (B) Less energy would be required
for the reaction to proceed. (C) There would be an increase in
the total enthalpy change of the reaction. (D) There would be a decrease in
the total enthalpy change of the reaction. (E) The reaction would release
energy to the surroundings, instead of absorbing energy.
This question is asking us about
activation energy. We can best understand activation
energy by looking at a reaction profile, which shows the energy level of the
molecules in the reaction from beginning to end. In the reaction profile, the
activation energy is the difference between the energy level of the reactants and
the energy level of the high-energy transition state in the middle of the
reaction. The activation energy is the
minimum amount of energy required for a reaction to occur. If less energy than this is added,
the reaction will not proceed.
As the question indicates, some
substances are able to reduce the activation energy. We call these substances
catalysts. In the presence of a catalyst, the
activation energy is reduced. In other words, the energy barrier
that the reaction needs to reach in order to proceed is lowered. Looking at our answer choices, this
description matches answer choice (B): less energy would be required for the
reaction to proceed. The catalyst changes the energetics
of the reaction and allows it to happen using less energy.
To be thorough, let’s eliminate the
other answer choices as well. Answer choice (A) is incorrect, as
it is the opposite of the observed effect of the catalyst. Answer choices (C) and (D) are
incorrect as well. The total enthalpy change, or Δ𝐻,
is the difference in energy between the reactants and the products. Since the catalyst only changes the
energy of the transition state, it does not affect the total enthalpy change from
the beginning to the end of the reaction.
Lastly, choice (E) is incorrect as
well. As we just noted, the catalyst does
not change the energy levels of the reactants or products, so it would not affect
whether the reaction gains or loses energy. Catalysts can be simple molecules
or complex biological structures. Vanadium oxide is a simple molecule
that is used as a catalyst in a variety of industrial processes, while amylase is an
example of a biological catalyst, or enzyme, that our bodies use to more efficiently
carry out necessary chemical reactions.
So, how would this affect the
energy changes during a chemical reaction? (B) Less energy would be required
for the reaction to proceed.