Which of the following factors does not affect the rate of a chemical reaction? (A) The concentration of reactants in a solution, (B) the pressure of reacting gases,
(C) the surface area of solid reactants, (D) the reversibility of the reaction, (E)
the presence of a catalyst.
According to collision theory, reactant particles must collide with sufficient energy
in order to form the products. This means that any factor that causes more collisions to occur between particles
will cause a chemical reaction to occur more quickly. In other words, the rate of the chemical reaction will increase. The rate of a chemical reaction describes how quickly the reactants are converted
into the products. This question wants us to determine which factor will not affect the rate of
Let’s start off with statement (A). Here, we have two solutions. On the left is a solution with a low concentration of reactants. And on the right, we have a solution with a high concentration of reactants. The particles in both solutions will move around in the container, resulting in
collisions with the container and with the other particles in the solution. Since there are more particles in the solution with a higher concentration, those
particles collide with each other more frequently. As a result, the solution with the higher concentration will have a greater rate of
reaction. So the concentration of reactants in a solution does affect the rate of a chemical
reaction, so answer choice (A) is not the right answer.
Now let’s move on to answer choice (B), the pressure of reacting gases. Say we have a container full of gas. There are two ways to change the pressure of the gas in the container. One way we can change the pressure is by changing the volume. If we decrease the volume of the container, the pressure of the gas will be
higher. Since the gas now occupies a smaller space, there will be more collisions between the
gas particles. As there are more collisions occurring between the particles, the reaction between
these two gases will occur more quickly.
Another way to increase the pressure of a gas is to increase the temperature. This will cause the gas particles to move more quickly. If the particles are moving faster, there will be more collisions between them. This will result in a faster reaction. So we’ve seen that the pressure of reacting gases can certainly affect the rate of a
chemical reaction, so answer choice (B) is not correct.
Answer choice (C) is the surface area of solid reactants. If we have the same mass of a solid, more solid particles are exposed when the
surface area is large. Particles that are not exposed to the surface cannot react until they are
exposed. This means a chemical reaction will occur more quickly if the surface area is
larger. So answer choice (C) isn’t the correct answer.
Now let’s think about statement (D), the reversibility of the reaction. If a reaction is reversible, it means the reaction can go in both the forwards and
backwards direction. In the forward reaction, the reactants form the products. In the backward reaction, the products form the reactants. When a reaction is reversible, the forward and backward reaction generally occur at
the same time until equilibrium is reached.
The factors we’ve discussed can influence how fast a reversible reaction reaches
equilibrium. However, the reversibility of a reaction does not affect the rate of the
reaction. So it seems that this is the correct answer.
But before we conclude this video let’s look at answer choice (E), the presence of a
catalyst, to make sure we understand this topic.
Catalyst affect the rate of a chemical reaction differently than the other factors
that we’ve looked at. We’ve said that a reaction occurs when particles collide with sufficient energy. When a catalyst is used, less energy is needed to convert the reactants into the
products. This increases the rate of reaction because more collisions result in a successful
reaction. So the presence of a catalyst does affect the rate of a chemical reaction.
The only factor we were given that does not affect the rate of a chemical reaction is
answer choice (D), the reversibility of the reaction.