Video Transcript
A student is measuring how the
temperature affects the rate of reaction when magnesium metal reacts with dilute
hydrochloric acid. Which of the following variables
should not be kept constant during this experiment? (A) The temperature of the
acid. (B) The concentration of the
acid. (C) The volume of the acid
added. (D) The size of the magnesium
strips. Or (E) the mass of magnesium.
The speed at which a chemical
reaction takes place is known as the rate of reaction. We can define the rate of reaction
as the change in reactant or product concentrations per unit of time. Many different factors can affect
the speed of a reaction.
One of these factors is
temperature, which is well known to affect the rate of reaction. For a chemical reaction to take
place between particles, the particles need to collide with enough energy. At higher temperatures, the
particles have greater energy, which cause them to move more quickly. This generally increases the number
of successful collisions and the rate of the reaction.
Concentration also affects the rate
of reaction. A sample with a higher
concentration has a greater quantity of particles in a given volume. This generally results in more
frequent collisions and a faster rate of reaction. Changing the volume of acid may
affect the overall amount of hydrogen gas produced. However, it would not likely change
the rate. If we change the size of the
magnesium strips, we would be altering the surface area of this reactant. An increased surface area increases
the rate of reaction as it increases the opportunity for particle collisions.
If we were to change the mass of
magnesium, we would alter the amount and surface area of a reactant. The student in this question is
measuring how the temperature affects the rate of a reaction. In order to measure how temperature
affects the rate of this reaction, it would be necessary to keep all of the other
variables constant.
Therefore, the variable that should
not be kept constant during this experiment is answer choice (A), the temperature of
the acid.
