### Video Transcript

The diagram below shows four
different reactions of a sample of metal with an acid. From slowest to fastest, which of
the following best predicts the order for the rate of reaction? Assume the total mass of the metal,
the concentration of the acid, and the temperature are the same for all
reactions. (A) A, C, B, D; (B) D, B, C, A; (C)
C, B, A, D; (D) B, D, C, A; or (E) D, A, C, B.

Each of the beakers shown
represents a reaction between a metal and an acid. We are told that the total mass of
the metal, the concentration of the acid, and the temperature are the same for all
of the reactions. But for each of the reactions, the
metal has been divided into a different number of pieces. We need to determine how this will
affect the rate of reaction, then list the reactions in order from slowest to
fastest.

Rate of reaction is a measure of
how reactant or product concentration, mass, or volume changes over time. In more simple terms, rate of
reaction can be thought of as the speed of a chemical reaction. Chemical reactions occur when
particles successfully collide with one another. If we can increase the number of
collisions between particles, then the chance of a successful collision increases
and the rate of reaction increases. In other words, increasing the
number of collisions leads to a faster reaction. Let’s consider a reaction between
orange and pink particles. In this example, the pink substance
is a solid with tightly packed particles.

In order for a reaction to occur,
the orange particles must successfully collide with the pink particles. Initially, the orange particles can
only collide with the outermost pink particles. The innermost particles can’t react
until the outermost particles have reacted, but let’s consider what would happen if
we cut the solid in half. The total number of pink particles
is the same. But by dividing the solid in half,
we have increased the number of outermost particles which can initially react. If we divide the solid again, then
even more pink particles can react with the orange particles. Each time the solid was divided,
the number of particles, which were initially available to react, increased. So the number of successful
collisions would increase and the rate of reaction would increase.

By dividing the solid, what we’ve
actually done is increase the surface area of the solid. So increasing the surface area
increases the number of collisions, which increases the rate of reaction. So the slowest reaction between a
metal and an acid will be the one in which the metal has the smallest surface
area. And the fastest reaction will be
the one in which the metal has the largest surface area. So to rank the reactions from
slowest to fastest, we should list the reactions in order from smallest metal
surface area to largest metal surface area.

The metal used in reaction A has
the smallest surface area, so this reaction should be the slowest. Dividing the solid into two pieces
increases the surface area, so reaction C should be faster than reaction A. The more times the metal is
divided, the larger the surface area becomes. So the metal in reaction B, which
has been divided into 20 pieces, should have a larger surface area than the metal in
reaction C. And the metal in reaction D, which
has been divided into 80 pieces, should have an even larger surface area. So we should expect that reaction B
is faster than reaction C and reaction D is the fastest of them all.

We have determined that the
reactions from slowest to fastest are A, C, B, D, which corresponds to answer choice
(A).