Factors affecting reaction rates | Kinetics | AP Chemistry | Khan Academy

- [Instructor] There are several factors

that can affect the rate of a reaction.

One factor is the
concentration of a reactant.

Most chemical reactions proceed faster

when the concentration of one
of the reactants is increased.

For example, let's look at
the reaction of solid zinc

with hydrochloric acid to
form an aqueous solution

of zinc chloride and hydrogen gas.

Let's say we put a piece of zinc metal.

So go ahead and draw in the
piece of zinc metal in here

in a flask that contains
three molar hydrochloric acid.

So in our flask here, we have three molar,

a solution of three
molar hydrochloric acid.

As the reaction proceeds,
hydrogen gas is formed.

So we could monitor the
rate of this reaction

by observing the amount
of hydrogen gas bubbles

that are coming out of the flask.

Let's say that we repeat the experiment.

This time instead of using
three molar hydrochloric acid

we're gonna use six
molar hydrochloric acid.

So we've increased the concentration

of our hydrochloric acid solution.

This time, when we add
our piece of solid zinc

to our six molar
hydrochloric acid solution,

we would observe more hydrogen gas bubbles

coming out of our flask.

So we have increased the concentration

of one of our reactants hydrochloric acid.

And we've observed an increase
in the rate of the reaction.

As the concentration of
hydrochloric acid increases,

there are more acid particles to collide

with the piece of zinc.

And therefore, as the concentration

of hydrochloric acid goes up,

the frequency of collisions increase

and the rate of the reaction increases.

Let's use the same reaction

to talk about another factor

that affects the rate of reaction.

And that factor is surface area.

You've already reacted
a piece of solid zinc

with hydrochloric acid.

We could use three molar
hydrochloric acid again.

And we saw some bubbles come
off of the piece of zinc

indicating that hydrogen gas was produced.

This time let's try
breaking the piece of zinc

into smaller pieces.

So instead of using one
large piece of zinc,

here we have a bunch of
small pieces of zinc.

And if we were to do the experiment again,

with three molar hydrochloric acid,

this time we would see more
bubbles of hydrogen gas

coming off of those small pieces of zinc.

So we have increased the
surface area of the solid

and we observed an increase
in the rate of the reaction.

When we had only one piece of zinc,

the rate of the reaction was limited

by the surface area of this one piece.

Therefore by breaking it
up into smaller pieces,

we were able to the rate of the reaction.

Temperature is another factor

that can affect the rate of a reaction.

So let's say on the left,
we have a glow stick

in a flask that contains some cold water.

And the glow stick,

let's say the glow stick
is already glowing here.

So let's draw in this color
here for our glow stick.

The glow from a glow stick

comes from a chemical reaction.

And if we were to
increase the temperature,

so let's say we heated
this flask on the left.

So we increase the temperature.

We would observe the glow to get stronger.

So let's go ahead and draw in here,

a more vibrant glow coming
from our glow stick.

So increasing the temperature

must have increased the
rate of the reaction.

The reason increasing the temperature

increases the rate of
the reaction in general,

is because increase in temperature means

the molecules are moving faster.

And therefore the molecules
are colliding with each other

with greater frequency
and with greater force

which increases the rate of the reaction.

The catalyst is another factor

that can affect the rate of a reaction.

Let's look at the balanced
equation for the decomposition

of hydrogen peroxide, which
turns into water and oxygen.

And let's say in our flask on the left,

we have a solution of hydrogen peroxide.

The hydrogen peroxide is
decomposing at room temperature,

but the reaction proceeds so slowly

that we don't see it even happening.

We can speed up the reaction
by adding a catalyst.

Let's say we have an aqueous
solution of potassium iodide

in our beaker here.

And we pour the solution
of potassium iodide

into our flask containing
the hydrogen peroxide.

The addition of the
iodide ion as a catalyst,

causes the decomposition
of hydrogen peroxide

to occur very quickly.

And we would see a huge plume of gas

come out of the reaction flask.

So the addition of a catalyst,

in this case it was the iodide, an ion,

increased the rate of the reaction.

A catalyst increases the rate of reaction

by effecting the kinds of collisions

that occur between particles.

And a catalyst increases
the rate of reaction

without being used up.