Video Transcript
In this video, we will learn how to
describe and explain the effect temperature and concentration have on the rate of
chemical reactions.
In a chemical reaction, reactants
are converted into products. As the reaction occurs, the
concentration of the reactants will decrease and the concentration of the products
will increase. The rate of reaction measures how
reactant or product concentration, mass, or volume changes per unit of time. This represents the speed at which
a chemical reaction takes place.
On a graph of concentration versus
time, the lines on the graph represent the rate of reaction. The steeper the line, the faster
the rate of reaction. The rate of reaction depends in
part on the number of collisions between particles. In order for two particles to
react, they need to collide with one another with a certain amount of energy. The greater the number of
collisions, the more likely a reaction will occur, and the rate of reaction will
increase. In this video, we will focus on two
factors that can increase the number of collisions between particles and therefore
increase the rate of reaction.
Let’s think about temperature. When we increase the temperature of
a substance, the particles gain energy and move faster. The faster the particles move, the
more collisions they are likely to have. So increasing the temperature will
increase the number of collisions between particles. And as the number of collisions
increase, the rate of reaction will increase. So increasing the temperature will
increase the rate of reaction. Let’s see how we can prove this to
be true with an experiment.
In this experiment, we will place
an effervescent tablet in water. When the tablet reacts with the
water, carbon dioxide gas will be produced. We can measure the volume of carbon
dioxide produced with a gas syringe. We’ll perform the experiment with
room-temperature water and boiling water. We will be able to see that the
experiment using boiling water will bubble more rapidly, the gas syringe will fill
more quickly, and the tablet will dissolve more quickly than the experiment using
room-temperature water. All of these signs indicate that
the rate of reaction at a higher temperature was faster. Eventually, both tablets will have
completely dissolved, the reactions will end, and the same volume of gas will be
produced.
Let’s analyze the results of these
experiments. We can see from the plots that
initially the line on the graph for the boiling water experiment is steeper than the
line for the room-temperature water experiment. So the rate of reaction using
boiling water is faster. We can also see that although the
reaction with boiling water is faster than the reaction with room-temperature water,
both reactions produced the same volume of gas.
In our everyday lives, we take
advantage of the effect that temperature has on the rate of reaction. We store our food in the
refrigerator. The lower temperature slows down
the chemical reactions that spoil food, making our food last longer. When we cook an egg, we can
increase the temperature to increase the rate of the reactions that cause the egg to
cook. This will allow us to cook the egg
faster.
Now that we’ve looked at how
temperature affects rate of reaction, let’s think about concentration. Concentration is a measure of the
amount of a substance in a particular volume. Increasing the concentration means
that there is more substance in the same volume. We might refer to this as being
concentrated. Decreasing the concentration means
that there is less substance in the same volume. We might refer to this as being
dilute.
So how does concentration affect
rate of reaction? Let’s consider a reaction between
orange and pink particles. We know that the particles must
collide with the right amount of energy in order for a reaction to occur. So there will be times when the
particles collide and don’t react. If we increase the concentration of
pink particles, then there will be more pink particles available to collide with the
orange particles. The number of collisions will
increase, and the chance of a successful collision to form the product will also
increase. This means that increasing the
concentration will increase the rate of reaction. Let’s see how we could prove this
to be true with an experiment.
In this experiment, we will burn
iron wool, also called steel wool. When iron wool is burned, the iron
reacts with oxygen. If we burn the iron wool using a
Bunsen burner, the oxygen involved in the reaction is coming from the air. Air is about 21 percent oxygen. The iron wool will burn but
relatively slowly. But what if we increased the
concentration of oxygen by placing some lit iron wool in a flask containing 100
percent oxygen? The reaction in 100 percent oxygen
would be more intense and rapid. We would see that the light given
off is brighter, and the iron wool would stop burning more quickly than the iron
wool burned over the Bunsen burner. These signs indicate that
increasing the concentration increased the rate of reaction.
Let’s analyze the results of a
different experiment. In this experiment, magnesium metal
is placed into a flask containing hydrochloric acid. When these two substances react,
one of the products is hydrogen gas. We can measure the volume of
hydrogen gas produced by attaching a gas syringe to the flask. We can perform this experiment with
dilute and concentrated hydrochloric acid. We will see that the experiment
using concentrated hydrochloric acid will bubble more rapidly, the gas syringe will
fill more quickly, and the magnesium will be used up more quickly than in the
experiment using dilute hydrochloric acid. All of these signs indicate that
the rate of reaction was faster when a greater concentration of hydrochloric acid
was used. Eventually, the magnesium in both
experiments will be used up, the reactions will end, and the same volume of hydrogen
gas will be produced.
Let’s analyze the results of these
experiments. We can see from the plots that
initially the line on the graph for the concentrated hydrochloric acid experiment is
steeper than the line for the dilute hydrochloric acid experiment. So the rate of reaction using
concentrated hydrochloric acid is faster. We can also see that although the
reaction with concentrated hydrochloric acid was faster, both reactions produced the
same volume of gas.
We’ve examined how temperature and
concentration affect rate of reaction. Let’s apply our understanding to
some questions.
The boxes below each contain an
equal number of reactant molecules. The boxes are heated to different
temperatures. Which box will have the greatest
frequency of collisions between molecules?
In order for two reactant molecules
to react with one another, they have to collide. There are several factors that can
affect the number of collisions between reactant molecules. Two of these factors are
concentration and temperature. Concentration is a measure of the
amount of substance in a given volume. We are told, and can see, that each
box contains the same number of red and blue reactant molecules. So the concentration of reactant
molecules is the same in each box. But the temperature of each box is
not the same.
Let’s consider this picture of
orange and pink particles. At this temperature, each particle
moves at a particular speed and collisions between particles are occurring. As we increase the temperature, the
particles gain energy and move faster. The faster they move, the more
likely they are to collide. So the frequency of collisions will
increase. This means that the box which will
have the greatest frequency of collisions between molecules is the box which has the
highest temperature. Box (D), at 80 degrees, has the
highest temperature and will therefore have the greatest frequency of
collisions. So the correct answer is (D).
A chemist performs a series of
experiments to determine the effect of concentration on the rate of a reaction. They pour an equal amount of
hydrochloric acid of different concentrations into four test tubes. Then they place an identical piece
of magnesium ribbon into each of the test tubes. The experiment setup is shown
below. From slowest to quickest, what is
the likely ordering of the rate of reaction for the four experiments? (A) A, C, D, B. (B) B, C, A, D. (C) C, D, B, A. (D) D, C, A, B. Or (E) C, A, B, D.
In each of the test tubes, an
identical piece of magnesium ribbon is submerged in the same volume of hydrochloric
acid. The test tubes differ in the
concentration of hydrochloric acid used, given here in moles per liter. Concentration is a measure of the
amount of substance in a given volume. We need to determine how the
different concentrations of hydrochloric acid in each test tube will affect the rate
of reaction. Let’s clear some space at the top
of the screen.
The rate of a reaction measures how
reactant or product concentration, mass, or volume changes per unit of time. We can think of this as the speed
of a chemical reaction. In order for a chemical reaction
between two particles to occur, the particles must collide with one another with a
certain amount of energy. This means that some collisions
won’t result in a reaction. But if we can increase the total
number of collisions, we can increase the chance that a reaction will occur, thus
increasing the rate of reaction.
So how does changing the
concentration affect rate of reaction? Let’s consider the sample of pink
and orange particles. These particles will have a certain
number of collisions with one another. Increasing the concentration of
pink particles means that there will be more pink particles available to collide
with the orange particles. The number of collisions will
increase, and the rate of reaction will increase. So increasing the concentration
increases the rate of reaction.
We want to order the experiments
from slowest to quickest. If increasing the concentration
increases the rate of reaction, then we should put the experiments in order from
lowest concentration to highest concentration. At 0.1 moles per liter, (B) has the
lowest concentration, followed by (C), then (A), then (D), with the highest
concentration at 5.0 moles per liter. So the correct answer for the
likely ordering of the rate of reaction for the four experiments from slowest to
quickest is answer choice (B) B, C, A, D.
Let’s wrap up this video by
reviewing what we’ve learned. The rate of a reaction measures how
reactant or product concentration, mass, or volume changes per unit of time. We can think of this as the speed
of a chemical reaction. Reactant particles must collide
with one another in order for a chemical reaction to occur. In general, as the number of
collisions between the reactant particles increases, the rate of reaction
increases.
As the temperature increases, the
particles gain more energy and the number of collisions increases. This causes the rate of reaction to
increase. We discussed how effervescent
tablets react faster with warmer water than colder water.
Increasing the concentration
increases the number of particles present, so the number of collisions
increases. This increases the rate of
reaction. We saw how iron wool burns faster
in pure oxygen than in air because the concentration of oxygen is lower in air. We also saw that magnesium reacts
faster with concentrated hydrochloric acid than with dilute hydrochloric acid.
