Video Transcript
In this video, we’ll learn about
the speed or rate of a chemical reaction. We will describe the rate of
reaction, learn how it’s measured, and understand what factors affect the rate of a
reaction.
During a chemical reaction, a
substance or substances, which are called the reactants, are converted into new
substances, which are called the products. For example, hydrogen and oxygen
gas react to form a brand new substance, which is water. Chemical reactions occur all around
us all the time. There’s a combustion reaction that
occurs in our cars. Fuel is burned to power the
vehicle. There’s lots of chemical reactions
involved in making food, such as the Maillard reaction, which is responsible for the
browning of bread and other foods.
Chemical reactions are involved
when something gets bleached, when a metal rusts, and when diamond is formed. Chemical reactions occur all the
time inside our body. Our cells are constantly undergoing
cellular respiration, which converts glucose into energy. Not all of these chemical reactions
occur at the same speed. Some occur faster than we could
time with a stopwatch. Some reactions take minutes to
complete or hours. Some reactions take days or even
weeks to complete. Some reactions are even slower and
take hundreds of years to finish. As chemical reactions take place on
a range of time scales, we’d like to be able to measure their speed. To do this, we’ll have to
understand what’s going on to the reactants and products during a chemical
reaction.
Say we have a chemical reaction
where two reactants combine to form a product. We place our reactants in a
beaker. We can express the initial amount
of our reactant in terms of a concentration, usually given in units of moles per
liter. Initially, we won’t have any
product because none has been formed yet. But the reactant particles will
begin to collide with each other, which forms the product. The concentration of the product
increases because products are being formed. And the concentration of the
reactant decreases as it gets used up. Eventually, the concentration of
the products stop changing over time. When this happens, the reaction has
finished. We can see this took about 50
seconds for this reaction.
We’d like some way to measure the
speed of this chemical reaction. We’ll measure the speed of our
reaction similarly to how we’d measure the speed of a car. We determine the speed of a car by
measuring the change in distance per unit time. And the speed of our chemical
reaction will also be a change in something per unit of time. The thing that’s changing during a
chemical reaction isn’t the distance but the amount of the products and
reactants. This amount can be a mass, volume,
or some other measure. But the most common measure is
probably concentration like we saw in this graph. So the speed of a chemical
reaction, which we call the rate of reaction, is the change in the concentration of
the reactants and products per unit time.
We can measure the rate of reaction
by measuring the disappearance of a reactant or the formation of a product over
time. Let’s say we have a copper sulfate
solution, which is a bright blue. We add some sodium hydroxide to the
copper sulfate. After some time, the solution will
become colorless and a solid will form on the bottom of the beaker, which is copper
hydroxide. There are two ways we could measure
the rate of this reaction because the reactant, copper sulfate, is colored but the
product isn’t. We can easily measure the
disappearance of the reactant by measuring how long it took the solution to become
colorless. We can also measure the rate of
reaction by determining how much copper hydroxide formed after a certain period of
time.
There are several factors that
affect the rate of reaction, such as the nature of the reactants, the surface area
of the reactants, the concentration of the reactants, the temperature of the
reaction, and the use of catalysts. In this video, we’ll focus on how
the nature of the reactants and the surface area affect the rate of reaction. We’ll start off by looking at the
nature of the reactants. Specifically, it’s the type of
bonding in a compound that affects the rate of reaction. Compounds with ionic bonding, such
as sodium chloride, can disassociate into positive and negatively charged ions. These ions can react quickly. We can compare this to reactions
between compounds with covalent bonding, like hydrogen and oxygen. Here, the bonds between the atoms
and the reactant molecules must break so that bonds can form between the atoms and
the products.
The making and breaking of bonds
takes time. So reactions between covalently
bonded compounds are often much slower than reactions between compounds with ionic
bonding. The surface area also affects the
rate of reaction. In a solid, only particles on the
surface can react. Particles in the middle of the
solid can’t react, until after the particles on the surface have reacted. Say that we have two samples that
contain the same amount of a solid. In one sample, the solid is in one
piece, but in the other, it’s broken up into smaller pieces. When the solid is in smaller
pieces, it has a larger surface area and more of the particles are exposed and
available to react than when the solid is in a large piece. So the reaction will occur more
quickly when the solid is in smaller pieces. In other words, the larger the
surface area, the faster the reaction.
We can see this in the iron filing
experiment. In this experiment, we have two
flasks. In one, we put a piece of iron
ribbon. In the other, we put the same mass
of iron, but in the form of iron filings. We put some hydrochloric acid in
each flask and stopper them. Iron and hydrochloric acid react to
form iron chloride and hydrogen. Hydrogen is a gas. As it’s formed, it will produce
bubbles in the flask. We can collect this hydrogen gas in
a gas syringe. And we can plot the volume of
hydrogen gas produced over time. For both the iron filings and the
iron ribbon, we can see that both flasks produce the same amount of hydrogen
gas. This makes sense since we started
with the same mass of iron in each flask.
Comparing the two lines, we see
that they reach the maximum amount of hydrogen gas at different times. The flask with the filings reached
the maximum amount much faster than the flask with the ribbon. So the reaction occurred faster in
the flask with the filings than the flask with the ribbon. Now we’ve learned about the rate of
reaction. So let’s work some problems before
we conclude this video.
Which of the following statements
best defines the rate of a chemical reaction? (A) The measure of change in the
concentration of the reactants or products per unit of time. (B) The difference in mass between
the reactants and the products. (C) The final concentration of the
products following a chemical reaction. (D) The speed at which particles
need to move in order to successfully collide. (E) Time at which the concentration
of the products and reactants are equal.
The rate of a chemical reaction
tells us its speed, in other words whether the reaction will be fast or slow. To define this quantity, we should
know that a rate in general is a change in some quantity per unit time. For example, the rate or speed of a
car is the change in distance per unit time. But during a chemical reaction,
it’s not the distance that’s changing. It’s the amount of the reactants
and products. Over time, the amount of reactants
decrease as they’re used up. And the amount of the products
increases as they’re formed. We can measure this amount as a
volume, a mass, or a concentration. But the most common measure is
probably the concentration of the reactants and products. Given what we’ve talked about,
answer choice (A) seems to best define the rate of a chemical reaction: the measure
of change in the concentration of the reactants or products per unit of time.
The graph below shows the
concentration of oxygen gas produced during the following chemical reaction: two
H2O2 reacts to form two H2O plus O2. How long does this reaction take to
reach completion? (A) 90 seconds, (B) 120 seconds,
(C) 80 seconds, (D) 100 seconds, (E) 20 seconds.
In the reaction in this question,
hydrogen peroxide decomposes to form water and oxygen gas. The concentration of oxygen gas has
been graphed over time. We need to determine how long it
takes this reaction to reach completion. At the start of the reaction, the
concentration of oxygen is zero because no oxygen gas has been produced yet. Then, the concentration gradually
increases until the concentration stops changing when it reaches a value of one mole
per liter. When the concentration of the
reactants and products stop changing, it means the reaction has reached
completion. The graph shows that the
concentration of oxygen stopped changing after 90 seconds. So answer choice (A) is the correct
answer. It took 90 seconds for this
reaction to reach completion.
In an experiment, a student added a
sample of a solid into a solution, resulting in a chemical reaction. The student repeated the experiment
five times but changed the surface area of the solid. The graph showing how the
concentration of one of the products changes over time for each experiment is shown
below. In which experiment was the surface
area of the solid the greatest?
In the experiment in this question,
a solid was added to a solution, and the solid reacted. The concentration of a product of
this reaction was graphed over time. Each of the lines on this graph
represents the same reaction with the same mass of solid. The only difference is the surface
area of the solid. We need to figure out which of
these lines represents the experiment where the surface area of the solid was the
greatest. In a solid, only particles on the
surface can react. If we increase the surface area,
more solid particles are exposed and available to react. This means the reaction will occur
more quickly if the surface area is larger. So the experiment where the solid
had the largest surface area will also be the experiment where the reaction occurs
the fastest.
Looking at this graph, we can see
that each line reaches the same maximum value for the concentration of the product,
about 1.5 moles per liter. But each line took a different
amount of time to reach 1.5 moles per liter. The red line got there first. The pink line took the longest to
get there. In other words, the red line was
the experiment where the reaction happened the fastest and the pink line was the
experiment where the reaction happened the slowest. As we said, the fastest reaction
will correspond to the greatest surface area. So the red line represents the
experiment where the surface area of the solid was the greatest, making answer
choice (B) the correct answer.
Now, let’s conclude this video with
the most important points. Chemical reactions occur on a range
of time scales. During a chemical reaction, the
concentrations of the reactants decrease over time, but the concentrations of the
products increase over time. The rate of a chemical reaction is
the change in the concentration of the reactants and products per unit of time. The rate of a chemical reaction is
affected by the nature of the reactants, the surface area, the concentration of the
reactants, the temperature of the reaction, and catalysts.
