Video Transcript
Neutralization Reactions
Hydrochloric acid is a pungent
solution that can erode living tissue. Lye is a white powder that in
solution is extremely corrosive and is used for industrial cleaning purposes. What would happen if we mixed these
two hazardous chemicals together? Well, we’d end up with salt
water. Please do not try this at home. This reaction is an example of a
neutralization reaction, where we mix an acid and a base to produce water and a
salt.
In this lesson, we will learn how
to define and identify a neutralization reaction and write an interpret
neutralization reaction equations. In general terms, a neutralization
reaction is what happens when we mix an acid and a base to produce water and a
salt. For the specific case of
hydrochloric acid and lye, also known as sodium hydroxide, the chemical equation
looks like this. HCl plus NaOH produces H2O plus
NaCl. In this case, the salt produced is
sodium chloride, which we know in the kitchen as common table salt. But in chemistry, a salt simply
refers to any ionic compound made from a positive ion and a negative ion.
At this point, we might wonder, how
do we know what is an acid and what is a base? Well, we can help answer that
question by seeing what happens when they dissolve. Acids are any substance that
dissolves to produce hydrogen ions. We sometimes call hydrogen ions
protons, as there are no electrons or neutrons present in this ion. In addition to the hydrogen ion,
acids will also produce a negatively charged anion, in the case of hydrochloric
acid, a chloride ion. Bases dissolve to produce hydroxide
ions, or OH− ions.
It’s worth noting that there are
several definitions of acids and bases that focus on different characteristics. The definition that focuses on the
production of hydrogen ions by acids and hydroxide ions by bases is called the
Arrhenius definition. Alongside the hydroxide ion, they
also produce a positive cation when dissolved. In the case of sodium hydroxide,
that positive cation is a sodium ion.
When the two solutions are mixed
together, the hydrogen ion from the acid and the hydroxide ion from the base combine
to form H2O, water. In our initial equation, we said
that an acid plus a base produces a water and a salt. We can see here with the breakdown
of ions that the end form of the salt is that it is dissolved in the water as a
solution. Because the sodium ions and the
chloride ions are dissolved ions at the beginning of the reaction and at the end of
the reaction, we leave them out of the net ionic equation.
Net ionic equations include only
the ions that participate in the reaction. In this case, the hydrogen ion from
the acid and the hydroxide ion from the base combine to form water. So, overall, in a neutralization
reaction, we will have an acid, supplying the hydrogen ion, that combines with the
base, supplying a hydroxide ion, to form water and a salt.
The pH scale is a measure of
acidity or basicity, and it runs from zero to 14. A strong acid, like 0.1-molar
hydrochloric acid, will have a very low pH value of about one. On the other side of the scale, a
strong base, like 0.1-molar sodium hydroxide, will have a pH value of about 13. Substances lower on the pH scale
are more acidic and have higher concentrations of hydrogen ions, while substances on
the higher end of the pH scale are more basic with higher hydroxide ion
concentrations. When we mix a strong acid and a
strong base, what will be the pH of the resulting neutralized solution?
The answer is that the resulting
solution’s pH will be right around seven. If we have the same amount of the
two acids, the hydroxide ions and the hydrogen ions will combine to form water until
there are no more hydroxide or hydrogen atoms left. However, not all acids and bases
are made equal. Some acids and bases are considered
weak acids or weak bases. This means that not all of the
available hydrogen or hydroxide ions will dissolve into solution.
This means that if we combine, say,
a strong base and a weak acid, more of the hydroxide ions from the strong base will
dissolve into solution than the corresponding hydrogen ions from the weak acid. The extra hydroxide ions will raise
the pH of the resulting solution.
It is also important to recognize
that one mole of hydrogen ions will neutralize one mole of hydroxide ions. This means that, in general, one
mole of acid neutralizes one mole of base. There are a couple exceptions. For example, there are some acids
and bases that produce more than one mole of ions per mole of the acid or base. And as we mentioned before,
depending on the strength of the acid or base, the solutions will neutralize but
maybe not to a perfectly neutral pH of seven.
Since the ions pair up particle by
particle to form water molecules, it is important to get the same number of moles of
acid and base and not, say, the same volume or mass. If we attempted to mix a basic
solution and an acidic solution of unequal amounts, the resulting solution would
have a pH further toward one end or the other of the pH scale. To end up with a neutral solution,
the amount in moles of acid needs to equal the amount in moles of base.
We can use this relationship to our
advantage in titration experiments. If we have an acid or base, in this
case let’s say a base of unknown concentration, we can use a burette to slowly add
in a solution of the opposite type with a known concentration. We will keep adding the second
solution until the solution in the beaker is just neutralized.
By taking the volume of the acid
added, we can calculate the amount in moles of the acid. This will be the same as the amount
in moles of the base in the beaker, which will let us calculate the concentration of
the starting basic solution. But how do we know when the
solution is neutralized?
At the beginning, when we were
first adding acid to the base, the solution will gradually change in pH up to a
certain point. Then the pH will rapidly change
when the solution has been neutralized, before leveling off once again. We call the point where the pH
changes rapidly the equivalence point. This is because at this point the
amount in moles of acid and the amount in moles of base in the solution in the
beaker are equal. Once there are no more hydroxide
ions in the solution, the addition of hydrogen ions from the acid changes the pH
rapidly.
If we include the right indicator
in the beaker at the beginning of the experiment, the solution will change color
when the equivalence point has been reached. A popular choice is
phenolphthalein, which will be pink when the solution is more basic and clear when
the solution is more acidic, changing color somewhere between 8.2 and 10 on the pH
scale. Overall, this neutralization
reaction that occurs during the process we call titration allows us to determine the
unknown concentration of an acid or base in solution.
One more thing to recognize about a
neutralization reaction is that the temperature will increase. The formation of bonds between the
hydroxide ions and the hydrogen ions to make water will release energy into the
surroundings that we can observe as heat. We call reactions that release heat
into the surroundings exothermic reactions.
Now that we’ve gone over the basics
of neutralization reactions, let’s go over one exceptional case to push our
understanding a little farther. The combination of magnesium oxide
and hydrochloric acid to form water and magnesium chloride is a neutralization
reaction. This might seem a little odd
considering that magnesium oxide is a base, but it does not have a hydroxide ion in
its chemical formula, nor is it soluble in water.
To explain the first of these
interesting characteristics, we can say that our previous definition of a base was a
simplification. While most bases have an oxygen and
a hydrogen in their chemical formula, as long as they accept protons, to make water,
they are considered bases. Defining an acid as something that
can donate protons and a base as something that can accept protons is called the
Brønsted–Lowry definition of acids and bases.
The other interesting
characteristic of this base, the fact that it’s insoluble in water, is explained by
the fact that this reaction occurs on the surface of magnesium oxide, as opposed to
occurring between the ions in the solution. This exceptional neutralization
reaction proves that not all bases have oxygen and hydrogen in their chemical makeup
and not all bases are soluble in water.
Now that we’ve learned about
neutralization reactions, let’s do some practice problems to review.
A neutralization reaction is shown
below. Aqueous HNO3 plus aqueous KOH turns
into aqueous KNO3 plus liquid H2O. What is the acid in this
equation? What is the base in this
equation? What is the salt in this
equation?
The general form of a
neutralization reaction is that an acid and a base added together produce water and
a salt. This multipart question is asking
us to look at the four compounds listed in the chemical equation and identify which
one is the acid, which one is the base, and which one is the salt.
If we look again at the general
equation for a neutralization reaction, we can see that the acid and base are the
reactants of the reaction on the left side of the equation, while the salt is one of
the products on the right side of the equation. Note that we cannot simply say that
the acid comes first, the base comes second, the water comes third, and the salt
comes fourth. The reactants on the left-hand side
of the equation could be listed in any order, and the products on the right-hand
side of the equation could be listed in any order.
Let’s start by identifying the
salt. In chemistry, a salt is an ionic
compound consisting of a positive ion and a negative ion. It’s a product of a neutralization
reaction. So it will appear on the right-hand
side of the equation. Thankfully, it is not too difficult
to identify which of the products is which. Liquid H2O is also known as water,
leaving aqueous KNO3, the ionic compound made up of the positive potassium ion and
the negative nitrate ion, to be the salt.
Next, we need to identify which is
the acid and which is the base. Based on the general formula, we
know that the acid and the base are the reactants of the neutralization
reaction. So of the two reactants on the
left-hand side of the equation, HNO3 and KOH, one will be the acid and one will be
the base. How do we determine which one is an
acid and which one is a base?
Well, acids produce hydrogen ions
when dissolved, while bases produce hydroxide or OH− ions when dissolved. If we break down these compounds
into their constituent ions, we can see that HNO3 produces hydrogen ions, while KOH
produces hydroxide ions. This means that HNO3 is our acid
and KOH is our base.
Note that there are some bases that
don’t produce hydroxide ions. However, the acid will still
produce hydrogen ions. So you can identify the acid and
base in this case by first identifying the acid. Neutralization reactions will
always follow the general formula of acid plus base produces water and a salt. If we can identify the reactant
that produces hydrogen ions, the reactant that produces hydroxide ions, and the
product that is an ionic compound, we can identify the acid, base, and salt,
respectively.
In the neutralization reaction
shown below, the acid is HNO3, the base is KOH, and the salt is KNO3.
What product or products are formed
during the following neutralization reaction?
This question is asking about a
neutralization reaction. The general form of neutralization
reactions is that a base and an acid combine to produce water and a salt. This question is specifically
asking about the products formed during the reaction. Or if we combine these two
substances on the left-side of the equation, what chemicals appear on the right-hand
side of the equation?
If we look at our general formula,
we can more specifically ask, water and what salt will be produced by this
reaction? To answer this question, we need to
know what ions are present in the reactants and how will they combine to form the
products. Barium hydroxide breaks down into a
barium ion and two hydroxide ions. H2CO3, also known as carbonic acid,
breaks down into two hydrogen ions and one carbonate ion.
Listing the ions gives us a good
breakdown of what will happen when the products are formed. The hydroxide ion supplied by the
base and the hydrogen ion supplied by the acid will combine to form water, H2O. Meanwhile, the positive cation
supplied by the base and the negative anion supplied by the acid will combine to
form the salt. Written out, the products of the
reaction look like this: 2H2O plus BaCO3.
Since the base and the acid each
supply two ions, the reaction as a whole will produce two molecules of water,
indicated by the two before H2O. It’s also worth noting that our
salt, barium carbonate, is formed with one barium ion and one carbonate ion, because
the two plus charge from barium cancels out the two minus charge from the carbonate
ion. If the two ions that make up our
salt had different charges, we would need to use different numbers of each of them
to balance out the charges.
In a neutralization reaction, the
acid and the base each supply one ion to help make water. And they each supply one ion to
help make the salt. In the neutralization reaction
given here, the products that are formed are two molecules of water and barium
carbonate salt.
Now that we’ve done some practice
problems, let’s review the key points of the video. The general form of a
neutralization reaction is that an acid and a base combine to form water and a
salt. The acid supplies the hydrogen ion,
also known as a proton, and the base typically supplies an OH− ion, or a hydroxide
ion. The net ionic equation gives the
ions that take part in the reaction. For a neutralization reaction, the
net ionic equation is H+ plus OH− produces H2O.
For the net ionic equation, the
ions that produce the salt are left out, as they remain aqueous ions in solution at
the beginning and end of the reaction. In some exceptional cases, we may
see a base that does not dissolve in water. We may also see a base that does
not supply a hydroxide ion. According to the Brønsted–Lowry
definition of a base, since it can still accept a proton to create water, it is
still considered a base.
Since the particles match up ion by
ion to produce water molecules, one mole of acid neutralizes one mole of base. Lastly, neutralization reactions
are used in titration experiments to determine the unknown concentration of an acid
or base.
