Video Transcript
In this video, we will learn what
an oxide is and which oxides are acidic, basic, amphoteric, and neutral. We’ll look at some chemical
equations for how some oxides react with acids. First, let’s ask ourselves, what is
an oxide? An oxide is a compound which
contains the element oxygen bonded to another element. For example, in carbon dioxide,
oxygen is bonded to carbon. Some commonly occurring oxides
which you may have heard of are iron(III) oxide, which is the main component of
rust, sulfur dioxide, which is sometimes used as a food preservative, dinitrogen
monoxide, which is the IUPAC name and whose common name is nitrous oxide, this is
laughing gas, water, silicon dioxide or silica, which is the main component of sand,
aluminum oxide, and magnesium oxide.
Perhaps you’ve produced magnesium
oxide in the lab. A pair of tongs holds magnesium
ribbon in a Bunsen burner flame. A highly exothermic reaction occurs
with much heat and light being given off as the magnesium reacts with oxygen in the
air to produce magnesium oxide. All oxides can be categorized into
one of four groups: acidic, basic, amphoteric, or neutral oxides. Let’s investigate this and start
with acidic oxides. Acidic oxides are those which when
they react with water produce acids. These oxides contain nonmetals from
groups 14 to 17 of the periodic table. The general equation is nonmetal
oxide plus water react to give an acid. Let’s look at some examples.
When sulfur dioxide gas reacts with
water, sulfurous acid is produced, H2SO3. When the nonmetal oxide, carbon
dioxide, reacts with water, carbonic acid or H2CO3 is produced. These two reactions can occur in
the environment when there is much SO2 and CO2 in the atmosphere. Liquid water and rain drops can
interact with carbon dioxide and sulfur dioxide in the atmosphere and produce the
two acids carbonic and sulfurous acid. These two acids are components of
acid rain. One last example of when a nonmetal
oxide interacts with water to produce an acid is the reaction of nitrogen dioxide
gas with water to produce nitric acid. If a few drops of universal
indicator were added to solutions of these acids, it would turn red-orange, proving
that the nonmetal oxides SO2, CO2, and NO2 when they react with water do indeed
produce acids.
Now let’s have a look at basic
oxides. A basic oxide is an oxide which
when it reacts with water forms a base or an alkali. In general, basic oxides contain
metals from groups one or two of the periodic table. The general equation is metal oxide
plus water react to give a base or alkali. An alkali is a substance containing
the hydroxide ion OH−. Here are two examples of equations
where metal oxides produce hydroxides or alkali solutions. In the first, sodium oxide reacts
with water to produce sodium hydroxide, and in the second, calcium oxide reacts with
water to produce calcium hydroxide. Sodium hydroxide is highly soluble
in water, while calcium hydroxide is only slightly soluble. Nevertheless, if a few drops of
universal indicator were added to these solutions, it would turn blue-purple,
confirming that the solutions are basic or alkaline and proving that these oxides do
indeed react with water to produce bases or alkalis.
So far, we have seen that nonmetal
oxides or acidic oxides react with water to produce acids and that metal oxides or
basic oxides react with water to produce a base. Acidic oxides can also act like
acids and react with a base to produce salt and water. And basic oxides can act like a
base by reacting with an acid to produce salt and water. The general rule applies: an acid
reacts with a base to produce salt and water. A bit later in this video, we will
look specifically at how basic oxides react with acids to produce salt and
water. For now, let’s move on to the third
type of oxide, amphoteric oxides.
Amphoteric oxides, unlike acidic
and basic oxides, usually don’t dissolve in or react with water. However, they do show both acidic
and basic properties. They behave like an acid when they
react with a base, and they behave like a base when they react with an acid. These oxides contain metals such as
copper, zinc, lead, beryllium, aluminum, and tin. We saw a moment ago that when an
acid and a base react, they produce salt and water. So because amphoteric oxides can
behave like acids or bases, there are two general equations to look at. When these oxides behave like an
acid, the equation is amphoteric oxide plus base, giving salt and water. And when they react like a base,
the equation is amphoteric oxide plus acid, giving salt and water.
Let’s look at an example for
each. Aluminum oxide is amphoteric. It does not dissolve in or react
with water and can act like an acid or a base. When it reacts with a base such as
sodium hydroxide, sodium aluminate, salt, and water are produced. Note that this formula is a
simplification. Aluminum can form quite complex
ions in solution. The formula here for sodium
aluminate is actually the formula for the solid anhydrous product. But sodium aluminate in the
presence of water is highly soluble, and so will react with water to form a hydrated
compound with a complex formula, which we will not look at here. When this amphoteric oxide reacts
with an acid, the salt aluminum chloride is produced. This dual nature of amphoteric
oxides is indicated by their name. The word amphoteric comes from the
Greek word amphoteroi, meaning both.
Let’s move on to the last type of
oxide, neutral oxides. Neutral oxides do not show acidic
or basic properties and do not react with acids or bases. There are only a few known neutral
oxides, and these include carbon monoxide, nitrous oxide, and nitric oxide. Again, neutral oxides do not
undergo reactions with acids or bases. Now, let’s have a look specifically
of how basic oxides react with acids to produce salt and water, as well as some more
examples of how amphoteric oxides can act as bases, react with acids, and produce
salt and water. When sodium oxide reacts with
hydrochloric acid, sodium chloride and water are the products. Notice that the anion in the acid
and the cation in the basic oxide determine which salt is produced.
Can you guess what the acid is in
this next equation? Magnesium oxide is largely
insoluble in water. However, in dilute, warmed acidic
solution, it can react to produce salt and water, in this case, magnesium nitrate
and water. The magnesium cation in the salt
product comes from the oxide, and the NO3 or the nitrate ion must have come from the
acid. There are two nitrate ions, which
means there must be two positive charges in the acid or two H+ ions. Combining these ions together, we
get, we get two HNO3, which is the acid nitric acid. We saw an example earlier of how an
amphoteric oxide can act as a base and react with an acid. Let’s look at one more example.
The reaction of zinc(II) oxide with
sulfuric acid produces the salt zinc sulfate and water. Again, the cation in the salt comes
from the oxide, and the anion in the salt comes from the acid. So far, we’ve looked at the types
of oxides, how they react, and we’ve looked at many equations. Before we do a practice example,
let’s do something a bit different. Let’s investigate how different
elements react with oxygen to produce oxides and how this gives us a basic idea of a
reactivity series for the elements.
Some elements react with oxygen
more vigorously than others. Gold has low to no reactivity with
oxygen. We say it’s inert and does not
react. Silver and mercury are very slow
and resistant to react with oxygen. More metals have been added to this
list in a specific order, and this is based on increasing reactivity with oxygen, in
other words, an increase in the ease with which these elements react when increasing
the vigor. The elements at the far right of
the series react easily and vigorously with oxygen, requiring little energy to
undergo this reaction, with potassium metal being the most vigorous or we say the
most reactive.
It’s important to know that all
these metals can be induced to react with oxygen under the correct conditions, even
gold. But here we are talking about their
natural reactivity. The more reactive an element is,
the more likely it is to be found in nature bonded to oxygen or other elements. This list here is called a
reactivity series. It shows the general trend or order
with which the elements react with oxygen. You’ll notice only metals written
on this reactivity series. But nonmetals can also react with
oxygen. The vigor with which hydrogen
reacts fits in between iron and zinc. Let’s look in some detail at a
specific reactivity series of four nonmetals. Bear in mind, though, that these
nonmetals can be placed into the top reactivity series in amongst the metals,
according to their relative reactivity with oxygen.
There are many known oxides of
chlorine. However, chlorine does not react
with oxygen in the air and in normal conditions. Energy input is needed to induce a
reaction. Of these four nonmetals, chlorine
is the least reactive. Carbon also does not generally
react spontaneously with oxygen. Think about a piece of charcoal on
a barbecue. It must first be heated till red
hot. Then it will react and burn with
oxygen in the air. Sulfur reacts a bit more
vigorously. It will catch fire when heated over
a Bunsen burner. Phosphorus, however, reacts quite
vigorously and ignites spontaneously in oxygen in the air. Phosphorus is the most reactive
towards oxygen from the elements in this series. So from their reaction with oxygen,
we can conclude an increasing reactivity from chlorine to carbon to sulfur to
phosphorus. Now it’s time to have a look at an
example before we summarize everything we have learnt.
To determine the pH of various
oxides, an experiment was set up. Three beakers were filled with 0.5
liters of deionized water and a few drops of universal indicator were added. A spatula of the following oxide
was then added to each beaker. What color will each solution
change to following the addition of the oxide? (A) A: blue, B: green, and C:
red. (B) A: green, B: red, and C:
blue. (C) A: blue, B: red, and C:
green. (D) A: red, B: green, and C:
blue. Or (E) A: red, B: blue, and C:
green.
An oxide is a compound consisting
of oxygen bonded to another element. P2O10, which was added to the first
beaker, is a nonmetal oxide because it is composed of the nonmetal phosphorus bonded
to oxygen. MgO and Al2O3 are both examples of
metal oxides because Mg, magnesium, is a metal and Al, aluminum, is also a
metal. And these metals are bonded to
oxygen. In general, when a nonmetal oxide
reacts with water, an acid is formed. This usually occurs when a nonmetal
is from groups 14 to 17 of the periodic table. When the metal in a metal oxide is
from groups one or two of the periodic table, for example, magnesium, and the oxide
reacts with water, a base or alkali solution is usually formed.
Note, however, that there are
always exceptions to the rule. For example, beryllium oxide is not
soluble in water and does not react with water under normal conditions. Other metal oxides containing
metals which are not in groups one or two of the periodic table, for example,
copper, zinc, lead, aluminum, and tin, when they are placed in water, they do not
usually react and are usually insoluble. Now, magnesium oxide is also not
very soluble in water, but tiny amounts of it will dissolve and react with water to
produce a base or alkaline solution.
We were told that universal
indicator was added to each beaker to determine the pH. Universal indicator is red in the
highly acidic region of the pH scale, then orange-yellow, then green around neutral
point, then blue, and at the far end of the spectrum in the highly basic region, it
is purple. We have seen that when a nonmetal
oxide reacts with water, an acid forms in the case of beaker A. So the indicator
will turn red in beaker A. In beaker B, a base or alkali will form as the metal
oxide reacts with water, so the indicator will turn a purple-blue color. And in beaker C, when the metal
oxide is added, no reaction occurs.
Aluminum oxide is an example of an
amphoteric oxide. And again, these are not usually
soluble, nor do they usually react with water, although they can react with acids
and bases. Because there is no reaction with
water, the pH of the water in this beaker will remain neutral, and the indicator
will appear green. So the color change in each beaker
because of the addition of an oxide will be A: red, B: blue, and C: green.
Let’s summarize what we have
learnt. An oxide is a compound containing
oxygen bonded to another element. When a nonmetal oxide reacts with
water, an acid is produced. When a metal oxide reacts with
water, a base is produced. When an amphoteric oxide is placed
in water, it does not dissolve or react. And when a neutral oxide is placed
in water, there is also no reaction. These are the general trends. When a nonmetal oxide reacts with a
base, salt and water are produced. Salt and water are also produced
when a metal oxide reacts with an acid. And for amphoteric oxides which can
react with an acid or a base because they act as acids or bases, again salt and
water are the products.