Lesson 17
Objectives:
Review: Ionic compounds involving polyatomic anions and cations; Arrhenius definition of acids and bases
Matter - overview of our understanding
Review:
1.
Arrhenius definition of acids: Acid is an ionic compound of hydrogen ion H+ and a negative ion.
a.
Binary acids consist of a hydrogen ion H+ and an -ide ion of a nonmetal atom. hydro___________ic acid
b.
Oxyacids that we will discuss in this course consist of:
i.
a hydrogen ion H+ and an -ate anion. Name: name of a nonmetal atom-ic acid
ii.
a hydrogen ion H+ and an -ite anion. Name: name of a nonmetal atom-ous acid
2.
Arrhenius definition of bases: Base is an ionic compound of a metal ion and a hydroxide ion OH-.
3.
Polyatomic ions are treated as “units” in compounds such as acids, bases and salts. If there are two or more such
units in a “molecule” of these compounds then we use brackets and subscripts to indicate the number of such units
e.g., the formula for magnesium hydroxide is written as Mg(OH)2 or the formula for iron(III) chlorate is written as
Fe(ClO3)3. When only a single polyatomic ion is present in a compound then we don’t use brackets e.g., the formula
for sodium hydroxide is written as NaOH and the formula ammonium nitrite is written as NH4NO2.
4.
Rules for writing names of compounds given their formula
a.
identify: acid (presence of H), base (presence of OH) or salt
b.
identify the formulas of ions and their charges - important to do so when dealing with transition metals Co,
Mn, Cr, Fe, Cu
c.
write the names of ions
d.
write the name of the compound
5.
Rules for writing formulas of compounds given their name
a.
identify: acid, base (hydroxide) or salt
b.
Identify the names of ions
c.
write formulas for ions with their charges
d.
write the formula using rules for writing formulas of ionic compounds
Matter
We have began our journey only a few weeks ago. Our goal was and still is to show what we know about
the matter. Our present understanding of matter in our Universe is a culmination of efforts and work of
countless chemists and physicists who struggled to understand their observations over a period of the last
two hundred years. A key idea, which probably led us to this understanding began with Dalton’s atomic
theory and the law of definite proportions.
It is a currently accepted theory that our Universe began with the Big Bang. We don’t know much about
the conditions in the Universe when the Big Bang happened some 15 billions years ago. But it appears
that free quarks and electrons were filling up space in the first millisecond after the Big Bang. Our Universe
was composed of particles that were very simple in the sense that they only had two properties: mass and
charge. They were indistinguishable from one another. What happened next is described in the diagram
on the left. Formation of neutrons and protons was followed by formation of nuclei and then atoms and
molecules were formed. We have discussed that we can understand this process in terms of just a few
rules of the game in this Universe. We used analogies to help us grasp these rules.
The analogy:
1. In our world, everything seems to fall down. It makes us think that all objects are attracted to earth.
Similarly, all particles in our Universe are attracted or repelled by other particles. The two major forces
acting between particles in the early Universe are called a nuclear force and an electrical force.
2. When an object falls down it loses something that we call energy. For example, when a rock rolls down
into a pit it gets stuck in there. We say that the rock sitting at the bottom of the pit has less energy than it
had before the fall. This is because it takes some energy to lift the rock out of the pit and bring it back
where it was before. The deeper the pit is, the more energy it takes to bring it up. Similarly, when quarks
combine to form neutrons and protons they get “stuck” in these configurations because they lost energy.
Some energy was lost at every stage of a process described in the diagram on the left. Approximately 10-10 J of energy is lost when
neutrons and protons combine to form nuclei. Approximately 10-15 -10-18 J of energy is lost when nuclei combine with electrons to form
atoms. The amount of energy lost is even smaller when atoms combine to form molecules or solid crystals.
It is interesting to note that as the Universe grew older the world of particles became progressively more complex. At the beginning
there were just three particles (we exaggerate a little here) with just two properties each. They formed 108 known different atoms.
Properties of atoms are more complex: some are metals while others are nonmetals. Some don’t interact with other atoms (noble
gases) while others are very reactive (e.g., halogens). In short, atoms have more properties than its constituent particles neutrons,
protons and electrons. Still the world of atoms is limited. For, example, atoms don’t have complex shapes. Although we visualize them
as colourful “balls” the shape or colour is not among their properties. Molecules, ions and crystals of ionic compounds represent the
next stage in the development. There does not seem to be an end to how many such compounds can be made out of 108 atoms.
Molecules have different shapes and many more properties than simple atoms (e.g., acids taste sour and bases taste bitter). The
interactions between molecules are also more complicated. Molecular forces that act between molecules depend on the shape of
molecules, their size etc.
When we discussed how particles combine to form more complex units (e.g.,atoms) we talked about collisions which resulted in
bonding of the two colliding particles. In the history of the Universe the bonded particles were also broken in collisions. The two
analogies described below can show you how it is possible to break the bond between particles:
1. How to get the rock out of the pit? Think of this observation. A collision with a fast moving car or a fast moving ball can set objects
(like windows etc.) into motion although they appeared to be previously stuck in a very stable position. We may get the rock out of the
pit by hitting it with a fast moving ball. Similarly, bonds between atoms can be broken in collisions with fast moving particles.
2. How to get fast moving particles? Consider a soup that you put on a stove and turn the heat on. The noodles that are sitting down at
the bottom of the pot begin to move as the soup is warming up. The hotter the soup is, the faster the noodles move. Similarly, the
speed of particles depends on the temperature.
In the early hot Universe particles bonded together in collisions but they were also broken up in collisions because the temperatures
were high and the particles were often moving fast. As the Universe expanded it cooled down. Soon after the birth of the Universe it
became impossible to break quarks out of the neutrons and protons. When the Universe was about 100 000 years old, it became
impossible to separate neutrons and protons out of nuclei. Eventually, it became impossible to break atoms. Once formed, even
molecules were rarely broken into its constituent atoms. Consequently, !5 billion years after the Big Bang, there does not seem to be a
limit what we find in our Universe: we observe clouds of hydrogen atoms in our Galaxy as well as fiery mixture of gases composed of
hydrogen, helium and other atoms in stars. We observe solids in small dust particles scattered throughout the Universe as well as in
large solid objects such as our planet. In the last decades of the twentieth century astronomers even identified presence of amino
acids (molecules that are essential to life) in molecular clouds.
Read the section Matter in the website VisionLearning.
As we said at the outset, chemists and physicists collaborated in the efforts to formulate our understanding of the Universe. As you
might guess, there are still more questions than answers when we think about the evolving Universe. The work continues. Chemists
focus in their work on interactions between atoms and molecules. They study chemical changes which involve formation of new
molecules or breaking up old molecules. Physicists focus in their work on the world of subatomic particles (nuclear and high energy
physics) and on the world of large objects. They either study formation or breaking up of nuclei or they focus on causes of motion of
large objects.
In the last few lectures of this course we will focus on the work of chemists. As we said before atoms and most of the molecules in our
Universe were born in random collisions over a very long period of time. They constitute matter that surrounds us. The matter exist in
three possible states:
•
•
•
solid: constituent particles are locked in a relatively stable position - see a simulation in VisionLearning - they only vibrate
around the same point in space. Most ionic compounds are solids. The average distance between ions in the solid depends
on the size of the ions but it is approximately 10-10 m.
liquid: constituent particles are moving freely - relative position of particles is constantly changing; their distances vary given
their motion but on average the distance between particles in liquids is still about 10-10 m. Collisions between particles are
quite likely.
gas: constituent particles are moving freely. The average distance between particles is at least 10 times bigger than the
average distance between particles in liquids. It is characteristic of gases that the interaction between particles is very weak
and collisions are less frequent than in liquids.
Chemists study atomic and molecular interactions through observations of reactions between molecules that occur when molecules
collide. Although reactions can take place randomly in any state of substances, chemists prefer to work with matter in a liquid state
where collisions are frequent and experiments take less time. Some substances are liquid to begin with but other substances may be
solids. In such cases, chemists dissolve these substances in a liquid to form solutions. We will discuss solutions in the next class.
Then we will discuss reactions and how to interpret observations of reactions.