Unit 4 Bonding Homework AP
Directions: For each question, provide your final answer below the question. For math-based answers,
work must be shown on this page or a separate sheet of paper attached to the back of this packet.
Without work, no credit will be given. Check significant figures and units.
1. State precisely what is meant by each of the following four terms. Then distinguish clearly between each of
the two terms in part (a) and between each of the two terms in part (b), using chemical equations or examples
where helpful.
(a) Bond polarity and molecular polarity (dipole moment)
(b) Hybridization and formal charge
2.
(a) Draw the Lewis electron-dot structures for CO32-, CO2, and CO, including resonance structures where
appropriate.
(b) Which of the three species has the shortest C-O bond length? Explain the reason for your answer.
(c) Predict the molecular shapes for the three species. Explain how you arrived at your predictions.
3. Butane, chloroethane, acetone, and 1-propanol all have approximately the same molecular weights. Data on
their boiling points and solubilities in water are listed in the table below.
Boiling Solubility
Compound
Formula
Pt.(ºC) in water
Butane
CH3CH2CH2CH3
0
insoluble
Chloroethane
CH3CH2Cl
12
insoluble
O
Acetone
56 completely
||
miscible
CH C C CH
3
1-Propanol
3
CH3CH2CH2OH
97
completely
miscible
Unit 4 Bonding Homework AP
On the basis of dipole moments (molecular polarities) and/or hydrogen bonding, explain in a qualitative way
the differences in the
(a) boiling points of butane and chloroethane.
(b)water solubilities of chloroethane and acetone.
(c) water solubilities of butane and 1-propanol.
(d) boiling points of acetone and 1-propanol.
Directions: Read the passage below and answer the summary questions that follow in complete sentences.
Adhesives
Chris Woodford
http://www.explainthatstuff.com/adhesives.html
Have you ever stopped to think why glue doesn't stick to its tube? Have you ever wondered why, when you
open up a jam sandwich, there's jam on both pieces of bread when you put it on only one slice to begin with? If
it's ever bothered you how adhesives work, and why they fail, you're not alone. That question has taxed some of
the world's best minds since ancient times. Even after all these years, scientists still don't fully understand how
gluey substances make one thing stick to another, though they've got some pretty good ideas.
According to historians and archeologists, adhesives have been used for thousands of years—probably since
Stone Age cave dwellers first applied bitumen (a tarry substance used to surface highways) to stick flint
axeheads to the tops of their wooden hunting spears. In ancient times, people made their glues from whatever
they found in the world around them—such things as sugar, fish skins, and animal products boiled in water. We
still use some of these natural adhesives today, though we're much more likely to use artificial adhesives made
in a chemical plant. It's obvious modern glues are chemical products from the horrible names they have—
polyvinyl acetate (PVA), phenol formaldehyde (PH), ethylene vinyl acetate (EVA), and cyanoacrylate ("super
glue") to name just four. Many modern adhesives are called synthetic resins for no good reason other than that
resin (a gooey substance found in pine trees and other plants) was one of the first widely used adhesives.
Unit 4 Bonding Homework AP
Knowing what something is called is a far cry from knowing how it works. That was a lesson the Nobel-Prizewinning American physicist Richard Feynman (1918–1988) often used to teach. So let's forget all about
adhesives, acetates, and acrylates and try to figure out why one thing will stick to another. If you want a short
answer, the word is "forces." People stick to Earth's surface even though the planet is rotating at high speed, and
even there's no glue on the soles of our feet. The reason is simply that gravity bonds us to the planet with
enough force to stop us whizzing off into space. But gravity isn't enough to keep us permanently in place. If we
supply bigger forces, for example by using our muscles to move our legs and jump in the air, we can "unstick"
ourselves and go somewhere else. Life on Earth is a bit like being a giant living Post-It® note—only with legs!
So you don't always need a blob of adhesive to stick things together. That much is blindingly obvious whenever
it rains on your window. Gravity tries to pull the water down to the bottom of the glass, and sooner or later it
usually wins, but two interesting things try to stop it. First, water molecules (two atoms of hydrogen and one
atom of oxygen joined together) naturally stick to one another, so they clump together in big droplets on the
window. The forces that make them do this are called cohesive forces (and the process involved is called
cohesion). Second, the water droplets also stick to the glass without any help or glue. Different forces are at
work here known as adhesive forces (the sticking process is called adhesion). Now the cohesive forces must be
bigger than the adhesive forces or the water wouldn't form droplets at all. Instead, it would just spread out in a
very thin layer on the glass—much as oil does when you spread it on water. But the adhesive forces are still
pretty strong: some of the water droplets that stick to your window are surprisingly big. Next time it rains,
watch how the water behaves. See how the rain naturally clumps into droplets (because of cohesion), which
remain on the glass (because of adhesion). The drops fall down the window only when they're too heavy for the
adhesive forces to keep them in place (when the gravitational force pulling them down is greater than the
adhesive force holding them up). Notice how they run down the window in distinct tracks, with droplets
following existing, watery paths. That's because the water drops that are falling are trying harder to stick to the
water that's already there rather than to the glass (cohesion at work again). Why does the rain form those streaky
channels? Because as drops fall down the glass, cohesive forces tear some of the water molecules away from
passing drops, leaving behind droplets that are small enough to stick to the glass (adhesion again).
What does all this have to do with adhesives? Adhesive and cohesive forces are also at work in glues. Let's say
you want to stick together two bits of wood, A and B, with an adhesive called C. You need three different forces
here: adhesive forces to hold A to C, adhesive forces to stick C to B, and cohesive forces to hold C together as
well. The first two are pretty obvious: the glue has to stick to each of the materials you want to hold together.
But the glue also has to stick to itself! If that's not obvious, think about sticking a training shoe to the ceiling.
The glue clearly has to stick both to the training shoe and to the ceiling. But if the glue itself is weak, it doesn't
matter how well it sticks to the shoe or the ceiling because it will simply break apart in the middle, leaving a
layer of glue behind on both surfaces. That's a failure caused when the adhesive forces are greater than the
cohesive ones and the cohesive forces aren't big enough to
overcome the pull of gravity. Jam sandwiches may not be the
first thing to spring to your mind when you think about
adhesives, but the jam is working as a kind of glue. It's made
of sugar and water: a classic adhesive recipe used since
ancient times. If you use fairly strong bread, you can pick up a
jam sandwich by just one corner of one slice and the whole
thing will stay together in your hand—thanks to the jammy
glue. Jam has pretty high cohesive forces (that's why jam can
be hard to dig out of the jar with your knife), but its adhesive
Unit 4 Bonding Homework AP
forces are high too. If you butter two pieces of bread and cover one slice with jam, then close up the sandwich,
then peel it apart, you'll find there's some jam left on both surfaces. As you pull apart the sandwich, you'll find
the jam breaking itself in two in lots of little strands. That's because the adhesive forces are stronger than the
cohesive ones. Your jam sandwich "fails" due to a failure of cohesion.
Now we know that adhesives work through adhesive and cohesive forces, we need to understand a bit more
about how those forces themselves work. Let's start with cohesive forces. As you can discover in our main
article about the magic of water, water molecules join together with one another because they're not
symmetrical. One end has a slight positive charge, the other end has a slight negative charge, and the positive
and negative ends of different molecules snap together like the opposite ends of magnets. That's a kind
of electrical or electrostatic bonding. In metals, the atoms are strongly held together in a rigid crystal structure
called a lattice (a bit like scaffolding or a climbing frame with atoms at the joins and invisible bars holding them
together). You can easily separate one "piece" of water from another (by lifting some out with a spoon): the
cohesive forces are quite weak. But you can't easily separate one bit of iron from another (with a spoon or
anything else) because the cohesive forces are incredibly strong. Water and iron are both pretty useless as glues,
but for quite different reasons. Water could be an excellent glue because it sticks quite well to other substances
(such as glass), but its cohesive forces are incredibly weak. You can stick paper to the wall by wetting it first,
but you can usually peel it off quite easily too. When you peel, you're breaking the weak cohesive forces that
hold one water molecule to another. Iron is no good as a glue because it's too preoccupied with sticking to itself
to stick to anything else. All its forces are occupied internally, fixing one iron atom to another in a strong
cohesive structure. There's nothing it can use to attach itself to other objects: its adhesive forces are virtually
nonexistent.
Now for the real question: what makes a gluey substance stick to something else? You may be surprised to hear
that there's no single, simple answer—but that's not so surprising if you consider how many different types of
glue there are and how many different ways in which we can use them. For each different glue, and each
different surface we use it on, scientists think a combination of different factors are at work holding the two
together. But the plain truth is: no-one exactly what's going on in every case. One of the main factors is
called adsorption. When you spread adhesive, it wets the surface you apply it to. Lots of very weak electrostatic
forces between the glue molecules and the molecules in the surface (called van der Waals forces for the
physicist Johannes Diderik van der Waals (1837–1923) who discovered them) hold the two things together. For
adhesives to work well like this, they have to spread thinly and wet the surfaces very well. There's no actual
chemical bond between the glue and the surface it's sticking to, just a huge number of tiny attractive forces. The
glue molecules stick to the surface molecules like millions of microscopic magnets. In some cases, adhesives
can make much stronger chemical bonds with the materials they touch. For example, if you use certain glues on
certain plastics, the glue and the plastic actually merge together to form a very strong chemical bond—they
effectively form a new chemical compound at the join. That process is called chemisorption.
Absorption and chemisorption are chemical connections between the glue and the surface. Glues can also
form physical (mechanical) bonds with the surface they're sticking to. Suppose the surface is porous (full of
holes). The glue can seep into those holes and grip through them, like a climber's fingers grabbing holes in a
rock face. That's called the mechanical theory of adhesives. Another theory of how glues work suggests the
adhesive can diffuse into the surface and vice-versa, with molecules swapping over at the join and mingling
together. This is called the diffusion theory. And one last explanation suggests adhesives can work by suction.
Exactly how Post-It® notes work is a closely guarded (and very profitable) commercial secret. But some
Unit 4 Bonding Homework AP
chemists who've studied the problem think their magic adhesive is made from masses of microscopic bubbles.
When you push a Post-It® onto a table, some of the bubbles burst and form microscopic vacuums that stick the
paper to the table like lots of tiny suction cups. When you peel away the note, you simply break the vacuum.
You can reuse a note because not all the bubbles burst each time. But, gradually, all the bubbles do burst and the
note loses its stick.
Adhesives are designed to work when they leave the tube—and not before. Different adhesives achieve this in
different ways. Some are dissolved in chemicals called solvents that keep them stable and non-sticky in the
tube. When you squeeze them out, the solvents quickly evaporate in the air or get absorbed by the surfaces
you're sticking to, freeing the adhesives themselves to do their job. Plastic modeling glue works like this. It
contains molecules of polystyrene in an acetone solvent. When you squeeze the tube, the glue spurts out and
you can usually smell the very strong acetone as it evaporates. Once it's gone, the polystyrene molecules lock
together to make strong chemical bonds. Glue doesn't smell when it's dry because all the solvent has vanished
into the air. Some glues (such as synthetic, epoxy resins) have to be mixed together before they work. They
come in two different tubes, one containing the synthetic resin and the other containing a chemical that makes
the resin harden. The two chemicals are useless by themselves but, mixed together, form a tough, permanent
adhesive.
Geckos have been baffling people for over 2000 years, ever since one of the very first scientists, Aristotle,
wondered why they can walk upside down on the ceiling. Now a gecko has mass, so it also has weight. Gravity
pulls it downwards like anything else. If you tried walking on the ceiling, you'd very quickly find yourself on
the floor. So how do geckos defy one of the most basic laws of physics? If a force acts on an object, but that
object doesn't move, there must be another force acting in the opposite direction. In other words, two forces
must be exactly balancing one another. Since the gecko doesn't fall, there must be another force acting upwards
that stops gravity from pulling it down.

Maybe the gecko has suction pads on its feet? Scientists thought this might be the explanation. So they put a
gecko in a tank and sucked all the air out of it. Strangely enough, the gecko still managed to walk on the
ceiling.
 Perhaps the gecko's body squeezes out some kind of glue? That was another theory the scientists tested. But
when they examined a tank that geckos had been climbing around in, they found no evidence of any sticky
stuff.
 Could static electricity explain it? Electricity can certainly stick things together. If you rub a balloon on a
woolen jumper, the balloon will eventually stick to you. Rubbing makes static electricity build up on the
balloon's surface. This creates an electrical force between the rubber and your jumper that makes the balloon
stick. Forces like this are called electrostatic because they use static (non-moving) electricity.
 Does static electricity make the gecko stick? Moisture tends to make static electricity disappear: static
electricity will flow away through the water and vanish. But if you put a gecko in a really humid tank, or one
with a moist ceiling, it can still do the upside-down trick. So static electricity doesn't quite explain what's
happening either.
So what is the trick? Each one of the geckos feet is covered in millions of tiny hairs called setae. Looking under
a powerful electron microscope, scientists found each hair is also a bit like a brush with hundreds of bristles
(called spatulae) at its end. When the gecko walks on the ceiling of a tank, hundreds of millions of bristles are
brushing against the glass. Each gecko bristle is made of organic (carbon-based) molecules, while the glass is
Unit 4 Bonding Homework AP
made up of molecules of a different substance, silicon dioxide. When the gecko moves its hairs over the glass,
the organic molecules brush past the silicon-dioxide molecules. When the two types of molecules are extremely
close together, tiny electrostatic forces (van der Waals forces) magically appear between them. Each of the
organic molecules sticks to a silicon dioxide molecule like a balloon sticks to your jumper. Every single bristle
provides a microscopic upward force that helps to stick the gecko to the ceiling. With hundreds of billions of
bristles all working as a team, there's more than enough sticking force to balance the gecko's weight.
1. Explain the difference between cohesion and adhesion.
2. Explain how the sole of your sneakers are attached to the top of the sneakers in terms of cohesion and
adhesion.
3. Define chemisorption with an example.
4. Are van der Waals forces strong or weak? Explain using your knowledge of chemistry and the gecko
example.