EXAM 1 Flashcards
Different for gravitational and electrostatic
Gravitational: proportional to mass of
objects
Electrostatic: proportional to charge
——————————————————
Gravitational: only attractive
Electrostatic: can be attractive or repulsive
Same for both gravitational and
electrostatic
Meditated by fields
Proportional to 1/r^2
require two objects
- You have an isolated system of two masses (for example, a ball and the ground).
3a. How do you think the gravitational force will change with respect to distance between the
two masses?
As the objects get closer together, the gravitational force (an attractive force) will increase.
3b. Explain in words what is happening to the potential energy as the two masses move
together. Why does the potential energy change in this way?
The potential energy decreases as the two masses get closer together because there is
an attractive force between the masses and they are moving with the force.
3c. Draw a graph to show how the potential energy of the system changes with distance
between the same two masses. Be sure to label your axes. We will define the potential
energy of the two masses at a great distance (where they don’t interact) as starting at the
distance axis.
PE increases as the distance increases
3d. Since the system is isolated, what do you think will happen to the kinetic energy as the
potential energy changes?
In an isolated system, the total energy is constant. As the potential energy decreases,
the kinetic energy will increase.
4Now you have an isolated system consisting of two oppositely charged species.
4a. How do you think the electrostatic force would change with respect to the distance
between the oppositely charges species?
As the opposite charges move closer together, the attractive electrostatic force will increase.
4b. Explain in words what is happening to the potential energy as the two opposite charged
species move together. Why does the potential energy change in this way?
The attractive electrostatic force causes the potential energy to drop as the two
opposite charged species approach each other because they are moving with the force.
4c. Draw a graph to show how the potential energy of the system changes with distance
between the two opposite charged species. Be sure to label your axes. We will define the
potential energy of the two charged objects at a great distance (where they do not interact)
as starting at the distance axis
PE increases as the distance between the objects increase.
4d. Since the system is isolated, what do you think will happen to the kinetic energy as the
potential energy changes?
Since the system is isolated, the total energy remains constant. As the potential energy
decreases, kinetic energy increases.
- Now you have an isolated system consisting of two charged species of the same sign.
5a. How do you think an electrostatic force would change with respect to the distance
between the two charged species with the same sign?
As the charges with the same sign move closer together, the repulsive electrostatic force will
increase.
5b. Explain in words what is happening to the potential energy as the two charged species of
the same sign move together. Why does the potential energy change in this way?
The repulsive electrostatic force causes the potential energy to increase as the two like
charged species approach each other because they are moving against the force.
5c. Draw a graph to show how the potential energy of the system changes with distance
between the two charged species of the same sign. We will define the potential energy of
the two charged objects at a great distance (where they do not interact) as starting at the
distance axis
PE decreases as distance increases.
5d. Since the system is isolated, what do you think will happen to the kinetic energy as the
potential energy changes?
In an isolated system, total energy is constant. As potential energy increases, kinetic
energy will decrease
Why do you think the potential energy graphs 4c and 5c look different? Explain.
3d is for a system with an attractive force whereas 4d is for a system with a repulsive
force.
Draw a molecular level picture to show what happens as two Neon atoms approach
each other and use it to help you explain why the two atoms are attracted to each
other.
The electron cloud on one atom randomly fluctuates causing an uneven distribution of charge within that atom. That instantaneous dipole induces a dipole in a neighboring atom. The partial positive end of one dipole attracts the partial negative end of the other dipole.
As the atoms get very close together their electron clouds overlap. Draw a picture and use it to help you explain why the potential energy rises as the atoms get very close.
As the atoms get very close, their electron clouds are essentially in the same space. Electrons from one atom will repel the electrons from the other atom. When the atoms move together, they are moving against the repulsive force so the potential energy will increase.
Describe the forces that are present and their respective strengths at the potential minimum.
At the potential minimum the strength of the attractive forces equals the strength of the repulsive forces. This results in a stable interaction.
What information does the depth of the potential well tell you about the strength of the interaction?
The deeper the potential well, the more energy would be required overcome the interaction between the atoms. Therefore, the deeper the well, the stronger the
interaction
What information can you get from the position of the potential minimum along the
x-axis (the distance axis)?
You can determine the distance between the nuclei when the atoms are most stable.
Now draw a PE curve for the interaction of two Ne atoms, and then on the same set
of axes draw a curve for the interaction of two Xe atoms. Explain the relative depths
of the potential wells and the relative positions of the minima along the x-axis.
The solid line represents the PE curve for Ne and dashed line represents the PE curve
for Xe. Ne has 10 electrons and Xe has 54 electrons.
The potential energy well is deeper for Xe because Xe has more electrons and
therefore bigger partial charges on the instantaneous dipoles. Bigger charges result
in stronger LDFs.
The potential well for Xe is farther right because Xe has a larger electron cloud than
Ne. This results in a longer internuclear distance at the potential minimum when the
edges of the electron clouds are just touching.
Draw a picture of solid Xe and liquid Xe. Use your picture to explain why you need to add energy to change a solid to a liquid.
To change solid Xe into liquid Xe, we have to add energy so that some of the forces
holding the atoms in place are overcome and the atoms can move relative to each
other. If enough energy is added to overcome all of the forces holding the atoms
together, the particles will separate completely to form gaseous Xe.
Explain the relationship between the strength of the interaction between particles and the melting point.
The stronger the attraction between the particles the more energy is needed to
overcome that attraction.
Predict and explain the relative melting points of Ne and Xe.
Xe has a higher melting (and boiling) point than Ne because it has stronger LDFs. Xe has stronger LDFs because of the larger distortion (greater partial
charges) of the larger electron clouds around each Xe. This makes the Xe atoms more strongly attracted to each other and therefore more energy (a
higher temperature) is needed to separate the atoms so they can move relative to each other.
What is the difference between an atom and a molecule?
An atom is the smallest unit of an element. A molecule is a collection of atoms chemically connected to each other.
Consider the question: Is there an attractive force between a pencil and the earth?
Is this a scientific question?
I. Yes
II. No
Because
III. It can be tested and answered through observations and experiments.
IV. The answer is already known and we find it online.
V. There is no way to design an experiment to answer this question.
a. I and III c. II and IV
b. I and IV d. II and V
a. I and III
Which of the following properties ascribed to atoms by the Greeks do we still consider
valid?
I. The size and the shape of the atoms determine a material’s properties.
II. Atoms are indivisible.
III. Atoms are in constant motion.
a. I only c. II and III e. III only
b. I and II d. II only
e. III only
Electrons are negatively charged. Which experiment provided evidence to support this
claim? What is the specific evidence used to support this claim?
Experiment:
I. The gold foil experiment.
II. The cathode ray tube experiment.
Evidence:
III. Most of the positively charged alpha particles shot at a thin gold foil passed
straight through.
IV. A small fraction of the positively charged alpha particles shot at a thin gold foil
were deflected.
V. The beam of particles emitted from the cathode bent toward the positively
charged plate.
VI. The beam of particles emitted from the cathode behaved the same no matter what
type of metal the cathode was made of.
a. I and III c. II and V
b. I and IV d. II and VI
c. II and V
Which of the following contain(s) atoms?
a. heat c. energy e. all of the above
b. light d. dust
d. dust
The diagram below shows alpha particles being fired at a piece of gold foil. Where will
the majority of the alpha particles be detected (point I, II, or III) and why?
Because:
IV. Alpha particles bounce off the nuclei within the gold atoms, because the nuclei
are much more massive than the alpha particles.
V. The alpha particles are repelled when they come close to the nuclei, because both
are positively charged.
VI. The alpha particles go straight through the atom, because it is mostly empty
space.
a. I and IV c. II and V e. III and IV
b. I and V d. II and VI f. III and VI
f. III and VI
Thomson’s cathode ray tube experiment proved false what part of Dalton’s atomic
theory?
a. All atoms are indivisible and indestructible.
b. All atoms of a given element are identical.
c. All matter is made of atoms.
d. Compounds are formed from the combination of two or more elements.
e. Chemical reactions are rearrangements of atoms.
a. All atoms are indivisible and indestructible
How would the strength of the electrostatic force change if the distance between two
charged objects decreased?
a. The electrostatic force would become stronger.
b. The electrostatic force would become weaker.
c. The electrostatic force would become stronger or weaker depending on whether the
force is attractive or repulsive.
d. It is impossible to say from the information given
a. The electrostatic force would become stronger.
Consider the following statement: “A nitrogen molecule is made of the same type of
atoms.” Is this statement true or false and why?
I. True
II. False
Because:
III. An element is made of one type of atom. Nitrogen is an element that exists as
diatomic molecules.
IV. Nitrogen is a compound that exists as diatomic molecules. Compounds are made
of different types of atoms.
V. Nitrogen is an element, and therefore must exist as separate atoms not molecules.
a. I and III c. I and V e. II and IV
b. I and IV d. II and III f. II and V
a. I and III
Why does the potential energy increase as two xenon atoms move extremely close
together?
a. Because the kinetic energy decreases as the atoms slow down.
b. Because the electrostatic attractive force, from the instantaneous dipoles, is dominant.
c. Because the repulsive force, between the overlapping electron clouds, is dominant.
d. Because there is more potential energy than kinetic energy when the atoms are close
together.
c. Because the repulsive force, between the overlapping electron clouds, is dominant
Which statement best describes the forces that exist between helium atoms in the solid
state?
a. The electron cloud on one helium atom is attracted to the electron cloud on the other
atom.
b. The nucleus of one helium atom is repelled by the nucleus of the neighboring atom.
c. The helium atoms share electrons, forming a bond.
d. The partial positive end of one atom’s instantaneous dipole attracts the partial
negative end of the neighboring atom’s induced dipole.
e. Both atoms have mass, so they are attracted to each other by gravitational forces
d. The partial positive end of one atom’s instantaneous dipole attracts the partial
negative end of the neighboring atom’s induced dipole
When a covalent bond is formed between two nitrogen atoms:
I. Energy is absorbed
II. Energy is released
III. A molecule is formed
IV. The nitrogen changes from the gaseous to liquid state.
a. I and II c. II and III e. I, II, III, and IV
b. I and III d. II, III, and IV
c. II and III
The London dispersion force between two neon atoms compared to the London
dispersion force between two argon atoms where both systems are the most stable is:
I. the same
II. stronger
III. weaker
Because:
IV. They are both noble gases.
V. Neon has a smaller electron cloud, therefore there will be a smaller separation of
charge resulting in a weaker force.
VI. Neon has a smaller electron cloud, therefore there will be a larger separation of
charge resulting in a stronger force.
a. I and IV c. II and VI e. III and VI
b. II and V d. III and V
d. III and V
Is the following statement true or false?
When chlorine freezes, London dispersion forces are formed.
This statement is
I. True
II. False
Because
III. When a substance freezes, covalent bonds are formed.
IV. There are LDFs in both the solid state and the liquid state, but more in the solid
state.
V. There are LDFs in the solid state, but not in the liquid state.
a. I and IV c. II and III e. II and V
b. I and V d. II and IV
a. I and IV
Which do you predict to have the higher boiling point, fluorine (F2) or neon (Ne) and
why?
a. Ne, because the smaller atoms are packed more tightly together and harder to break
apart.
b. Ne, because the LDFs between neon atoms are stronger, requiring more energy to
overcome.
c. F2, because the covalent bonds between fluorine atoms are stronger, requiring more
energy to overcome.
d. F2, because the LDFs between fluorine molecules are stronger, requiring more energy
to overcome.
d. F2, because the LDFs between fluorine molecules are stronger, requiring more energy
to overcome.
Consider a system of two atoms with a stable interaction
between them (the atoms are stuck in the bottom of the
potential energy well). The potential energy curve for this
system is shown to the right. What would happen to the
system for the potential energy to change from Y to Z?
I. A third atom from the surroundings would collide with
the system.
II. Energy would transfer from the surroundings to the system.
III. Energy would transfer from the system to the surroundings.
IV. The interaction between the atoms in the system would be formed.
V. The interaction between the atoms in the system would be overcome.
a. I, II, and IV c. I, III, and V e. II only
b. I, II, and V d. I, III, and IV f. V only
b. I, II, and V
Use the potential energy curve in question 18 to explain the change in kinetic energy as
two atoms move from Z to Y.
a. The kinetic energy decreases because the potential energy decreases.
b. The kinetic energy does not change because the two atoms are an isolated system.
c. The kinetic energy decreases because the atoms are repelling each other and are
slowing down.
d. The kinetic energy increases because the atoms are attracted to each other and are
speeding up.
e. The kinetic energy increases because the potential energy decreases
d. The kinetic energy increases because the atoms are attracted to each other and are
speeding up.
The electron was the first sub-atomic particle that was discovered. Why do you think it was
the first?
Electrons are located on the outside of the atom (the nucleus is in the middle).
Therefore, it is easier to remove the electrons. Electrons are also charged so they are
easier to detect than an uncharged particle (like a neutron) because charged particles
can be manipulated with electrical and magnetic fields
Describe the evidence that was used to support the discovery of the electron. Why was it
assumed that electrons exist within atoms, rather than are created in the course of the
experiment?
A beam of particles (electrons) was emitted from cathodes (the negative end of the
apparatus) composed of many different metals. These “cathode rays” were the same
regardless of the types of atoms in the cathode. Therefore, electrons must be a sub-
atomic particle that exists in all of the metals that were tested.
Draw a picture of Thomson’s proposed atomic structure and indicate how it differs from
Dalton’s model of the atom.
The electrons were embedded in a positively charged blob,
together making up the atom. This is similar to how
chocolate chips are embedded in a cookie to make up a
chocolate chip cookie.
Dalton’s model of the atom was an indivisible
atom that had no subatomic particles.
Rutherford discovered the existence of a small positively charged nucleus within each
atom. What observation was this conclusion based on? How did this change Thomson’s
atomic model?
Rutherford observed that when alpha particles are shot at a piece of gold foil, most go
straight through, and a tiny fraction were deflected. He concluded (claimed) that the
atom is mostly empty space and contains a very small, positively charged nucleus.
Because the nucleus is so small, only a tiny fraction of the positively charged alpha
particles come close enough to the positively charged nucleus to be repelled and change
direction.
Draw a picture of Rutherford’s model of the atom. What is wrong with it?
A negatively charged particle moving in an electric field
will emit radiation and spiral into the nucleus, causing the atom to implode.
Fluorine (F2) and Chlorine (Cl2) both exist as diatomic molecules. Fluorine has a boiling
point of 85 K, and chlorine has a boiling point of 239 K.
a. Draw a graph showing the change in potential energy as two molecules of fluorine get closer
to each other. Label the position where the molecules are most stable. Explain why the
potential energy changes (either increases or decreases) as the two molecules approach
As the molecules initially approach, an instantaneous dipole on one molecule will induce a
dipole on the other molecule. The partial positive end of one dipole will attract the partial
negative end of the other dipole, causing the molecules to come together. Since the molecules
are moving together with the attractive force, the potential energy decreases. When the
molecules get very close together their electron clouds overlap and repulsive force between
the electrons becomes dominant. Now, the molecules are moving together against the
repulsive force, causing the potential energy to increase.
Now using a dotted line draw a similar curve – on the same set of axes. Show the change in
potential energy when two chlorine molecules approach each other. Compare the energy
changes and position of greatest stability for chlorine to the curve you drew for fluorine.
Explain why they are different.
The minimum for Cl2 is lower than the minimum for F2 which tells us that it takes more
energy to break the interaction (LDF) between two Cl2 molecules. The interaction is stronger
for Cl2 because it has a larger electron cloud than F2, leading to larger partial charges on
the instantaneous dipoles, and a stronger electrostatic attraction. Stronger LDFs require
more energy to overcome.
The position of the well along the x-axis gives information about the distance between the
centers of the molecules. Because Cl atoms are bigger that F atoms, Cl2 molecules are larger
than the F2 molecules. When two Cl2 molecules interact through LDFs, the molecule centers
cannot get as close together as they can when two F2 molecules interact through LDFs. This
is reflected by the minimum for Cl2 being farther to the right.
Why do molecules of fluorine attract each other? (That is, why can fluorine exist as a liquid
or a solid?)
The electron cloud on a fluorine molecule is not stationary but fluctuates. When the electron
cloud shifts to one side of a fluorine molecule, it creates an instantaneous dipole. The side to
which the electron cloud has shifted has a partial negative charge whereas the other side has
a partial positive charge. This induces a dipole in neighboring molecules. The partial
positive end of one molecule attracts the partial negative end of a neighboring molecule.
When the molecules are touching (like they are in the solid and liquid states), we call this
attractive force the London Dispersion Force (LDF).
Why does adding thermal energy make fluorine change from a liquid to a gas?
Thermal energy is transferred to the molecules upon collision which increases their kinetic
energy. When the molecules have enough thermal energy to overcome the attractive forces
between them, they move apart from each other.
As the atoms move
toward each other, which
force is strongest?
A. Electrostatic
Attraction (As the atoms move together, the PE decreases (because
they are moving with the attractive force).
B. Electrostatic
repulsion
C. Gravitation
D. ES attraction =
ES repulsion
A. Electrostatic
Attraction (As the atoms move together, the PE decreases (because
they are moving with the attractive force).
As the atoms move even closer,
which force is strongest?
A. Electrostatic attraction
B. Electrostatic repulsion (As the atoms move even closer, the PE increases (because they are
moving against the repulsive force). When they get too close together,the repulsive force becomes strong enough to push the atoms apart.
C. Gravitation
D. ES attraction = ES repulsion
B. Electrostatic repulsion (As the atoms move even closer, the PE increases (because they are
moving against the repulsive force). When they get too close together,the repulsive force becomes strong enough to push the atoms apart.
At the potential
minimum, which
force is strongest?
A. Electrostatic
attraction
B. Electrostatic
repulsion
C. Gravitation
D. ES attraction =
ES repulsion (At the potential minimum, the system is most stable. The
attractive forces = the repulsive forces.
D. ES attraction =
ES repulsion (At the potential minimum, the system is most stable. The
attractive forces = the repulsive forces.
Bonds and IMFs
Which type of interaction is stronger?
a. Covalent bond
b. LDFs
c. Same
a. Covalent Bond
Which statement about the breaking of a single chemical bond is true?
a. Energy is absorbed
b. Energy is released
c. Energy is absorbed or released depending on the polarity of the bond being broken
d. Energy is released or absorbed depending on the strength of the bond being broken
Energy is absorbed
Which statement about the forming of a single chemical bond is true?
a. Energy is absorbed
b. Energy is released
c. Energy is absorbed or released depending on the polarity of the bond being broken
d. Energy is released or absorbed depending on the strength of the bond being broken
b. Energy is released
