EXAM 1

Question 1

Different for gravitational and electrostatic

Answer 1

Gravitational: proportional to mass of
objects
Electrostatic: proportional to charge
——————————————————
Gravitational: only attractive
Electrostatic: can be attractive or repulsive

Question 2

Same for both gravitational and
electrostatic

Answer 2

Meditated by fields
Proportional to 1/r^2
require two objects

Question 3

3. 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?

Answer 3

As the objects get closer together, the gravitational force (an attractive force) will increase.

Question 4

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?

Answer 4

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.

Question 5

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.

Answer 5

PE increases as the distance increases

Question 6

3d. Since the system is isolated, what do you think will happen to the kinetic energy as the
potential energy changes?

Answer 6

In an isolated system, the total energy is constant. As the potential energy decreases,
the kinetic energy will increase.

Question 7

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?

Answer 7

As the opposite charges move closer together, the attractive electrostatic force will increase.

Question 8

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?

Answer 8

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.

Question 9

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

Answer 9

PE increases as the distance between the objects increase.

Question 10

4d. Since the system is isolated, what do you think will happen to the kinetic energy as the
potential energy changes?

Answer 10

Since the system is isolated, the total energy remains constant. As the potential energy
decreases, kinetic energy increases.

Question 11

5. 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?

Answer 11

As the charges with the same sign move closer together, the repulsive electrostatic force will
increase.

Question 12

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?

Answer 12

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.

Question 13

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

Answer 13

PE decreases as distance increases.

Question 14

5d. Since the system is isolated, what do you think will happen to the kinetic energy as the
potential energy changes?

Answer 14

In an isolated system, total energy is constant. As potential energy increases, kinetic
energy will decrease

Question 15

Why do you think the potential energy graphs 4c and 5c look different? Explain.

Answer 15

3d is for a system with an attractive force whereas 4d is for a system with a repulsive
force.

Question 16

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.

Answer 16

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.

Question 17

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.

Answer 17

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.

Question 18

Describe the forces that are present and their respective strengths at the potential minimum.

Answer 18

At the potential minimum the strength of the attractive forces equals the strength of the repulsive forces. This results in a stable interaction.

Question 19

What information does the depth of the potential well tell you about the strength of the interaction?

Answer 19

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

Question 20

What information can you get from the position of the potential minimum along the
x-axis (the distance axis)?

Answer 20

You can determine the distance between the nuclei when the atoms are most stable.

Question 21

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.

Answer 21

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.

Question 22

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.

Answer 22

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.

Question 23

Explain the relationship between the strength of the interaction between particles and the melting point.

Answer 23

The stronger the attraction between the particles the more energy is needed to
overcome that attraction.

Question 24

Predict and explain the relative melting points of Ne and Xe.

Answer 24

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.

Question 25

What is the difference between an atom and a molecule?

Answer 25

An atom is the smallest unit of an element. A molecule is a collection of atoms chemically connected to each other.

Question 26

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

Answer 26

a. I and III

Question 27

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

Answer 27

e. III only

Question 28

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

Answer 28

c. II and V

Question 29

Which of the following contain(s) atoms? a. heat c. energy e. all of the above b. light d. dust

Answer 29

d. dust

Question 30

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

Answer 30

f. III and VI

Question 31

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.

Answer 31

a. All atoms are indivisible and indestructible

Question 32

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

Answer 32

a. The electrostatic force would become stronger.

Question 33

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

Answer 33

a. I and III

Question 34

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.

Answer 34

c. Because the repulsive force, between the overlapping electron clouds, is dominant

Question 35

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

Answer 35

d. The partial positive end of one atom’s instantaneous dipole attracts the partial negative end of the neighboring atom’s induced dipole

Question 36

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

Answer 36

c. II and III

Question 37

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

Answer 37

d. III and V

Question 38

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

Answer 38

a. I and IV

Question 39

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.

Answer 39

d. F2, because the LDFs between fluorine molecules are stronger, requiring more energy to overcome.

Question 40

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

Answer 40

b. I, II, and V

Question 41

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

Answer 41

d. The kinetic energy increases because the atoms are attracted to each other and are speeding up.

Question 42

The electron was the first sub-atomic particle that was discovered. Why do you think it was the first?

Answer 42

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

Question 43

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?

Answer 43

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.

Question 44

Draw a picture of Thomson’s proposed atomic structure and indicate how it differs from Dalton’s model of the atom.

Answer 44

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.

Question 45

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?

Answer 45

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.

Question 46

Draw a picture of Rutherford’s model of the atom. What is wrong with it?

Answer 46

A negatively charged particle moving in an electric field will emit radiation and spiral into the nucleus, causing the atom to implode.

Question 47

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

Answer 47

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.

Question 48

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.

Answer 48

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.

Question 49

Why do molecules of fluorine attract each other? (That is, why can fluorine exist as a liquid or a solid?)

Answer 49

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).

Question 50

Why does adding thermal energy make fluorine change from a liquid to a gas?

Answer 50

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.

Question 51

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

Answer 51

A. Electrostatic Attraction (As the atoms move together, the PE decreases (because they are moving with the attractive force).

Question 52

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

Answer 52

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.

Question 53

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.

Answer 53

D. ES attraction = ES repulsion (At the potential minimum, the system is most stable. The attractive forces = the repulsive forces.

Question 54

Bonds and IMFs Which type of interaction is stronger? a. Covalent bond b. LDFs c. Same

Answer 54

a. Covalent Bond

Question 55

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

Answer 55

Energy is absorbed

Question 56

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

Answer 56

b. Energy is released