Chem Exam 1

What is wrong with the following statement? As the ball falls, its potential energy decreases.

-A single object cannot possess potential energy -There is no mention of a field

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 potential energy well is deeper for Xe because Xe has more electrons and therefore has stronger LDFs. -The potential well for Xe has a longer distance between the nuclei than Ne because Xe is a bigger atom (more electrons and larger electron cloud)

Similarities between gravitational force and electrostatic force

-mediated by fields -proportional to 1/r^2 -requires 2 objects

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.

Which of these statements are true: I. All neutral atoms and molecules attract each other. II. London dispersion forces increase as the size of the electron cloud increases. III. It takes energy to overcome the attractions between particles. IV. Energy is released to the surroundings when atoms form a stable interaction

All are true.

If you put bromine liquid in a pan on the stove and heat it up the bromine will boil. What is the mechanism by which this happens?

As more bromine molecules get more energy, they can overcome the electrostatic forces of attraction and the bromine molecules can escape into the gas phase.

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. The electrostatic repulsion between the electrons means that the atoms are forced apart. Since energy had to be used to "push" the electrons together (in the form of kinetic energy) this energy must be stored in the electrostatic field as potential energy.

Now you have an isolated system consisting of two charged species of the same sign. a. 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.

You have an isolated system of two masses (for example, a ball and the ground). a.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.

Now you have an isolated system consisting of two oppositely charged species a. 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.

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 allows a stable interaction to occur.

You are asked to compare the interaction between atoms of two substances, substance X and substance Y. When you look at the graph showing how potential energy changes as distance between the atoms changes, you notice that substance Y has a much deeper potential energy well than substance X. From this information, you conclude...

Atoms of Y interact to form covalent bonds; covalent bonds are stronger than London dispersion forces and require more energy to break.

Choose the best set of claim, evidence, and reasoning that can be used to explain the results of Rutherford's experiments

Claim: The atom is mostly empty space with a very small dense nucleus. Evidence: Most alpha particles went straight through, only a few were deflected. Reasoning: The positively charged alpha particles were deflected by the repulsive interaction with the positive nucleus; since only a few were deflected the nucleus must take up a very small volume of the atom.

The electron was the first sub-atomic particle that was discovered (by Thomson.) 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 also have a charge so they are easier to detect than an uncharged particle (like a neutron).

Thomson's experiment allowed him to make the claim that all atoms have a sub structure and contain smaller particles (electrons) embedded in the structure of the atom. What evidence and reasoning allowed him to make this claim?

Evidence: Electrons were ejected from the cathode, regardless of what metal the cathode was made Reasoning: Since the particles are emitted regardless of the nature of the cathode, they must be part of the sub-structure of the atom.

The atomic theory of matter is an example of a scientific theory. What distinguishes it from a scientific law such as the law of gravity? The atomic theory...

Explains the underlying cause of a range of phenomena.

Is this statement (one of Dalton's atomic theory tenets) true or false and why? Elements are composed of small indivisible, indestructible particles called atoms.

False; Atoms have a sub-structure.

The boiling point of hydrogen is 20K. Describe the particles present in a container of hydrogen at 25K.

Gaseous hydrogen molecules.

Differences between gravitational force and electrostatic force

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

Which of the following are correct statements? I. Covalent bonds are interactions within molecules II. Covalent bonds are interactions between separate molecules. III. LDFs are typically interactions between separate molecules IV. LDFs are within molecules

I. Covalent bonds are interactions within molecules III. LDFs are typically interactions between separate molecules

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. The kinetic energy increases because the objects accelerate toward each other (the stronger force causes the velocity to increase).

Consider the following four objects: M1 = 5 g M2 = 5 g M3 = 10 g M4 = 10 g Assume that the objects are separated by equal distance. Between which two objects is the gravitational force the strongest?

M3 and M4

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 theory?

Most alpha particles (nucleus of the helium atom) passed straight through the gold foil. Only a small fraction were deflected by the + nucleus. Therefore, only a small part of the volume of the atom contains the + charge.

Consider the question: Do dragons have wings? Is this question a scientific question? Why or why not

No; There is no way to design an experiment to answer this question.

Why do you think the potential energy graphs look different? Explain.

PE decreasing is for a system with an attractive force whereas PE increasing is for a system with a repulsive force.

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?

Particles (electrons) were emitted from many different cathodes (the negative end of the apparatus). These "cathode rays" were the same regardless of the types of atoms in the cathode. Therefore, electrons must be a sub-atomic particle.

Design an experiment that involves what happens when cold milk is added to hot coffee.

Question: How does the temperature of hot coffee change when cold milk is added? Start with 100mL of hot coffee. Add small volumes (5mL) of cold milk. Monitor the temperature as the milk is added. Collect the temperature of the hot coffee, cold milk, and the coffee/milk mixture. Plot the temperature vs. the volume of milk added.

The boiling point of neon is -246°C and the melting point of neon is -249°C. What state is neon at -260°C?

Solid.

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.

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 to get the atoms to separate. Therefore, the deeper the well, the stronger the interaction.

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 that the electron cloud has shifted to 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. This attractive force holds the molecules together in the liquid and solid states.

What is the cause of the attractive force between the two He atoms?

The electron clouds of the He atoms may fluctuate randomly causing a transient dipole that induces a dipole in other atoms which causes them to attract each other.

Two atoms approach and get close enough so that their electron clouds overlap. Why does the potential energy increase?

The electron clouds repel each other.

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 the atom (like chocolate chips in a cookie). Dalton's model of the atom was an indivisible atom that had no subatomic particles.

How would the strength of the electrostatic force change if the distance between two charged objects increased?

The electrostatic force would become weaker.

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 because Cl2 is bigger than F2 (bigger electron clouds). The induced dipoles in Cl2 are bigger and the force of attraction is more. The depth of the PE well tells you how much energy is needed to separate the molecules from each other, so Cl2 is more stable than F2. The position of the well along the x-axis gives information about internuclear distance. Because Cl atoms are bigger that F atoms, Cl2 has a greater internuclear distance.

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.

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 potential energy increases as the two objects move closer together. This is because of the repulsive force between the two objects.

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 the attraction. As energy is added, the particles (atoms) vibrate more and more strongly until they have enough energy to separate.

In an isolated system, when two atoms collide, which is always true?

The total energy of the system will not change.

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

Thermal energy is transferred to the molecules (by collisions) increasing their kinetic energy. Eventually, the molecules have enough thermal energy to over come the attractive forces and move apart from each other.

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 the forces holding the atoms in place are overcome and the atoms can more relative to each other. If more energy is added, the particles will separate completely to form gaseous

The distance between the two helium atoms where the system would be the most stable is...

Where the potential energy is at a minimum.

The diagram to the right shows the PE curves for two atoms of X and two atoms of Y. Which is true?

X has a longer stronger interaction than Y

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 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 mover relative to each other, which is what melting the solid does.

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

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