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Chemistry LibreTexts

Exercises

  • Page ID
    54337
  • Section 3-1

    Q1-1

    Where do you think the atoms in your body come from (go back as far as you can)?

    Q1-2

    Is the number of atoms in the universe a constant? Why or why not?

    Q1-3

    Can an atom of one element change into an atom of another element? How?

    Q1-4

    How do you know atoms exist?

    Q1-5

    How old are the atoms in your body?

    A) ~20 years

    B) ~600 years

    C) ~60,000 years

    D) ~10,000,000,000 years

    E) ageless (eternal)

    Section 3-2

    Q2-1

    Generate a graph that estimates the number of atoms in the universe as a function of time (beginning with the big bang, up to the present day).

    Now please explain your reasoning for why you constructed your graph the way you did.

    Q2-2

    Draw another graph to illustrate the number of elements in the universe as a function of time.

    Now please explain your reasoning for why you constructed your graph the way you did.

    Q2-3

    Can an isolated atom have a melting point or boiling point? Please explain why or why not?

    Q2-4

    Can an isolated atom exist in a state? (solid, liquid, gas?) Please explain why or why not?

    Q2-5

    Why do bonds between atoms form?

    Section 3-3

    Q3-1

    The melting point of molecular hydrogen (H2) is ~ 14K (-259 °C). Draw a molecular level picture of what molecular hydrogen looks like below this temperature (ie as a solid). Why are the molecules of hydrogen sticking together?

    Q3-2

    The boiling point of molecular hydrogen (H2) is ~ 20K (-253 °C). Draw a molecular level picture of what molecular hydrogen looks like above this temperature (ie as a gas)

    Q3-3

    At high temperatures eg > 6000K molecular hydrogen dissociates. Draw a picture of what you imagine this might look like. Why do you think it takes such a high temperature to bring about this change?

    Section 3-4

    Under normal circumstances Hydrogen atoms form molecules of H2, but Helium atoms do not form stable molecules of He2.

    • Please draw diagrams and explain why Hydrogen does form molecules of H2 and Helium does not form molecules of He2.

    Section 3-5

    Q5-1

    What would you predict for the type of bonding and intermolecular forces (IMF) in the chart below for each element:

    Elemental form

    H2

    He

    Li(s)

    Be(s)

    B(s)

    C(s)

    N2

    O2

    F2

    Ne

    Melting point

    13.81 K

    0.95 K

    453.65 K

    1560 K

    2348 K

    3823 K

    63.15 K

    54.36 K

    53.53 K

    24.56 K

    Boiling point

    20.28 K

    4.22 K

    1615 K

    2744 K

    4273 K

    4098 K

    77.36 K

    90.20 K

    85.03 K

    27.07 K

    BP-MP

    6.47 K

    3.27 K

    1161 K

    1184 K

    1925 K

    275 K

    14.21 K

    35.84 K

    31.5

    2.51 K

    Name

    hydrogen

    helium

    lithium

    beryllium

    boron

    carbon

    nitrogen

    oxygen

    fluorine

    Neon

    What kinds of bonds are present in each elemental form above?

    What IMFs are present if you have a mole of each elemental form?

    Q5-2

    What is the difference between a covalent bond and London Dispersion Forces?

    Section 3-6

    Q6-1

    Explain how the bonding in metals explains:

    A) Why metals are shiny

    B) Why metals are malleable

    C) Why metals conduct electricity

    Q6-2

    Diamond and Graphite have very different properties; discuss how the bonding differs between the two and how it affects its properties.

    Q6-3

    Why is nitrogen a gas at room temperature and carbon is a solid?

    Section 3-7

    Listed below are some properties of three elements: Na(s) Si(s), and Br2(l)

    Explain how the type of bonding in the element leads to its observed properties. Draw a picture at the atomic/molecular level of the element showing how the atoms are bonded, that can explain these observed properties

    Sodium: Na, mp 370 K, grey, shiny, malleable and ductile. Conducts electricity.

    Silicon: Si, mp 1687 K, grey, shiny, hard and brittle, can be chipped.

    Bromine: Br2, mp 266 K, brown liquid, easily vaporized (bp 332 K). At temperatures above 1000K, Br2 dissociates.

    Section 3-8

    Q8-1

    1. In the box below draw a molecular representation of solid water, ice, H2O(s).

    Explain what your diagram shows:

    • When ice is heated it liquefies. Draw a picture of what that looks like at the molecular level.\

    • Explain what is happening at the molecular level during this process

    • Now imagine the liquid is heated up to its boiling point (100 C). Draw a picture of what that looks like at the molecular level.
    • Explain what is happening at the molecular level during this process

    Photoelectron Effect

    Download the applet on the photoelectric effect from

    http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect

    Q1

    Draw and label a schematic of the set-up. Explain what each part is for (ignore the voltmeter, and everything on the right hand side where there are various settings - we will come back to that).

    Q2

    Set the wavelength to 400 nm and turn up the intensity to 100% - what do you observe? What are you changing when you increase the intensity (the energy of an individual photon or the number of photons)?

    Q3

    Now keep the intensity at 100% and change the wavelength of the light from 400 to 700 nm. What do you observe? What are you changing when you change the wavelength (the energy of an individual photon or the number of photons)?

    Q4

    Choose a wavelength of light where electrons are emitted and change the intensity – what happens? Explain in terms of photons why this happens.

    Q5

    Choose an intensity and wavelength where electrons are emitted, and increase the wavelength. What happens? Explain in terms of photons why this happens.

    Q6

    Draw a graph that represents the number of electrons emitted as a function of wavelength of light illuminating the target at 100% intensity (change the wavelength with the slider from 400 to 700 nm).

    Q7

    Explain why the photoelectric effect provides evidence for the quantized nature of light.

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