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Senanu's Bio class

Ch 2.2 Reading Guide – Water and life

If you are using the 6th edition of Freeman, please also read “How do Amino Acids link to form proteins” on p 81. The principles there are general, and not limited to proteins.

OBJECTIVE: Understand the structure of the polar covalent bonds in a molecule of water and explain how they enable water molecules to form hydrogen bonds.

  1. What is the molecular formula for water?
  2. Draw one molecule of water using Structural Formula format. Is this molecule bent or straight? Why? What do you think takes up space where there is no atom?
  3. Review: Why is the bond between hydrogen and oxygen considered to be ‘covalent’?
  4. Review: Why is the bond between hydrogen and oxygen considered to be ‘polar’? Explain.
  5. In your drawing of a water molecule above, identify which atom is more electronegative by labeling it with a partial negative charge.
    • What is the partial charge on the hydrogen atom?
  6. Draw two molecules of water.
    • Label each polar covalent bond in both molecules.
    • Write the partial charges on all atoms involved in polar bonds.
    • Identify where one hydrogen bond could form with a dashed line. (Why isn’t a solid line used?)

    OBJECTIVE: Describe how the emergent properties of water are the result of hydrogen bonding & what makes a molecule hydrophobic vs. hydrophilic.

  7. List 4 main properties of water that emerge from its ability to form hydrogen bonds. Briefly explain what is meant by each.

    Properties of water: Water is a versatile solvent

  8. In a solution of salt and water, _____________ is the solvent and _____________ is the solute.
  9. Fig 3.8 to explain how a large salt crystal can be dissolved by water molecules. Make sure you recognize how the polar covalent bonds in water dictate the orientation of the water molecules with respect to the ions.
  10. What happens if you mix a cup of water with a cup of oil?

  11. What is it about the bonds in oil that makes this so?

    Properties of water: Water Has Strong Cohesion and Adhesion

  12. What is the difference between cohesion and adhesion?

  13. How does the cohesive/adhesive property of water allow it to move against gravity in the xylem of a tree?

    Properties of water: Moderation of temperature by Water

  14. What is the difference between thermal energy and temperature?

  15. You are conducting an experiment in which you heat up a beaker of 200ml vegetable oil and a separate beaker of 200ml water on the same hotplate. You measure the temperature each minute and get the following results:

    Time (min) Temp Oil Temp Water
    0 20 20
    1 24 22
    2 29 25
    3 34 27
    4 38 30
    5 42 33
    6 47 35
    7 52 38
    8 57 41
  16. Use a different color to plot these data on a graph. Graph paper
    • Decide which variable should be on each axis. Read BioSkills 2 from your text.
    • Label the axes
    • Include units on your axis labels
    • Use a different color for each data series
    • Use a ruler for any straight lines you are drawing
    • Make sure you graph is neat and large
    • Give the graph a title or caption
    • Make sure you do all of these things when drawing any graphs in this class, regardless of whether you are drawing them by hand or with a computer!
  17. Explain why two substances in the graph above differ in their specific heat. Make sure you explain in terms of the hydrogen bonds that exist in water but don’t exist in oil.
  18. Do all molecules in a container of water have the same kinetic energy? Explain
  19. Which molecules are most likely to evaporate from the surface as water vapor?
  20. What happens to the average kinetic energy of the remaining molecules after evaporation?
  21. What is the term we use for “average kinetic energy”? Does it go up or down as water evaporates?

  22. Explain how evaporative cooling helps you maintain your body temperature on a hot day.

    Properties of water: Ice floats (rather than sinks) on liquid water.

  23. What happens to the density of most substances as they cool?
  24. What happens to the density of water as it cools below 4°C to freezing point (0°C)

  25. Explain how ice protects the habitat of freshwater fish.

    OBJECTIVE: Define acid, base, the pH scale, what pH measures & demonstrate the consequences of changing the pH to molecules.

  26. Write the reversible reaction of two water molecules dissociating into the Hydronium ion (H3O+) and the Hydroxide ion (OH-).
  27. When pure water reaches a state of dynamic equilibrium, which molecule is in the highest concentration? (H2O, H3O+, or OH- _____________
  28. What is the concentration of each ion in pure water?
  29. What exactly does the pH scale measure?
  30. Write the reversible reaction of sodium hydroxide (NaOH) dissociating into its ions.
  31. How does this dissociation allow sodium hydroxide to act as a base?

  32. What happens at the molecular level when an acid such as HCl is added to pure water?

    OBJECTIVE: Explain why a change in pH is harmful to cells and the function of buffers.

  33. What is the function of a buffer?
  34. Write the reversible reaction of carbonic acid (H2CO3) dissociating into bicarbonate ion (HCO3-) and a hydrogen ion (H+).
  35. Indicate which molecule donates H+ by labeling it an acid.

  36. Indicate which molecule removes H+ from the solution by labeling it a base.

    OBJECTIVE: Distinguish between spontaneous and non-spontaneous chemical reactions.

  37. What do you understand by the First Law of Thermodynamics?
  38. Considering what you wrote above about the First Law, what is the relationship between potential and kinetic energy?
  39. Which would you expect to have more energy stored in the bond – two atoms with similar electronegativities or two atoms with very different electronegativities? Explain.

  40. If electrons between atoms with equal electronegativities reacted in a chemical reaction to form bonds with 2 different atoms with different electronegativities, would this be an exothermic or endothermic reaction. Explain. Would the resulting bond contain more or less energy?

    OBJECTIVE: Understand how macromolecules are assembled and disassembled in condensation reactions and hydrolysis.

  41. Using Fig 2.24 (Fig 3.4 in 6th ed), what happens to the H on one reactant and the OH on the other reactant during a condensation reaction?
  42. Why do you think this is called a condensation reaction?
  43. Use the table in BioSkills 15 to understand the 2 parts of the word “hydrolysis”. How does this pertain to the reaction described?

Do the following in or after class…

  1. Fill a test tube (or glass) completely full of water, or put drops of water on a penny, what shape is the top surface of the water (convex or concave)? How do you explain this?
  2. If you partially fill a test tube (or glass) with water, what shape is the top surface? How do you explain this, and how is it different from the question above?
  3. Use your calculator to calculate the pH if the concentration of hydronium ions is 0.001M. What about if the concentration was 0.000001M? Which of these has more hydronium ions? Describe the relationship between hydronium ions and pH.
  4. Do you expect methane (CH4) to be polar or non-polar? What about oxygen gas (O2)? Draw these two molecules, indicating whether they have long or short bonds, and if those bonds contain a lot or relatively little energy.
  5. When methane and oxygen react, they form CO2 and 2H2O. Draw these molecules, indicating their relative bond length and if these bonds are expected to contain a lot or relatively little energy.
  6. Based on your drawings above, do you think the combustion of methane is spontaneous or not?