chem hw 21

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chem hw 21
2010-05-12 13:21:27
chem hw

chem hw 21
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  1. ch 18 rq 9
    What does the pH of a solution indicate?
    The pH indicates if a solution is acidic, neutral, or basic, by the concentration of hydronium ions.
  2. rq 10
    As the hydronium-ion concentration of a solution increases, does the pH of the solution increase or decrease?
    As the hydronium concentration increases, the solution is more acidic, and the pH decreases.
  3. rq 11
    What is the product of the reaction between carbon dioxide and water?
    Carbonic acid, H2CO3
  4. rq 12
    What does sulfur dioxide have to do with acid rain?
    Sulfur dioxide in the atmosphere mixed with rain water produce sulfuric acid, which is acid rain.
  5. rq 13
    How do humans generate the air pollutant sulfur dioxide?
    The pollutant sulfur dioxide is produced from the combustion of oil and coal.
  6. ex 4
    What happens to the corrosive properties of an acid and a base after they neutralize each other? Why?
    The corrosive properties are no longer present because the acid and base no longer exist. Instead they have chemically reacted with each other to form completely new substances - salt and water - that are not so corrosive.
  7. ex 5
    Why do we use the pH scale to indicate the acidity of a solution rather than simply stating the concentration of hydronium ions?
    The concentration of hydronium ions is typically so small it needs to be stated using scientific notation. The pH scale, therefore, is one of convenience.
  8. ex 8
    What happens to the pH of an acidic solution as pure water is added?
    As water is added to an acidic solution, the hydronium ions becoem more dilute, that is, less concentrated. thus the pH increases.
  9. ex 9
    Can an acidic solution be made less acidic by adding an acidic solution?
    Yes, provided that the added acidic solution is less concentrated with hydronium ions. For example, a 1 M solution of HCl will become less acidic if you were to add to it a .01 M solution of HCl. The .01 M solution has the effect of diluting the 1 M solution.
  10. ex 12
    Pour vinegar onto beach sand from the Caribbean and the result is a lot of froth and bubbles. Pour vinegar onto beach sand from California, however, and nothing happens. Why?
    Beach sand from the Caribbean and many other tropical cliamtes is made primarily of the calcium carbonate remains of coral and shelled creatures. Vinegar is an acid and the calcium carbonate is a base. The reaction between the two results in the formation of carbon dioxide, which creates bubbles as it is formed. California beach sand primarily comes from the erosion of rocks and minerals, which are mostly made of inert silicon dioxide, SiO2.
  11. ex 17
    What happens to the pH of soda water as it loses its carbonation?
    As the soda water loses carbon dioxide molecules if is losing the carbonic acid that these carbon dioxide molecules form when in solution. Thus, the pH of a flat soda is typically higher than the pH of the same soda when carbonated.
  12. ex 18
    How might you tell whether or not your toothpaste contained calcium carbonate, CaCO3, or perhaps baking soda, NaHCO3, without looking at the ingredients label?
    Add it some vinegar. If the toothpaste starts to bubble, there is a good chance it contains calcium carbonate.
  13. ex 19
    Why do lakes lying in granite basins tend to become acidified by acid rain more readily than lakes lying in limestone basisn?
    It is the alkaline character of limestone (also known as calcium carbonate) that serves to neutralize waters that might be acidified in the mid wester United States.
  14. pr 1
    Show that the hydroxide-ion concentrationin an aqueous solution is 1 x 10 ^-4 M when the hydronium-ion concentration is 1 x 10 ^-10 M.
    The product of the hydroxide ion and hydronium ion concentration is always equal to 1 x 10 ^-14. So if the hydroxide ion concentration equals 1 x 10 ^-14 M, then the hydronium concentration must be equal to 1 x 10 ^-10 M, because these two values multiply together to equal 1 x 10^14.
  15. Pr 2
    When the hydronium-ion concentration of a solution is 1 x 10^10 M, what is the pH of the solution? Is the solution acidic or basic?
    • pH = -log (10 ^-10) = -(-10) = 10
    • This solution is basic.
  16. Pr 3
    When the hydronium-ion concentration of a solution is 1x 10 ^-4 M, what is the pH of the solution? Is the solution acidic or basic?
    pH = -log (10^4) = - (-4) = 4 This solution is acidic.
  17. rq 27
    What metal coats a galvanized nail?
    Zinc coats the nail, and oxidizes to form a thin layer of zinc oxide, and inert, insoluble substance that protects the iron underneath.
  18. rq 24
    What is electrolysis, and how does it differ from what goes on inside a battery?
    Electrolysis is the use of electrical energy to produce chemical change. Recharging a car battery is an example of electrolysis. It is the opposite of the oxidation-reduction reaction that goes on inside a battery. It is using electrical energy to reverse the oxidation-reduction reaction.
  19. rq 21
    What type of reaction occurs at the cathode?
    Chemicals are reduced at the cathode (negative end of the battery).
  20. rq 20
    What is the purpose of the salt bridge in a voltaic cell?
    The salt bridge completes the electrical circuit, by allowing ions to migrate such that neither container builds up any positive or negative charge.
  21. rq 19
    What is eletrochemistry?
    Electrochemistry is the study or the relationship between electrical energy and chemical change.
  22. rq 18
    What elements have the greatest tendency to behave as reducing agents?
    The elements in the lower left hand corner.
  23. rq 17
    What is the difference between an oxidating agent and a reducing agent?
    An oxidizing agent is actually reduced in the reaction, but it causes the oxidation of the reducing agent in the reaction.
  24. rq 15
    Which elements have the greatest tendency to behave as oxidizing agents?
    The elements in the upper right hand corner.
  25. Ex 23
    What element is oxidized in the following equation and what element is reduced?

    I2 + 2 Br -> 2 I + Br2
    The iodine atoms, I, gain electrons and are reduced, while the bromine ions, Br, lose electrons and are oxidized.
  26. Ex 24
    What element behaves as the oxidizing agent in the following equation and what element behaves as the reducing agent?
    Sn++ + 2 Ag -> Sn + 2Ag+
    The tin ion, Sn++, is the oxidizing agent because it causes the silver, Ag, to lose electrons. Meanwhile, the silver atoms, Ag, are the reducing agents because they cause the tin ion to gain electrons.
  27. Ex 25
    Hydrogen sulfide, H2S, burns in the presence of oxygen, O2, to produce water, H2O, and sulfur dioxide, SO2. Through this rection, is sulfer oxidized or reduced?
    2 H2S + 3 O2 -> 2 H2O + 2 SO2
    The sulfur is oxidized as it gains oxygen atoms for sulfur dioxide.
  28. Ex 29
    What correlation might you expect between an element's electronegativity and its ability to behave as an oxidizing agent? How about its ability to behave as a reducing agent?
    An oxidizing agent causes other materials to lose electrons. It does so by its tendency to gain electrons. Atoms with great electronegativity tend to have a strong attraction for electrons and, therefore, also behave as strong oxidizing agents. Conversely, a reducing agent causes other materials to gain electrons. It does so by its tendency to lose electrons. Atoms with great electronegativity, therefore, have little tendency to behave as reducing agents.
  29. Ex 30
    Iron atoms, Fe, are better reducing agents than coper ions, Cu^+2. In which direction do electrons flow when an iron nail is submerged in a solution of Cu^+2 ions?
    The electrons flow from the submerged nail to the copper ions in solution.
  30. Ex 34
    A chemical equation for the combustion of propane, C3H8, is shown below.Through this reaction, is the carbon oxidized or reduced?
    C3H8 + 5O2 -> 3 CO2 + 4 H2O
    As the carbon of propane, C3H8, forms carbon dioxide, CO2, it is losing hydrogen and gaining oxygen, which tells us that the carbon is being oxidized, which is opposite of what happens to a carbon during photosynthesis.
  31. 35
    Chemical equations need to be balanced not only in terms of the number of atoms, but also by the charge. In other words, just as teh same number of atoms should appear before and after the arrow of an equation, so should the same charge. Take this into account to balance the following chemical equation:
    Sn^+2 + Ag -> Sn +Ag ^+
    Recognize that there is a total charge of +2 with the reactants, but a total charge of +1 with the products, which is not balanced. To balance the charges, we need two silver ions, Ag +. But to account for TWO silver ions, we also need two silver atoms, Ag.
  32. 38
    Why are combustion reactions generally exothermic?
    Combustion reactions are generally exothermic because they involve the transfer of electrons to oxygen, which of all atoms in the periodic table has one of the greatest tendencies for gaining electrons.
  33. 43
    Why is the air over an open flame always moist?
    One of the products of combustion is water vapor.