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Prentice Hall Chemistry (c) 2005

Prentice Hall Chemistry (c) 2005. By Daniel R. Barnes Init: 3/30/2011. Section Assessment Answers Chapter 18.

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Prentice Hall Chemistry (c) 2005

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  1. Prentice Hall Chemistry(c) 2005 By Daniel R. Barnes Init: 3/30/2011 Section Assessment Answers Chapter 18 WARNING: some images and content in this presentation may have been taken without permission from the world wide web. It is intended for use only by Mr. Barnes and his students. It is not meant to be copied or distributed. Reaction Rates and Equilibrium

  2. 1. How is the rate of a chemical reaction expressed? “The rate of a chemical reaction is the amount of reactant changing per unit of time.” Reaction rate can also be expressed as the speed at which products are formed. This makes sense because reactants change by turning into products. 18.1 Section Assessment

  3. 2. What are four factors that affect the rate of a chemical reaction?” “temperature, concentration, particle size, use of a catalyst.” Reaction rate also depends upon pressure, particularly if one of the reactants is a gas. Partial pressure is one way of expressing the concentration of a gas, so, in a sense, this is just another example of concentration affecting reaction rate. 18.1 Section Assessment

  4. 3. Suppose a thin sheet of zinc containing 0.2 mol of the metal is completely converted to zinc oxide (ZnO) in one month. How would you express the rate of conversion of the zinc. “Rate = 0.2 mol Zn/month” 18.1 Section Assessment

  5. 4. Does every collision between reacting particles lead to products? Explain. “No; the collision must have sufficient energy to break and form bonds.” Also, particles must be facing the right direction. If tab A needs to go into slot B, then tab A and slot B better be pointing toward each other when they collide. 18.1 Section Assessment

  6. 5. Refrigerated food stays fresh for long periods. The same food stored at room temperature quickly spoils. Why? “Chemical reactions involved in food spoilage occur faster at higher temperatures because more energy is available.” Also, also, at higher temperatures, particles wander around more quickly, and, therefore, bump into each other more often. Anything that increases the rate of collisions will tend to increase reaction rate. 18.1 Section Assessment

  7. 11. How do the amounts of reactants and products change after a reaction has reached chemical equilibrium? “They do not change.” Remember though, that even at equilibrium, reactants are still turning into products and products are still turning back into reactants. It’s just that these two processes are happening at equal rates, so they cancel each other out. 18.2 Section Assessment

  8. 12. What are three stresses that can upset the equilibrium of a chemical system? “changes in concentrations of reactants and products, changes in temperature, and changes in pressure” . . . In other words, changes in anything that affects reaction rate. 18.2 Section Assessment

  9. 13. What does the value of the equilibrium constant tell you about the amounts of reactants and products present at equlibrium? “Keq > 1 favors products; Keq favors reactants.” 18.2 Section Assessment

  10. 14. How can a balanced chemcal equation be used to write an equilbrium-constant expression? “An equilibrium constant expression is a ratio. In the numerator, product concentrations are multiplied. In the denominator, reactant concentrations are multiplied. Each concentration is raised to a power equal to the coefficient for that species in the balanced equation.” 18.2 Section Assessment

  11. 15. Can a pressure change shift the equilibrium position in every reversible reaction? Explain. “no; only in reversible reactions in which the mole ratios of gaseous reactants and products are unequal.” Increases in pressure tend to shove the equilibrium into the direction where there are fewer gas molecules. Decreases in pressure tend to shove the equilibrium into the direction where there are more gas molecules. If, in the balanced equation for a reaction, there are just as many gas molecules on the left (reactants) side of the arrow as there are on the right (products) side of the arrow, pressure changes will not affect equilibrium position. 18.2 Section Assessment

  12. 16. Using the following equilibrium constants for several reactions, determine in which reactions the products are favored. Why? • Keq = 1 x 102 b. Keq = 0.003 c. Keq = 3.5 “Products are favored in reactions a and c because Keq > 1. 18.2 Section Assessment

  13. 21. What is the relationship between the solubility product constant and the solubility of a compound? “the smaller the Ksp, the lower the solubility of the compound.” 18.3 Section Assessment

  14. 22. How can you predict whether a precipitate will form when two salt solutions are mixed? “If the product of the concentrations in a mixture is greater than the Ksp of the compound formed from the ions, a precipitate will form.” 18.3 Section Assessment

  15. 23. Write the soulbility product expression for Ag2CO3. “[Ag+]2 x [CO32-] = Ksp” This is based on the idea that Ag2CO3 dissolves according to the following equation: Ag2CO3(cr)  2Ag+(aq) + CO32-(aq) The concentration of Ag2CO3 does not figure into the expression because it is a solid. The concentrations of solids are regarded as constant, and, therefore, are not included in any kind of equilibrium expression. The concentration of Ag+ is squared in the expression because of the coefficient of “2” in the dissociation equation. 18.3 Section Assessment

  16. 24. What is the concentration of lead ions and sulfide ions in a saturated solution of lead sulfide (PbS) solution at 25oC? (Ksp = 3.0 x 10-28) “2 x 10-14 M” I have a sneaking suspicion that “1.7 x 10-14 M” is a better answer. 18.3 Section Assessment If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

  17. 25. The Ksp of barium sulfate is 1.1 x 10-10. What is the sulfate-ion concentration of a 1.00 L saturated solution of BaSO4 to which 0.015 mol of Ba(NO3)2 has been added? “7.3 x 10-9 M” 18.3 Section Assessment If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

  18. 26. Would precipitation occur when 500 mL of a 0.02 M solution of AgNO3 is mixed with 500 mL of a 0.001 M solution of NaCl? Explain. “yes; the product [Ag+] x [Cl-] = 0.01 M x 0.0005 M = 5 x 10-6 >> Ksp AgCl = 1.8 x 10-10” 18.3 Section Assessment

  19. 27. Which compound, FeS (Ksp = 8.0 x 10-19) or CuS (Ksp = 8.0 x 10-37) has the higher solubility? “FeS” FeS and CuS both dissociate into two ions each, so their Ksp values are directly comparable. Whichever compound has the higher Ksp value is more soluble. 10-19 is bigger than 10-37, so FeS wins. 18.3 Section Assessment

  20. 28. What is the Ksp of nickel(II) sulfide if the equilibrium concentrations of Ni2+ and S2- in a saturated solution of NiS are each 1 x 10-10 M? The teacher’s edition says “4 x 10-20” . . . This answer is disturbing. I think that it should be 1 x 10-20. Either the book is wrong or I’m wrong, and I don’t like the idea of either one of those being true. NiS(cr)  Ni2+(aq) + S2-(aq), so Ksp = [Ni2+] x [S2-] Ksp = (1 x 10-10)(1 x 10-10) = 1 x 10-20 Am I missing something here? Barnes is scared. UPDATE: Zurbano agrees with me. I’m pretty sure the book is wrong. 18.3 Section Assessment

  21. 29. What are two characteristics of spontaneous reactions? “They produce substantial amounts of products at equlibrium and release free energy.” 18.4 Section Assessment

  22. 30. What part does entropy play in determining whether a chemical reaction is spontaneous or nonspontaneous? “An increase favors a spontaneous reaction; a decrease favors a nonspontaneous reaction.” 18.4 Section Assessment

  23. 31. What two factors determine the spontaneity of a reaction? “The size and direction of enthalpy and entropy changes together to determine whether a reaction is spontaneous.” If a reaction causes a decrease in enthalpy, it tends to be spontaneous. If a reaction cases an increase in entropy, it tends to be spontaneous. 18.4 Section Assessment

  24. 32. What is the sign (+ or -) of the Gibbs free-energy change in a spontaneous reaction? “negative (-) sign” 18.4 Section Assessment

  25. 33. Where does released free energy typically end up? Does free energy lost as heat ever serve a useful function? Explain. “As heat; it can be useful; for example, waste heat from chemcial reactions in the body maintains body temperature at 37oC, and waste heat from industrial processes and electrical generation can be used to heat water and buildings.” 18.4 Section Assessment

  26. 34. Suppose the products in a spontaneous process are more ordered than the reactants. Is the entropy change favorable or unfavorable? “unfavorable” The universe loves to become more disorderly. 18.4 Section Assessment

  27. 35. Explain why, under certain circumstances, a non-spontaneous reaction becomes spontaneous? “A change in state of a reactant, due to change in temperature or pressure, may change the balance between DS and DH. 18.4 Section Assessment

  28. 38. How does the size of the specific rate constant (k) for a reaction relate to the speed of the reaction? “The larger the specific rate constant k, the faster the reaction.” 18.5 Section Assessment

  29. 39. What do the hills and valleys in a reaction progress curve represent? “Peaks correspond to energies of activated complexes. Valleys correspond to energies of intermediates.” 18.5 Section Assessment

  30. Define each of the following terms as applied to chemical reactions. • a. elementary reaction • intermediate • c. reaction mechanism • d. activation energy “converts reactants to products in a single step” “a product of one reaction that becomes the reactant in a second reaction” 18.5 Section Assessment “the sequence of elementary reactions of a complex reaction” “minimum energy required for conversion of a reactant to a product (or product to a reactant) in an elementary reaction

  31. 41. Show that the unit k for a reaction that is second-order in A and second-order overall can be expressed in L/(mol•s) “Rate = k[A][B] and rates are moels per liter per second. [A] and [B] are moles per liter. k = rate/[A][B] k = 1/M•s or L/mol•s” Yay! Science is fun! (he said with bitter Fe-y) 18.5 Section Assessment

  32. 42. The rate law for this reaction is first order in NO and O3, and second-order overall. NO(g) + O3(g)  NO2(g) + O2(g) Write the complete rate law for the reaction. “rate = k[NO][O3]” 18.5 Section Assessment

  33. The rest of this presentation is UNDER CONSTRUCTION Let Mr. Barnes know if you really need a certain part of it to get done. barnesd@centinela.k12.ca.us

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