Chemistry
Chapter Problems 649
610 Chapter 16 The Process of Chemical Reactions
Chapter Problems 649
643
610 Chapter 16 The Process of Chemical Reactions
34. Assume that the following reaction is a single step reaction in which a C−Br bond is broken as the C−I bond is formed. The heat of reaction is +38 kJ/mol.
I−(aq) + CH3Br(aq) + 38 kJ → CH3I(aq) + Br−(aq)
H H
HH
C
I
C
B
r
H H
a. With reference to collision theory, describe the general process that takes place as this reaction moves from reactants to products.
b. List the three requirements that must be met before a reaction between I− and CH3Br is likely to take place.
c. Explain why an I− ion and a CH3Br molecule must collide before a reaction can take place.
d. Explain why, in the process of this reaction, it is usually necessary for the new C−I bonds to form at the same time as the C−Br bonds are broken.
e. Draw a rough sketch of the activated complex. (You do not need to show bond angles. Be sure to show the bond that is breaking and the bond that is being formed.)
f. Explain why a collision between an I− ion and a CH3Br molecule must have a certain minimum energy (activation energy) in order to proceed to products.
g. The activation energy for this reaction is 76 kJ/mol. Draw an energy diagram for this reaction, showing the relative energies of the reactants, the activated complex, and the products. Using arrows show the activation energy and heat of reaction.
h. Is this reaction exothermic or endothermic?
i. Explain why an I− ion and a CH3Br molecule must collide with the correct orientation if a reaction between them is going to be likely to take place.
reactants
s
product
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Potential Energy (kJ)
Reaction 1
Reaction 2
reactants
products
10
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Potential Energy (kJ)
40. If both systems described by the energy diagrams in the previous problem are at the same temperature, if the concentrations of initial reactants are equivalent for each reaction, and if the orientation requirements for each reaction are about the same, which of these reactions would you expect to have the greatest forward reaction rate? Why?
Section 16.3 Reversible Reactions and Chemical Equilibrium
44. Reversible chemical reactions lead to dynamic equilibrium states. What is dynamic about these states? Why are they called equilibrium states?
55. Predict whether each of the following reactions favor reactants, products, or neither at the temperature for which the equilibrium constant is given.
a. 2COF2(g) CO2(g) + CF4(g) KP = 2 at 1000 °C
b. ⅛S8(s) + O2(g) SO2(g) KP = 4.2 × 1052 at 25 °C
c. C2H6(g) C2H4(g) + H2(g) KP = 1.2 × 10−18 at 25 °C
63. Ethylene, C2H4, used to make polyethylene plastics, can be made from ethane, C2H6, one of the components of natural gas. The heat of reaction for the decomposition of ethane gas into ethylene gas and hydrogen gas is 136.94 kJ per mole of C2H4 formed, so it is endothermic. The reaction is run at high temperature, in part because at 800-900 °C, the equilibrium constant for the reaction is much higher, indicating that a higher percentage of products forms at this temperature. Explain why increased temperature drives this reversible chemical reaction in the endergonic direction and why this leads to an increase in the equilibrium constant for the reaction.
C2H6(g) C2H4(g) + H2(g)
67. Acetic acid, which is used to make many important compounds, is produced from methanol and carbon monoxide (which are themselves both derived from methane in natural gas) by a process called the Monsanto process. The endothermic reaction is run over a rhodium and iodine catalyst at 175 °C and 1 atm of pressure. Predict whether each of the following changes in the equilibrium system will shift the system to more products, to more reactants, or neither. Explain each answer in two ways, (1) by applying Le Chatelier’s principle and (2) by describing the effect of the change on the forward and reverse reaction rates.
Rh/I2
CH3OH(g) + CO(g) + 207.9 kJ CH3CO2H(g)
175 °C 1 atm
a. The concentration of CO is increased by the addition of more CO.
b. The concentration of CH3OH is decreased.
c. The concentration of CH3CO2H(g) is decreased by removing the acetic acid as it forms.
d. The temperature is decreased from 300 °C to 175 °C.
e. The Rh/I2 catalyst is added to the equilibrium system.