Thermochemistry

61. (Equilibrium) Consider the bonds that must be made or broken for the reaction

O2N-NO2 (g) = 2 NO2 (g)

The reaction will be

endothermic, exothermic

62. (Extended structures, discrete molecules, solubility) The reaction for dissolving gaseous HCl in water is HCl (g) --> H+ (aq) + Cl- (aq).

How many bonds are broken per HCl formula unit when a sample of gaseous HCl dissolves?

zero, one, many

The layer sequence of LiCl is shown below. The reaction for dissolving the extended solid in water is LiCl (s) --> Li+ (aq) + Cl- (aq).

How many bonds are broken per LiCl formula unit when a chunk of LiCl dissolves?

zero, one, many

66. (Enthalpy, equilibrium) N2(g) + 3H2(g) = 2 NH3(g) + reaction energy; Which are collectively stronger bonds?

those in the reactants, those in the products


What effect will spraying H2O into the system have if NH3 is far more soluble in H2O than N2 and H2?

no effect, increase product, increase reactants

75. (Polymers, thermodynamics) Consider the polymerization of ethylene in which, neglecting the ends of the polymer, many C=C double bonds worth ~600 kJ/mol each are converted to twice as many C-C single bonds in polyethylene worth ~350 kJ/mol each.

n H2C=CH2 (g) --> -[-CH2-CH2-]-n

This reaction is

exothermic, endothermic, thermoneutral

What happens to the entropy in this reaction?

increases, decreases, remains the same

191. When graphite is burned to yield CO2, 394 kJ of energy are released per mole of C atoms burned. When C60 is burned to yield CO2 approximately 435 kJ of energy is released per mole of carbon atoms burned. Would the buckyball-to-graphite conversion be exothermic or endothermic?

exothermic, endothermic


If heat were added to the graphite/C60 system, would the equilibrium shift toward graphite or C60?

graphite, C60

204. Which of the following processes will have a change in entropy less than 0?

A, B, C

205. The following graph of the change in free energy versus temperature (T) corresponds to which of the following situations?

A, B, C

210. For the reaction in which solid carbon dioxide (dry ice) sublimes to become gaseous carbon dioxide, CO2(s) --> CO2(g), which of the following is true for the reaction as written?



216. Many processes have a likelihood of occurring that is proportional to exp(-E/RT). In this relationship, E is the energy required for the process, R is the gas constant, and T is the absolute temperature. According to this expression, the process is most probable for the combination of:

LargeE and low temperature
LargeE and high temperature
SmallE and low temperature
SmallE and high temperature


222. Plotted below are graphs of heat absorbed versus temperature for two systems. Which system has the larger heat capacity?

 

A, B

236. Dissecting a Polymerization Reaction

 

Polyethylene is formed when the molecule ethylene is polymerized. Approximately how many C-C single bonds are formed from n molecules of ethylene? [The instructor may want to draw approximately half a dozen H2C=CH2 molecules in a line, sketch electron movement with arrows, and draw the resulting alkane to help students visualize this process.]

 

n

2n

3n

 

For the net reaction

nC2H4(g) ---> -[CH2-CH2]-n(s)

 

What is the sign of S?

S>0

S<0

S=0

 

Given that a mole of C=C double bonds is worth approximately 600 kJ/mol and a C-C single bond is worth approximately 350 kJ/mol, what is the sign of H?

 

H>0

H<0

H=0

 

Does H or S drive this reaction?

 

H

S

 




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