Professor Bunsen Honeydew

 

 

Chemistry 141 - Problem Session 3

  1. The specific heat of iron is 0.451 J/g· K. When a 200.0 g iron slug is heated to 368oC and added to 60.0 g of water at 20.0oC, all of the heat lost by the iron is gained by the water. If the heat of vaporization of water is 2260 J/g, how many grams of water will vaporize?

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  2. When gasoline burns in a car engine, the heat released causes the products CO2 and H2O to expand, which pushes the pistons outward. Excess heat is removed by the car’s cooling system. If the expanding gases do 451 J of work on the pistons and the system loses 325 J to the surroundings as heat, calculate the change in energy (D E) in kcal.

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  3. The standard enthalpy of combustion of liquid heptane (C7H16) is - 4816.0 kJ/mol. The products of this combustion are liquid water and carbon dioxide gas. Calculate the standard enthalpy of formation of liquid n-heptane.

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  4. Acetylene reacts with bromine according to the following equation:

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    C2H2 + 2 Br2 ¾ ® C2H2Br4

     

    From bond energies, calculate the standard enthalpy change for the reaction.

     

     

     

     

     

  5. Calculate the standard enthalpy of formation of solid magnesium hydroxide from the following data:

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    2 Mg(s) + O2(g) ¾ ® 2 MgO(s) D Ho = - 1203.7 kJ

     

    MgO(s) + H2O(l) ¾ ® Mg(OH)2(s) D Ho = - 36.7 kJ

     

    2 H2O(l) ¾ ® 2 H2(g) + O2(g) D Ho = +571.6 kJ

     

     

     

     

     

     

  6. Is a tree growing a spontaneous process? If so, what drives it, entropy or enthalpy? Explain.

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    1. Why does a tree grow??
  7. Fill in the chart with a plus (+) or minus (- ) sign.

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    D H

    D S

    		  

    spontaneous at all temperatures

    spontaneous at high temperatures

    spontaneous at low temperatures

    nonspontaneous at all temperatures

    H2O(l) ® H2O(s)

    CaCO3(s) ® CaO(s) + CO2(g)

    Ag+(aq) + Cl- (aq) ® AgCl(s)

    spreading perfume smell through a room

    separating O2 and N2 from each other in air

    mending a broken clock

    dissolution of sugar in hot coffee

    reaction of N atoms to form N2 molecules at 25oC and 1 atm

     

     

  8. Ethanol is a major ingredient of a camping fuel called "Sterno". If the energy released in the combustion of 5.00 g of ethanol (C2H6O) is transferred without loss to 200.0 g of liquid water at 22.0oC, what will be the final state and temperature of the water? (Combustion of one mole of ethanol yields 1235 kJ of energy)

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  9. Aspirin is produced commercially from salicylic acid, C7O3H6. A large shipment of salicylic acid is contaminated with boric oxide, which like salicylic acid is a white powder. The heat of combustion of salicylic acid at constant volume is known to be –3.00 x 103 kJ/mol. Boric oxide, because it is fully oxidized, does not burn. When a 3.556 g sample of contaminated salicylic acid is burned in a bomb calorimeter, the temperature increases 2.556oC. From previous measurements, the heat capacity of the calorimeter is known to be 13.62 kJ/K. What is the amount of boric oxide in the sample, in terms of mass percent?

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  10. A sample of neon gas has a volume of 3.15L and a pressure of 0.951 atm at 21° C. If the pressure increases to 1.564 atm and temperature remains constant, what is the final volume?

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  11. Calculate the volume of O2 at 2.89 atm and 15oC required for the complete combustion of 125 g octane (C8H18) to CO2 and H2O.

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  12. Consider the three boxes, all at 27oC, in the diagram below. Assuming the connecting tubes have negligible volumes, what is the partial pressure of each gas and the density of the final mixture when both of the stopcocks are opened?

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  13. A 0.700 g sample of lithium metal is placed in an evacuated 1.00 L flask connected by a stopcock to a 1.00 L flask containing oxygen at a temperature of 25.0oC and 5.00 atm pressure. The stopcock is opened and the exothermic reaction allowed to proceed to completion. When the temperature of the system returns to 25.0oC, what will be the pressure? (assume that the volumes of Li(s) and Li2O(s) are negligible).

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  14. The gaseous reaction below is followed by monitoring the total gas pressure in the reaction vessel. The temperature is held constant by a large water bath at 100oC. The initial pressure of CS2 and H2 was 0.558 torr. The final pressure was 0.480 torr. The limiting reactant, CS2 was entirely used up. Using Dalton’s Law of Partial Pressure, calculate the partial pressures of the other gases when the reaction was finished and the partial pressures of the two initial gases.

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    Total Pressure

    CS2(g) + 4 H2(g) ® CH4(g) + 2 H2S(g)

    Initial

    0.558 torr

    		    

    0 torr

    0 torr

    D

    		          

    Final

    0.480 torr

    0 torr

    		      

     

     

     

     

  15. If at a temperature T, the average speed of a CO2 molecule is 3.5 x 104 ms- 1, what would be the average speed of a methane (CH4) molecule at the same temperature?

 

 

  16.  Explain/show the structure of a hydrogen bond.

 

 

 

 

 

 

   17.Write the Lewis structures and give the shapes of each of the following molecules; predict which substance of each pair has the higher boiling point. Explain your reasoning.

     

    1. Ethanol, CH3CH2OH or dimethyl ether, CH3-O-CH3?

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    2. Butane, C4H10 or octanol, C8H18?

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    3. PF3 or PCl3?

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    4. SO2 or CO2?

     

     

     

     

  1. The phase diagram of an unknown substance is shown below:

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    1. Label the solid liquid and gas regions of the phase diagram.
    2. Label the triple point and the critical point.
    3. Label the normal freezing and boiling points.
    4. Which phase is more dense the liquid or the solid? Explain your reasoning.

     

     

     

     

     

  2. Explain why CH3OH is completely soluble in water but not completely soluble in toluene, C6H5CH3.

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  3. The vapor pressure of ethyl bromide (C2H5BBr) is 100.0 torr at –10.0oC and 1520 torr at 60.2oC. What is the normal; boiling point of ethyl bromide?