CHEMISTRY 253

Spring, 2015 - Dixon

Homework Set 3.2 Solutions

Problems to be turned in for grading

Ch. 6  Problems: 1

1.  If the new energy policies discused by countries at the Copenhagen 2009 meeting were to be implemented, the IEA estimates that annual global energy growth would be reduced to 1.2% over the 2008-2035 period.

a) What would then be the percentage growth in energy by 2035?

Starting in year 2008, this would be 27 years or E/E_o = e^kt = e^0.012(27) = 1.38 or 38%

b) In order to keep the total growth to 20% by 2035, to what percentage would the average annual growth rate have to be reduced?

E/E_o = 1.20 = e^k(27)  or ln(1.2) = k(27) or k = 0.182/27 = 0.0068 or 0.68%

Additional Problems: 1, 4

1.  Over the time period 1980 to 2000, global energy use increased by 40%.  What was the average annual increase, assuming growth was compounded?

1.4 = E/E_o (40% increase) = e^k(20) ln1.4 = k(20) or k = 0.0168 or 1.7%

By what percentage would energy use have grown by 1990?

E/E_o = e^(0.0168)(10) = 1.18 or 18%

4.  The replacement by natural gas of oil or coal used in power plants has been proposed as a mechanism by which CO₂ emissions can be reduced.  However, much of the advantage of switching to gas can be offset since methane escaping into the atmosphere from gas pipelines is 23 times as effective, on a molecule-per-molecule basis, in causing global warming as is carbon dioxide.  Calculate the maximum percentage of CH₄ that can escape if the replacement of oil by natural gas is to reduce the rate of global warming.

The ratio of the energy generated by combustion to mol of CO₂ produced is roughly 4 to 3 for CH₄ vs. CH₂ (considered to be oil) – see hint and p.269 or Ch. 6 review problem 8 solutions for an example calculation).

Thus the replacement of X mol of oil by natural gas will result in a ~33% decrease in CO₂ emissions.  The leakage of CH₄ needs to be held to 33%/23 = 1.4%

Ch. 7

Problems: 1

1.  Show by comparison of oxidation numbers that the carbon atoms in ethanol are more reduced – and therefore better fuels upon their oxidation – than  they were in the glucose molecules from which they originated before fermentation.  Show also that there is no net change in the oxidation number of the carbon in going from reactants to products in the fermentation reaction.

reaction:           C₆H₁₂O₆ → 2C₂H₆O + 2CO₂

The averaged oxidation state in glucose is given by x in: 6x +12(+1) +6(-2) = 0

or x = 0

In ethanol, the averaged oxidation state is: 0 = 2x + 6(+1) -2 or 2x = -4 or x = -2 which is more reduced than 0.

The net oxidation state of the products is: 4(-2) (4 carbons in 2 moles of ethanol) + 2(+4) (2 carbons of +4 oxidation state in CO₂) = 0 – the same as in glucose

Additional Problems: 1

1.  The burning for energy production of scavenged wood and wood-chip waste has been proposed as a way of reducing CO₂ emissions, even though the combustion produces larger amounts of carbon dioxide per unit heat produced than burning fossil fuels.  Explain the rationale behind this proposal.

Wood is a renewable resource so that land cleared will re-grow wood that can absorb the carbon dioxide out of the atmosphere that was emitted upon combustion of wood.  Additionally, scrap wood would otherwise eventually decay to CO₂ so providing energy has an overall benefit.