OCG 123 Lab 5, Answers
Carbon Dioxide and Climate Change – Calculating Carbon Budgets
Humans have significantly altered the natural carbon cycle through the burning of fossil fuels and deforestation. These activities have increased the release of carbon to the atmosphere by a much faster rate than occurs under natural conditions. The concentration of CO2 in the atmosphere has increased by 30% since the onset of the industrial revolution. Because carbon dioxide is a greenhouse gas, this dramatic increase is of great concern. In order to predict the effect that past and future anthropogenic emissions of carbon dioxide on global climate, scientists must create a carbon budget by calculating the amounts of carbon in the reservoir and the rates at which they are exchanges between reservoirs. For this lab, we will estimate some rates of carbon exchange with the atmosphere.
Estimating the Release of CO2 due to the Combustion of Gasoline
The internal combustion engine converts high-energy bonds in gasoline into mechanical energy. Gasoline is burned, oxidized, by combining it with oxygen from air and adding a small activation energy (a spark from a spark plug). A similar process occurs when oil or natural gas is burned to heat buildings and homes and to produce electricity.
A simplified equation to represent this process is as follows:
Gasoline + Oxygen -> Carbon dioxide + Water
Which can be written as the balanced equation:
2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O
where C8H18 is a simplified formula for gasoline, O2 represents oxygen, CO2 represents carbon dioxide, and H2O represents water. Two molecules of gasoline and 25 molecules of oxygen are converted to 16 molecules of carbon dioxide and 18 molecules of water.
For determining global carbon budgets, scientists usually do not refer to the number of molecules (or atoms which compose molecules) involved because of the difficulty presented by the large numbers involved. Instead, we use the molecular weight of the molecules and then look at the mass of molecules or element that we are interested in.
To simplify the problem further, we make calculations using known molar mass of elements (given in a periodic table). Remember from high school chemistry that 1 mole = 6.022 * 10 23 atoms (or molecules) -Avogadro’s Number? We don’t need to use this here, but its helpful for understanding calculations involving chemical equations.
For the simplified gasoline compound, the molecular weight is calculated below:
Molecular Wt. of C8H18 = (8 moles C * 12 g C/mole C) +(18 moles H * 1 g H/mole H)
= 96 g C + 18 g H = 114 g C8H18
Since there are 2 C8H18, we multiply 114 g C8H18 * 2 = 228 g C8H18
For CO2:
Molecular Wt. of CO2 = (1 mole C * 12 g C/mole C) + (2 moles O * 16 g /mole H)
= 12 g C + 32 g H = 44 g CO2
Since there are 16 CO2 we multiply 44 g * 16 CO2 = 704 g CO2
So for every 228 g of C8H18 burned, 704 g of CO2 are produced.
704 g CO2/114 g C8H18 = 3.09 CO2 /1 C8H18
So there is a ratio of 3.09 units of carbon dioxide per unit of C8H18.
How much carbon dioxide is produced by burning a gallon of gasoline?
(1 gal gas = about 2.65 kg gas)
1 gal gas * 2.65 kg gas * 3.09 kg CO2 = 8.2
kg CO2 released
1 gal gas
1 kg gas
Now that you know how to estimate the CO2 emissions from the combustion of gasoline, carry out the following calculations…
Problems (please show all work and write in complete sentences for full credit):
1. Determine the amount of CO2 you personally will contribute to the atmosphere by driving your car for one year. Please give the answer in units of kg CO2 per year.
To solve this problem you will need to determine:
- (estimate) the number of miles you drive per week (mile/week) 20 mi/week
- (estimate) the fuel efficiency of your car (miles/gallon) 25 mi/gal
- number of weeks in a year (52)
20
mi (7
day) (52 week) =
7280 mi
day week
year
year
7280
mi (1
gal) = 291.2
gal
year
25 mi year
291.2
gal (8.2
kg CO2) = 2387.84
kg CO2 (answers
will vary somewhat)
year
gal
year
2. Humans respire approximately 20.48 g CO2 per hour. How much does one human respire in one year (kg CO2 /year)?
20.48
g CO2
24 hr
365 day =
179404.8 g CO2
= 179.4 kg CO2
hour
day
year
year
year
3. What is your total output of CO2 over one year assuming driving and respiration are your only CO2 producing activities (kg CO2 /year)?
2387.84
kg CO2 +
179.4 kg CO2 =
2567.2
kg CO2
year year
year
(answers will vary somewhat)
4 a) The human population is 1,048,319 according to the latest (2000) census. Estimate the production of CO2 for the population of RI based on the estimate for yourself from #3 above. Express in kg CO2 /year.
1,048,319
people *
2562.2 kg CO2/year
= 2686002941.8 kg CO2
person
year
2.868
109 kg CO2
year
(again answers will vary somewhat but should be around 109 kg CO2/year )
b) Is this a valid approach (i.e. using calculations from one individual and generalizing to the entire population of RI)?
No, this is not a valid approach. To be valid, we would need a larger sample size that is more representative of the population. Respiration rates may vary according to a person’s size and level of activity. People have different kinds and sizes of cars which have different mileage rates. Also, people drive different amounts. Many other carbon-producing activities have been excluded. However, we have to start from somewhere, and estimates are necessary for budget calculations.
c) How could we make the calculation more accurate?
We could obtain information from a larger sample size. We could obtain information from gas stations within the state on how many gallons of gas were sold per year. We could include other carbon producing activities (production of electricity, heating of home and businesses, etc).
5. Assume a corn plant consumes 11.8 grams of CO2 per hour for photosynthesis and that there are 2.3 million corn plants in RI during the summer. What is the estimated consumption rate of CO2 by corn plants in a year (kg CO2 /year). For your calculation, assume a plant lives for 5 months for the year and that they carry out photosynthesis for 12 hours each day.
11.8
g CO2 12
hours 30 day 5
months = 21240 g CO2 or
21.24 kg CO2
hour
day month
year
year
year
21.24
kg CO2
2.3 106 plants
= 48.852 10 6
kg CO2 or 4.8852 107
kg CO2
1 plant * 1year
year
year
6. How does the CO2 consumption by corn plants compare to the CO2 released due to human respiration and driving? Is this simplified system close to being balanced?
Subtract 5 from 4.
Carbon dioxide consumption is about 264 kg CO2/year less than production.
The system is not close to being in balance.