Classroom Resources

Published on November 27th, 2012 | by Simi

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# Can our family car really be emitting tonnes of greenhouse gases each year? Surely not!

One of the hydrocarbons in petrol is octane C8H18. This burns in oxygen in our engine to form carbon dioxide (a greenhouse gas) and water vapour, as follows:

2C8H18 + 25O2 → 16CO2 + 18H2O.

Using knowledge of relative molecular masses from Chemistry, the above equation tells us that 228 grams of octane C8H18 uses 800 grams of oxygen O2 to form 704 grams of carbon dioxide CO2 and 324 grams of water vapour H2O.

Essentially, the equation tells us that every 228 grams of octane produce 704 grams of carbon dioxide.

The density of octane is close to 700 grams/L, so we can re-write the last statement as:

[latex]\dfrac{228}{700}[/latex]  litres of octane cause 704 grams of carbon dioxide to be released into the atmosphere.

The average car travels about 16,000 km each year, and the average consumption for a smaller car (4-cylinder) is about 7.5L/100km.

So this car would use 160 × 7.5 L in the year, or 1,200 L.

[We’re making the simplifying assumption that the fuel is 100% octane, which isn’t so, but it will give us a good estimate of the amount of pollutants. We’re also assuming that we have a perfectly “clean burn”, which also is far from the truth].

1. How much carbon dioxide is produced by this small car in a year?

Extensions

2. Using the fact that 44 grams of CO2 occupy 25L of space at normal atmospheric conditions, show that the amount of gas calculated above occupies about 1474m3, which is close to the volume of ten ordinary-sized classrooms. But of course the gas doesn’t stay pure – it spreads out and pollutes everyone’s air.

3. Estimate the amount produced in your city for a year, and also for a single day.

Question created by Robert Nelder, Toowoomba State High School

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