Hi, I'd appreciate some help for my chemistry assignment. I've missed a lot of Year 11 due to illness and I'm sure they explained this while I was away. What are the photosynthetic origins of...

Hi,

I'd appreciate some help for my chemistry assignment. I've missed a lot of Year 11 due to illness and I'm sure they explained this while I was away. What are the photosynthetic origins of chemical energy in coal, petroleum and natural gas?

Thanks Cassie

Asked on by cassash

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sciftw's profile pic

sciftw | High School Teacher | (Level 1) Educator Emeritus

Posted on

Let me start by saying that energy is a flow, not a cycle. The reason it is a flow is because it has a starting place and an ending place.  Fossil fuels all originally got their energy from the sun.  The sun sends light to Earth, plants use it to power photosynthesis to make ATP and glucose, animals eat plants to get energy from them, both organisms eventually die, their stored carbon can get converted into coal or other fossil fuels after millions of years of heat and pressure.  Those fuels can be used for a variety of things as they are burned.  But at no point is the original energy of the sun being returned to the sun.  That is why it is a flow, not a cycle. 

The key to making fossil fuels from photosynthetic organisms (and organisms that eat the photosynthetic organisms) is the lack of oxygen.  The simplest way to get oxygen free zones for dead organic material is to find places that have intense heat or pressure or both. Ocean depths are good because it's already high pressure and anything else that dies and sinks adds to the pressure (think millions of years of dead stuff). Quickly buried organic material will have the oxygen issue too, since normal decomposers didn't have access.  Volcanic burials, mud slides, etc.  Whatever the case may be, those carbon based organisms, after millions of years of heat and pressure carbonize and become fossil fuels like coal.  

But without the original photosynthetic energy obtained from the sun, none of those organisms could have been converted into fossil fuels.  

gsarora17's profile pic

gsarora17 | (Level 2) Associate Educator

Posted on

Coal , Petroleum and natural gas; all are formed from the remains of plants and animal.Plants harvest solar energy to live and grow, and animals live eating plants. Normally, when plants and animals die, they are decomposed and they are converted back into carbon dioxide, water and nutrients which are released back into the environment. 

However in certain locations, instead of being fully decomposed to carbon dioxide and water, some plant and animal material which got buried in the earth due to volcanic eruptions or other changes in the earth , and transformed into fossil fuels . The conversion took place due to intense heat and pressure and lack of oxygen and the process takes million of years . This is known as carbonization.

The aquatic plants and animals died and their remains deposited on the bottom of large lakes or seas . This lack of oxygen prevents complete decomposition by aerobic bacteria which would result in the production of carbon dioxide and water. This partially decomposed material is then buried by sediment and over millions of years is subjected to heat and pressure in the Earth’s crust. This converts the plant and animal remains into oil and/or natural gas. 

 Hence, the energy value of each is directly related to the photosynthesis reaction providing the stored chemical energy in each source.

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