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# Hess's Law of Heat Summation

## Energy change in a chemical reaction is equal to the sum of the energy in the individual reactions

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Practice Hess's Law of Heat Summation

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Credit: Daniel Ramirez
Source: http://www.flickr.com/photos/danramarch/4909398403/

In many situations, algae can be a real problem. It covers the water and cuts off sunlight below the surface. The excess plant material leads to increased decay, causing an increase in oxygen utilization. However, algae is increasingly becoming a plant of interest because of its potential to provide useful biofuels, cutting down on our dependence on oil.

#### Why It Matters

• What will we do when the oil runs out? What will we use for fuels? A number of groups are taking a close look at this question and running up against some basic chemistry issues. It’s one thing to calculate or measure the potential energy in ethanol or some other biofuel, but there are other energy considerations that need to be factored in.
• How do we get the energy in a given molecule from its original source to the final product ready for the gas tank? What energy costs are involved in the process? Ethanol has been widely considered to be a viable alternative to gasoline (or at least a supplement to the petroleum-based fuel). However, the energy available from ethanol is about 40% less than that from an equivalent weight of gasoline. Add in the energy costs of production and any benefits of ethanol become questionable.
• Oil from algae is looking very feasible. The algae can be grown in water, while using somewhat less water than is needed to grow corn for making ethanol. The yield per acre is considerably higher for algae than it is for corn. Carbon dioxide, a waste product of many manufacturing processes, can be fed into the algae-growing system and provide starting materials for algal growth. The algae can be used to produce ethanol (there are additional energy costs associated with this process) or the cells can be disrupted by drying the material and adding hexane to break up the cell wall. Energy is involved in these extractive processes, so these costs need to be factored in.
• Credit: Mike Licht
Source: http://www.flickr.com/photos/notionscapital/2399120307/

Many plants can be made into ethanol. Here is a chart showing how ethanol can be made from opium [Figure2]

• Doing the calculation to determine the energy needed to form ethanol is far different from the real-world examination of all the energy costs and yields involved in the process of manufacture. Energy in the bonds is only a part of the overall energy equation.
• Watch a video about algae as biofuel at the link below:

#### Show What You Know

1. Describe the energy flow in an exothermic reaction.
2. What is the significance of a negative heat of formation?
3. How efficient is algal biofuel extraction using hexane?
4. What is the advantage of the fast-heat process for obtaining oil from algae?

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