0620 · 11.6Alcohols

Fermentation of ethanol

0:00The same substance can be made from sugar grown in a field or from chemicals obtained from petroleum. That substance is ethanol, an alcohol used as both a solvent and a fuel. The molecules are identical, but the manufacturing processes are very different. Understanding those differences helps us explain their conditions, advantages and disadvantages. Ethanol belongs to the family of organic compounds called alcohols. Each ethanol molecule contains two carbon atoms and an OH group bonded to one of them.

0:30This OH group is the functional group of all alcohols. We will focus on ethanol rather than treating every alcohol as if it has exactly the same properties. The first manufacturing method is fermentation. We start with aqueous glucose, which means glucose dissolved in water. Yeast is added, and the mixture is kept between twenty-five and thirty-five degrees Celsius. Oxygen must be absent, so the yeast breaks down glucose anaerobically.

0:56Yeast is a living organism whose enzymes help bring about this conversion. Enzymes are biological catalysts, so they speed up reactions without being used up overall. During fermentation, glucose is converted into ethanol and carbon dioxide. The carbon dioxide escapes as a gas, while ethanol remains mixed with the water. The word equation is glucose gives ethanol plus carbon dioxide. The balanced symbol equation is on the board. One glucose molecule produces two ethanol molecules and two carbon dioxide molecules.

1:26The atoms are rearranged into new substances, rather than being created or destroyed. The temperature range matters because the reaction depends on enzymes in living yeast. If the mixture is too cold, fermentation becomes slow. Excessive heat can denature the enzymes, changing their shape so they no longer work properly. So raising the temperature further does not keep making fermentation faster. The absence of oxygen is also essential to the manufacturing description.

1:53We need the conditions that allow yeast to produce ethanol through fermentation. Do not confuse this with the later combustion of ethanol, which requires oxygen. Making a fuel and burning that fuel involve different reactions with different conditions. Fermentation does not produce a container of pure ethanol. The product contains water, along with other substances from the mixture. Ethanol must therefore be separated and purified if a more concentrated product is needed.

2:19Distillation provides that separation, but its apparatus and operation belong in the video on separation and purification.

Hydration of ethene

2:26The second route starts with ethene, an alkene containing a carbon to carbon double bond. Ethene reacts with steam, which is water in the gaseous state. The conditions are three hundred degrees Celsius and six thousand kilopascals, or sixty atmospheres. An acid catalyst is also required. The word equation is ethene plus steam gives ethanol. This process is called the catalytic addition of steam to ethene. Addition means that the reactants combine to form a single product.

2:54The carbon to carbon double bond becomes a single bond as the elements of water are added. One ethene molecule combines with one water molecule to form one ethanol molecule. The reaction is reversible, as the arrows on the board show. Unreacted starting materials can be returned to the reactor rather than discarded. This recycling helps the factory use its raw materials efficiently. The high temperature helps the reaction proceed at a useful rate.

3:19High pressure brings the gaseous reactant particles closer together. The acid catalyst speeds up the reaction without being used up overall. Unlike fermentation, this route uses an industrial catalyst rather than enzymes inside a living organism.

Comparing ethanol production methods

3:32This next part is for the Extended paper. Fermentation has the advantage of using renewable raw materials because more sugar-producing crops can be grown. Its relatively low temperature also means less heating is needed during the reaction. However, separating ethanol from the watery product still requires energy, so low reaction temperature does not mean no energy use. Fermentation is slow and is normally operated as a batch process. A batch is prepared, allowed to ferment, and then removed before another is started.

4:02This creates interruptions between production runs. The ethanol is also impure, so further separation is needed before many uses. The ethene route is fast and can operate continuously. Reactants enter the system while products are removed, allowing steady production rather than separate batches. This route produces pure ethanol, because ethanol is the only product of the addition reaction. Any unreacted ethene and steam are separated off and recycled. The disadvantages come mainly from the raw material and operating conditions.

4:33Ethene is usually obtained from petroleum, which is non-renewable on a human timescale. Maintaining a high temperature and pressure also requires considerable energy. The faster process therefore trades renewable starting materials and mild conditions for industrial speed and continuous production.

Ethanol uses and combustion

4:49A solvent is a substance that dissolves another substance to form a solution. Ethanol is useful because it dissolves many substances that do not dissolve readily in water. For example, it can dissolve fragrance ingredients in perfumes. Its role here is to provide a liquid medium, not to burn or undergo fermentation. Ethanol is also used as a fuel because its combustion releases energy. Combustion means burning through reaction with oxygen.

5:15With a sufficient oxygen supply, complete combustion produces carbon dioxide and water. This reaction is exothermic, meaning energy is transferred to the surroundings. Let us build the balanced combustion equation rather than simply memorising it. Begin with ethanol plus oxygen gives carbon dioxide plus water. Ethanol contains two carbon atoms, so we need two carbon dioxide molecules on the product side. This balances carbon while leaving hydrogen and oxygen to check.

5:43An ethanol molecule contains six hydrogen atoms, including the hydrogen in its alcohol group. Each water molecule contains two hydrogen atoms. We therefore need three water molecules to account for all six. Changing the number of molecules is allowed, but changing a substance’s formula would describe a different chemical. The products now contain seven oxygen atoms altogether. Two carbon dioxide molecules contribute four, while three water molecules contribute another three.

6:10Ethanol already supplies one oxygen atom, leaving six to come from oxygen gas. Three oxygen molecules provide those six atoms, completing the balanced equation. The final equation means one ethanol molecule reacts with three oxygen molecules to produce two carbon dioxide molecules and three water molecules. All atoms are conserved. A common mistake is forgetting the oxygen already present in ethanol. Counting that atom prevents the incorrect oxygen coefficient that comes from considering only the products.

6:40An exam may ask you to describe the manufacture of ethanol by fermentation. A precise answer begins with aqueous glucose, rather than just saying sugar. Yeast is added to the solution. The temperature is maintained between twenty-five and thirty-five degrees Celsius. The reaction takes place in the absence of oxygen. For an Extended question, you may need to describe advantages and disadvantages of the two routes. Compare matching features rather than giving vague statements such as one method is better.

7:08Fermentation uses renewable raw materials but is slow, runs in batches and gives impure ethanol. In contrast, catalytic addition is fast, continuous and gives pure ethanol, yet depends on petroleum and much energy. Keep the conditions attached to the correct process. Yeast belongs with fermentation, whereas the acid catalyst belongs with steam addition to ethene. Steam is a reactant, not the catalyst. Also, renewable describes the source of the raw material; it does not mean burning the resulting ethanol produces no carbon dioxide.

7:39Ethanol can be made biologically from aqueous glucose or industrially from ethene and steam. Fermentation needs yeast, no oxygen, and a temperature between twenty-five and thirty-five degrees Celsius. For catalytic addition of steam to ethene, the conditions are three hundred degrees Celsius, six thousand kilopascals or sixty atmospheres, and an acid catalyst. These contrasting conditions explain many of the manufacturing advantages and disadvantages. The ethanol molecules have the same properties whichever route produced them.

8:09Ethanol is a useful solvent because it dissolves many substances, and it is a fuel because its combustion releases energy. Complete combustion produces carbon dioxide and water, with the balanced equation following directly from conserving every atom.