(O Level Chem) Alcohols and Carboxylic Acids Teaching & Learning Notes

1. We have seen in the chapter on alkenes that ethene C₂H₂ can be changed to ethanol C₂H₅OH by passing a mixture of ethene and steam over H₃PO₄ catalyst at 300 ºC and 70 atm. Ethanol is used as a solvent in perfumes and varnishes, and a useful fuel in many cars manufactured in the US. The ethanol that is present in alcoholic drinks such as wine and beer is made by fermentation of glucose.

2. Ethanol CH₃CH₂OH is the second member of a homologous series of alcohols which has a general formula of C_nH_2n+1OH. All alcohols contain the functional group -OH which is responsible for the chemical properties of alcohols. The first member of alcohols is methanol CH₃OH which contains only one carbon atom (n = 1). The third (n = 3) and fourth (n = 4) members of the alcohols are propanol and butanol respectively. When n is greater than 2, isomerism exists. Complete the following table and try to draw out all the full structural formulas of these alcohols including the isomers of propanol and butanol. 

Name	Number of carbon atoms, n	Molecular formula	Relative molecular mass	Structural formula
Methanol			32
Ethanol	2			CH3CH2OH
Propanol		C3H8O
Butanol				CH3CH2CH2CH2OH

3. Generally, down the series when the molecular mass of the alcohol increases, the boiling point increases. For example, methanol (n = 1) boils at 68 ºC, ethanol (n = 2) boils at 78 ºC and butanol (n = 4) boils at 118 ºC. Isomers however have different boiling points even though they have the same molecular formula. For example, propan-1-ol boils at 97.2 ºC but propan-2-ol boils at 82.3 ºC.

4. Alcohols, like alkanes and alkenes, are flammable. Ethanol, for example, undergoes complete combustion in sufficient supply of oxygen to produce carbon dioxide and water. Write down the balanced equation for the combustion of 1 mole of ethanol. Use the following information to calculate the enthalpy of combustion of 1 mole of ethanol. You should get -1020 kJ/mol. Because ethanol is runny and volatile and its combustion is so exothermic, ethanol can be used as  a fuel for motor vehicles. What is one advantage of using ethanol over fossil fuels to run motor vehicles?

Bond	Bond energy (kJ/mol)
H-C	413
C-C	348
C-O	743
O-H	463
O=O	498
C=O	743

5. Laboratory experiments sometimes use ethanol. For example, in Biology experiments, ethanol is used as a solvent to dissolve the green pigment, chlorophyll, in leaves. Care must be taken to keep ethanol from open Bunsen flames because ethanol is flammable. If you happen to spill ethanol on the laboratory bench and it catches fire, do not pour water because, water, being denser than ethanol would spread the ethanol over a larger area. Instead, you should use the fire blanket to put out the fire.

6. Besides undergoing combustion, alcohols can also be oxidized by atmospheric oxygen to form weak acids called carboxylic acids. This oxidation can also be achieved by other oxidizing agents such as acidified potassium dichromate or acidified potassium permanganate. For example, ethanol C₂H₅OH can be oxidized by acidified potassium dichromate to ethanoic acid CH₃COOH. In this redox reaction, the orange dichromate is changed to green Cr³⁺(aq). This reaction is used in breathalyzers to test for alcoholic breathe in drivers. Why is this a redox reaction? If potassium permanganate is used, what do you think would be observed?

7. Ethanoic acid is the second member of another homologous series called carboxylic acids which has the general formula, C_nH_2n+1COOH. The first member, methanoic acid HCOOH (n = 0), commonly called formic acid, boils at 101 ºC. Note that in this case, n for the first member is not 1. Ethanoic acid CH₃COOH (n = 1), commonly called acetic acid, boils at 118 ºC while propanoic acid CH₃CH₂COOH (n = 2) boils at 141 ºC. Therefore, like other homologous series, the boiling point of carboxylic acids increases as relative molecular mass increases. Try to draw out the full structural formulas of the first few members of the carboxylic acids.

8. The first few members of carboxylic acids are soluble in water. Down the series, however, the solubility of carboxylic acids decreases. From n = 9 onwards, the carboxylic acids become insoluble in water. The insoluble carboxylic acids which have long chain of carbon atoms are often referred to as fatty acids.

9. Fatty acids are said to be saturated if they contain only C-C single bonds, and are said to be unsaturated if they contain C=C double bonds. Do you still remember, from the topic on alkenes, how you can change unsaturated fatty acid to saturated fatty acid? What catalyst do you use?

10. The functional group of carboxylic acid is -COOH which is responsible for its acidic properties. Therefore, carboxylic acids can react with alkalis to form a salt and water, with metals to evolve hydrogen gas, and with carbonates to evolve carbon dioxide. Make sure you know how to describe the tests for hydrogen gas and carbon dioxide gas. Try to write out the chemical equations for the reaction between carboxylic acid with sodium hydroxide, with magnesium, and with calcium carbonate. Note that ethanoate CH₃COO^- has a charge of +1.

11. Carboxylic acids are, however, weak acids meaning that they dissociate partially to give H⁺(aq). In an aqueous solution of ethanoic acid, for example, only about 0.4% of the ethanoic acid molecules dissociate to become ethanoate ions CH₃COO^-(aq) and protons H⁺(aq). Vinegar, which contains about 1.0 mol dm^-3 of ethanoic acid, has a pH of 2.4. 

(12 and 13 are only for Pure Chemistry)

12. If we heat ethanoic acid and ethanol in the presence of sulfuric acid catalyst under reflux as shown below, we will get a sweet-smelling compound, called ethyl ethanoate CH₃COOC₂H₅, that floats on top of the reaction mixture. Ethyl ethanoate is a class of compounds called esters which contain the -COO- ester linkage. The reaction that forms esters is called esterification. At the same time, water is formed as a by-product. Since a small molecule is formed as a by-product, esterification is also described as a condensation reaction. Note that the reaction is reversible. To prevent the reaction from going backward reforming the reactants, concentrated sulfuric acid, which is a dehydrating agent, is used to absorb the water.

13. We can produce different esters by heating different alcohols and carboxylic acids using the same conditions. Try writing chemical equations for the formations of different esters. Esters tend to smell very pleasant and so they are used as perfumes and flavorings. They are also used as solvents such as in inks and adhesives as they can dissolve many organic compounds. 

Now, try to draw out the flow chart of reactions involving ethane, ethene, ethanol and ethanoic acid.

(A Level GP) Is science unreliable?

I saw this 2011 General Paper essay question and decided to give my two-cent view.

Question: "Science is unreliable, being based as much on theory as on fact."Is this a fair comment? (500-800 words)

The statement "Science is unreliable" is born out of a lack of faith in science. It completely disregards the fact that today's technology is based on the foundation of science if this statement is made in modern times. The reason provided in this statement being "[science] is based as much on theory as on fact" illustrates a misunderstanding of how science works.

Science is based on observations... 

Science has been based on observations of nature, of our surroundings. Scientists are curious and so ask questions about what they observe without being affected by other's opinions and preconceptions. During Galileo's time, many people, even scientists, when being posed a question, asked back "What did Aristotle say?" They took everything Aristotle had said as being true without carrying out investigations or experiments. Because Aristotle said that heavier objects fall faster and therefore reach the ground earlier than lighter objects if they are released from the same height, people believed him. Only the skeptical Galileo carried out experiments to test that claim and discovered that all objects fall at the same rate of increase in speed we call acceleration due to gravity. During Francesco Redi's time, people believed that maggots came from rotten meat. But Redi carried out experiments to dispel that belief. With careful consideration of variables by set-ups that changed only the accessibility of the meat by houseflies, he found that maggots did not appear on meat that was sealed tightly in containers.

How do theories arise? 

If the lack of faith in science is due to that science is based on theories, we need first to understand what a theory is, its use and how it comes about. The kinetic theory of particles, for example, zoom into the microscopic world of invisible perpetually-moving particles of matter. This theory, with the aid a model of particles, helps us understand and explain phenomena and experimental results in the macroscopic world. So far it works as it predicts other phenomena accurately even though we cannot directly observe particles.

Theories may be right or wrong... 

Yes, there have been theories that seem to be based on speculations instead of experimental verifications. Some of these theories would be directly or indirectly supported by new evidence. Einstein's impressive Theory of Relativity which tells the deflection of light by gravity, and the slowing down of time, shortening of length and increase in mass as a person travels near the speed of light is created using rigorous mathematics without experimental support. Later experiments by other scientists showed results that were predicted by this theory. And this theory is even being applied in the construction of the Global Positioning System. Some theories, however, did not stand the test of time. The phlogiston theory stating that fuels contain a fire-like element called phlogiston was proved false by Antoine Lavoisier. Today, we understand that combustion is a chemical reaction between a fuel and oxygen in the air to produce carbon dioxide and water vapor. The theory that all space including vacuo is filled with a material called aether in which light needs to move through space was held to be true for a very long time until the Michelson-Morley experiment showed there was no such thing as aether.

Why science needs theories? 

The fact that theories could be false should not erode our faith in science because we need theories to aid understanding and make predictions. This is especially so pertaining to the unobservable nano world of atoms and molecules, or to the vast expanse of time beyond Earth's existence to the notion of the Big Bang created to explain recent observations such as that light from distant stars tend to have longer wavelength. Science is after all a quest of knowledge, the search for truth through a process that necessitates a continuous cycle of the creation and negation of hypotheses and theories.

"The science of today is the technology of tomorrow" Edward Teller

Last but not least, today's advanced technologies in communication, construction, transport, medicine, food, entertainment and more will not be what they are today without the foundation of science.

Theories will be tested by experiments... 

We can see from the above that even if scientists come out with theories without experimentation, their theories would need to be tested by experiments before they can be widely accepted. It is thus more precise to say science is based on facts rather than on theories. Theories are tools in science for understanding and predictions. The comment that science is unreliable as it is based on theories as on facts is therefore unfair.

"Facts are the air of scientists. Without them you can never fly." Linus Pauling

(725 words)



Other Science-related General Paper essay questions:
- Is it possible to protect the environment when many countries require increasing amounts of energy to progress? (2007)
- Should research into expensive medical treatments be allowed when only a few can afford them? (2007)
- "The more science advances, the more religion will decline." To what extent do you agree? (2006)
- Should every country have the right to carry out unlimited scientific research? (2009)
- To what extent has technology had an impact on both privacy and security in your country? (2009)
- Can mathematics be seen as anything more than a useful tool in everyday life? (2010)
- To what extent has technology had a negative impact on the skill levels of people? (2010)
- Can space research be justified nowadays? (2011)
- How far is it acceptable for technology to be used only for financial benefit? (2012)
- Consider the view that mathematics possesses not only truth, but supreme beauty. (2012)
- "People in the Arts, living or dead, receive far more recognition than those in the Sciences, even though it is less deserved." Consider this claim. (2012)