Ch. 2 Critical Thinking

Once dietary fats are digested and absorbed, why can they not be released directly into the bloodstream?

All lipids are hydrophobic and unable to dissolve into the watery environment of blood. They are packaged into lipoproteins, whose outer protein envelope enables them to transport fats in the bloodstream.

Explain why CH4 is one of the most common molecules found in nature. Are the bonds between the atoms ionic or covalent?

A carbon atom has four electrons in its valence shell. According to the octet rule, it will readily participate in chemical reactions that result in its valence shell having eight electrons. Hydrogen, with one electron, will complete its valence shell with two. Electron sharing between an atom of carbon and four atoms of hydrogen meets the requirements of all atoms. The bonds are covalent because the electrons are shared: although hydrogen often participates in ionic bonds, carbon does not because it is highly unlikely to donate or accept four electrons.

Could two atoms of oxygen engage in ionic bonding? Why or why not?

Identical atoms have identical electronegativity and cannot form ionic bonds. Oxygen, for example, has six electrons in its valence shell. Neither donating nor accepting the valence shell electrons of the other will result in the oxygen atoms completing their valence shells. Two atoms of the same element always form covalent bonds.

AB+CD --> AD+BE Is this a legitimate example of an exchange reaction? Why or why not?

It is not. An exchange reaction might be AB+CD --> AC+BD or AB+CD --> AD+BC. In all chemical reactions, including exchange reactions, the components of the reactants are identical to the components of the products. A component present among the reactants cannot disappear, nor can a component not present in the reactants suddenly appear in the products.

The pH of lemon juice is 2, and the pH of orange juice is 4. Which of these is more acidic, and by how much? What does this mean?

Lemon juice is 100x more acidic than orange juice. This means that lemon juice has 100 fold greater concentration of hydrogen ions.

During a party, Eli loses a bet and is forced to drink a bottle of lemon juice. Not long after, he begins complaining of having difficulty breathing, and his friends take him to the ER. There, he is given an intravenous solution of bicarbonate. Why?

Lemon juice, like any acid, releases hydrogen ions in a solution. As excessive H+ enters the digestive tract and is absorbed into blood, Eli's blood pH falls below 7.35. Bicarbonate is a buffer, a weak base that accepts access hydrogen ions. By administering bicarbonate intravenously, the ER physician helps raise Eli's blood pH back toward neutral.

Magnesium is an important element in the human body, especially in bones. Magnesium's atomic number is 12. Is it stable or reactive? Why? If it were to react with another atom, would it be more likely to accept or to donate one or more electrons?

Magnesium's 12 electrons are distributed as follows: 2 in the first shell, 8 in the second shell, and two in its valence shell. According to the octet rule, magnesium is unstable (reactive) because its valence shell has just two electrons. It is therefore likely to participate in chemical reactions where it donates 2 electrons.

If the disaccharide maltose is formed from two glucose monosaccharides, which are hexose sugars, how many atoms of carbon, hydrogen, and oxygen does maltose contain and why?

Maltose contains 12 atoms of carbon, but only 22 atoms of hydrogen and 11 atoms of oxygen, because a molecule of water is removed during its formation via dehydration synthesis.

Oxygen, whose atomic number is eight, has three stable isotopes: 16O, 17O, and 18O. Explain what this means in terms of the number of protons and neutrons.

Oxygen has eight protons. In its most abundant stable form, it has eight neutrons too, for a mass number of 16. In contrast, 17O has nine neutrons, and 18O has 10 neutrons.

When you do a load of laundry, why do you not just drop a bar of soap into the washing machine? In other words, why is laundry detergent sold as a liquid or powder?

The greater the surface area of reactants, the more quickly they will interact. It takes energy to separate particles of a substance. Powder and liquid laundry detergents, with relatively more surface area per unit, can quickly dissolve into their reactive components when added to the water.

The most abundant elements in the foods and beverages you consume are oxygen, carbon, hydrogen, and nitrogen. Why might having these elements in consumables be useful?

These four elements (oxygen, carbon, hydrogen, and nitrogen) together make up more than 95% of the mass of the human body, and the body cannot make elements, so it is helpful to have them in consumables.

In a hurry one day, you merely rinse your lunch dishes with water. As you are drying your salad bowl, you notice that it still has an oily film. Why was the water aloe not effective in cleaning the bowl?

Water is a polar molecule. It has a region of weakly positive charge and a region of weakly negative charge. These regions are attracted to ions as well as to other polar molecules. Oils are nonpolar, and are repelled by water.