FAQ Unit 2


Lecture Questions	Enzyme Laboratory Questions

Lecture Questions
	Help! I'm reading the study guide and question 12, Unit 2-15, lists a number of compounds and asks whether they're formed by ionic or covalent bonds, I have no idea how to determine this!
	Is there an atom with a complete outer shell, and if so, what type of bond does it form with other atoms of the same type?
	Why does an electron leave the outer shell of one atom and go to another? Is there a greater "attraction" somewhere?
	One Student noted a chemistry resource she found helpful: Introduction to Chemistry for Biology Students, 7th Edition, by Sackheim: The Benjamin/Cummings Publishing Company, Inc. This is located in the bookstore under the ELI BIO 141 section
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Enzyme Laboratory Questions
	During this experiment, the paper towel did not rise to the surface on glass 5 (No H2O2). I started over with new yeast, thinking I had taken too long and my yeast had "died." I even tried using different temperatures of water. But got the same results. Has anyone else had the same results/problem?
	How do I write the chemical equation that shows hydrogen peroxide breaking down into water and oxygen gas?
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Help! I'm reading the study guide and question 12, Unit 2-15, lists a number of compounds and asks whether they're formed by ionic or covalent bonds, I have no idea how to determine this! The video on bonding develops this idea in detail. Thus, I would first suggest that if at all possible watch the video. However, here is the short answer; maybe this will get you on the right track. If the electron affinities of the atoms are the same or similar, the bond type in the molecule is a covalent bond. If the electron affinities of the two atoms are very different, the bond type will be ionic. In order to determine this, you need to know how many electrons are in the outer shell of each atom. Ionic bonds are formed between two atoms when one of the atoms has a relatively low affinity for electrons (only 1, 2, or 3 electrons in the outer shell) and the other has a relatively high affinity for electrons (5, 6, or 7 electrons in the outer shell). Thus, the atom type with a low affinity for electrons will donate electrons to the atom with a high affinity for electrons. If the electron affinities of the atoms are the same or similar, the bond type in the molecule is a covalent bond. All the diatomic molecules (such as O2 and Cl2) are formed by covalent bonds since the atoms in the molecule are atoms of the same element and have exactly the same number of electrons in their outer shells and hence the same affinity for electrons. In addition, atoms that have an intermediate number of electrons in their outer shells often form covalent bonds with atoms of other elements. A prime example of this is carbon that has four electrons in its outer shell. In this course, we will assume that any carbon containing compound is formed by covalent bonds (true in the vast majority of cases). When I try to predict the bond type, I first determine whether the molecule is a diatomic molecule (atoms in the molecule are of atoms of the same element) or is a carbon containing molecule (assume that if the molecule contains carbon atoms the bond type is covalent). If either of these situations is the case, the bond type is covalent. If that is not the case, then you must diagram the atoms. One needs to know the atomic numbers of each atom in order to do this. If the pattern is atom A with 1, 2, or 3 electrons in its outer shell and atom B with 5, 6, or 7 electrons in its outer shell, then one would predict that the atoms are united by an ionic bond. Top
Is there an atom with a complete outer shell, and if so, what type of bond does it form with other atoms of the same type? Yes, There are atoms with complete outer shells. Any atom with an atomic number of 2 or 10 has a complete outer shell for example (He: atomic number of 2; Ne: atomic number of 10). Remember there are 2 electrons in the first shell, eight in the second, eight in the third (able to accommodate up to 18 but will be stable with 8). For a look at all the elements that are nonreactive (inert) because their outer shell is filled, look at the periodic table at this site: http://pearl1.lanl.gov/periodic/default.htm All the atoms of elements in the far right column are inert. Since these atoms all have a filled outer shell they don't react with other atoms. Thus, except for helium (with an atomic number of 2 and 2 electrons in its outer shell), all the inert elements have atoms with 8 electrons in their outer shell. Since these atom types are nonreactive, they will not play a role in our study of biochemistry. However, it is important to understand why the inert atoms are inert (this idea when turned around helps one understand why atoms do react -namely to obtain a filled outer shell). If this idea still doesn't seem crystal clear to everyone, be sure to watch videos #1 and #2. Top
Why does an electron leave the outer shell of one atom and go to another? Is there a greater "attraction" somewhere? Atoms that donate electron(s) donate because they have a relatively low affinity for electron(s) compared to the atom that "steals" the electron away. Atoms with 1, 2, or 3 electrons in their outer shell tend to be electron donors to atoms with 5, 6, or 7 electrons in their outer shell. The general rule: An atom with low electron affinity loses its electron(s) to atom types with a high electron affinity. Again, videos #1 and #2 deal with this in detail. Also, study sections 2.6 through 2.8 in your text and steps 43-50 in the inorganic program in the Study Guiide (pages Unit 2 - 8 and 9). I don't understand how to determine the location in which atoms are bonding within structural formula. For example: In the structural formula for Glucose, the first carbon is double bonded to oxygen and the third carbon has the hydrogen and hydroxyl opposite the other carbons. How is this determined? In question 15 of Unit 2-21, you noted that fructose differs from glucose only because the second carbon is double bonded rather than the first carbon. Again, I don't understand how to determine this. You do not need to memorize the structural formula for glucose - it will be given to you; you just need to be able to recognize that it is glucose and to understand what the lines represent in a structural formula (a pair of shared electrons). You would not be required to determine that fructose is structurally different from glucose because of the position of the double bonded oxygen on the carbon chain. This was pointed out to emphasize the importance of structural formulas in biochemistry - subtle differences in molecular configuration can result in totally different compounds. You should be able to look at a structural formula and determine if it is possible, i.e., carbon always shares 4 pairs of electrons so note that four lines (represents 4 covalent bonds) always comes away from carbon. Also, you should know that oxygen in organic compounds will share two pairs of electrons (hence 2 lines) and hydrogen 1 pair. Objectives 15 and 16 discuss the differences between molecular and structural formulas and why structural formulas are important in biochemistry. As you continue in biochemistry, you will find out that it is the SHAPE of a molecule that often dictates its function (a great example of that in this unit is enzyme activity). Top
One Student noted a chemistry resource she found helpful: Introduction to Chemistry for Biology Students, 7th Edition, by Sackheim: The Benjamin/Cummings Publishing Company, Inc. This is located in the bookstore under the ELI BIO 141 section. Yes, some students do find it helpful. I would suggest viewing the tapes before purchasing more books however - the tapes will help to focus your attention on the points that are essential in this course. Basically, you don't need to become a biochemist for this course; you just need enough biochemistry so that the cell physiology you encounter later in the course is understandable! If after viewing the tapes, you are still confused, call me at ELI or, if possible, make an appointment to discuss the points you are having difficulty with. Also, all students that are taking a course at ELI can use the campus tutoring service. Call the campus nearest you to find out more about tutoring if you think you need a tutor. Top
. During this experiment, the paper towel did not rise to the surface on glass 5 (No H2O2). I started over with new yeast, thinking I had taken too long and my yeast had "died." I even tried using different temperatures of water. But got the same results. Has anyone else had the same results/problem? One would NOT expect the towel to rise in the glass that lacked hydrogen peroxide. Why? Think about why the towel is rising in the glasses where it does rise. Reread the last paragraph on page Lab 3-5. Underline the last three sentences in the paragraph and reread them. Why did you do an experiment that lacked hydrogen peroxide?? Who can answer this? Top
How do I write the chemical equation that shows hydrogen peroxide breaking down into water and oxygen gas? In a chemical reaction the reactants (starting materials) are on the left side of the ARROW (not an = sign) and the products (what is obtained at the end of the reaction) are on the right side of the arrow. Thus, the reactant, hydrogen peroxide, is on the left side of the arrow and the products, oxygen gas and water, are on the right side of the arrow. The enzyme, catalase, is written on top of the arrow. This indicates that the enzyme is not changed by the reaction. Note that the yeast cells are the SOURCE of the catalase. The equation for this reaction is written in the blanks on Exercise IIIA#4 and Exercise IV#1. Top

FAQs

Unit 2