02 Textbook Answers

Chapter 2 Answers pg. 1

Biological Science, 3e

Answers to Questions

Chapter 2: Water and Carbon: The Chemical Basis of Life

2.1 The Building Blocks of Chemical Evolution

Figure 2.3 Exercise Unpaired electrons in an unfilled shell can form chemical bonds. Write down the

number of bonds each highlighted atom is capable of forming.

ANSWER:

Atom # of Bonds

Hydrogen 1

Carbon 4

Nitrogen 3

Oxygen 2

Sodium 1

Magnesium 2

Phosphorus 3

Sulfur 2

Chlorine 1

PracticeIt (page 21)

If you understand this concept, you should be able to explain what the “children,” the “toy,” and the

children’s hold on the toy actually represent, and why the children want the toy.

ANSWER: In this analogy, the children represent the nuclei of the atoms participating in the covalent

bond. The toy represents the shared electrons, and the children want the toy because they want to fill

their valence shell with electrons.


If you understand this concept, you should be able to extend the analogy of a covalent bond being

represented by two children clutching the same toy. What would this situation look like if one of the

children wanted the toy more than the other? In this case, which of the two bonds would be weaker—

meaning, easier to break?

ANSWER: In this analogy, if one of the children wanted the toy more than the other, that child would

pull the toy harder and thus bring it closer to himself, just like an electronegative atom in a polar

covalent bond. A nonpolar covalent bond is easier to break than a polar covalent bond, because the

greater the sharing of electrons between two atoms, the stronger the bond between them.

Figure 2.7 Exercise Why do most polar covalent bonds involve nitrogen or oxygen?


ANSWER: The electrons participating in a polar covalent bond are shared unequally between the

bonded atoms. Electronegativity is a measure of how strongly an atom holds electrons in a covalent

bond. Elements with high electronegativity, such as oxygen and nitrogen, form polar covalent bonds

with less electronegative elements such as carbon and hydrogen.

Figure 2.8 Exercise Why can carbon participate in four single bonds, whereas oxygen can participate in

only two and hydrogen in only one?

ANSWER: The number of single bonds formed by an element is determined by the number of unpaired

electrons in its valence shell (outer shell). The difference in unpaired electrons in the valence shell in

these elements—four in carbon, two in oxygen, and one in hydrogen—accounts for the difference in the

number of single bonds formed by carbon, oxygen, and hydrogen.

Figure 2.9 Exercise Label which molecule is bent and planar, which forms a tetrahedron (having four

identical faces), and which forms a pyramid.

ANSWER:

(a) Methane (CH4): tetrahedron

(b) Ammonia (NH3): pyramid

(c) Water (H2O): bent and planar

Check Your Understanding (page 25)

If you understand that . . .

• Covalent bonds are based on electron sharing, while ionic bonds are based on electrical attraction

between ions with opposite charges.

• Covalent bonds can be polar or nonpolar, depending on whether the electronegativities of the two

atoms involved are the same or different.

You should be able to…

1) Draw the structural formulas of methane (CH4) and ammonia (NH3) and add dots to indicate the

relative locations of the covalently bonded electrons, based on the relative electronegativities of C,

H, and N.

2) Draw the electron shells around sodium ions (Na+) and chloride ions (Cl

–) and explain why table salt

(NaCl) exists.


ANSWER:

1) See figure below. The paired electrons in the C–H bonds of methane are shared equally between the

atoms, because the electronegativities of C and H are about the same. In contrast, because of the high

electronegativity of N, the paired electrons in the N–H bonds of ammonia are shared asymmetrically,

being held much more tightly by the N atom.

2) See figure below. Sodium is more stable if it loses an electron (leaving it with a full second shell), and

chloride is more stable if it gains an electron (filling its outer shell). When sodium loses an electron, it

has a net electric charge of +1 (11 protons – 10 electrons = +1); when chloride gains an electron, it has a

net electric charge of –1 (17 protons – 18 electrons = –1). The positively charged sodium is electrically

attracted to the negatively charged chloride, forming an ionic bond. When many sodium and chloride

ions are present, they will pack together in a crystal (table salt) held together by numerous ionic bonds.

2.2 The Early Oceans and the Properties of Water

Figure 2.12 Exercise Label the hydrogen bond in part (b).

Figure 2.12 Exercise Water molecules routinely form multiple hydrogen bonds. Add two hydrogen

bonds to the H2O molecule on the right side of part (b).

ANSWER:



If you understand how water’s polarity makes hydrogen bonding possible, you should be able to (1)

draw a version of Figure 2.12b that shows water as a linear (not bent) molecule, with partial charges on

the oxygen and hydrogen atoms; and (2) explain why electrostatic attractions between water molecules

would be much weaker, or even nonexistent, as a result.

ANSWER:

(1) !+H — O!–

— H!+

(2) If water were a linear molecule, then the partial positive charges on hydrogen would balance out the

partial negative charge on oxygen. There would be no negative end and no positive end. As a result,

water would not be a polar molecule. If it were not polar, then there would be few if any electrostatic

attractions between water molecules.

Figure 2.13 Question Explain the physical basis of the expression, “Oil and water don’t mix.”

ANSWER: Oils are nonpolar. They have long chains of carbon atoms bonded to hydrogen atoms, which

share electrons evenly because their electronegativities are similar. When an oil and water are mixed, the

polar water molecules will interact with each other via hydrogen bonding much more strongly than they

will interact with the nonpolar oil molecules, which will interact with themselves instead.

Figure 2.15 Exercise In ice, each molecule can form four hydrogen bonds at one time. (Each oxygen

atom can form two; each hydrogen atom can form one.) Choose two molecules in part (a), and circle the

four H bonds. If only three bonds are shown, draw in the fourth—where another water molecule would

be added to the crystal.

ANSWER:



If you understand this concept, you should be able to explain the difference in hydrogen ion

concentration between a slightly basic solution with pH 8 and a slightly acidic solution with pH 6.

ANSWER: A slightly basic solution with pH 8 has 1 proton per 100,000,000 (108) water molecules. An

acidic solution with pH 6 has 1 proton per 1,000,000 (10–6

) water molecules. The pH 8 solution has 100

times fewer protons than the pH 6 solution.

Figure 2.16 Exercise How is the pH of black coffee affected if you add milk?

ANSWER: The coffee becomes less acidic because milk is more alkaline (pH 6.5) than black coffer (pH

5).

Summary Table 2.2 Exercise You should be able to fill in the missing cells in this table. You should

also be able to make a concept map relating water’s structure to the properties listed here. (For an

introduction to concept mapping, see BioSkills 6.)

ANSWER: In the table below, the unfilled cells are completed in blue type

SUMMARY TABLE 2.2 PPrope rt ies of Water

Property CCause BBiological consequences

Solvent for charged or polar compounds

El e c t ro s t at i c a t t rac t i o n b e t we e n part ia l c ha rge s o n wat e r m o l e c u l e s and o ppo s i t e c ha rge s o n i o ns ; hy d ro ge n b o nd s b e t we e n w at e r and o t he r po l ar m o l e c u l e s

Most chemical reactions important for life take place in aqueous solution.

High ccohesion Hydrogen bonds form between water molecules. Creates surface tension; also important in water transport in plants.

High aadhesion Int e rac t i o ns b e t we e n wat e r m o l e cu l e s and s urf a ce s w it h po l ar o r ch ar ge d c o m po ne nt s

Plays a role in water transport in plants.

High ssurface tension High cohesion makes a water surface resist forces that increase surface area.

Important in water transport in plants; small organisms can walk on water.

Denser as a liquid than a solid

Hydrogen bonding leads to formation of low-density crystal structure in ice.

Be ca us e i c e f l o a t s o n d e ns e r l i q uid w at e r , o c e ans a nd o t he r b o d ie s o f wat e r d o no t b e co m e pe rm a ne nt l y f ro ze n .


High sspecific heat Water molecules must absorb lots of heat energy to break hydrogen bonds and experience increased movement (and thus temperature).

Oceans absorb and release heat slowly, moderating coastal climates.

High hheat of vaporization Ext e ns iv e hy d ro ge n b o nd ing b e t we e n wat e r m o l e c u l e s in l i q uid phas e m e a ns t h at l o t s o f he at e ne r gy has t o b e ab s o rb e d f o r l i q uid wat e r t o ch an ge t o a gas .

Evaporation of water from an organism cools the body.

One possible concept map relating the structure of water to its properties is shown below.


2.3 Chemical Reactions, Chemical Evolution, and Chemical Energy

Figure 2.19 Exercise In part (a), label which electrons have relatively low potential energy and which

have relatively high potential energy.

ANSWER:


If you understand these concepts, you should be able to explain why the same reaction can be

nonspontaneous at low temperature but spontaneous at high temperature. You should also be able to

explain why some exothermic reactions are nonspontaneous.

ANSWER: The Gibbs free-energy change, symbolized by !G, is calculated using the equation !G = !H

– T!S. If G, H, and S are the same for a given reaction and T (temperature) is low, the !G will be

positive. Positive !G indicates that the reaction is nonspontaneous. However, if T is high, then !G will

be negative, indicating that the reaction is spontaneous. At higher temperatures, molecules are moving

more quickly and are more reactive. Exothermic reactions may be nonspontaneous if they result in a

decrease in entropy—meaning that the products are more ordered than the reactants and !S is negative.

Check Your Understanding (page 38)

If you understand that. . .

• Chemical reactions tend to be spontaneous if they lead to lower potential energy and higher entropy

(more disorder).

• The combined effects of potential energy and entropy changes are summarized in the equation for

the Gibbs free-energy change.

You should be able to…

1) Write out the Gibbs equation and define each of the components.

2) Explain why potential energy might decrease as the result of a chemical reaction and why entropy

might increase.

ANSWER:

1) Gibbs equation: !G = !H – T!S

• !G symbolizes the change in the Gibbs free energy.


• !H represents the difference in potential energy between the products and the reactants. If the

products have more potential energy than the reactants, then !H will be positive, if the reactants

have more potential energy than the products, then !H will be negative.

• T represents the temperature (in degrees Kelvin) at which the reaction is taking place.

• !S symbolizes the change in entropy (disorder). When the products are less ordered than the

reactants, !S is positive because entropy (disorder) increased. If the products are more ordered

than the reactants, !S is negative because entropy (disorder) decreased.

2) When a molecule such as glucose, which has a high potential energy because of the energy stored in

its chemical bonds, is broken down into smaller parts, energy is released. The products (carbon dioxide

and water) have less potential energy than the original glucose molecule. They are also considered more

disordered, because they are smaller and more numerous than the larger reactant molecule; therefore,

entropy increases. According to the Gibbs equation, if a chemical reaction results in a decrease in

potential energy and increase in entropy, then !G will be <0. Such a reaction, called exergonic, will

occur spontaneously

2.4 The Importance of Carbon

Summary Table 2.3 Exercise Predict whether each functional group is polar or nonpolar based on the

electronegativities of the atoms involved.

ANSWER: All the functional groups in Table 2.3, except the sulfhydryl group (–SH), are highly polar.

The sulfhydryl group is only very slightly polar.

Chapter Review (page 41)

You should be able to draw the electron-sharing continuum and place molecular oxygen (O2), carbon

dioxide (CO2), and calcium chloride (CaCl2) on it.

ANSWER:

You should be able to draw how water interacts with ammonia in solution.


ANSWER:

You should be able to explain why cells cannot stay alive without constant inputs of energy.

ANSWER: Spontaneous chemical reactions are ones that lead toward disorder and a release of energy.

Cells are full of very ordered molecules that contain high-energy bonds. These molecules slowly

degrade as these bonds spontaneously break and new energy must be added to restore them and

therefore maintain cellular life.

You should be able to explain why molecules with carbon-carbon bonds have more potential energy

and lower entropy than carbon dioxide.

ANSWER: Electrons in carbon-carbon bonds are held loosely and equally between the carbon atoms,

whereas electrons in a carbon-oxygen bond are held tightly by the oxygen. Because of this difference,

molecules with carbon-carbon bonds have more potential energy than carbon dioxide with its two

carbon-oxygen bonds. Carbon-carbon bonds are often present in large, highly ordered molecules. The

entropy of a group of such molecules, a measure of their disorder, is much less than the entropy of a

group of simple, less-ordered molecules such as carbon dioxide.

QUESTIONS

Answers to the Test Your Knowledge multiple-choice questions appear in the textbook.

Test Your Understanding (page 42)

1. Consider the reaction between carbon dioxide and water, which forms carbonic acid:

CO

2(g) + H

2O(l)! H

2CO3(aq)

In aqueous solution, carbonic acid immediately dissociates to form a proton and the bicarbonate ion,

as follows:

H

2CO

3(aq)! H+ (aq) + HCO

3

! (aq)


Does this second reaction raise or lower the pH of the solution? Does the bicarbonate ion act as an

acid or a base? If an underwater volcano bubbled additional CO2 into the ocean, would this sequence

of reactions be driven to the left or the right? How would this affect the pH of the ocean?

ANSWER: The reaction between carbon dioxide and water lowers the pH of the solution by releasing

extra H+ into the solution. The bicarbonate ion acts as a base. If additional CO2 is added, the sequence of

reactions would be driven to the right, which would make the ocean more acidic. (As discussed in

Chapter 44, this same reaction occurs in the bloodstream, when CO2 enters the blood from active

muscles. The H+

that is generated inside red blood cells acidifies the cell, which causes hemoglobin to

change shape slightly and release more oxygen. The bicarbonate ion diffuses into the blood plasma and

is a major buffer component in the blood).

2. When chemistry texts introduce the concept of electron shells, they emphasize that shells represent

distinct potential energy levels. In introducing electron shells, this chapter also emphasized that they

represent distinct distances from the positive charges in the nucleus. Are these two points of view in

conflict? Why or why not?

ANSWER: The two views are not in conflict. Shells that are farther from the protons in the nucleus

house electrons that have greater potential energy than shells closer to the nucleus, as evidenced by

observation that electrons in outer shells will fall into inner shells if an opportunity arises. To fall to a

lower (closer) level, electrons must release energy in the form of light or heat.

3. Draw a ball-and-stick model of the water molecule, and explain why this molecule is bent. Indicate

the location of the partial electric charges on it. Why do these partial charges exist?

ANSWER: As shown in the figure below, water molecules are bent with an angle of about 109 degrees

between the two O–H bonds. They have this shape because of the geometry of the covalent bonds that

join the hydrogen atoms to the oxygen atom. The electron orbitals surrounding oxygen are oriented in

the shape of a tetrahedron. Two of these orbitals are shared with hydrogen atoms, so the resulting water

molecule has a bent shape. The partial charges exist on water molecules because the electrons are not

shared equally between oxygen and hydrogen. Due to its greater electronegativity, oxygen attracts the

bonding electrons more strongly than does hydrogen. For this reason, oxygen has a partial negative

charge, and the hydrogen atoms have a partial positive charge.

4. Hydrogen bonds form because the opposite, partial electric charges on polar molecules attract.

Covalent bonds form as a result of the electrical attraction between electrons and protons. Covalent

bonds are much stronger than hydrogen bonds. Explain why, in terms of the electrical attractions

involved.


ANSWER: A covalent bond is a much stronger bond than a hydrogen bond because in a covalent bond,

the electrical attractions between (+) and (–) charges are shared over both participating atoms. Each

electron participating in the covalent bond is attracted to both nuclei of the atoms linked by the bond. In

a hydrogen bond, the (+) and (–) charges are only partial, and the attraction between them does not

involve sharing of electrons. Instead, the bond is simply the result of attraction between opposite,

relatively weak charges. Hydrogen bonds between any two polar molecules can, therefore, be easily

broken by the presence of another polar molecule.

5. Explain why extensive hydrogen bonding gives water an extraordinarily high specific heat.

ANSWER: Specific heat is a measure of how much energy is required to raise the temperature of 1 gram

of a substance by 1ºC. Temperature is a measure of how fast molecules in a substance are moving. For

water molecules to move more quickly, hydrogen bonds must be broken. Although not much heat is

required to break one hydrogen bond, water in its liquid or solid state contains a tremendous number of

hydrogen bonds. Therefore, water must absorb a large amount of heat to break those hydrogen bonds in

order to allow enough molecular movement to increase the temperature of a body of water by 1ºC.

6. Explain the relationship between the carbon atoms in an organic molecule and the functional groups

on the same molecule.

ANSWER: The overall shape of an organic molecule, as a linear or ring structure, is determined by how

its carbon atoms are linked together. However, the functional groups (phosphate, carbonyl, amino, etc.)

attached to this carbon backbone determine the chemical behavior of an organic molecule since these

groups are reactive and available to interact with other molecules.

Applying Concepts to New Situations (page 42)

1. Why isn't CO2 bent and polar, like H2O? Why is H2O much more likely to participate in chemical

reactions than CO2?

ANSWER: CO2 is not bent and polar like H2O because the double bonds between the carbon and each

of the two oxygen atoms of CO2 lock the structure of CO2 into a linear molecule. Even though each

oxygen is more electronegative than carbon and does pull the shared electrons closer to its nucleus, the

resulting partial charges cancel one another out because of the linear arrangement of the atoms. As a

result, a CO2 molecule has no overall charge and thus is nonpolar. Because of the bent structure of a

water molecule, on the other hand, the partial (–) charge on the oxygen atom and partial (+) charges on

the hydrogen atoms do not cancel each other, so the entire molecule is polar. These partial charges allow

water to form hydrogen bonds with other polar or charged molecules, whereas carbon dioxide cannot. In

addition, the single covalent bonds in water are much easier to break than the double covalent bonds in

carbon dioxide. These two properties of water make it much more chemically reactive than carbon

dioxide.

2. Oxygen is extremely electronegative, meaning that its nucleus pulls in electrons shared in covalent

bonds. Because these electrons are close to the oxygen nucleus, they have lower potential energy.


Explain the changes in electron position that are illustrated in Figure 2.19a based on oxygen’s

electronegativity.

ANSWER: The changes in electron position that are illustrated in Figure 2.19a can be explained by the

electronegativity of oxygen, which is much greater than the electronegativity of either carbon or

hydrogen. When oxygen is covalently bonded to carbon or hydrogen, the bond electrons will spend

more time near the oxygen atom. Because the shared electrons are closer to oxygen’s nucleus, they

occupy a lower orbital and therefore have lower potential energy. In contrast, carbon and hydrogen have

roughly similar electronegativities, and they tend to share the electrons of a covalent bond more equally.

Electrons shared by carbon and hydrogen occupy a higher orbital and therefore have higher potential

energy than electrons in an O–H or C–O bond. As a result, during the chemical reaction pictured in

Figure 2.19a, in which C–H bonds are exchanged for O–H and C–O bonds, the overall potential energy

of the products is less than that of the reactants.

3. When nuclear reactions take place, some of the mass in the atoms involved is converted to energy.

The energy in sunlight is created during nuclear fusion reactions on the Sun. Explain what

astronomers mean when they say that the Sun is burning down and that it will eventually burn out.

ANSWER: The mass of the atoms in the Sun is converted to radiant energy during nuclear fusion

reactions, and this energy is released in the form of sunlight. So, the Sun will burn out when the mass of

its component atoms is finally depleted.

4. Why do coastal regions tend to have climates with moderate temperatures and lower annual variation

in temperature than do inland areas at the same latitude?

ANSWER: Coastal regions tend to have climates with moderate temperatures because they are close to

large bodies of water. Because water has a high specific heat and heat of vaporization, it tends to absorb

heat, thus moderating temperature. The hydrogen bonds between water molecules give water a

moderating influence on temperature.

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02 Textbook Answers