Quiz 0 (Propeties of Atoms, Molecules and Cell Structure)

Water is very unusual in that its solid phase (ice) is less dense than its liquid phase. Thus ice floats on water. This unusual property is the result of: water molecules in ice form a greater number of hydrogen bonds than water molecules in liquid water

True - Each water molecule is capable of forming a maximum of four hydrogen bonds with other molecules. Solid water (ice) has the maximum number of hydrogen bonds (four per water), water vapor (gas) has no hydrogen bonds, and liquid water is intermediate with about 3.2 hydrogen bonds per water molecule. With four bonds per water, ice forms a crystalline structure with the maximum distance between each water. In liquid water, the loss of 0.8 bonds per water means the individual water molecules can be a little closer together, making liquid water more dense than ice.

Two hydrogen (H) atoms are located adjacent to each other in a large biological molecule. Each of the H atoms forms a covalent bond with another atom in the same large molecule (such as C, N, or O). Which types of interactions could occur between these two H atoms? Recall that H atoms contain a single valence electron in a shell that is filled when it contains two elecrons. a hydrophobic (van der Waals) interaction

True - Hydrophobic interactions are very common between adjacent H atoms in biological molecules when both H atoms are bound to (different) C atoms. The C-H bond is not polar, so there are no partial + or - charges as is the case with hydrogen bonds. However, hydrophobic interactions can occur among all sorts of uncharged atoms in molecules. In biological molecules, the most common case is among H atoms in lipids.

Two hydrogen (H) atoms are located adjacent to each other in a large biological molecule. Each of the H atoms forms a covalent bond with another atom in the same large molecule (such as C, N, or O). Which types of interactions could occur between these two H atoms? Recall that H atoms contain a single valence electron in a shell that is filled when it contains two elecrons. a hydrogen bond

false - A hydrogen bond occurs between one atom with a partial negative charge (usually because that atom is very electronegative) and a second atom with a partial positive charge. It is common for H atoms, when they are bound to an O atom, to have a partial positive charge. However there are no atoms found in biological molecules that are less electronegative than H. Thus it is not possible for an H atom to have a partial negative charge, and thus a hydrogen bond cannot form between two adjacent H atoms in a biological molecule.

A solution contains water and a single type of solute. The orientation of the water molecules that are located closest to the solutes is such that all of the water molecules have their oxygen (O) atom pointing toward the solute. Which are properties that can be used to describe the solute? the solute has a negative charge

false - Although biological molecules can have negative charges, the O atom of water will never be attracted toward a negative charge because the O atom of water also has a negative charge, a partial negative charge due to its polar covalent bonds with the two H atoms.

Sulfur is in the same column of the periodic table as oxygen, but has an electronegativity that is similar to carbon. Compared to water molecules, molecules of H2S: will have a stronger cohesion to other molecules of H2S than the cohesion between two water molecules

false - Because the O-H bond of water is more polar than the S-H bond of H2S, there is a more uneven sharing of the bonding electons in water than in H2S. This means that two water moledules are more strongly attracted to each other than two H2S molecules. Attraction between molecules is the basis of cohesion.

A solution contains water and a single type of solute. The orientation of the water molecules that are located closest to the solutes is such that all of the water molecules have their oxygen (O) atom pointing toward the solute. Which are properties that can be used to describe the solute? the solute is hydrophobic

false - Hydrophobic molecules do not "like" water because they are not polar or charged. That is, there are no partial or full + charges on a hydrophobic molecule to specifically be attracted to the partial negative charge on the O atom of water.

Sulfur is in the same column of the periodic table as oxygen, but has an electronegativity that is similar to carbon. Compared to water molecules, molecules of H2S: will more readily undergo the reaction H2S → HS- + H+

false - In order for H2S to release a proton, the remaining H and S atoms must be able to stabilize the negative charge that is left behind when the proton leaves. If S is less electronegative than O, the HS- anion will be less stable than the OH- (hydroxyl) ion that forms when water releases a proton. Thus H2S will not release a proton (ionize) as readily as water.

Water is very unusual in that its solid phase (ice) is less dense than its liquid phase. Thus ice floats on water. This unusual property is the result of: the covalent bonds in the water molecule are more polar in liquid water than in ice

false - The polarity of the O-H bond does not change between liquid water and ice. The polarity of any covalent bond depends only on the relative electronegativities of that two atoms joined by the bond.

Two hydrogen (H) atoms are located adjacent to each other in a large biological molecule. Each of the H atoms forms a covalent bond with another atom in the same large molecule (such as C, N, or O). Which types of interactions could occur between these two H atoms? Recall that H atoms contain a single valence electron in a shell that is filled when it contains two elecrons. a non-polar covalent bond

false - The premise of the question tells you that each of these two H atoms is already bound to another atom in the molecule. Because H can only form on covalent bond, it would not be possible to have any type of covalent bond between the two H atoms.

An oxygen (O) atom is present in one of the amino acids exposed on the surface of a large, water-soluble protein. Could this O atom be involved in the following types of interactions? an ionic interaction

true - Acidic groups in proteins can ionize (lose a proton) leaving an O atom with a full negative charge. An O atom with a full negative charge can attract other atoms with a full positive charge (for example, the N atom on a protonated base) via an ionic interaction. For further information, please see the annotation in Section 2.3 of your online text which talks about ionic interactions in biological systems.

Sulfur is in the same column of the periodic table as oxygen, but has an electronegativity that is similar to carbon. Compared to water molecules, molecules of H2S: will experience a greater increase in temperature when absorbing the same amount of heat

true - As heat is added to a substance, some of the energy goes to raise the temperature and some goes to disrupt interactions between the molecules of the substance. The stronger the interactions are among the molecules, the more of the heat energy that goes to disrupt interactions and less goes to raising the temperature. Because the interactions between two water molecules are stronger than the interactions among two molecules of H2S, adding heat energy to H2S will cause a larger increase in temperature than in water.

A solution contains water and a single type of solute. The orientation of the water molecules that are located closest to the solutes is such that all of the water molecules have their oxygen (O) atom pointing toward the solute. Which are properties that can be used to describe the solute? The solute has a positive charge

true - Biological molecules typically have full + charges because the molecule contains at least one basic group (such as an amine group -NH2) which has been protonated (bound a proton, or H+). Because water is polar and the O atom has a partial - charge, the O atom of water will be attracted to any + charge on a solute molecule.

Sulfur is in the same column of the periodic table as oxygen, but has an electronegativity that is similar to carbon. Compared to water molecules, molecules of H2S: will form weaker hydrogen bonds with each other (H2S hydrogen bonding to another H2S) compared to the hydrogen bonds between two water molecules

true - If S has the same electronegativity as C, than the S-H bond will be less polar than the O-H bond of water. This means that the partial + and - charges in H2S will be smaller than the corresponding partial charges in on the H and O atoms of water. If the charges are smaller, the attraction between two H2S moleules will be less than the attraction between to H2O molecules.

An oxygen (O) atom is present in one of the amino acids exposed on the surface of a large, water-soluble protein. Could this O atom be involved in the following types of interactions? a hydrogen bond

true - If an O atom in a protein forms a covalent bond with any of the other common atoms in biological molecules (H, N, and C), the bond between them will be polar because of the O atom's high electronegativity. Thus the O atom in any polar covalent bond will have a partial negative charge associated with it. This partial negative charge can attract partial positive charges on other atoms joined by polar covalent bonds. This is the basis of hydrogen bonding. For further information, see section 2.3 ands Figure 2.16 in your text.

A solution contains water and a single type of solute. The orientation of the water molecules that are located closest to the solutes is such that all of the water molecules have their oxygen (O) atom pointing toward the solute. Which are properties that can be used to describe the solute? the solute is polar

true - In polar solutes, the net charge on the molecule is zero. The molecule's polarity arises because of the presence of covalent bonds between atoms with different affinites for electrons (electronegativity). The atom at one end of a polar covalent bond (the one with the higher electronegativity) will have a partial negative charge, while the atom at the other end of the polar covalent bond (with lower electronegativity) will have and equal but opposite positive charge. In this situation, only some of the water molecules around this polar solute would have their O atoms pointing toward the solute (toward the partial + charge). The rest would have their H atoms pointing toward the partial - charge. The statement asks the characteristic of a solute in whihc all of the O atoms point toward the solute.

An oxygen (O) atom is present in one of the amino acids exposed on the surface of a large, water-soluble protein. Could this O atom be involved in the following types of interactions? a polar covalent bond

true - The O atoms is the most electronegative atom commonly found in biological molecules. This means that it has a higher affinity for electrons that all of the other atoms typically found in biological molecules (such as H, N, and C). Thus a covalent bond between O and any of these atoms will be a polar covalent bond. See section 2.3 for further information.