College Chemistry Exam Questions

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21.

What is the difference between an aldehyde and a ketone?

  • An aldehyde has at least one H attached to the C=O double bond, while a ketone has only C attached to the C=O double bond

  • An aldehyde has a C-O single bond, while a ketone a C=O double bond

  • An aldehyde has two H attached to the C=O double bond, while a ketone has at least one C attached to the C=O double bond

  • The C=O double bond in an aldehyde is adjacent to an alkene, while the C=O double bond in a ketone cannot resonate

  • The C-O single bond in an aldehyde is protonated, while the C-O single bond in a ketone is not

Correct answer: An aldehyde has at least one H attached to the C=O double bond, while a ketone has only C attached to the C=O double bond

This means that an aldehyde always appears at the end of a molecule, while a ketone may appear in the middle of a carbon chain. The aldehyde needs only one H adjacent to C=O to be considered an aldehyde, but it is possible to have two adjacent H atoms; this is called "formaldehyde."

22.

On a phase diagram, where can all three phases co-exist?

  • At the triple point

  • At a critical point

  • Below the lowest line

  • Above the highest line

  • The three phases can never co-exist

Correct answer: At the triple point

The "triple point" is so named because the phase transition lines (solid/liquid, liquid/gas, solid/gas) all come together.

Just as a cup of ice water contains solid and liquid water co-existing at the same temperature and pressure, it is possible for solid, liquid, and gas to exist together under the same condition. For water, the triple point occurs at 0.01°C and 611.2 kPa (almost 6x the atmospheric pressure at sea level).

23.

What image best describes the Bohr atomic model?

  • Electrons circling the nucleus in well-defined orbits

  • Electrons occupying clouds of space around the nucleus

  • Electrons occupying shells around the nucleus

  • Electrons all circling the nucleus in a single orbit

  • Electrons staying in one spot, vibrating in place, at variable distances from the nucleus

Correct answer: Electrons circling the nucleus in well-defined orbits

In the Bohr model, the nucleus is surrounded by concentric orbitals around which electrons are dispersed. These electrons orbit the nucleus much like planets orbiting a sun. Electrons are held more tightly when orbiting closer to the nucleus.

24.

In a Voltaic/galvanic electrochemical cell:

  • Reduction occurs at the cathode, oxidation occurs at the anode, and voltage is positive

  • Reduction occurs at the cathode, oxidation occurs at the anode, and voltage is negative

  • Reduction occurs at the anode, oxidation occurs at the cathode, and voltage is positive

  • Reduction occurs at the anode, oxidation occurs at the cathode, and voltage is negative

  • Reduction occurs at the cathode, oxidation occurs at the anode, and voltage is zero

Correct answer: Reduction occurs at the cathode, oxidation occurs at the anode, and voltage is positive

If you struggle to remember the half-cells, think "Red Cat and An Ox." (REDuction = CAThode, OXidation = ANode). Voltaic cells contain spontaneous reactions so, by definition, the voltage is positive.

25.

What type of bond is most commonly found in organic chemistry?

  • Covalent

  • Ionic

  • Coordinate

  • Hydrogen

  • Double

Correct answer: Covalent

Organic chemistry overwhelmingly focuses on non-metals: C, H, O, N, P, etc. These atoms are similar enough in terms of electronegativities that they form covalent, rather than ionic, bonds. Hydrogen bonding is important especially for many molecules in biochemistry, but much less common than covalent bonds.

26.

What occurs in a redox reaction?

  • Exchange of electrons between species

  • Exchange of hydrogen ions between species

  • The exothermic release of carbon dioxide

  • Two substances combine to form a single substance

  • One substance separates into two or more different substances

Correct answer: Exchange of electrons between species

Redox reactions (short for oxidation-reduction reactions) constitute a broad category of reaction types. Many more specific types of reactions, such as combustion and replacement reactions, are types of redox. In redox, one species loses electrons (gets oxidized) while another species gains those electrons (gets reduced).

27.

What is unusual about the electron behavior of Cr (Group 6) and Cu (Group 11)?

  • They prefer to be 4s1 rather than 4s2

  • They do not fill the 4s orbital at all

  • They fill the 4d orbital rather than 3d

  • They rapidly lose one electron and cannot exist in the unoxidized state naturally

  • They can form many different oxidation states

Correct answer: They prefer to be 4s1 rather than 4s2

The rules of electron shells dictate that one subshell gets fully filled before moving onto the next subshell. Cr and Cu are exceptions to this rule. Both are more stable with 4s1 than with 4s2 because this allows a nice distribution of electrons in their 3d subshell, with exactly one (3d5) or exactly two (3d10) electrons in each orbital.

28.

As you proceed down the Group VA column (N, P, As, Sb, Bi), you expect to find all of the following trends, except:

  • Polarizability decreases

  • Atomic mass increases

  • Metallic character increases

  • Electronegativity decreases

  • Melting point increases

Correct answer: Polarizability decreases

As you move down the column, the atomic radius increases. When electrons are farther away from the nucleus, they are held more loosely and therefore can "slosh around," spending more time on one side or the other of the orbit. This tendency causes decreased electronegativity and increased melting/boiling points.

29.

What type of reaction is this:

3Cl2 (g) + 6OH− (aq) → 5Cl− (aq) + ClO3- (aq) + 3H2O (l)

  • Decomposition

  • Metathesis

  • Neutralization

  • Synthesis

  • Combustion

Correct answer: Decomposition

Though more complex than many decomposition reactions, the core idea is the same: a smaller number of more complicated molecules are decomposing into a larger number of simpler molecules. Though water forms, it is not a neutralization because the water molecule does not form from an acid and a base.

30.

Which of the following statements is/are always true in of an aqueous solution at 298K?

i. pH = 7

ii. pH + pOH = 14

iii. pH = pOH

iv. KW = [H3O+][OH-] = 10-14

  • ii. and iv.

  • iii. and iv.

  • i., ii., and iii.

  • ii., iii., and iv.

  • iv. only

Correct answer: ii. and iv.

Though the pH, of course, varies with what has been added to the aqueous solution, and the relative concentrations of H3O+and OH- are variable, these two conditions are stable by definition.

31.

Of the following hydrocarbons, which would you expect to have the shortest C-C bond length?

C2H6, C2H4, C2H2

  • C2H2

  • C2H4

  • C2H6

  • All C-C bonds are equivalent in length

  • The C-C bonds of C2H6 and C2H4 are equivalent in length; C2H2 does not exist

Correct answer: C2H2

All of these hydrocarbons can exist because carbon can form single (C2H6), double (C2H4), and triple (C2H2) bonds. The higher the bond order, the shorter the bond.

32.

When does a neutralization reaction occur?

  • When an acid and base are mixed together

  • When an oxidant and reductant are mixed together

  • When delta G goes to zero

  • When water is formed in a reaction

  • When a reaction ends at pH=7

Correct answer: When an acid and base are mixed together

This reaction does produce water, but so do other types of reactions (such as combustion). The final pH depends on the relative strengths of acid and base and may reach equilibrium at a pH other than 7.

33.

One calorie is:

  • The amount of energy required to increase the temperature of 1g of water by 1ºC

  • The amount of energy required to increase the temperature of 1L of water by 1ºF

  • The amount of energy required to melt 1g of water

  • The amount of energy required to vaporize 1L of water

  • A measure of the amount of energy in food

Correct answer: The amount of energy required to increase the temperature of 1g of water by 1ºC

Though “calories” are used colloquially to discuss food, it does have a strict definition. Interestingly, what we call a “food calorie” is actually a kilocalorie (1000 calories). This means that a food calorie can actually increase the temperature of 1L (rather than 1g) of water by 1ºC.

34.

If the reaction quotient Q = ∞, how does the reaction shift?

  • To the left (toward reactants)

  • To the right (toward products)

  • It doesn't (already in equilibrium)

  • Depends on delta G

  • Depends on equilibrium constant K

Correct answer: To the left (toward reactants)

Though it sounds strange to write Q = ∞, this is analogous to writing Q >>> K. If a reaction consists of essentially all products and no reactants, it should be expected to shift back toward reactants to form a more balanced equilibrium. The exact resting place (K) is unknown.

35.

What can X-Ray diffraction tell us about a substance?

  • Lattice structure of a crystalline solid

  • Ratio of a particle’s mass to charge

  • Average kinetic energy of a gas

  • Composition of a liquid mixture

  • Composition of a gas mixture

Correct answer: Lattice structure of a crystalline solid

Some solids form crystals, and these crystals may adopt one of seven basic patterns. Each pattern reflects light in a unique way that can be interpreted by a computer as one of the seven lattice patterns.

36.

Oxygen has six valence electrons. Which atom is it most likely to bond with?

  • Calcium (tends to form the ion Ca2+)

  • Strontium (tends to form the ion S2-)

  • Fluorine (tends to form the ion F-)

  • Aluminum (tends to form ions Al+ and Al3+)

  • Argon (does not form ions)

Correct answer: Calcium (tends to form the ion Ca2+)

From the ion information, you can infer that calcium must have two valence electrons, which nicely complements oxygen's lack of two valence electrons. Indeed, calcium and oxygen bond to form calcium oxide or CaO.

Oxygen can also bond with aluminum, but this is more complicated because multiple atoms (Al2O3) are required to fulfill the octet rule. Oxygen does not readily bond with fluorine or strontium, as both atoms also tend to gain rather than lose electrons. Noble gases like argon do not bond with anything.

37.

Which set of cations make halides (Cl-, Br-, I-) insoluble in aqueous solution?

  • Ag+, Pb+, and Hg22+

  • Ag+, Na+, and Hg22+

  • Pb+, Sr2+, and Hg22+

  • Pb2+, Ba2+, Ca2+ and Sr2+

  • Pb2+, Ba2+, Na+, and Ag+

Correct answer: Ag+, Pb+, and Hg22+

It is useful to memorize these sets of exceptions to general solubility rules. Notably, the answer choice Pb2+, Ba2+, Ca2+ and Sr2+ lists the exceptions for sulfate solubility.

38.

Some molecules can readily isomerize between the cis and trans forms. What must be true for isomerization to occur?

  • The atoms must be rotating around a single bond

  • It must be happening at or below the molecule's freezing point

  • It must be happening at or above the molecule's boiling point

  • The atoms must be rotating around a central atom that is more electronegative

  • The atoms must be rotating around a central atom that is larger

Correct answer: The atoms must be rotating around a single bond

While sigma bonds rotate easily, pi bonds (found in double bonds) "lock" the molecule in place, preventing free rotation. For isomerization to occur around a double bond, the electrons in the pi bond must temporarily dissociate somewhere else. This can happen if the double bond is in resonance with another, adjacent double bond.

39.

At what temperature is water the most dense? (Assume standard pressure with standard melting/boiling points.)

  • 4°C

  • Below 0°C

  • Above 100°C

  • 98°C

  • 70°C

Correct answer: 4°C

This answer choice can be reasoned out without having memorized the number in advance. Though most substances are densest in the solid state (in this case, below 0°C), that is not true for water; this is why ice floats. Water is clearly denser as a liquid than as a gas (above 100°C). In general, substances in all phases become denser as they cool because molecules move less and therefore can be closer together. Therefore, liquid water must be the least dense at 98°C, followed by 70°C, and the most dense at 4°C.

40.

All of the following should decrease the rate of reaction, except:

  • Decreased solvent viscosity

  • Decreased temperature

  • Increased hydrogen bonding between reactant and solvent

  • Decreased surface area between a solid reactant and aqueous reactant

  • Decreased solubility of all reactants

Correct answer: Decreased solvent viscosity

The rate of reaction decreases when there is less opportunity for reactants to collide and interact. When a solvent is more viscous (thicker), it is harder for dissolved reactants to move around; when the solvent is less viscous (thinner), movement through the solvent is easier. Rate of reaction does indeed decrease at lower temperature (molecules move slower), with hydrogen bonding (disrupts interactions between reactants), with less surface area for interactions to occur, and with less of a reactant successfully dissolved in solution.