GENERAL CHEMSITY TEST 2 AND SOME OF 1

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1

What is responsible for the decrease in atomic size across the periodic table?

The increasing nuclear charge .

2

Colligative properties

Boiling Point
Freezing Point
Osmotic Pressure
Vapor Pressure

3

What is the reason for the increase in atomic size moving down the periodic table?

electron shielding effect

4

All of the following are assumptions of an ideal gas EXCEPT one. Which one is the EXCEPTION?

The average kinetic energy is proportional to the number of gas molecules -FALSE

Assumptions of ideal gas law are:
The volume of the gas molecules is negligibly small compared to the volume of the entire gas.
Gas molecules are in constant, random motion.
Gas molecules collisions are perfectly elastic.
There are no intermolecular forces occurring between gas molecules. Hence, they are assumed to be nonpolar (i.e. no attraction/repulsion between molecules).
The average kinetic energy of the gas molecules depends only on the temperature of the system (not the number of gas molecules!)Click again to see term

5

All of the following statements accurately describe why mercury forms a convex meniscus in a glass thermometer EXCEPT one. Which one is the EXCEPTION?

Mercury tends to become ionized and the net charge repels the glass.

6

Which of the following best describes a compound that reacts as both an acid and a base?

amphoteric

7

Which statement explains why HI is a stronger acid than HBr, which is a stronger acid than HCl?

The bond strength between H-I is lower than that of H-Cl or H-Br.

8

N2(g) + 3H2(g) ⇌ 2NH3(g) ; ΔH° = -92.22 kJ•mol-1

Which change would decrease the Keq in the above reaction?

A. Adding an iron catalyst.

B. Decreasing the volume of the vessel.

C. Injecting H2 gas.

D. Cooling ammonia gas into a liquid

.E. Increasing the temperature.

DAT pro-tip: The trick to this question is to realize the only thing that can change the Keq is changing the temperature

WE SEE IT HAS TWO WAY ARROW

9

Acetic acid (Ka = 1.8 x 10-5) is titrated with KOH. Which pH indicator should you use to signal the end point of the titration?

For the titration of a weak acid with a strong base, the equivalence point will be a weak base. For the pH indicator to properly signal the end point (or the “equivalence point”), the indicator’s pH range must be similar to the pH at the equivalence point.

CRESOL-RED PH 7.5-9

10

A chemist mixes two solutions in a flask and notices the flask becomes cold. It can be concluded that the reaction:

A. is exothermic.

B. is endothermic.

C. produces a precipitate.

D. decreases in entropy.

E. is a neutralization.

ANSWER = B. is endothermic.

A. is exothermic.

If the flask became hot, the reaction would be exothermic. [A] is incorrect.

C. produces a precipitate.

This is unknown. [C] is incorrect.

D. decreases in entropy.

This is unknown. [D] is incorrect.

E. is a neutralization.

11

The ∆Hf for Br(g) is +193 kJ/mol. What is the bond dissociation energy of a Br-Br bond?

The ∆Hf for Br(g) is:

(1/2) Br2(l) -> Br(g) ; ∆Hf = +193 kJ / mol

If we simply double this reaction, we find:

Br2(l) → 2Br(g) ; ∆Hf = (2)(+193) = +386 kJ/mol

12

All of the following will affect the rate of an irreversible chemical reaction EXCEPT one. Which one is this EXCEPTION?

E. concentration of products

The concentration of products will not affect the rate of an irreversible reaction. Product molecules colliding into each other will not help the forward reaction to occur quicker. Choice E is the EXCEPTION. [E] is the answer.

A. pressure.

Increasing the pressure will increase collision frequency, resulting in a change in the rate of the reaction. Eliminate [A].

B. concentration of reactants

Increasing the concentration of the reactants will also affect the frequency of molecular collisions. Eliminate [B].

C. presence of a catalyst.

A catalyst will lower the activation energy required and speed up a reaction. Eliminate [C].

D. surface area of reactant solid

More surface area of a reactant will give more opportunities for molecules to strike to cause a reaction. Eliminate [D].

13

A first order reaction has a half-life of 90 minutes. What is the rate constant in hour-1?

t1/2 = 0.693/k

We are given time in minutes and the final rate constant should be in hours. There are 1.5 hours in 90 minutes. Substitute in the values given:

k = (0.693)/1.5

14

How many electrons are transferred in this half-reaction?
Mn2+ → MnO4-

ASKING FOR OXIDATION NUMBER

15

Calcium has a larger atomic radius than magnesium because of the:

INCREASING AS YOU GO DOWN THE PERIOD IS DUE TO ELECTRON SHIELDING EFFECT

16

Beta Decay (β- decay)

Neutron -> Proton + Electron emitted

17

Alpha Decay (α-decay)

Emits a Helium atom. Reduces mass number.

18

Electron capture

Proton + Electron -> Neutron

19

Positron emission (β+ decay)

Proton -> Neutron

20

A student finds that the average number of cells over 10 samples is 2130, with a standard deviation of ±6. However, the professor determines the true average number of cells is 2850. The student’s measurement was:

Precise but not accurate

21

Which statement best explains why bromine is a liquid and iodine is a solid at STP?

E. London dispersion intermolecular forces increase as molar mass increases

As molecules get heavier, they exhibit a greater amount of London dispersion forces (nonpolar forces), and these forces become so powerful that iodine actually turns into a solid.

22

All of the following statements accurately describe why mercury forms a convex meniscus in a glass thermometer EXCEPT one. Which one is the EXCEPTION?

Solve using the process of elimination.

A. The molecular forces of cohesion exceed the forces of adhesion.

The forces of cohesion exceed the forces of adhesion, causing the convex meniscus for mercury. Mercury is more attracted to itself than the sides of a test tube. [A] is true.

C. Nonpolar and polar molecules tend to repel each other.

Mercury is a nonpolar substance and glass is a polar substance (silicon dioxide, or SiO2). According to “like attracts like”, mercury and glass will not attract and try to stay as far apart from each other as possible. [C] is true.

D. The forces between mercury molecules are stronger than the forces between the mercury and glass.

The force between the mercury molecules is much stronger than the force between the mercury and glass. [D] is true.

E. Capillary action pulls the edges of the mercury down.

Capillary action is partly responsible for forming a concave meniscus with water and glass. In this scenario, capillary action helps form a convex meniscus. [E] is true.

B. Mercury tends to become ionized and the net charge repels the glass.

Mercury does not tend to ionize or gain a charge if left alone in glass. [B] is the only statement left, and is false. [B] is the answer.

23

Pure elemental gases, such as O2 and N2, have very low melting points, at around -220°C. What is one reason that this is the case?

C. Pure gases are purely covalently bonded and are nonpolar

24

An aqueous solution of potassium chloride is heated from 20C to 60C (no vaporization occurred). Which of the following occurs while the solution is being heated?A. The mole fraction of solute decreases.

B. The mole fraction of solvent increases.

C. The density of the solution is constant.

D. The molarity of the solution is constant

.E. The molality of the solution is constant.

E. The molality of the solution is constant.

the volume of the solution increases due to added heat (thermal expansion). An example of this effect is a mercury thermometer in which the mercury expands as it heats up, revealing the temperature.

E. The molality of the solution is constant.

Molality is the number of moles of solute/kg of solution. Neither the moles of the solute nor the mass of the solvent changes with temperature or volume. [E] is the answer.

A. The mole fraction of solute decreases.

Mole fractions are unaffected by volume changes. A mol fraction is measured as mol of solute/total mol of solution. KCl wasn’t added or removed. Eliminate [A].

B. The mole fraction of solvent increases.

Mole fractions are unaffected by volume changes. No water was vaporized. Eliminate [B].

C. The density of the solution is constant.

The density of the solution does not stay constant because the volume changes. Density is measured by mass/volume. Eliminate [C].

25

If the conjugate base of a molecule has a pKb of 1.4, what would you expect the molecule to be?

WEAKLY ACIDIC

26

Consider the below chemical equilibrium in a closed vessel at a constant temperature.

Na2CO3(s) ⟷ CO2(g) + Na2O(s)

The easiest method to measure the equilibrium constant for this system is to measure:

...

27

Two half-reactions of silver(I) solution are set up in an galvanic cell with differing concentrations. In this concentration cell, which of the following is driving the current?

D. Tendency for the two differing concentrations to equalize due to entropy

In a typical galvanic cell, two different half reactions are used. In this case, the same half reaction is used, silver(I), but the two half reactions have different concentrations. Two unequal concentrations will tend to equalize due to entropy. [D] is the answer.

28

Which of the following best describes the bond character for hydrochloric acid?

Hydrochloric acid may appear ionic because it is made from hydrogen and chlorine which appear on opposite sides of the periodic table. Based the Lewis structure of HCl and general electronegativity trends, it is clear that the hydrogen and chlorine share an electron to complete each other's valence shells. Because chlorine is much more electronegative than hydrogen, it will hog the electron more than the hydrogen, creating polarity, creating a polar covalent bond.

29

Single, double, and triple covalent bonds exist in chemistry. Why does N2 have a triple covalent bond whereas O2 only has a double covalent bond?

The number of possible bonds that can be formed is determined by the number of vacancies in the valence shell. Nitrogen has 5 electrons, and is 'missing' 3 electrons for a full valence shell, enabling it to form 3 bonds in order to fill the valence shell. [B] is the answer.

30

Which compound(s) listed below would result in a basic solution when placed in water?

1. BaO
2. CO2
3. Na2O
4. P4O1

Answer is C. 1 and 3

1. Metal oxides produce basic solutions in water. 2. Non-metal oxides produce acidic solutions in water.

Compounds 1 and 3 are metal oxides and will produce basic solutions. Compounds 2 and 4 are non-metal oxides and react with water to form acids. [C] is the answer.

1. BaO + H2O → Ba2+ + 2 OH-

2. CO2 + H2O → H2CO3

3. Na2O + H2O → 2 Na+ + 2 OH-

4. P4O10 + 6 H2O → 4 H3PO4

31

Filtration is

an effective technique to separate a solid from a liquid.

32

Distillation i

is used to separate two liquids in a solution based on differences in boiling point.

33

Crystallization

is used to purify solid compounds on the basis of differences in solubility.

34

Liquid column chromatography

is used to differentiate liquids in a solution on the basis of differences in polarity.

35

The boiling point of a 1.0m aqueous solution is 101.5°C. The molal boiling point constant for water is 0.512°C•m-1. Which of the following substances is most likely dissolved in the water?

Use the boiling point elevation formula:

Where ΔTb represents the change in temperature, “i” represents the van’t Hoff factor, m represents the molality, and Kb represents the molal boiling point constant for water.

Plug in the given values:

The van’t Hoff factor is equal to 3. Look to the answer choices to determine which species will dissolve into 3 parts.

Na2SO4 will break down into two Na+ ions and one SO4-2 ion, for a total of 3 ions. [B] is the answer.

36

What volume of 0.5M H2SO4 is needed to neutralize 250mL of 0.5M KOH?

H2SO4 has two moles of H+ ions (n1=2), while OH only has one mole of OH-(n2=1). Therefore, we must remember to use the normality of the acid in this dilution reaction.

(n1)(M1)(V1) = (n2)(M2)(V2)

(2)(0.5)(V1) = (1)(0.5)(250)

V1 = 125 mL