General chemistry test (2)

1st Ionization 2nd Ionization Energy eV Energy eV From the information given in the table above, which of the following is most probably a Group IA metal?. a negative change in enthalpy

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General Chemistry Subject Test 2

1 Which one of the following reactions will be

accompanied by an increase in entropy?

A Na(s) + H2O(l) ∅ NaOH(aq) + H2(g)

B I2(g) ∅ I2(s)

C H2SO4(aq) + Ba(OH)2(aq) ∅ BaSO4(s) +

H2O(l)

D H2(g) + 1/2O2(g) ∅ H2O(l)

E None of the above

2 Which one of the following has the largest ionic

radius?

B K+

D Al3+

E Cl–

3 The heat of combustion of gaseous ammonia,

NH3(g), is 81 kcal/mole How much heat is released

in the reaction of 34 grams of ammonia with excess

oxygen?

A 40.5 kcal

B 60.3 kcal

C 75.8 kcal

D 81 kcal

E 162 kcal

4 What is the percent composition by mass of Al in

Al2(SO4)3?

A 7.9 %

(kcal/mole, 25 °C, 1 atm)

CO(g) –26.4

Based on the values contained in the table, what is the heat of combustion of one mole of ethylene at 298K and 1 atm pressure?

A 316.1 kcal

B 12.5 kcal

C –291.1 kcal

D –316.1 kcal

E –337.3 kcal

6 The reaction below is carried out at constant temperature in a 2.0 liter vessel

N2 + O2 ∅ 2NO

At equilibrium, the vessel is found to contain 8 moles

of NO, 2 moles of N2 and 4 moles of O2 What is the value of Keq for this reaction under these conditions?

A 1

B 8

C 0.25

D 2

E 0.5

7 I A  slow   →

fast

   →

fast

   →

B

Given the above series of reactions leading to the formation of products D and E, which is the rate-determining step?

A I

B II

C III

D I and II

E II and III

8 According to VSEPR Theory, the H-Te-H bond angle

in H2Te should be closest to

A 90°

B 120°

C 109.5°

D 149.5°

E 180°

9 A 300 mL flask containing nitrogen at a pressure of

100 torr and a 200 mL flask containing oxygen at a

pressure of 200 torr are connected such that the gases

are allowed to fill the combined volume of the two

flasks What is the partial pressure of nitrogen in the

combined volume?

A 60 torr

B 80 torr

C 100 torr

D 150 torr

E 200 torr

10 Which of the following statements is true with regard

to the diagram below?

U W X Y Z Reaction Coordinate

Free

Energy

A Going from U to W requires less energy than going

form X to Y

B Going from U to X occurs more readily than going

from X to Z

C Although in the forward direction the reaction X to

Z will occur more rapidly than from U to X, in the

reverse reaction Z to X will occur more slowly than

X to U

D The reaction U to X is exothermic while the

reaction from X to Z would be endothermic

E The overall reaction U to Z is endothermic

11 Which of the elements below is the least

electronegative?

A F

B Cl

C Br

D K

E C

12 Which one of the electronic configurations below represents the atom with the highest ionization energy?

A 1s22s22p63s1

B 1s22s22p63s23p64s23d104p65s14d10

C 1s22s22p63s23p64s23d104p65s24d105p5

D 1s22s22p63s23p63s23p5

E 1s22s22p63s23p6

13 Electron density studies have revealed that X and Y have an equal number of electrons Which of the following could be X and Y respectively?

A Ca+ and K

B H+ and He

C Cl and F

D O– and S+

E None of the above

14

1st Ionization 2nd Ionization Energy (eV) Energy (eV)

From the information given in the table above, which

of the following is most probably a Group IA metal?

A U

B V

C X

D Y

E Z

15 2 HCl + Na2CO3 ∅ 2 NaCl + H2O + CO2(g)

(Atomic weights: H = 1, Cl = 35, Na = 23, O=16,

C=12)

36 g of HCl are mixed with excess Na2CO3 If the

reaction is carried out at STP, how many liters of

CO2 are formed?

A 5.60

B 11.2

C 16.8

D 22.4

E 44.8

16 Which of the following elements would be the most

reactive with fluorine?

A Ar

B Ti

C Cl

D Li

E I

Questions 17 -18 refer to the following kinetic rates:

17 What is the reaction order with respect to A?

A 1

B 1.5

C 2

D 3

E 0

18 What would be the initial rate of the reaction in a

fourth trial if [A]o = 4 x 10– 2 M and [B]o = 3 x 10–3

M?

A 1 x 10–3 M/min

B 2 x 10–3 M/min

C 8 x 10–3 M/min

D 8 x 10 –2 M/min

E Cannot be determined from the information given

19 A spontaneous reaction is always accompanied by

A a negative change in enthalpy

B a positive change in enthalpy

C a negative change in enthalpy and a positive change in entropy

D a positive change in enthalpy and a negative change in entropy

E a negative change in Gibbs free energy

20 Combustion of a 50 gram sample of an unknown hydrocarbon yields 132 grams of CO2 and 126 grams

of H2O Approximately how many grams of carbon were contained in the original sample?

21 Under which of the following sets of conditions (temperature and pressure) will the ideal gas law be least likely to apply?

A 250 K, 4 atm

B 250 °C, 400 atm

C 25 °C, 4 torr

D 25 K, 400 torr

E 25 K, 400 atm

22 The following reaction is first order with respect to W and first order with respect to X If the concentration

of W and X are doubled what happens to the rate of the reaction?

W + X ∅ Y + Z

A It is doubled

B It is quadrupled

C It is squared

D It is increased but it cannot be determined how much from the information given

E It remains the same

23 If atom A has a valence of 7 and atom B has a valence of 2, which compound consisting of atoms A and B would be the most likely to form?

A BA

B B2A

D B2A2

E B3A2

24 In moving down column II A of the Periodic Table

A electron affinity increases

B ionization energy increases

C electronegativity remains constant

D the oxidation state remains constant

E the shielding effect decreases

25 Which of the following choices lists elements, from

left to right, with decreasing atomic radii?

A Li, Be, B, C, N

B Be, Mg, Ca, Sr, Ba

C Li, Mg, Sc, Zn, Ta

D Au, Ag, Cu

E Two of the above

26 Under which of the following conditions will a

reaction always be in thermodynamic equilibrium?

A ∆ H

∆ S > 1

B ∆ H

∆ S < 1

C ∆ H

∆ S = 1

D ∆ H

T ∆ S = 1

E ∆ H

∆ S < 0

27 An ideal gas is contained in a one-liter vessel at 350

K What additional information is sufficient to

determine the pressure of the gas?

A the molecular weight of the gas

B the number of gas molecules

C the molecular weight and the mass of the gas

D B or C

E A, B, or C

28 An ideal gas is placed in a 10 liter container that is free to expand If the temperature (K) is doubled and the pressure is changed from 60 mmHg to 20 mmHg, what is the new volume in liters?

A 20

60 x 1

2 x 10

B 20

60 x 2 x 10

C 60

20 x 1

2

D 60

20 x 1

( 2 x 10 )

E 60

20 x 2 x 10

29 The concentration of which of the following substances would NOT decrease as a result of an increase in the temperature?

B both AB and P

C X2Y only

D both X2Y and BD

E Cannot be determined without a value for the change in entropy

30 Boron found in nature has an atomic weight of 10.811 and is made up of the isotopes 10B (mass 10.013 amu) and 11B (mass 11.0093 amu) What percentage of naturally occurring boron is made up of

10B and 11B respectively?

A 30; 70

B 25; 75

C 20; 80

D 15; 85

E 10; 90

STOP! END OF TEST.

THE ANSWER KEY AND EXPLANATIONS BEGIN ON THE FOLLOWING PAGE

GENERAL CHEMISTRY SUBJECT TEST 2

ANSWER KEY

1 A

2 E

3 E

4 C

5 E

6 B

7 A

8 C

9 A

10 C

11 D

12 E

13 A

14 C

15 B

16 D

17 E

18 B

19 E

20 C

21 E

22 B

23 C

24 D

25 E

26 D

27 D

28 E

29 D

30 C

EXPLANATIONS

1 A

Qualitative thermodynamics questions can be answered by applying the definitions of the various state functions This question asks about entropy, which can be defined as the energetic equivalent of randomness, or disorder The question thus translates into “Which of the following will have an increase in disorder as the reaction proceeds from left

to right?” Recall that the general order of increasing entropy goes: solid<liquid<ions in solution<gas Choice A is thus correct since it shows one mole of a highly ordered, crystalline solid, metallic sodium, and one mole of a somewhat well-ordered, hydrogen-bonding liquid, water, transforming into two moles of aqueous ions, Na+ and OH-, and one mole of gas, H2 The relatively well-ordered reactants are replaced by highly disordered products as the reaction proceeds, and a net increase in disorder, or entropy, is the result As for the wrong choices, choice B shows the deposition of gaseous iodine This represents a large decrease in entropy as the random motion of the gas phase molecules is replaced by the highly ordered crystal structure of the solid phase Choice C shows a reaction between aqueous sulfuric acid and aqueous barium hydroxide; this reaction is simultaneously a precipitation, as the barium ions combine with the sulfate ions to form the ionic solid, barium sulfate, and a neutralization, as the aqueous acid reacts with the aqueous base to form water Precipitation takes aqueous ions to ionic solid and thus represents a decrease in entropy Neutralization takes aqueous ions

to molecules of liquid and is accompanied by a net decrease in disorder, therefore C is accompanied by a decrease in entropy Choice D shows the formation of liquid water or, alternatively, the combustion of gaseous hydrogen By either description, this reaction proceeds with a large decrease in entropy as the mole and a half of mixed gas phase molecules transform into one mole of pure liquid

2 E

In general, a cation is smaller than a neutral atom of the same element since the removal of an electron or electrons

to form the cation leaves fewer electrons to shield one another from the attractive force of the nucleus and to repel one another The remaining electrons on the cation are drawn closer to the nucleus, resulting in a reduction in size By this same reasoning, it follows that the greater the positive charge, the smaller the ion Conversely, anions are generally larger than the corresponding neutral atoms due to an increased number of electrons A size trend is also observed in ions of equal charge: as one moves down a group, or column of the Periodic Table, the atomic and ionic radii increase due to the larger size of the highest occupied orbital Therefore sodium atoms are smaller than potassium atoms, and Na+ ions are smaller than K+ ions If we apply these facts and place the ions into a partial order by size, we have:

K+ > Na+ > Mg2+ > Al3+

Thus choices A, C, and D can be eliminated, and we must now compare the remaining species, K+ and Cl-.When potassium, atomic number 19, gives up an electron to form the cation K+, it is left with 18 electrons and is thus

isoelectronic to argon, atomic number 18 Chlorine, atomic number 17, takes on an electron to form Cl-, and thus it likewise becomes isoelectronic to argon Each species is isoelectronic to the other, and the relative radii will be affected most by the attractive force exerted on the outer electrons by the positive charge of the nucleus It follows that the more protons the nucleus contains, i.e., the larger the atomic number is, the smaller the ionic radius will be The final

comparison is thus Cl- > K+, verifying choice E as the credited choice

3 E

This question combines some basic stoichiometry with thermodynamic values for a reaction We are given the

∆Hcomb per mole of ammonia and the mass of ammonia in grams Applying dimensional analysis, we must convert the mass of ammonia into moles, then multiply by kcal/mol to find the total number of kcal Ammonia has a formula weight

of 14 + 3(1) = 17 g/mol, therefore:

(34 g)/(17 g/mol) = 2 mol

(2 mol).(81 kcal/mol) = 162 kcal

Note that it is not necessary to come up with a balanced equation for the combustion reaction to do this problem

4 C

The percent by mass of an element in a compound is calculated by multiplying the atomic mass of that element by the number of atoms of that element in one formula of the compound, then dividing by the total formula weight of the compound and, finally, converting the result to a percentage In this example there are two atoms of aluminum (atomic weight 27) and three sulfate ions (formula weight = 32 + 4(16) = 96) per formula of aluminum sulfate, so the calculation goes:

2(27)/(2(27) + 3(96)) = 54/342

which rounds off and converts to 15.8% Calculation can, as usual, be avoided by noticing that 54/342 will be

approximately equal to 50/350, or roughly 15% Choice A is the result of inserting one aluminum atomic weight into the top of the calculation, while choices B, D, and E are the results of other random errors in calculation

5 E

This question simultaneously tests the basic definition of combustion and thermodynamic calculations involving heats of formation, along with a bit of basic organic nomenclature To calculate thermodynamic values for a reaction or process from given heats of formation, we must set up a correctly balanced equation for the process and then subtract the sum of the heats of formation, ∆Hf, of the reactants from the sum of the heats of formation of the products To do so for this problem, we must first identify ethylene as the C2H4(g) entry in the table and then write a balanced equation for its combustion:

C2H4(g) + 3 O2(g) ∅ 2 CO2(g) + 2 H2O(l)

Adding the values for the ∆Hf(products) = 2 ∆Hf(CO2(g)) + 2 ∆Hf(H2O(l)) and subtracting the sum of the

∆Hf(reactants) = ∆Hf(C2H4(g)) + 3 ∆Hf(O2(g)), the result is :

∆Hcomb(C2H4(g)) = ∆Hf(products) - ∆Hf(reactants)

= [2 ∆Hf(CO2(g)) + 2 ∆Hf(H2O(l))]- [∆Hf(C2H4(g)) + 3 ∆Hf(O2(g))

= 2(-94) + 2(-68.4) -(12.5) -3(0) = -337.3 kcal/mol

(Recall that the heat of formation of any standard state element, like O2(g) here, is zero.)

As for the wrong choices, notice first that choices A and B could have been immediately eliminated since

combustion, by nature, is exothermic; a correct answer to this question must therefore be negative Choice D results from using the heat of formation of gaseous water rather than that of the liquid water that should be expected at 25oC, and choice C is the result obtained by adding 12.5 (when it should have been subtracted) in addition to using the value for gaseous rather than liquid water

6 B

We can calculate the equilibrium constant for this reaction by setting up the reaction quotient in terms of the concentrations of the reactants and products, then solving based on the numbers given in the problem Since we are told that the vessel is at equilibrium the reaction quotient is the equilibrium constant For the given reaction, the reaction quotient is:

Qc = Keq = [N[NO]2

2][O2] Since the reaction takes place in a 2.0 liter container, we can convert the given numbers of moles into moles per liter

by dividing each by two, then plug the results into the expression for Qc:

[NO] = 8 mol/2 L = 4 mol/L

[N2] = 2 mol/2 L = 1 mol/L

[O2] = 4 mol/2 L = 2 mol/l