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Introductory Chemistry — Mid-term Exam 1

Name:________________________________

Student number:_______________________

Autumn Semester

Basic Chemistry for Engineering/ Fundamental Inorganic Chemistry

Mid-term Exam

05 December 2011

Time: 45 minutes

Instructions: Answer any 4 questions out of 6 possible questions. You may answer more than 4 questions if you wish. Your marks will be taken from your top 4 questions. Show all relevant working. Write all answers and final working in pen. 10 pages including: 1 top page, 1 data page, 1 periodic table, 6 question pages, 1 page for rough working Hand in all pages at the end of the exam. Permissible materials: Calculator. English or English bilingual Dictionary. Course text book. Mobile phones and/or devices with internet connectivity are NOT allowed.


Equations and Data

pV = nRT where p = pressure, V = volume, n= no. of moles, R = 8.3144 J K–1mol–1 or R = 0.082057 L atm K–1mol–1, T = temperature Avagadro number 6.022 x 1023

Heat of fusion of H2O at 0°C: 334 J g–1

Heat of vapourisation of H2O at 100°C: 2.26 x 103 J g–1 Specific heat capacity of H2O(s) (ice): 2.09 J g–1 K–1 Specific heat capacity of H2O(l) (liquid water): 4.184 J g–1 K–1


Question 1 A) i) Which element has an atomic number of 40? Zr

ii) Which element has a relative atomic mass of approximately 40? Ar or K or Ca [any one

or more of these is fine]

iii) Which of these elements has the highest total number of electrons? Zr

iv) Which of these elements has the most valence electrons? [Depending on answer to ii)

Zr or Ar]

[2] B) Label the following substances as being ‘mixture’, ‘compound’ or ‘element’. i) wood: mixture

ii) copper: element

iii) air: mixture

iv) iron chloride: compound

[2]

C) i) Label the following diagram of an atom. The atom is neutral and all particles are visible.

ii) Identify the element: Be

iii) This element easily forms ions. What charge (oxidation state) is this element most likely to form

in ions? +2

iv) This diagram is a simplification of an atom. Name one way this diagram is simplified, when

compared to a real atom.

[There are many answers, but they include:] nucleus/nucleons too big, electrons too big, electrons

appear to orbit nucleus like planets around the Sun.

[6]

protonneutron

electron


Question 2 A) Determine the relative molecular mass of the following compounds:

i) CH3CH2CO2H 74 g mol–1

ii) KMnO4 158 g mol–1

[2]

B) How many atoms of oxygen would be found in 5 g of Na2CO3?

Na2CO3 => 104g mol–1 => 5g Na2CO3 equiv. to 0.05 mol => 0.15 mol of oxygen atoms is 8 x 1022

oxygen atoms

[3]

C) i) Balance the following equation:

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) (‘(g)’ means ‘gas’)

[1]

ii) This reaction is conducted in a sealed 200L reaction vessel at an initial pressure of 0.9 atm and a

temperature of 560 ˚C. Assuming the temperature and volume is kept constant over the course of

the reaction, what is the pressure in the reaction vessel once the reaction has completed?

9 mol of gas becomes 10 mol of gas => pressure is 10/9 x 0.9 atm = 1 atm

[1]

iii) A little time after the reaction has finished, the vessel is allowed to cool below 100 ˚C,

whereupon the pressure in the vessel suddenly drops significantly, even though no further reaction

occurs. What is the reason for this sudden drop in pressure?

The H2O gas condenses into a liquid.

[1]

What is the total number of molecules in the reaction vessel at the end of the reaction?

n = 1 atm x 200L/0.082 L atm K–1 mol–1 x 833 K = 2.9 mol => 1.8 x 1023 molecules

[2]


Question 3 A) i) What mass of sodium hydrogen carbonate (NaHCO3) is required to make 150 ml of a 5M

solution?

Mr NaHCO3 = 83 g mol–1 => 0.15 dm3 x 5 mol x 83 g mol–1 = 62.3 g (in 150 ml)

[2]

ii) NaHCO3 reacts with HNO3 according to the following equation:

NaHCO3 + HNO3 NaNO3 + H2CO3

What kind of reaction is this? Acid-base [displacement is acceptable]

[1]

iii) What is the minimum amount of 5M NaHCO3 solution we need to completely react with 18 ml

of 3M HNO3 solution?

0.054 mol of HNO3 requires 0.054 mol NaHCO3, equiv to 0.01 dm3 = 11 ml [or 18ml x 3M/5M]

[1]

B) i) Can we easily make O3– ions? Explain why.

No we cannot. According to the octet rule, oxygen only has space for 2 extra electrons.

[1]

ii) How does the arrangement of elements in the periodic table, according to periods (rows) and

groups (columns) correspond to the arrangement of electrons in their atoms.

The periods correspond to the valence shell which is being filled, the groups correspond to the

number of valence electrons.

[2]

C)Draw simple Lewis diagrams of the following compounds/ions: [Any suitable variation of the

following is acceptable]

i) CO32– ii) PH3

iii) AlCl3 iv)I–

[2]

v) Which compound(s) is/are likely to be a Lewis acid? AlCl3

[1]

CO

O O

P HH

H

Al ClCl

Cl I–


Question 4 A) Which equations are correct, which are incorrect? Correct the incorrect equations.

i) NaHCO3 + HNO3 NaNO3 + H2CO3 correct

ii) CO2 + NaOH Na2CO3 + H2O CO2 + 2NaOH Na2CO3 + H2O

iii) 2Na + 2H2O 2Na + 2OH– + H2 2Na + 2H2O 2Na+ + 2OH– + H2 [2NaOH is

acceptable]

iv) 3NO H2O + NO2 3NO N2O + NO2 [any balanced correction is

acceptable]

[2]

B) Classify the following reactions as combination, decomposition or displacement:

i) 2NO2 2NO + O2 decomposition

ii) CuO + H2 Cu + H2O displacement

iii) N2O2 + Br2 2NOBr combination

iv) C4H9Br + OH– C4H9OH + Br– displacement

[2]

C) Classify the following reactions as being either Bronsted-Lowry (write B-L) or Lewis acid-base

reactions. Identify the acids and bases.

i) 3NH3 (base) + H3PO3 (acid) (NH4)3PO3 B-L

ii) PtCl2 (acid) + 4NH3 (base) PtCl2(NH3)4 Lewis

iii) B(OH)3 (acid) + Cl– (base) [B(OH)3Cl]– Lewis

iv) CuO (acid) + H2O (base) Cu(OH)2 B-L

[4]

D) Sodium is an extremely reactive metal that catches fire when it reacts with water and chlorine is

an extremely reactive and toxic gas. Sodium chloride, however, is an unreactive solid that we can

eat in small amounts. Briefly explain why sodium chloride is so different to sodium metal and

chlorine gas.

Sodium chloride has a different arrangement of valence electrons to those of its constituent

elements. This gives sodium chloride different chemical and physical properties to its constituent

elements.

[2]


Question 5 A) Calculate the formal charge on the indicated atoms. Assume all valence shells are filled.

[2]

B) Calculate the formal oxidation states of the indicated elements:

i) PO43– P: +5 ii) AlCl3 Al: +3 iii) KMnO4 Mn: +7 iv) H2Se Se: –2

[2]

C) Which of the following statements are true or false? Why?

i) Assuming constant pressure, a sample of gas cooled from 42 ˚C to 21˚C will halve its volume.

Not true. The temperature change in Kelvins is not half so the ideal gas equation does not predict a

halving of volume.

ii) At 1 atm and 25 ˚C, 3.5 moles of SF6 (Mr = 146 g mol–1) will occupy a greater volume than 3.5

moles of F2 (Mr = 38 g mol–1).

Not true. The nature of the molecules is not important to the ideal gas equation.

iii) At 35K and a pressure of 10 atm, the ideal gas equation cannot be used to predict the properties

of gaseous H2.

True. The ideal gas equation cannot be used at high pressures and/or low temperatures.

[3]

D) A researcher drops and breaks a bottle containing 50 ml of liquid chloroform in a corridor. The

corridor has a volume of 187 m3. Assume that all of the chloroform evaporates and disperses evenly

throughout the whole volume. Will the concentration of chloroform vapour pass the acceptable

limit of 240 mg m–3? (Assume the liquid chloroform had a density of 1.5g cm–3 at the time it was

dropped). Show your working.

Yes it will. 50 ml CHCl3 = 75 g CHCl3 => 0.40 g m–3 = 400 mg m–3 CHCl3 in corridor

[3]

P O–

–O–O

–O +1

H O

H

H

+1 Br Al

Br

Br

Br

–1

H3C C

O

O –1


Question 6 A) Explain, in terms of vapour pressure, why a visible mist forms over hot cups of coffee.

The air immediately above the coffee has a high temperature and so lots of water evaporates. As the

air rises, it cools and the vapour pressure of water is reduced below the actual partial pressure of

water in that space. The extra water then condenses as a mist.

[2]

B) A domestic solar panel heats water at a rate of 2.9 kJ s–1. How many hours will it take the solar

panel to heat 273 L of water from 12 ˚C to 65 ˚C?

∆T = 53 K Energy required: 53K x 4.184 J g–1 K–1 x 273 000 g = 60.5 x 106 J = 60 500 kJ

Time required in hours = (60 500 kJ / 2.9 kJ s–1)/3 600 = 5.8 hours

[4]

C) i) Ceasium chloride has a crystal lattice made of two interpenetrating simple cubic lattices of Cs+

and Cl– ions. Which of the following diagrams represents the unit cell of CsCl? (Spheres with the

same pattern represent the same element, different patterns represent different elements)

a) b) c) d)

Answer: c)

ii) What is the total sum of Cs+ and Cl– ions inside the unit cell? Cs+: 1 Cl–: 1

iii) Lithium fluoride (LiF), sodium chloride (NaCl) and potassium bromide (KBr) have similar

crystal structures, however, their densities are in the order: KBr (2.75 g cm3) > LiF (2.63 g cm3) >

NaCl (2.61 g cm3).

How can we best explain this order?

KBr is the densest crystal because the ions are heavier than those of the other crystals. But LiF is

denser than NaCl because the atoms are packed more closely.

[4]


Rough Working

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