Worked solutions to textbook questions 16
Chapter6 Ionic compounds
Q1.
Sodium chloride does not conduct electricity in the solid state but it does conduct when molten.
a How would you use Figure 6.3 on page 94 to explain why solid sodium chloride does not conduct electricity?
b Explain why molten (liquid) sodium chloride conducts electricity.
A1.
a The diagram shows that in solid sodium chloride, the sodium and chloride ions are held in fixed positions in the crystal lattice and are not free to move and conduct electricity.
b Molten sodium chloride contains sodium and chloride ions that are free to move and, therefore, it can conduct electricity.
Q2.
Write the formulas of the ions most likely to form when the following metal atoms react with non-metals:
a Li
b Na
c Al
d Cs
A2.
a Li+
b Na+
c Al3+
d Cs+
Q3.
Write the formulas of the ions most likely to form when the following non-metal atoms react with metals:
a I
b S
c N
d F
A3.
a I–
b S2–
c N3–
d F–
Q4.
Write the electronic configurations for the ions most likely to form from the following atoms. In each case, state which noble gas has the same electronic configuration as the ion formed.
a Cl
b Mg
c Ca
A4.
a 1s2 2s2 2p6 3s2 3p6 (Ar)
b 1s2 2s2 2p6 (Ne)
c 1s2 2s2 2p6 3s2 3p6 (Ar)
Q5.
Suppose that the atoms of elements with the following electronic configurations form ions by losing or gaining one or more electrons. In each case, write the formula of the ion that is most likely to form. (The symbols shown for the elements are not their real ones.)
a Element M: 1s22s22p4
b Element N: 1s22s1
c Element P: 1s22s22p63s23p1
d Element Q: 1s22s22p63s23p5
e Element R: 1s22s22p63s23p64s1
A5.
The atoms of the metal and the non-metal form ions that have the same configuration as the nearest noble gas or group 8 element.
a M2–
b N+
c P3+
d Q–
e R+
Q6.
Use diagrams similar to Figure 6.6 on page 97 to show the formation of ions in the reactions between:
a potassium and fluorine
b magnesium and sulfur
c aluminium and fluorine
e sodium and oxygen
f aluminium and oxygen
A6.
a
b
c
d
e
Q7.
Explain why potassium chloride has the formula KCl whereas the formula of calcium chloride is CaCl2.
A7.
The electronic configuration of an atom of potassium is 1s22s22p63s23p64s1 and that of an atom of chlorine is 1s22s22p63s23p5. Because an atom of K has one more electron than an atom of a noble gas and an atom of Cl has one less, in a reaction one K atom can donate one electron to one Cl atom to give K+ (1s22s22p63s23p6) and Cl– (1s22s22p63s23p6), both of which have the electronic configuration of the noble gas argon. The formula of this compound is, therefore, KCl.
Calcium, however, has the electronic configuration of 1s22s22p63s23p64s2. A Ca atom will lose two electrons to gain a noble gas configuration. Because each Cl atom will gain only one electron, there will be two chlorine atoms for each Ca atom. This reaction will therefore produce Ca2+ (1s22s22p63s23p6) ions and Cl– (1s22s22p63s23p6) ions. The formula of the compound is, therefore, CaCl2.
Q8.
When atoms of potassium react with atoms of oxygen, the changes that occur in the electronic configurations of each atom may be represented by an equation:
2K(1s22s22p63s23p63s1) + O(1s22s22p4) ® 2K+(1s22s22p63s23p6) + O2–(1s22s22p6)
Write similar equations for the reactions between:
a sodium and chlorine atoms
b magnesium and oxygen atoms
c aluminium and sulfur atoms
A8.
a Na(1s22s22p63s1) + Cl(1s22s22p63s23p5) ®
Na+(1s22s22p6) + Cl–(1s22s22p63s23p6)
b Mg(1s22s22p63s2) + O(1s22s22p4) ® Mg2+(1s22s22p6) + O2–(1s22s22p6)
c 2Al(1s22s22p63s23p1) + 3S(1s22s22p63s23p4) ®
2Al3+(1s22s22p6) + 3S2–(1s22s22p63s23p6)
Q9.
Using the information in Table 6.3 on page 101, write formulas for the following compounds:
a sodium chloride
b potassium bromide
c zinc chloride
d potassium oxide
e barium bromide
f aluminium iodide
g silver bromide
h zinc oxide
i barium oxide
j aluminium sulfide
A9.
a NaCl
b KBr
c ZnCl2
d K2O
e BaBr2
f AlI3
g AgBr
h ZnO
i BaO
j Al2S3
Q10.
Using the information in Table 6.3 on page 101, write formulas for the following compounds:
a sodium carbonate
b barium nitrate
c aluminium nitrate
d calcium hydroxide
e zinc sulfate
f potassium hydroxide
g potassium nitrate
h zinc carbonate
i potassium sulfate
j barium hydroxide
A10.
a Na2CO3
b Ba(NO3)2
c Al(NO3)3
d Ca(OH)2
e Zn(SO4)2
f KOH
g KNO3
h ZnCO3
i K2SO4
j Ba(OH) 2
Q11.
Using the information in Table 6.3 on page 101, write formulas for the following compounds:
a copper(I) chloride
b iron(III) oxide
c copper(II) oxide
d chromium(III) sulfate
e iron(II) oxide
f lead(II) nitrate
g lead(IV) oxide
h tin(II) nitrate
A11.
a CuCl
b Fe2O3
c CuO
d Cr2(SO4) 3
e FeO
f Pb(NO3)2
g PbO2
h Sn(NO3) 2
Q12.
Use the information in Table 6.3 on page 101 to name the ionic compounds with the following empirical formulas:
a KCl
b CaO
c MgS
d NaF
e Mg(OH)2
f K2O
g FeSO4
h Ba(NO3)2
i NH4Cl
j Na2CO3
k CuSO4
A12.
a potassium chloride
b calcium oxide
c magnesium sulfide
d sodium fluoride
e magnesium hydroxide
f potassium oxide
g iron(II) sulfate
h barium nitrate
i ammonium chloride
j sodium carbonate
k copper(II) sulfate
Q13.
Use the information in Table 6.3 on page 101 to write an empirical formula for each of the following ionic compounds:
a lithium chloride
b calcium carbonate
c ammonium carbonate
d iron(III) sulfide
e sodium dichromate
A13.
a LiCl
b CaCO3
c (NH4)2CO3
d Fe2S3
e Na2Cr2O7
Q14.
This language of chemists is universal. Can you identify the formulas in the figure below?
Chemical symbols are the same throughout the world.
A14.
Pt; Al2O3; ZnO; O; KOH; HgCl2; NaOH
Q15.
‘Diet-Rite’ Lite Salt, which contains 50% less sodium than table salt, is actually a mixture of sodium chloride and potassium chloride.
a Write the electronic configuration of the potassium ion in potassium chloride.
b To what group of the periodic table do sodium and potassium both belong?
c Suggest why potassium chloride is a suitable substitute for some of the sodium chloride.
A15.
a 1s22s22p63s23p6
b group 1
c Both elements are from the same group, so the chloride salts should react similarly.
Q16.
Sodium chloride, or common salt, is an essential part of our diet. Throughout history, it has also been used to preserve food in the absence of refrigeration. So important was sodium chloride to daily life that many words or expressions in the English language have been derived from the word salt. Salary and salinity are two of them. Can you think of any others?
A16.
Salad; salami; saline; expressions such as ‘salt of the Earth’, ‘take with a pinch of salt’, ‘worth one’s salt’; and superstitions, such as throwing salt over one’s shoulder to keep away evil spirits.
Chapter review
Q17.
Explain why an ionic compound will not conduct electricity in the solid state even though it contains charged particles.
A17.
In the solid state, ions are held in position by strong ionic bonds and are not free to move.
Q18.
Describe what happens to the forces between particles as sodium chloride is heated and melts.
A18.
The strength of the forces remains unchanged, but the kinetic energy of the ions increases until the forces can no longer hold the ions in the solid lattice, and the lattice breaks up as the solid melts.
Q19.
The melting temperature of sodium chloride is 801°C whereas that of magnesium oxide is 2800°C.
a What particles are present in the two solids?
b Which solid has the stronger forces between its particles?
c Give some possible differences in the structure and bonding of the two solids that would explain the large difference in melting temperature.
A19.
a Na+, Cl–; Mg2+, O2–
b MgO has the stronger forces between its particles. More energy is required to overcome the stronger forces. The higher melting temperature therefore reflects the solid with stronger forces between particles.
c The strength of electrostatic attraction between ions will depend on the size of the ions and on their charge. The Mg2+ ion is slightly smaller than the Na+ ion, and the O2– ion is much smaller than the Cl– ion. More importantly, the Mg2+ ion and the O2– ions each have twice the charge of the Na+ ion and the Cl– ion. The attraction between the ions in MgO is therefore much stronger than in NaCl. Magnesium oxide therefore has a much higher melting temperature.
Q20.
Describe an experiment you could carry out to demonstrate each of the following properties of the compounds given. In each case you should:
i Sketch the equipment you would use.
ii Describe what you would expect to observe.
a Solid magnesium chloride does not conduct electricity.
b Molten sodium chloride is a good electrical conductor.
c Solid sodium chloride is hard and brittle.
A20.
a Assemble equipment to test conductivity, such as that shown in Figure 6.5 (page 95). Add a globe to the circuit. When the electrodes are touching the solid magnesium chloride, the globe will not light up.
b Using the same equipment with molten sodium chloride, the globe will glow. Care is needed, as sodium chloride melts at 801°C.
c If a crystal of sodium chloride was hit firmly with a hammer, it would shatter. Again, care is needed—safety glasses must be worn.
Q21.
Use the ionic bonding model to explain the following properties of ionic compounds:
a They generally have high melting temperatures.
b They are hard and brittle.
c They do not conduct electricity in the solid state but will conduct when molten or dissolved in water.
A21.
The information in Table 6.1 (page 94) is an excellent summary.
a The electrostatic forces of attraction between the positive and negative ions are strong and will be overcome only at high temperatures.
b The strong electrostatic forces of attraction between the ions mean that a strong force is needed to break up the lattice, giving the ionic crystals the property of hardness. However, the crystal lattice will shatter when a strong force is applied, suddenly causing ions of like charge to become adjacent to each other and be repelled.
c In the solid state, the ions are not free to move. However, when the solid melts or dissolves in water, the ions are free to move and conduct electricity.
Q22.
The elements X, Y and Z form ionic compounds when reacted with other elements. The following compounds are formed: Ca3X2, Y2CO3 and Al2Z3.
a What is the electrovalency (charge) of the ion formed by:
i element X?
ii element Y?
iii element Z?
b Use these charges on the ions to write correct chemical formulas for the following compounds:
i the sulfate salt of Y
ii the potassium salt of Z
iii the ionic compound formed between X and Y
iv the ionic compound formed between Y and Z
A22.
a i –3
ii +1
iii –2
b i Y2SO4
ii K2Z
iii Y3X
iv Y2Z
Q23.
Write an empirical formula for each of the following ionic compounds:
a copper(I) nitrate
b silver oxide
c chromium(III) fluoride
d lithium nitride
e potassium carbonate
f magnesium hydrogen carbonate
g nickel(II) phosphate
A23.
a CuNO3. Copper(I) ion has a charge of +1, nitrate ion has –1.
b Ag2O. Silver ion has a charge of +1, oxide ion has –2.
c CrF3. Chromium(III) ion has a charge of +3, fluoride ion has –1.
d Li3N. Lithium ion has a charge of +1, nitride ion has –3.
e K2CO3. Potassium ion has a charge of +1, carbonate ion has –2.
f Mg(HCO3)2. Magnesium ion has a charge of +2, hydrogen carbonate ion has –1.
g Ni3 (PO4) 2. Nickel ion has a charge of +2, phosphate ion has –3.
Q24.
Name the ionic compounds with the following chemical formulas:
a KF
b LiOH
c ZnCl2
d AlN
e (NH4)2CO3
f Cu(NO3)2
g CrBr3
A24.
a potassium fluoride
b lithium hydroxide
c zinc chloride
d aluminium nitride
e ammonium carbonate
f copper(II) nitrate
g chromium(III) bromide
Q25.
Write the empirical formula for the ionic compound formed in the reaction between:
a potassium and bromine
b magnesium and iodine
c calcium and oxygen
d aluminium and fluorine
e calcium and nitrogen
A25.
a KBr. Potassium ion has a charge of +1, bromide ion has –1.
b MgI2. Magnesium ion has a charge of +2, iodide ion has –1.
c CaO. Calcium ion has a charge of +2, oxide ion has –2.
d AlF3. Aluminium ion has a charge of +3, fluoride ion has –1.
e Ca3N2. Calcium ion has a charge of +2, nitride ion has –3.
Q26.
Use diagrams to show the electron transfer that occurs when:
a lithium reacts with chlorine
b magnesium reacts with fluorine
c potassium reacts with sulfur
d magnesium reacts with nitrogen
A26.
a
b
c
d
Q27.
The electronic configurations of some metallic and non-metallic elements are given below. (The symbols shown for the elements are not their real ones.) Write formulas for the compounds they are most likely to form if they react together. The first example has been done for you.
a A: 1s22s1 B: 1s22s22p4 A2B