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NSSCAS Chemistry (8224) practice questions

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Question 1Atomic structure and the periodic table

Which statement describes the number of neutrons in an atom of chlorine-37?

  1. 17, because the neutron number equals the atomic number
  2. 37, because the mass number equals the neutron number
  3. 20, because the mass number minus the proton number gives the neutron number Correct answer.
  4. 54, because the mass number plus the proton number gives the neutron number

Explanation

Chlorine has proton number 17, so an atom of chlorine-37 contains 37 − 17 = 20 neutrons. The mass number is the total of protons and neutrons, so subtracting the proton number gives the neutron count.

Question 2Atomic structure and the periodic table

Which species contains the same number of electrons as a potassium ion, K⁺?

  1. Ca²⁺ Correct answer.
  2. Na⁺
  3. Cl⁻
  4. Ne

Explanation

A potassium atom has 19 electrons, so K⁺ has 18 electrons, the argon configuration. A calcium atom has 20 electrons, so Ca²⁺ also has 18 electrons, making it isoelectronic with K⁺.

Question 3Atomic structure and the periodic table

Which sub-shell is filled immediately after the 3p sub-shell in the build-up of electron configurations?

  1. 3d
  2. 4s Correct answer.
  3. 4p
  4. 3s

Explanation

After 3p (which completes at argon) the next sub-shell filled is 4s, because it lies at lower energy than 3d. Only after 4s is occupied does the 3d sub-shell fill, from scandium onwards.

Question 4Atomic structure and the periodic table

Which statement correctly describes the trend in first ionisation energy across Period 3 from sodium to argon?

  1. It generally increases because nuclear charge increases while shielding stays similar Correct answer.
  2. It stays constant because the number of shells is unchanged
  3. It decreases steadily because shielding increases
  4. It increases only where a new sub-shell begins

Explanation

Across Period 3 the nuclear charge rises by one for each element while electrons enter the same shell, so shielding changes little. The greater effective nuclear charge holds the outer electron more tightly, so first ionisation energy generally increases.

Question 5Atomic structure and the periodic table

The first ionisation energy of aluminium is lower than that of magnesium. Which explanation is correct?

  1. The outer electron of aluminium is in a 3p orbital, which is higher in energy and better shielded than the 3s orbital Correct answer.
  2. Aluminium has a larger atomic radius than magnesium
  3. Aluminium has more neutron-rich nuclei than magnesium
  4. Aluminium has one more proton than magnesium

Explanation

Magnesium loses a 3s electron, whereas aluminium loses a 3p electron, which lies higher in energy and is shielded by the 3s electrons. This outweighs the small increase in nuclear charge, so aluminium has the lower first ionisation energy.

Question 6Atomic structure and the periodic table

Which ion has the electron configuration 1s² 2s² 2p⁶?

  1. Cl⁻
  2. Mg²⁺
  3. Na⁺ Correct answer.
  4. F⁻

Explanation

A sodium atom has 11 electrons, so Na⁺ has 10 electrons, matching the neon configuration shown. Magnesium has 12 electrons, so Mg²⁺ has 10 electrons as well, so check which option fits the given configuration.

Question 7Atomic structure and the periodic table

Which property increases down Group 2 from magnesium to barium?

  1. Electronegativity
  2. Nuclear charge per unit volume
  3. First ionisation energy
  4. Atomic radius Correct answer.

Explanation

Each element down Group 2 has one more occupied shell, so the atomic radius increases even though nuclear charge rises. The outer electrons are further from the nucleus and better shielded, so ionisation energy and electronegativity decrease.

Question 8Atomic structure and the periodic table

Which statement explains why the first ionisation energy of sulfur is lower than that of phosphorus?

  1. Sulfur has a smaller nuclear charge than phosphorus
  2. Sulfur has one fewer proton than phosphorus
  3. In sulfur, two electrons occupy the same 3p orbital and repel each other, easing removal Correct answer.
  4. Sulfur's outer electron is in a 3s orbital

Explanation

Phosphorus has a half-filled 3p set (one electron in each orbital), whereas sulfur has a paired set in one 3p orbital. The repulsion between the paired electrons makes one easier to remove, so sulfur has the lower first ionisation energy.

Question 9Atomic structure and the periodic table

Which pair of elements shows the most similar chemical properties?

  1. Li and Na
  2. Na and Mg
  3. Ne and Na
  4. Na and K Correct answer.

Explanation

Sodium and potassium are both Group 1 alkali metals, so they have the same number of outer-shell electrons and similar chemical behaviour. Elements in the same group show the closest resemblance in properties.

Question 10Atomic structure and the periodic table

Which statement about the periodic table is correct?

  1. Groups are horizontal rows of elements with similar masses
  2. Periods are vertical columns containing elements with the same number of shells
  3. Elements are arranged in order of increasing relative atomic mass, with no exceptions
  4. Elements are arranged in order of increasing proton number Correct answer.

Explanation

The modern periodic table arranges elements by increasing proton (atomic) number, which places them in order of electronic structure. This ordering resolves the anomalies that arose when elements were arranged by relative atomic mass alone.

Question 11Atomic structure and the periodic table

Which species has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵?

  1. Fe³⁺
  2. V²⁺
  3. Cr³⁺
  4. Mn²⁺ Correct answer.

Explanation

Manganese has 25 electrons, so Mn²⁺ has 23, giving the configuration ending 3d⁵ after removal of the two 4s electrons. The half-filled 3d⁵ arrangement is consistent with the species shown.

Question 12Atomic structure and the periodic table

Which statement explains why the atomic radius of chlorine is smaller than that of sodium?

  1. Chlorine has a full outer shell, which shrinks the atom
  2. Chlorine has fewer protons than sodium
  3. Chlorine has a greater effective nuclear charge pulling the same shell closer Correct answer.
  4. Chlorine has fewer shells of electrons than sodium

Explanation

Sodium and chlorine both have electrons in the third shell, but chlorine has seven more protons, so the effective nuclear charge is greater. The outer electrons are pulled closer to the nucleus, so the radius decreases across the period.

Question 13Atomic structure and the periodic table

Which statement about isotopes of an element is correct?

  1. They have the same mass number but different proton numbers
  2. They have the same number of neutrons but different numbers of protons
  3. They have different chemical symbols and belong to different elements
  4. They have the same number of protons but different numbers of neutrons Correct answer.

Explanation

Isotopes are atoms of the same element, so they have identical proton numbers, but they differ in neutron number and therefore in mass number. Because the electronic configuration is the same, their chemical properties are almost identical.

Question 14Atomic structure and the periodic table

Which statement describes the shape of the p orbitals in an atom?

  1. A ring lying in the plane perpendicular to the nucleus
  2. Spherical, centred on the nucleus
  3. A single lobe pointing in one direction only
  4. Dumb-bell shaped, with two lobes on opposite sides of the nucleus Correct answer.

Explanation

Each p orbital consists of two lobes arranged along one axis, giving the characteristic dumb-bell shape. The three p orbitals are oriented at right angles to one another along the x, y and z axes.

Question 15Atomic structure and the periodic table

Which statement explains why the second ionisation energy of an element is always greater than its first ionisation energy?

  1. The second ionisation energy involves removing a neutron from the nucleus
  2. The second electron comes from a shell closer to the nucleus, which is easier to remove
  3. The second ionisation energy is measured at a higher temperature
  4. The second electron is removed from a positively charged ion, so more energy is needed Correct answer.

Explanation

After the first electron is removed, the remaining ion carries a positive charge, so the electrostatic attraction on the next electron is stronger. More energy is therefore required to remove a second electron from the same shell.

Question 16Atomic structure and the periodic table

Which statement correctly describes the trend in electronegativity across Period 3 from sodium to chlorine?

  1. It remains constant because the number of shells is unchanged
  2. It decreases because atomic radius increases
  3. It increases only for elements with a full outer shell
  4. It increases because the effective nuclear charge increases Correct answer.

Explanation

Across Period 3 the nuclear charge increases while the number of shells stays the same, so the attraction for bonding electrons increases. Electronegativity therefore rises from sodium to chlorine.

Question 17Atomic structure and the periodic table

Which statement about the ions formed by Group 1 and Group 2 elements is correct?

  1. Group 1 elements form 1+ ions and Group 2 elements form 1+ ions
  2. Group 1 elements form 2+ ions and Group 2 elements form 1+ ions
  3. Group 1 elements form 1+ ions and Group 2 elements form 2+ ions Correct answer.
  4. Both groups form 1− ions by gaining one electron

Explanation

Group 1 atoms have one outer electron, which they lose to form a 1+ ion with a noble-gas configuration. Group 2 atoms have two outer electrons, which they lose to form a 2+ ion.

Question 18Atomic structure and the periodic table

An element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which group and period of the periodic table is it found?

  1. Group 6, Period 3
  2. Group 16, Period 3 Correct answer.
  3. Group 4, Period 3
  4. Group 14, Period 2

Explanation

The highest occupied shell is the third, so the element is in Period 3. There are six outer electrons (two in 3s and four in 3p), which places it in Group 16, the oxygen group.

Question 19Atomic structure and the periodic table

Which statement explains why the ionic radius of a cation is smaller than the atomic radius of its parent atom?

  1. The cation contains fewer protons than the parent atom
  2. The cation has gained electrons, which increases the shielding
  3. The cation has lost an outer shell and has a greater effective nuclear charge per electron Correct answer.
  4. The cation has a smaller nuclear charge than the parent atom

Explanation

When a metal atom loses its outer electrons to form a cation, an entire shell is often removed and the remaining electrons are pulled in more strongly by the unchanged nuclear charge. The result is a smaller radius than in the parent atom.

Question 20Atomic structure and the periodic table

Which statement about the emission spectrum of hydrogen is correct?

  1. It is identical in appearance to the absorption spectrum of hydrogen
  2. It consists of discrete lines produced by transitions within the nucleus
  3. It consists of a series of discrete lines produced by electron transitions between energy levels Correct answer.
  4. It consists of a continuous band of all wavelengths

Explanation

Electrons in a hydrogen atom occupy discrete energy levels, so transitions between them emit photons of specific energies. Each energy difference corresponds to one wavelength, giving a line spectrum rather than a continuous one.

Question 21Atomic structure and the periodic table

Which statement explains why the first ionisation energy of xenon is lower than that of helium?

  1. Xenon has a larger nuclear charge than helium
  2. Xenon has a full outer shell whereas helium does not
  3. The outer electron of xenon is in a much higher shell, so it is further from the nucleus and better shielded Correct answer.
  4. Xenon has fewer protons than helium

Explanation

The outer electron of xenon lies in the fifth shell, far from the nucleus and shielded by many inner electrons. Although xenon has a larger nuclear charge, distance and shielding dominate, so its first ionisation energy is much lower than that of helium.

Question 22Atomic structure and the periodic table

Which statement about the mass spectrum of a pure sample of magnesium is correct?

  1. It shows peaks at mass numbers 24, 25 and 26 corresponding to its isotopes Correct answer.
  2. It shows a single peak because magnesium has only one isotope
  3. It shows peaks only for ions of charge 2+
  4. It shows peaks at the same mass number but different charges

Explanation

Magnesium has three stable isotopes with mass numbers 24, 25 and 26, so its mass spectrum shows three peaks at those mass-to-charge ratios. The relative heights of the peaks reflect the relative abundances of the isotopes.

Question 23Atomic structure and the periodic table

Which statement about the trend in melting point across Period 3 from sodium to argon is correct?

  1. Melting point increases steadily across the whole period
  2. Melting point is constant because all elements are solids
  3. Melting point rises to a maximum at silicon and then falls sharply towards argon Correct answer.
  4. Melting point is highest at sodium and decreases steadily

Explanation

Sodium, magnesium and aluminium are metals whose melting points rise as the metallic bonding strengthens. Silicon has a giant covalent structure with a very high melting point, after which phosphorus, sulfur, chlorine and argon have simple molecular or monatomic structures with low melting points.

Question 24Atomic structure and the periodic table

Which statement correctly describes the relative abundance of isotopes in a sample of an element?

  1. The relative atomic mass equals the mass number of the most abundant isotope
  2. The relative atomic mass is the weighted average of the isotope masses according to their abundances Correct answer.
  3. All isotopes of an element are always present in equal abundance
  4. The relative atomic mass is the unweighted average of the isotope mass numbers

Explanation

The relative atomic mass is calculated by taking each isotope's mass, multiplying by its fractional abundance, and summing the contributions. This weighted average lies between the lightest and heaviest isotope masses.

Question 25Chemical bonding, structure and properties

Which statement best describes the metallic bonding present in a solid sample of magnesium?

  1. Electron pairs are shared between adjacent magnesium atoms in directional covalent bonds.
  2. Neutral magnesium atoms are held together by weak instantaneous dipole forces.
  3. Magnesium cations are electrostatically attracted to a lattice of magnesium anions.
  4. Positive metal ions are held in a lattice by a sea of delocalised electrons. Correct answer.

Explanation

Metallic bonding consists of a regular lattice of cations surrounded by a delocalised pool of electrons donated by the metal atoms; the electrostatic attraction between these holds the structure together. This delocalisation explains both electrical conductivity and malleability.

Question 26Chemical bonding, structure and properties

A crystal of sodium chloride is struck with a hammer and shatters. Which feature of the ionic lattice best accounts for this behaviour?

  1. The covalent bonds within each NaCl unit are broken by the applied force.
  2. Layers of ions can slide, and like charges then come into alignment and repel. Correct answer.
  3. Weak intermolecular forces between NaCl molecules break when stress is applied.
  4. Delocalised electrons are displaced and set up a repulsive field within the lattice.

Explanation

When a force displaces one plane of the ionic lattice, ions of the same charge can be brought adjacent to one another. The resulting electrostatic repulsion splits the crystal, so ionic solids are brittle rather than malleable.

Question 27Chemical bonding, structure and properties

Which species has a bond angle of approximately 104.5 degrees?

  1. H2O Correct answer.
  2. NH3
  3. CO2
  4. CH4

Explanation

Water has two bonding pairs and two lone pairs on oxygen, giving a bent shape with an angle compressed below the tetrahedral value to about 104.5 degrees. The two lone pairs repel more strongly than bonding pairs.

Question 28Chemical bonding, structure and properties

Which of the following substances is held together in the solid state by hydrogen bonding?

  1. Ice Correct answer.
  2. Solid iodine
  3. Solid carbon dioxide
  4. Solid sodium chloride

Explanation

In ice each water molecule has O-H bonds, so the partially positive hydrogen of one molecule is attracted to a lone pair on the oxygen of a neighbour. This hydrogen bonding produces the open hexagonal lattice of ice.

Question 29Chemical bonding, structure and properties

Which property is characteristic of a simple molecular substance such as hexane, C6H14?

  1. It has a high melting point because covalent bonds must be broken throughout the lattice.
  2. It dissolves readily in water because it can form hydrogen bonds with water molecules.
  3. It has a low melting point because only weak intermolecular forces must be overcome. Correct answer.
  4. It conducts electricity when molten because electrons become delocalised.

Explanation

Simple molecular solids melt when the weak London dispersion forces between molecules are overcome; the strong intramolecular C-H and C-C covalent bonds remain intact. This is why such substances are volatile and melt at low temperature.

Question 30Chemical bonding, structure and properties

Which statement about the electrical conductivity of a molten ionic compound is correct?

  1. It does not conduct because the lattice is still held together by strong ionic bonds.
  2. It does not conduct because the ions are fixed in position in the liquid.
  3. It conducts because electrons are released from the anions on melting.
  4. It conducts because the ions become free to move and carry charge. Correct answer.

Explanation

In the solid, ions are locked in the lattice and cannot migrate. Melting frees the ions, which then move towards the electrodes and carry current, so the molten compound conducts.

Question 31Chemical bonding, structure and properties

Which molecule contains a bond that is polar but the molecule overall is non-polar?

  1. Boron trifluoride, BF3 Correct answer.
  2. Ammonia, NH3
  3. Water, H2O
  4. Hydrogen chloride, HCl

Explanation

BF3 is trigonal planar with three identical polar B-F bonds arranged at 120 degrees, so the bond dipoles cancel and the molecule has no net dipole. A molecule can therefore contain polar bonds yet be non-polar overall.

Question 32Chemical bonding, structure and properties

Which factor is the main reason that the boiling point of hydrogen fluoride is higher than that of hydrogen chloride?

  1. Hydrogen chloride molecules are held together by ionic attractions in the liquid.
  2. Hydrogen fluoride has a larger relative molecular mass than hydrogen chloride.
  3. Hydrogen fluoride molecules form hydrogen bonds, whereas hydrogen chloride molecules do not. Correct answer.
  4. The H-F bond is weaker than the H-Cl bond, so less energy is needed to vaporise HF.

Explanation

Fluorine is highly electronegative and small, so HF forms strong intermolecular hydrogen bonds; chlorine is too large and poorly electronegative to support them. The extra energy needed to break these hydrogen bonds raises the boiling point of HF.

Question 33Chemical bonding, structure and properties

Which statement explains why magnesium oxide has a much higher melting point than sodium chloride?

  1. Both ions in MgO carry double the charge, giving stronger electrostatic attraction. Correct answer.
  2. MgO contains delocalised electrons that hold the lattice together more tightly.
  3. MgO has a simple molecular structure with strong covalent bonds between units.
  4. The ions in MgO are much larger, so the lattice energy is greater.

Explanation

Mg2+ and O2- carry twice the charge of Na+ and Cl-, and the ions are also smaller, so the electrostatic attraction is much stronger. More energy is therefore needed to separate the lattice, giving a higher melting point.

Question 34Chemical bonding, structure and properties

In a molecule of ethyne, C2H2, what are the hybridisation of each carbon atom and the H-C-C bond angle?

  1. sp3, with a bond angle of 109.5 degrees
  2. sp, with a bond angle of 180 degrees Correct answer.
  3. sp2, with a bond angle of 180 degrees
  4. sp2, with a bond angle of 120 degrees

Explanation

Each carbon in ethyne is bonded to two atoms and has two electron domains, giving sp hybridisation and linear geometry. The triple bond uses the two remaining p orbitals, and the H-C-C angle is 180 degrees.

Question 35Chemical bonding, structure and properties

Which sequence shows the species arranged in order of increasing bond angle?

  1. H2O < CH4 < NH3
  2. H2O < NH3 < CH4 Correct answer.
  3. CH4 < NH3 < H2O
  4. NH3 < H2O < CH4

Explanation

All three have four electron domains around the central atom, but lone pairs repel more strongly than bonding pairs. Water has two lone pairs (about 104.5 degrees), ammonia one (about 107 degrees) and methane none (109.5 degrees).

Question 36Chemical bonding, structure and properties

Which pair of substances both have giant covalent structures?

  1. Iodine and graphite
  2. Carbon dioxide and water
  3. Sodium chloride and magnesium oxide
  4. Diamond and silicon dioxide Correct answer.

Explanation

Diamond is a three-dimensional network of carbon atoms and silicon dioxide is a continuous network of SiO4 tetrahedra sharing corners. Both are giant covalent lattices with very high melting points.

Question 37Chemical bonding, structure and properties

Graphite conducts electricity along its layers, whereas diamond does not. What is the best explanation?

  1. In graphite each carbon uses three of its four valence electrons in sigma bonds, leaving a delocalised electron per atom. Correct answer.
  2. Diamond contains no valence electrons available for conduction because all are in lone pairs.
  3. In graphite the layers are held by covalent bonds that allow electron transfer along the sheets.
  4. Graphite contains mobile ions between the layers that carry the current.

Explanation

Each carbon in graphite is trigonal planar, bonded to three neighbours, so the fourth valence electron occupies a p orbital that overlaps to give a delocalised pi system. These electrons move within the layers and carry current, whereas in diamond all four electrons are localised in sigma bonds.

Question 38Chemical bonding, structure and properties

Which observation provides the strongest evidence that a compound has a giant covalent rather than a giant ionic structure?

  1. The solid conducts electricity but the molten compound does not.
  2. The compound is brittle and shatters when struck.
  3. The solid does not conduct, but the molten compound also fails to conduct. Correct answer.
  4. The compound dissolves readily in water to give a conducting solution.

Explanation

Giant covalent substances contain no mobile ions or delocalised electrons in either the solid or molten state (unless they are like graphite), so neither conducts. Ionic compounds conduct when molten because their ions become mobile.

Question 39Chemical bonding, structure and properties

Why does the boiling point of the halogens increase from chlorine to iodine?

  1. The covalent X-X bond strength increases down the group.
  2. The molecules have more electrons and a larger surface area, so London dispersion forces are stronger. Correct answer.
  3. The halogen molecules become more polar as the atoms get larger.
  4. Hydrogen bonding between molecules becomes stronger down the group.

Explanation

All the halogens are non-polar diatomic molecules held by London dispersion forces. Down the group the molecules gain electrons and become more polarisable, so the induced dipoles are larger and more energy is needed to separate the molecules.

Question 40Chemical bonding, structure and properties

Which molecule is expected to have the largest permanent dipole moment?

  1. Methane, CH4
  2. Chloromethane, CH3Cl Correct answer.
  3. Carbon dioxide, CO2
  4. Tetrachloromethane, CCl4

Explanation

In CH3Cl the polar C-Cl bond is not balanced by the three C-H bonds, so the bond dipoles add to give a net dipole. The tetrahedral symmetry of CCl4 and CH4 and the linear symmetry of CO2 cause their dipoles to cancel.

Question 41Chemical bonding, structure and properties

Which statement correctly describes the bonding in the ammonium ion, NH4+?

  1. The four N-H bonds are unequal, with one much longer than the others.
  2. Three N-H bonds are covalent and one is a dative covalent bond from nitrogen to a proton. Correct answer.
  3. The ion contains two lone pairs on nitrogen and four covalent bonds.
  4. All four N-H bonds are ionic because the ion carries a positive charge.

Explanation

Ammonia donates its lone pair into the empty 1s orbital of a proton, forming a dative (coordinate) bond. Once formed, all four N-H bonds are equivalent in length and strength, but the origin of one is dative.

Question 42Chemical bonding, structure and properties

Which property is explained by the delocalised electrons in a metal?

  1. The metal is malleable and conducts electricity. Correct answer.
  2. The metal has a low density and a low melting point.
  3. The metal is a poor conductor of heat.
  4. The metal is brittle and shatters when hammered.

Explanation

Delocalised electrons can move through the lattice, carrying charge, and they also transmit kinetic energy effectively, making metals good conductors. Their non-directional bonding allows layers of cations to slide without breaking, which gives malleability.

Question 43Chemical bonding, structure and properties

Which statement about the relative strength of the forces broken when each substance is boiled is correct?

  1. Boiling water breaks hydrogen bonds between molecules, not the O-H covalent bonds. Correct answer.
  2. Boiling water breaks the O-H covalent bonds within each molecule.
  3. Boiling iodine breaks the covalent I-I bonds.
  4. Boiling sodium chloride breaks covalent bonds within NaCl units.

Explanation

A change of state involves overcoming the forces between particles, not the strong intramolecular bonds. In water the hydrogen bonds between molecules are broken, while the O-H bonds remain intact.

Question 44Chemical bonding, structure and properties

A solid has a high melting point, does not conduct electricity in the solid state, and its molten form conducts. Which type of structure is most consistent with these observations?

  1. A simple molecular solid
  2. A metallic lattice
  3. A giant covalent network such as diamond
  4. A giant ionic lattice Correct answer.

Explanation

High melting point plus conduction only when molten is the classic signature of an ionic solid: the lattice is strong, and the ions become mobile only after melting. The absence of conduction in the solid shows there are no delocalised electrons.

Question 45Chemical bonding, structure and properties

Which statement best explains why the ionic radius of Mg2+ is smaller than that of Na+?

  1. Mg2+ has a smaller nuclear charge than Na+, so the outer electrons are held less tightly.
  2. Mg2+ has more electron shells than Na+, so the outer electrons are further from the nucleus.
  3. Mg2+ has a full outer shell whereas Na+ has a partially filled shell.
  4. Mg2+ has the same electron configuration as Na+ but a greater nuclear charge, so the electrons are pulled in more strongly. Correct answer.

Explanation

Both ions are isoelectronic with neon, so they have the same number of shells. Magnesium has one more proton, so the greater effective nuclear charge contracts the electron cloud.

Question 46Chemical bonding, structure and properties

The successive ionisation energies of an element show a very large jump between the third and fourth values. What does this indicate about the element?

  1. The element is in Group 2 of the periodic table.
  2. The element has four electrons in its outermost shell.
  3. The element has three electrons in its outermost shell. Correct answer.
  4. The element has a full outer shell of eight electrons.

Explanation

A large jump occurs when an electron is removed from a shell closer to the nucleus, after all the outer-shell electrons have gone. A jump after the third ionisation therefore means three electrons were in the outermost shell.

Question 47Chemical bonding, structure and properties

Why is the melting point of silicon dioxide much higher than that of carbon dioxide, even though both contain strong covalent bonds?

  1. SiO2 contains ionic bonds between silicon and oxygen that require more energy to break.
  2. SiO2 is a giant covalent network, so many strong covalent bonds must be broken, whereas CO2 is molecular and only weak intermolecular forces are overcome. Correct answer.
  3. SiO2 has a simple molecular structure with very strong intermolecular forces.
  4. CO2 molecules are held together by hydrogen bonds that are easily broken on melting.

Explanation

In silica each silicon is covalently bonded to four oxygens in an extended network, so melting requires breaking many strong covalent bonds. Carbon dioxide consists of discrete molecules held together only by London dispersion forces.

Question 48Chemical bonding, structure and properties

Which statement correctly compares the bond lengths and strengths of the C-C bond in ethane and the C=C bond in ethene?

  1. The C-C and C=C bonds are the same length because both are single sigma bonds.
  2. The C=C bond is shorter but weaker because pi bonds are weaker than sigma bonds.
  3. The C=C bond is shorter and stronger because the additional pi overlap increases the electron density between the nuclei. Correct answer.
  4. The C=C bond is longer and weaker because the pi bond pushes the carbon atoms apart.

Explanation

A double bond consists of one sigma plus one pi bond, so there is more electron density between the carbon nuclei. This increases the attraction and shortens the bond, and more energy is needed to break two bonds than one.

Question 49Stoichiometry and the mole concept

What is the mass of one mole of carbon-12 atoms, expressed to the appropriate number of significant figures?

  1. 6.02 × 10^23 g
  2. 12.0 g
  3. 12 g Correct answer.
  4. 12.011 g

Explanation

By definition the mole is the amount of substance containing as many entities as there are atoms in exactly 12 g of carbon-12, so one mole of carbon-12 atoms has a mass of exactly 12 g. The value 12.011 g refers to naturally occurring carbon, which contains carbon-13.

Question 50Stoichiometry and the mole concept

A sample contains 0.250 mol of hydrated copper(II) sulfate, CuSO4·5H2O. How many water molecules does the sample contain?

  1. 7.53 × 10^23 Correct answer.
  2. 3.01 × 10^23
  3. 1.51 × 10^23
  4. 1.20 × 10^24

Explanation

Each formula unit contains five water molecules, so the amount of water is 0.250 × 5 = 1.25 mol. Multiplying by Avogadro's constant gives 1.25 × 6.02 × 10^23 = 7.53 × 10^23 molecules.