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IB Chemistry SL practice questions

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Question 1Structure 1 - Models of the particulate nature of matter

Which statement best describes the arrangement and motion of particles in a gas at room temperature?

  1. Particles are closely packed in fixed positions and vibrate about those positions.
  2. Particles are closely packed but slide past one another in a random arrangement.
  3. Particles lie in a regular lattice with long-range order and cannot move.
  4. Particles are far apart, move rapidly and randomly, and interact only weakly. Correct answer.

Explanation

In a gas the average separation of the particles is much greater than the particle diameter, so the forces between them are negligible and they travel rapidly in random directions, filling the whole container. Fixed positions with vibration describe a solid, while close packing with particles able to slide describes a liquid.

Question 2Structure 1 - Models of the particulate nature of matter

Solid carbon dioxide (dry ice) left at room pressure turns directly into a gas without forming a liquid. Which term names this change of state?

  1. Condensation
  2. Fusion
  3. Sublimation Correct answer.
  4. Deposition

Explanation

Sublimation is the direct conversion of a solid into a gas without passing through the liquid state, which is how dry ice behaves at 1 atm. The reverse process, gas to solid, is called deposition, and solid to liquid is fusion or melting.

Question 3Structure 1 - Models of the particulate nature of matter

A neutral atom of the isotope chlorine-37 has atomic number 17. How many neutrons and electrons does this atom contain?

  1. 20 neutrons and 17 electrons Correct answer.
  2. 20 neutrons and 20 electrons
  3. 17 neutrons and 17 electrons
  4. 37 neutrons and 17 electrons

Explanation

The mass number counts protons plus neutrons, so neutrons = 37 − 17 = 20. Because the atom is neutral, its number of electrons equals its number of protons, which is 17.

Question 4Structure 1 - Models of the particulate nature of matter

A solid is heated at a steady rate until it has melted, then heating continues until the liquid boils. At the two plateaus of the heating curve, which statement is correct?

  1. The average kinetic energy is constant while the potential energy of the particles increases. Correct answer.
  2. Neither the kinetic energy nor the potential energy of the particles changes.
  3. The average kinetic energy of the particles increases while the potential energy stays constant.
  4. Both the average kinetic energy and the potential energy increase steadily.

Explanation

A plateau on a heating curve is a change of state occurring at constant temperature, and temperature measures average kinetic energy, so that quantity does not change. The energy supplied is used instead to weaken and overcome intermolecular forces, which raises the potential energy of the particles.

Question 5Structure 1 - Models of the particulate nature of matter

How many molecules are present in 4.40 g of carbon dioxide? (M(CO2) = 44.01 g mol^-1; Avogadro constant L = 6.02 x 10^23 mol^-1)

  1. 6.02 x 10^22 Correct answer.
  2. 6.02 x 10^23
  3. 1.20 x 10^24
  4. 2.65 x 10^23

Explanation

The amount of substance is n = m / M = 4.40 / 44.01 = 0.100 mol, and the number of molecules is N = nL = 0.100 x 6.02 x 10^23 = 6.02 x 10^22. A full mole would need 44.0 g, ten times the mass given.

Question 6Structure 1 - Models of the particulate nature of matter

A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. What is its empirical formula? (Ar: C = 12.0, H = 1.0, O = 16.0)

  1. C2H4O2
  2. CHO
  3. C2H6O
  4. CH2O Correct answer.

Explanation

Dividing each percentage by the appropriate atomic mass gives 40.0/12.0 = 3.33 mol C, 6.7/1.0 = 6.7 mol H and 53.3/16.0 = 3.33 mol O. Dividing through by the smallest value, 3.33, gives the ratio 1 : 2 : 1, so the empirical formula is CH2O.

Question 7Structure 1 - Models of the particulate nature of matter

A sealed flask of volume 2.00 dm^3 contains 0.0500 mol of an ideal gas at 300 K. What is the pressure of the gas? (R = 8.31 J K^-1 mol^-1)

  1. 125 kPa
  2. 24.9 kPa
  3. 6.23 kPa
  4. 62.3 kPa Correct answer.

Explanation

Rearranging pV = nRT gives p = nRT / V = (0.0500 x 8.31 x 300) / 2.00 x 10^-3 m^3 = 6.23 x 10^4 Pa, which is 62.3 kPa. The volume must be converted from dm^3 to m^3 before substitution so that the answer comes out in pascals.

Question 8Structure 1 - Models of the particulate nature of matter

What is the ground-state electron configuration of the Fe2+ ion? (Z(Fe) = 26)

  1. [Ar] 3d6 4s2
  2. [Ar] 3d4 4s2
  3. [Ar] 3d6 Correct answer.
  4. [Ar] 3d8

Explanation

An iron atom is [Ar] 3d6 4s2, and when a transition metal is ionized the 4s electrons are removed first because in the cation the 4s orbital lies higher in energy than 3d. Removing two electrons therefore leaves [Ar] 3d6, with the 3d subshell untouched.

Question 9Structure 1 - Models of the particulate nature of matter

Two identical rigid flasks contain equal amounts of the same ideal gas. Flask X is at 300 K and flask Y is at 600 K. Which comparison of the two samples is correct?

  1. The mean kinetic energy in Y is twice that in X, and the molecules in Y hit the walls more often. Correct answer.
  2. The mean kinetic energy is the same in both flasks, but the molecules in Y move faster on average.
  3. The molecules in Y have twice the average speed of those in X because the temperature is doubled.
  4. The pressure in Y equals the pressure in X because the amount of gas and the volume are unchanged.

Explanation

The mean kinetic energy of the molecules of an ideal gas is directly proportional to the absolute temperature, so doubling the temperature from 300 K to 600 K doubles the mean kinetic energy. The average speed rises only by a factor of the square root of 2, and the more energetic molecules collide with the walls more often and more forcefully, so the pressure also doubles at constant volume.

Question 10Structure 1 - Models of the particulate nature of matter

A sample of magnesium contains 79.0% Mg-24, 10.0% Mg-25 and 11.0% Mg-26 by amount of substance. What is the relative atomic mass of magnesium in this sample?

  1. 24.00
  2. 25.00
  3. 24.10
  4. 24.32 Correct answer.

Explanation

The relative atomic mass is the weighted mean of the isotope masses: (0.790 x 24) + (0.100 x 25) + (0.110 x 26) = 18.96 + 2.50 + 2.86 = 24.32. The value lies close to 24 because that isotope accounts for 79% of the atoms present.

Question 11Structure 1 - Models of the particulate nature of matter

A fixed mass of gas is compressed at constant temperature. Which statement explains the increase in pressure?

  1. More frequent wall collisions per unit area. Correct answer.
  2. The number of gas particles increases.
  3. The particles themselves become smaller.
  4. The particles move with a higher average speed.

Explanation

At constant temperature the average kinetic energy of the particles is unchanged. Compressing the gas packs the same number of particles into a smaller volume, so they strike each unit of wall area more frequently, raising the pressure.

Question 12Structure 1 - Models of the particulate nature of matter

A substance is heated at constant pressure and its temperature remains constant while it changes from liquid to gas. What happens to the energy supplied?

  1. It increases the number of particles present.
  2. It lowers the potential energy of the particles.
  3. It breaks intermolecular attractions. Correct answer.
  4. It raises the average kinetic energy of the particles.

Explanation

During boiling the temperature stays constant, so the average kinetic energy of the particles does not change. The energy supplied is used to overcome the attractive forces between particles as they separate into the gas phase.

Question 13Structure 1 - Models of the particulate nature of matter

Which sample contains the same number of particles as 1.00 mol of helium atoms?

  1. 2 mol of hydrogen atoms
  2. 0.5 mol of oxygen molecules
  3. 8.0 g of oxygen gas
  4. 1 mol of water molecules Correct answer.

Explanation

One mole of any specified entity contains the Avogadro number of particles, 6.02 × 10^23. One mole of water molecules therefore contains the same number of particles as one mole of helium atoms.

Question 14Structure 1 - Models of the particulate nature of matter

A mass spectrum of an element shows peaks at m/z = 63 and m/z = 65. What is given directly by the relative heights of these peaks?

  1. The molar mass of the element
  2. The relative abundance of each isotope Correct answer.
  3. The total number of isotopes in the sample
  4. The number of neutrons in each isotope

Explanation

The position of a peak on the m/z axis indicates the mass of an isotope, while the height of the peak is proportional to the relative abundance of that isotope in the sample. The heights do not give the total number of isotopes or the number of neutrons directly.

Question 15Structure 1 - Models of the particulate nature of matter

A compound has empirical formula CH2O and relative molecular mass 180. What is its molecular formula?

  1. CH2O
  2. C3H6O3
  3. C6H12O6 Correct answer.
  4. C2H4O

Explanation

The empirical formula mass of CH2O is 12.0 + 2(1.0) + 16.0 = 30.0. Dividing the relative molecular mass by this value gives 180/30 = 6, so the molecular formula is C6H12O6.

Question 16Structure 1 - Models of the particulate nature of matter

What is the total number of ions present in 1.00 mol of aluminium sulfate, Al2(SO4)3?

  1. 3.01 × 10^24 Correct answer.
  2. 1.20 × 10^24
  3. 6.02 × 10^23
  4. 1.81 × 10^24

Explanation

Each formula unit of Al2(SO4)3 gives two Al3+ ions and three SO4^2- ions, a total of five ions. One mole of formula units therefore gives five moles of ions, which is 5 × 6.02 × 10^23 = 3.01 × 10^24 ions.

Question 17Structure 1 - Models of the particulate nature of matter

Which electron configuration represents an atom in an excited state?

  1. 1s2 2s2 2p6
  2. 1s2 2s2 2p6 3s1
  3. 1s2 2s2 2p5 3s1 Correct answer.
  4. 1s2 2s2 2p6 3s2

Explanation

In a ground-state neon atom the configuration is 1s2 2s2 2p6. Promoting one 2p electron to the 3s orbital gives 1s2 2s2 2p5 3s1, which has the same number of electrons but is an excited state.

Question 18Structure 1 - Models of the particulate nature of matter

A sample of gas has mass 0.500 g and occupies 0.250 dm^3 at 300 K and 1.00 × 10^5 Pa. What is the molar mass of the gas? (R = 8.31 J K^-1 mol^-1)

  1. 50 g mol^-1 Correct answer.
  2. 100 g mol^-1
  3. 25 g mol^-1
  4. 12.5 g mol^-1

Explanation

Use n = PV/RT with V = 0.250 dm3 = 2.50 × 10^-4 m3. This gives n = (1.00 × 10^5)(2.50 × 10^-4)/(8.31 × 300) = 0.0100 mol, so M = 0.500/0.0100 = 50 g mol^-1.

Question 19Structure 1 - Models of the particulate nature of matter

A balloon contains 2.0 m^3 of gas at 1.0 atm and 300 K. It rises to a height where the pressure is 0.50 atm and the temperature is 250 K. Assuming ideal behaviour, what is the new volume?

  1. 5.3 m^3
  2. 2.0 m^3
  3. 1.2 m^3
  4. 3.3 m^3 Correct answer.

Explanation

For a fixed amount of gas, P1V1/T1 = P2V2/T2. Rearranging gives V2 = V1 × (P1/P2) × (T2/T1) = 2.0 × (1.0/0.50) × (250/300) = 3.3 m3.

Question 20Structure 1 - Models of the particulate nature of matter

A 5.0 dm^3 flask at 300 K contains 0.20 mol N2 and 0.30 mol O2. What is the partial pressure of N2? (R = 8.31 J K^-1 mol^-1)

  1. 50 kPa
  2. 150 kPa
  3. 100 kPa Correct answer.
  4. 250 kPa

Explanation

The partial pressure of a gas in a mixture is calculated from its own amount using p = nRT/V. For nitrogen, p = (0.20 × 8.31 × 300)/(5.0 × 10^-3) = 9.97 × 10^4 Pa, which is about 100 kPa.

Question 21Structure 1 - Models of the particulate nature of matter

A sample contains two different types of atoms that are chemically bonded in a fixed ratio. At the particulate level, how is this sample best classified?

  1. a homogeneous mixture
  2. an element
  3. a heterogeneous mixture
  4. a compound Correct answer.

Explanation

A compound consists of two or more different elements chemically bonded in a fixed ratio, so its particles are molecules or formula units made from different atoms. An element would contain only one type of atom, and a mixture would not have a fixed chemical bonding ratio.

Question 22Structure 1 - Models of the particulate nature of matter

Which statement is an assumption of the kinetic molecular theory for an ideal gas?

  1. Gas particles attract each other with strong forces.
  2. The average kinetic energy of gas particles depends on pressure.
  3. Collisions between gas particles are inelastic.
  4. Gas particles have negligible volume compared with the container. Correct answer.

Explanation

The kinetic molecular theory assumes that the volume occupied by gas particles themselves is negligible compared with the container volume. It also assumes no intermolecular forces and elastic collisions, so attraction, inelastic collisions and pressure-dependent kinetic energy are not assumptions.

Question 23Structure 1 - Models of the particulate nature of matter

When a molecular liquid boils, which forces are overcome?

  1. ionic bonds in a lattice
  2. covalent bonds within the molecules
  3. nuclear forces within the atoms
  4. intermolecular forces between the molecules Correct answer.

Explanation

Boiling is a change of state from liquid to gas, so the particles separate from one another. The covalent bonds within each molecule remain intact; the intermolecular forces holding the molecules together in the liquid are overcome.

Question 24Structure 1 - Models of the particulate nature of matter

Two rigid flasks of equal volume are at the same temperature and pressure. One contains argon and the other contains methane. Which quantity is the same for both samples?

  1. the mass of each sample
  2. the number of molecules Correct answer.
  3. the average molecular speed
  4. the density of each gas

Explanation

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain the same number of molecules. The identity of the gas affects mass, density and average molecular speed, but not the number of molecules under these conditions.

Question 25Structure 1 - Models of the particulate nature of matter

How many oxygen atoms are present in 0.500 mol of aluminium sulfate, Al2(SO4)3? (Avogadro constant L = 6.02 × 10^23 mol^-1)

  1. 3.61 × 10^24 Correct answer.
  2. 1.81 × 10^24
  3. 7.22 × 10^24
  4. 6.02 × 10^23

Explanation

Each formula unit of Al2(SO4)3 contains three sulfate ions, and each sulfate ion has four oxygen atoms, giving 12 oxygen atoms per formula unit. Therefore 0.500 mol of formula units contains 0.500 × 12 = 6.00 mol of oxygen atoms, which is 6.00 × 6.02 × 10^23 = 3.61 × 10^24 atoms.

Question 26Structure 1 - Models of the particulate nature of matter

In the mass spectrum of an organic compound, what does the peak at the highest m/z value, excluding any isotope peaks, usually represent?

  1. a doubly charged ion
  2. the molecular ion Correct answer.
  3. the base peak
  4. a fragment ion

Explanation

The molecular ion peak arises when the whole molecule loses one electron but does not fragment, so its m/z value gives the relative molecular mass. Fragment ions appear at lower m/z values, and the base peak is simply the most intense peak in the spectrum.

Question 27Structure 1 - Models of the particulate nature of matter

A gas has a density of 1.96 g dm^-3 at 273 K and 1.00 × 10^5 Pa. What is the molar mass of the gas? (R = 8.31 J K^-1 mol^-1)

  1. 28.0 g mol^-1
  2. 64.0 g mol^-1
  3. 44.0 g mol^-1 Correct answer.
  4. 22.4 g mol^-1

Explanation

Rearranging the ideal gas equation gives M = ρRT/p. Converting the density to 1.96 kg m^-3 and using p = 1.00 × 10^5 Pa, T = 273 K and R = 8.31 J K^-1 mol^-1 gives M ≈ 44 g mol^-1.

Question 28Structure 1 - Models of the particulate nature of matter

Under which set of conditions does a gas behave least like an ideal gas?

  1. low temperature and high pressure Correct answer.
  2. low temperature and low pressure
  3. high temperature and low pressure
  4. high temperature and high pressure

Explanation

A real gas deviates most from ideal behaviour when its particles are close together and intermolecular forces are significant. This occurs at low temperature, where particles have less kinetic energy, and high pressure, where they are forced closer together; under these conditions the finite volume of particles and attractions cannot be ignored.

Question 29Structure 1 - Models of the particulate nature of matter

10.0 cm^3 of ethyne, C2H2(g), is burned completely in 50.0 cm^3 of oxygen at constant temperature and pressure. What volume of carbon dioxide is produced?

  1. 25.0 cm^3
  2. 10.0 cm^3
  3. 20.0 cm^3 Correct answer.
  4. 40.0 cm^3

Explanation

The balanced equation is 2 C2H2 + 5 O2 → 4 CO2 + 2 H2O. At constant temperature and pressure, gas volumes are proportional to amounts in moles, so 10.0 cm^3 of C2H2 needs 25.0 cm^3 of O2 and produces 20.0 cm^3 of CO2; oxygen is in excess.

Question 30Structure 1 - Models of the particulate nature of matter

Which ground-state electron configuration is correct for a chromium atom (Z = 24)?

  1. [Ar]3d6
  2. [Ar]3d4 4s2
  3. [Ar]3d5 4s1 Correct answer.
  4. [Ar]4s2 3d4

Explanation

Chromium has Z = 24, and its ground-state configuration is an exception to the usual filling order. A half-filled 3d subshell and a singly occupied 4s orbital give extra stability, so the configuration is [Ar]3d5 4s1 rather than [Ar]3d4 4s2.

Question 31Structure 1 - Models of the particulate nature of matter

Which statement describes the attraction that holds the ions together in a crystal of sodium chloride?

  1. Sharing of electron pairs between sodium and chlorine atoms
  2. Electrostatic attraction between oppositely charged ions Correct answer.
  3. Delocalised electrons attracting the sodium and chloride ions
  4. Dipole–dipole attraction between neighbouring NaCl molecules

Explanation

Ionic bonding is the electrostatic attraction between oppositely charged ions, here Na⁺ and Cl⁻, packed into a giant three-dimensional lattice. Electrons are transferred from sodium to chlorine rather than shared, so the solid contains no discrete NaCl molecules and no delocalised electron sea.

Question 32Structure 1 - Models of the particulate nature of matter

According to VSEPR theory, what is the shape of a methane molecule, CH₄?

  1. Tetrahedral Correct answer.
  2. Square planar
  3. Bent (V-shaped)
  4. Trigonal pyramidal

Explanation

Carbon in methane has four bonding pairs and no lone pairs, so the four electron domains repel equally and adopt the arrangement of a regular tetrahedron. The resulting H–C–H bond angle is 109.5°, and the four hydrogen atoms lie at the corners of a tetrahedron around the carbon.

Question 33Structure 1 - Models of the particulate nature of matter

Which of these compounds forms hydrogen bonds between its molecules in the liquid state?

  1. Hydrogen chloride, HCl
  2. Methane, CH₄
  3. Hydrogen sulfide, H₂S
  4. Ammonia, NH₃ Correct answer.

Explanation

Hydrogen bonding requires hydrogen attached to a highly electronegative N, O or F atom, so ammonia qualifies through its N–H bonds. The lone pair on nitrogen then attracts the partially positive hydrogen of a neighbouring molecule. Sulfur and chlorine are not electronegative enough, and methane has no lone pair.

Question 34Structure 1 - Models of the particulate nature of matter

Carbon dioxide contains two polar C=O bonds, yet CO₂ molecules are non-polar overall. What is the best explanation?

  1. Each oxygen atom carries a lone pair that cancels the dipole of the other oxygen
  2. Carbon dioxide contains only sigma bonds, which cannot produce a dipole
  3. The C=O bonds are not actually polar because oxygen and carbon have similar electronegativities
  4. The molecule is linear, so the two bond dipoles are equal and opposite and cancel Correct answer.

Explanation

Bond polarity comes from electronegativity difference, and the polarity of the whole molecule is the vector sum of its bond dipoles. Because CO₂ is linear with two identical C=O bonds pointing in opposite directions, the dipoles cancel exactly and the net dipole moment is zero.

Question 35Structure 1 - Models of the particulate nature of matter

Graphite conducts electricity while diamond does not. Which statement explains this?

  1. Each graphite carbon contributes a delocalised electron, so mobile electrons are present Correct answer.
  2. Graphite contains mobile ions between its layers, whereas diamond contains no free ions at all
  3. Diamond has stronger covalent bonds, which prevent electrons from moving
  4. Graphite has a lower melting point, so its electrons gain energy more easily

Explanation

In graphite each carbon is bonded to three neighbours, leaving one electron in a p orbital that is delocalised across the layer; these mobile electrons carry current along the layers. In diamond all four valence electrons are used in localised sigma bonds, so no delocalised electrons are available to conduct.

Question 36Structure 1 - Models of the particulate nature of matter

Which of these ionic compounds has the most exothermic (most negative) lattice enthalpy?

  1. MgCl₂
  2. NaCl
  3. KCl
  4. MgO Correct answer.

Explanation

The magnitude of the lattice enthalpy increases with higher ionic charges and smaller ionic radii, because the electrostatic attraction between the ions becomes stronger. MgO contains 2+ and 2− ions with small radii, so considerably more energy is released when its gaseous ions form the lattice than for the other three compounds.

Question 37Structure 1 - Models of the particulate nature of matter

Methane and ammonia each have four electron domains around the central atom. Which statement correctly compares their H–X–H bond angles?

  1. Both molecules have angles of exactly 109.5° because both have four electron domains around the central atom
  2. NH₃ has the larger angle, about 109.5°, because the lone pair pushes the N–H bonds apart
  3. NH₃ has the smaller angle, about 107°, because a lone pair repels more strongly than a bonding pair Correct answer.
  4. CH₄ has the smaller angle because it has no lone pair to hold the H–C–H bonds open

Explanation

Repulsion strength follows the order lone pair–bond pair > bond pair–bond pair, so the single lone pair in ammonia squeezes the three N–H bonding pairs closer together. The H–N–H angle therefore falls to about 107°, whereas methane, with four equal bonding pairs, keeps the ideal tetrahedral angle of 109.5°.

Question 38Structure 1 - Models of the particulate nature of matter

In one ethene molecule, C₂H₄, how many sigma (σ) and pi (π) bonds are present in total?

  1. 4 σ and 1 π
  2. 4 σ and 2 π
  3. 6 σ and 2 π
  4. 5 σ and 1 π Correct answer.

Explanation

Each of the four C–H bonds is a single sigma bond, and the C=C double bond is made up of one sigma bond plus one pi bond formed by sideways overlap of p orbitals. Adding these gives five sigma bonds and one pi bond in the molecule.

Question 39Structure 1 - Models of the particulate nature of matter

In the nitrate ion, NO₃⁻, all three N–O bond lengths are identical. What is the accepted explanation?

  1. The pi electrons are delocalised over all three N–O positions, giving each bond a bond order of 1.33 Correct answer.
  2. One N=O double bond and two N–O single bonds are fixed in position by the geometry of the ion
  3. Nitrogen expands its octet to form four equivalent single bonds to oxygen, so no multiple bonding is present
  4. Each N–O bond is a single bond, and the extra electron pair sits entirely on nitrogen

Explanation

Three valid resonance structures can be drawn for NO₃⁻, differing only in which oxygen carries the double bond, and the real ion is a weighted average of them. The pi electrons are spread over all three N–O positions, so each bond has a bond order of about 1.33 and partial double-bond character, which is why all three lengths are equal.

Question 40Structure 1 - Models of the particulate nature of matter

Magnesium has a much higher melting point than sodium. Which explanation is best?

  1. Magnesium forms a giant covalent lattice, whereas sodium is held together by metallic bonds
  2. Mg²⁺ ions and two delocalised electrons per atom give stronger metallic bonding than in sodium Correct answer.
  3. Sodium has a full outer shell and therefore cannot release electrons into a delocalised sea
  4. Magnesium atoms are larger than sodium atoms, so the delocalised electrons have more space to move

Explanation

Melting a metal requires overcoming the attraction between the cations and the delocalised electrons. Magnesium supplies two delocalised electrons per atom and forms 2+ ions that are also smaller than Na⁺, so the electrostatic attraction is stronger and more energy is needed to melt the solid.

Question 41Structure 1 - Models of the particulate nature of matter

Which statement best describes the structure of a solid metal such as copper?

  1. A lattice of negative ions surrounded by a sea of positive ions.
  2. A network of covalently bonded atoms with no free electrons.
  3. A lattice of positive ions surrounded by a sea of delocalized electrons. Correct answer.
  4. Discrete molecules held together by London dispersion forces.

Explanation

Metals consist of a regular lattice of positive metal ions. The valence electrons are delocalized and free to move throughout the structure, forming an electron sea that holds the ions together. This model accounts for the characteristic conductivity and malleability of metals.

Question 42Structure 1 - Models of the particulate nature of matter

According to VSEPR theory, what is the shape of a boron trifluoride molecule, BF3?

  1. Trigonal pyramidal
  2. Trigonal planar Correct answer.
  3. Bent
  4. Tetrahedral

Explanation

BF3 has three bonding pairs and no lone pairs on the central boron atom. The three electron domains repel each other to positions 120° apart in a plane. The molecular shape is therefore trigonal planar.

Question 43Structure 1 - Models of the particulate nature of matter

Which intermolecular force is present between all molecules, whether polar or non-polar?

  1. Hydrogen bonding
  2. Dipole-dipole attraction
  3. Ionic bonding
  4. London dispersion forces Correct answer.

Explanation

London dispersion forces arise from temporary fluctuations in electron distribution. They exist in all molecules, including non-polar ones. The other forces require permanent dipoles or specific H–F, H–O, or H–N bonds.

Question 44Structure 1 - Models of the particulate nature of matter

A pi (π) bond is formed by which type of orbital overlap?

  1. End-to-end overlap of an s and a p orbital
  2. End-to-end overlap of two s orbitals
  3. Head-on overlap of two p orbitals
  4. Sideways overlap of two p orbitals Correct answer.

Explanation

Pi bonds form when parallel p orbitals overlap sideways above and below the internuclear axis. End-to-end or head-on overlap produces sigma bonds instead. The sideways overlap gives a pi bond its characteristic electron density on two sides of the bond axis.

Question 45Structure 1 - Models of the particulate nature of matter

Which of these molecules has a permanent dipole moment overall?

  1. BF3
  2. CCl4
  3. CO2
  4. NH3 Correct answer.

Explanation

NH3 has a trigonal pyramidal shape with a lone pair on nitrogen. The N–H bond dipoles do not cancel, so the molecule is polar overall. CCl4 and BF3 are symmetric, and CO2 is linear, so their bond dipoles cancel.

Question 46Structure 1 - Models of the particulate nature of matter

What is the molecular geometry of sulfur tetrafluoride, SF4?

  1. Tetrahedral
  2. See-saw Correct answer.
  3. Trigonal bipyramidal
  4. Square planar

Explanation

SF4 has five electron domains around sulfur: four bonding pairs and one lone pair. The lone pair occupies an equatorial position in a trigonal bipyramidal arrangement. The molecular geometry is therefore see-saw.

Question 47Structure 1 - Models of the particulate nature of matter

Which statement correctly compares a C–C single bond with a C=C double bond?

  1. The double bond is shorter and stronger than the single bond. Correct answer.
  2. The two bonds have equal length and strength.
  3. The double bond is longer and stronger than the single bond.
  4. The single bond is shorter and stronger than the double bond.

Explanation

A double bond contains a sigma bond plus a pi bond. The additional pi bonding pulls the carbon nuclei closer and increases the bond strength. Therefore a C=C double bond is shorter and stronger than a C–C single bond.

Question 48Structure 1 - Models of the particulate nature of matter

Which molecule has a bond angle of approximately 104.5°?

  1. H2O Correct answer.
  2. CO2
  3. CH4
  4. BF3

Explanation

Water has two bonding pairs and two lone pairs on oxygen. The lone-pair repulsion compresses the H–O–H angle from the tetrahedral 109.5° to about 104.5°. BF3 is 120°, CH4 is 109.5°, and CO2 is 180°.

Question 49Structure 1 - Models of the particulate nature of matter

Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Which statement best explains this difference?

  1. In the solid, ions are fixed in a lattice; when molten, the ions become free to move. Correct answer.
  2. In the solid, molecules are held by hydrogen bonds; when molten, these bonds break.
  3. In the solid, sodium atoms are covalently bonded; when molten, metallic bonding appears.
  4. In the solid, electrons are delocalized; when molten, the electrons become localized.

Explanation

Ionic solids conduct electricity only when their ions are mobile. In solid NaCl, the ions are locked in a rigid lattice and cannot carry charge through the structure. Melting releases the ions, allowing them to move and conduct electricity.

Question 50Structure 1 - Models of the particulate nature of matter

Why are metals malleable, meaning they can be hammered into thin sheets without shattering?

  1. Their molecules are held by weak London dispersion forces that readily break.
  2. Their ions are held by strong directional covalent bonds that can rotate.
  3. Delocalized electrons let layers of ions slide without breaking the metallic bond. Correct answer.
  4. Their positive and negative ions repel each other when a force is applied.

Explanation

In a metal, the delocalized electron sea is not directional. When a force is applied, layers of positive ions can slide over each other while the electron sea continues to hold them together. This allows metals to deform rather than fracture.