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IB Chemistry HL study materials
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IB Chemistry HL practice questions

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Question 1Structure

Which statement about the mass number of an atom is correct?

  1. It equals the number of electrons in the highest occupied energy level.
  2. It equals the number of protons plus the number of neutrons. Correct answer.
  3. It equals the number of protons plus the number of electrons.
  4. It equals the number of neutrons minus the number of protons.

Explanation

The mass number A counts the nucleons in the nucleus, so it is the sum of protons (Z) and neutrons (N). Electrons are far lighter and are not counted in the mass number.

Question 2Structure

A sample of element X contains two isotopes, ⁶³X and ⁶⁵X, in a 3:1 ratio by abundance. What is the relative atomic mass of X?

  1. 63.5 Correct answer.
  2. 64.0
  3. 64.5
  4. 65.0

Explanation

The weighted mean is (63 × 3 + 65 × 1)/4 = (189 + 65)/4 = 254/4 = 63.5. The more abundant isotope dominates, so the value lies closer to 63.

Question 3Structure

Which electron configuration represents the ground state of a chromium atom (Z = 24)?

  1. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²
  2. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
  3. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³ 4s² 4p¹
  4. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ Correct answer.

Explanation

Chromium is a special case in which a half-filled 3d subshell gives extra exchange stability, so one electron is promoted from 4s. The configuration [Ar] 3d⁵ 4s¹ is therefore the ground state.

Question 4Structure

Which property increases across Period 3 from sodium to chlorine?

  1. First ionisation energy Correct answer.
  2. Atomic radius
  3. Number of occupied principal energy levels
  4. Metallic character

Explanation

Across a period the nuclear charge rises while electrons enter the same shell, so the effective nuclear charge increases and the outer electron is held more tightly. First ionisation energy therefore increases overall from Na to Cl.

Question 5Structure

Which description of an ionic bond is consistent with the structure of sodium chloride?

  1. A directional bond formed by overlap of half-filled orbitals
  2. Delocalised electrons surrounding a lattice of sodium cations
  3. Electrostatic attraction between a lattice of cations and a lattice of anions Correct answer.
  4. Sharing of electron pairs between sodium and chlorine atoms

Explanation

Ionic bonding is the electrostatic attraction between oppositely charged ions, and in a solid the ions are arranged in a regular three-dimensional lattice. Each ion is surrounded by ions of opposite charge rather than by a single partner.

Question 6Structure

Which molecule has a trigonal pyramidal shape?

  1. CH₄
  2. BF₃
  3. CO₂
  4. NH₃ Correct answer.

Explanation

Ammonia has three bonding pairs and one lone pair on nitrogen, giving a trigonal pyramidal shape. The lone pair repels the bonding pairs more strongly, compressing the H–N–H angle below the tetrahedral value.

Question 7Structure

Which intermolecular force is responsible for the relatively high boiling point of water compared with hydrogen sulfide?

  1. Hydrogen bonding Correct answer.
  2. Ion–dipole attraction
  3. London dispersion forces only
  4. Metallic bonding

Explanation

Water has O–H bonds in which hydrogen is attached to a highly electronegative atom, allowing extensive hydrogen bonding between molecules. These bonds are much stronger than the dipole–dipole and dispersion forces present in H₂S.

Question 8Structure

What is the oxidation state of manganese in the permanganate ion, MnO₄⁻?

  1. +8
  2. +4
  3. +2
  4. +7 Correct answer.

Explanation

Each oxygen is assigned −2, giving a total of −8 from the four oxygen atoms, and the ion carries an overall charge of −1. Solving x + (−8) = −1 gives x = +7 for manganese.

Question 9Structure

Which statement about the trend in first ionisation energy down Group 1 is correct?

  1. It increases because the nuclear charge increases down the group.
  2. It decreases because the atoms have more protons in the nucleus.
  3. It increases because the outer electron is in a higher principal energy level.
  4. It decreases because the outer electron is further from the nucleus and shielded by more inner shells. Correct answer.

Explanation

Going down Group 1 adds a principal energy level, so the outer electron is further from the nucleus and shielded by more inner electrons. Despite the greater nuclear charge, the increased distance and shielding weaken the attraction, so ionisation energy decreases.

Question 10Structure

Which species is isoelectronic with the argon atom?

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

Explanation

Argon has 18 electrons. A chloride ion has 17 protons plus one extra electron, giving 18 electrons, so it is isoelectronic with argon.

Question 11Structure

Which statement best explains why the melting point of magnesium is higher than that of sodium?

  1. Magnesium forms a covalent network solid rather than a metallic lattice.
  2. Magnesium has weaker metallic bonding because its atoms are smaller.
  3. Magnesium has a higher nuclear charge per atom, but the key factor is that it contributes two delocalised electrons per atom, strengthening the metallic lattice. Correct answer.
  4. Magnesium has more isotopes than sodium, raising its melting point.

Explanation

Both metals have metallic bonding, but magnesium atoms release two electrons into the delocalised sea compared with one for sodium. The greater charge and number of delocalised electrons give stronger electrostatic attraction to the cations, raising the melting point.

Question 12Structure

Which molecule is non-polar despite containing polar bonds?

  1. HCl
  2. NH₃
  3. H₂O
  4. CCl₄ Correct answer.

Explanation

In CCl₄ the four C–Cl bond dipoles are arranged tetrahedrally and cancel by symmetry, so the molecule has no net dipole. Individual bonds are polar, but the vector sum is zero.

Question 13Structure

Which statement about the bonding in a molecule of ethene, C₂H₄, is correct?

  1. The molecule contains only sp³ hybridised carbon atoms.
  2. The two carbon atoms are joined by a sigma bond formed by end-on overlap and a pi bond formed by sideways overlap of p orbitals. Correct answer.
  3. The carbon–carbon bond is a single sigma bond with no pi component.
  4. The two carbon atoms are joined by two identical sigma bonds.

Explanation

Ethene has a C=C double bond consisting of one sigma bond from end-on overlap of hybrid orbitals and one pi bond from sideways overlap of unhybridised p orbitals. The carbons are sp² hybridised and the molecule is planar.

Question 14Structure

Which factor explains why the first ionisation energy of aluminium is lower than that of magnesium?

  1. Aluminium has fewer protons than magnesium.
  2. Magnesium has a half-filled 3p subshell.
  3. Aluminium's outer electron is in a 3p orbital, which is higher in energy and more shielded than the 3s orbital occupied in magnesium. Correct answer.
  4. Aluminium has a larger nuclear charge that repels the outer electron.

Explanation

For aluminium the electron removed comes from a 3p orbital, which lies higher in energy and is shielded by the 3s electrons. This outweighs the increase in nuclear charge, so Al has the lower first ionisation energy.

Question 15Structure

A gaseous ion X²⁺ has the electron configuration 1s² 2s² 2p⁶. Which element is X?

  1. Neon
  2. Aluminium
  3. Magnesium Correct answer.
  4. Sodium

Explanation

The ion has 10 electrons, and X²⁺ means the neutral atom had two more, giving 12 protons. The element with Z = 12 is magnesium.

Question 16Structure

Which statement about the shapes of electron domains around a central atom is correct according to VSEPR theory?

  1. Four electron domains always give bond angles of exactly 109.5°, regardless of lone pairs.
  2. Lone pairs occupy more space than bonding pairs, so they reduce adjacent bond angles. Correct answer.
  3. Double bonds occupy less space than single bonds and increase adjacent bond angles.
  4. Two electron domains produce a bent shape with a bond angle of about 104.5°.

Explanation

A lone pair is held by only one nucleus, so its charge cloud spreads out more than a bonding pair. This greater repulsion pushes bonding pairs closer together, reducing bond angles below the ideal values.

Question 17Structure

Which property is characteristic of a typical transition element such as iron?

  1. It forms only one oxidation state in all of its compounds.
  2. It is a non-metal that forms acidic oxides.
  3. It has a completely filled d subshell in all of its compounds.
  4. It forms coloured compounds and exhibits variable oxidation states. Correct answer.

Explanation

Transition elements have partially filled d orbitals and can lose different numbers of electrons, giving variable oxidation states. Electronic transitions involving d orbitals also produce coloured compounds.

Question 18Structure

Which arrangement of ions is found in a crystal of caesium chloride?

  1. Cs⁺ ions form a metallic lattice with delocalised electrons.
  2. Each Cs⁺ ion is surrounded by six Cl⁻ ions in an octahedral arrangement.
  3. Cs⁺ and Cl⁻ ions are covalently bonded in pairs.
  4. Each Cs⁺ ion is surrounded by eight Cl⁻ ions in a body-centred cubic arrangement. Correct answer.

Explanation

Because Cs⁺ is large, the caesium chloride structure has an 8:8 coordination in which each ion sits at the centre of a cube of eight oppositely charged ions. This differs from the 6:6 octahedral coordination of sodium chloride.

Question 19Structure

Which statement explains why graphite conducts electricity while diamond does not?

  1. Diamond has a higher density, which prevents electron movement.
  2. Graphite contains mobile ions between its layers.
  3. Graphite has delocalised electrons in p orbitals perpendicular to each layer, whereas diamond's electrons are all localised in sigma bonds. Correct answer.
  4. Graphite has weaker covalent bonds, allowing electrons to break free.

Explanation

Each graphite carbon is sp² hybridised with one unhybridised p electron, and these p electrons form a delocalised system within each layer. In diamond every valence electron is used in a localised sigma bond, so there are no mobile charge carriers.

Question 20Structure

Which pair of elements would be expected to form an ionic compound with the formula XY₂?

  1. Calcium and chlorine Correct answer.
  2. Sodium and chlorine
  3. Carbon and oxygen
  4. Hydrogen and oxygen

Explanation

Calcium is a Group 2 metal that forms Ca²⁺, while chlorine forms Cl⁻, so charge balance gives CaCl₂. The large electronegativity difference between a reactive metal and a halogen favours ionic bonding.

Question 21Structure

Which statement about the trend in electronegativity across Period 3 is correct?

  1. It increases because the effective nuclear charge increases while shielding remains roughly constant. Correct answer.
  2. It remains constant because all Period 3 elements have the same number of valence electrons.
  3. It decreases because atomic radius decreases.
  4. It increases because the atoms have more electron shells.

Explanation

Across Period 3 the number of protons increases while electrons are added to the same shell, so effective nuclear charge rises. A stronger pull on bonding electrons increases electronegativity from sodium to chlorine.

Question 22Structure

A sample of copper contains 69% of ⁶³Cu and 31% of ⁶⁵Cu. Which value is closest to its relative atomic mass?

  1. 65.0
  2. 64.0
  3. 64.4
  4. 63.6 Correct answer.

Explanation

The weighted mean is (0.69 × 63) + (0.31 × 65) = 43.47 + 20.15 = 63.62, which rounds to 63.6. Because the lighter isotope is more abundant, the value lies below 64.

Question 23Structure

Which explanation accounts for the very low first ionisation energy of potassium compared with calcium?

  1. Potassium has a half-filled 4s subshell that stabilises the atom.
  2. Calcium's outer electron is in a 3d orbital, which is harder to remove.
  3. Potassium has a smaller atomic radius than calcium, making electron removal easier.
  4. Potassium's outer electron is in the 4s orbital and is removed from a lower effective nuclear charge than calcium's, so less energy is required. Correct answer.

Explanation

Potassium has one 4s electron and a nuclear charge of 19, whereas calcium has two 4s electrons and a nuclear charge of 20. The extra proton in calcium increases the attraction for the outer electrons, so potassium has the lower first ionisation energy.

Question 24Structure

Which statement about hydrogen bonding is correct?

  1. It occurs whenever hydrogen is present in a molecule, including in CH₄.
  2. It is a covalent bond formed between two hydrogen atoms.
  3. It is weaker than London dispersion forces in all cases.
  4. It requires hydrogen bonded to a highly electronegative atom such as N, O or F and acts between molecules. Correct answer.

Explanation

Hydrogen bonding arises when H is covalently bonded to N, O or F, creating a strong dipole that attracts a lone pair on a neighbouring electronegative atom. It is an intermolecular attraction, stronger than ordinary dipole–dipole and dispersion forces.

Question 25Structure

Which statement about the delocalised model of benzene is correct?

  1. Benzene's carbons are sp³ hybridised, giving a puckered ring.
  2. Benzene's stability arises from strong London dispersion forces between its molecules.
  3. Each carbon is sp² hybridised, and six p electrons are delocalised over the ring, giving extra stability compared with a hypothetical cyclohexatriene. Correct answer.
  4. Benzene contains three localised double bonds that alternate rapidly between positions.

Explanation

In benzene each carbon is sp² hybridised with one unhybridised p orbital, and the six p electrons are delocalised over the planar ring. This delocalisation lowers the energy relative to a structure with three isolated double bonds, explaining benzene's resistance to addition.

Question 26Structure

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which block of the periodic table does this element appear?

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

Explanation

The highest-energy electrons occupy the 3p subshell, and the block is named after the subshell receiving the last electron. Elements whose final electrons enter a p subshell are placed in the p block, which here corresponds to sulfur.

Question 27Structure

Which statement correctly describes the relative masses of the three subatomic particles?

  1. A proton is roughly twice as heavy as a neutron, and an electron is roughly half the mass of a neutron.
  2. An electron has about the same mass as a proton, and both are far heavier than a neutron.
  3. A neutron has about the same mass as an electron, and both are far heavier than a proton.
  4. A proton has about the same mass as a neutron, and both are far heavier than an electron. Correct answer.

Explanation

Protons and neutrons each have a mass close to one atomic mass unit, so they dominate the mass of an atom. The electron is roughly 1/1836 the mass of a proton, so its contribution to atomic mass is negligible.

Question 28Structure

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

  1. Ar⁺
  2. K⁺
  3. S²⁻
  4. Cl⁻ Correct answer.

Explanation

A chloride ion has 17 protons plus one extra electron, giving 18 electrons arranged as 1s² 2s² 2p⁶ 3s² 3p⁶. Counting electrons is the reliable method, since the configuration depends only on total electron number.

Question 29Structure

Which type of bonding is present in a solid sample of sodium chloride?

  1. Electrostatic attraction between oppositely charged ions in a lattice Correct answer.
  2. Hydrogen bonding between chlorine atoms
  3. Attraction between delocalised electrons and positive ions
  4. Sharing of electron pairs between sodium and chlorine atoms

Explanation

Sodium chloride is formed from Na⁺ and Cl⁻ ions held in a giant ionic lattice by electrostatic attraction. The transfer of an electron from sodium to chlorine produces these ions rather than shared pairs.

Question 30Structure

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

  1. The radius decreases because nuclear charge increases while electrons enter the same shell. Correct answer.
  2. The radius stays constant because the number of shells does not change.
  3. The radius increases because extra electron shells are added.
  4. The radius decreases because the shielding by inner electrons increases strongly.

Explanation

Across a period, protons are added to the nucleus while the added electrons enter the same principal shell, so effective nuclear charge rises. The stronger attraction pulls the electron cloud inward and the atomic radius falls.

Question 31Structure

Which statement correctly describes the number of significant figures in the measurement 0.004560 g?

  1. Two significant figures
  2. Four significant figures Correct answer.
  3. Three significant figures
  4. Six significant figures

Explanation

Leading zeros before the first non-zero digit are placeholders and are not significant. The digits 4, 5, 6 and the trailing zero after the decimal point are all significant, giving four.

Question 32Structure

Which observation provides evidence that the first ionisation energy of an element is lower than its second ionisation energy?

  1. The second electron removed is always a core electron.
  2. Removing an electron from a positive ion requires more energy than removing one from the neutral atom. Correct answer.
  3. Successive ionisation energies always increase by the same amount.
  4. The second ionisation energy involves a smaller atom with a larger radius.

Explanation

Once an electron has been removed, the remaining species is positively charged and holds its electrons more tightly, so further removal costs more energy. This electrostatic argument explains why the second ionisation energy exceeds the first.

Question 33Structure

Which compound contains bonding that is best described as polar covalent with the partial negative charge on the more electronegative atom?

  1. NaCl
  2. CH₄
  3. HCl Correct answer.
  4. Cl₂

Explanation

In hydrogen chloride the electronegativity difference between chlorine and hydrogen is large enough to polarise the bond, placing δ⁻ on chlorine. The bond remains covalent because the electrons are shared, not transferred.

Question 34Structure

Which statement correctly describes the geometry of the electron domains around the central atom in a molecule with five bonding pairs and no lone pairs?

  1. The domains adopt a square pyramidal arrangement with bond angles of 90°.
  2. The domains adopt a trigonal bipyramidal arrangement with bond angles of 90° and 120°. Correct answer.
  3. The domains adopt an octahedral arrangement with bond angles of 90°.
  4. The domains adopt a trigonal planar arrangement with bond angles of 120°.

Explanation

Five electron domains minimise repulsion by adopting a trigonal bipyramidal arrangement, with axial–equatorial angles of 90° and equatorial–equatorial angles of 120°. With no lone pairs, the molecular shape matches this domain geometry.

Question 35Structure

Which statement about the bonding in a molecule of ammonia, NH₃, is correct?

  1. The nitrogen atom forms three single covalent bonds and retains two lone pairs.
  2. The nitrogen atom forms three single covalent bonds and retains one lone pair. Correct answer.
  3. The nitrogen atom forms one double bond and one single bond to hydrogen.
  4. The nitrogen atom forms four single covalent bonds using all five valence electrons.

Explanation

Nitrogen has five valence electrons; three are shared with hydrogen atoms and the remaining two form a lone pair. This lone pair is responsible for the trigonal pyramidal shape and the basic character of ammonia.

Question 36Structure

Which statement about the structure of a metal such as magnesium is consistent with its electrical conductivity?

  1. Electrons are transferred permanently from one metal atom to another.
  2. Covalent bonds extend throughout the lattice and allow electron transfer.
  3. Outer electrons are delocalised and can move through a lattice of positive ions. Correct answer.
  4. Layers of ions are held by hydrogen bonds that allow electron flow.

Explanation

In the metallic model, valence electrons are delocalised over a regular lattice of cations. When a potential difference is applied, these mobile electrons drift and carry the current.

Question 37Structure

Which pair of elements would be expected to form a compound with the empirical formula XY?

  1. Aluminium and oxygen
  2. Magnesium and chlorine
  3. Carbon and hydrogen
  4. Potassium and bromine Correct answer.

Explanation

Potassium forms K⁺ and bromine forms Br⁻, so the charges balance in a 1:1 ratio, giving KBr with empirical formula XY. The other pairs involve ions whose charges do not match one to one.

Question 38Structure

A sample of an element consists of two isotopes. The lighter isotope has abundance 75% and relative isotopic mass 35.0, and the heavier isotope has abundance 25% and relative isotopic mass 37.0. Which value is closest to the relative atomic mass?

  1. 36.5
  2. 35.5 Correct answer.
  3. 37.0
  4. 36.0

Explanation

The relative atomic mass is the abundance-weighted mean: (0.75 × 35.0) + (0.25 × 37.0) = 26.25 + 9.25 = 35.5. The value lies closer to the mass of the more abundant isotope.

Question 39Structure

Which statement explains why the ionic radius of a chloride ion is larger than the atomic radius of a chlorine atom?

  1. The ion has more protons, which pulls the electrons outward.
  2. The ion has fewer shells of electrons than the neutral atom.
  3. The ion has lost an electron, so the remaining electrons occupy a larger shell.
  4. The ion has gained an electron, increasing electron–electron repulsion within the same shell. Correct answer.

Explanation

Adding an electron to a chlorine atom increases electron–electron repulsion while the nuclear charge stays the same, so the electron cloud expands. The ion therefore has a larger radius than the parent atom.

Question 40Structure

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

  1. Melting point increases because the number of delocalised electrons per atom increases. Correct answer.
  2. Melting point increases because the bonding changes from metallic to ionic.
  3. Melting point is unchanged because all three are metals.
  4. Melting point decreases because atomic radius decreases.

Explanation

Sodium contributes one delocalised electron per atom, magnesium two and aluminium three, so the electrostatic attraction between cations and the electron sea strengthens. Stronger metallic bonding raises the melting point across the series.

Question 41Structure

Benzene, C₆H₆, has all six carbon–carbon bond lengths equal at about 139 pm, intermediate between a typical C–C single bond and a C=C double bond. Which model best accounts for this observation?

  1. The molecule alternates rapidly between two Kekulé structures with distinct single and double bonds.
  2. The ring contains three localised double bonds that are longer than single bonds.
  3. Each carbon forms two σ bonds and one π bond localised to a single neighbour.
  4. Six p orbitals overlap to give delocalised π electrons above and below the ring. Correct answer.

Explanation

In the delocalised model, the six p orbitals combine to form π molecular orbitals spread over the ring. The π electrons are shared equally by all six carbons, making every C–C bond equivalent with intermediate length.

Question 42Structure

A gaseous ion X³⁺ has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶. Which element is X?

  1. Argon
  2. Titanium
  3. Calcium
  4. Scandium Correct answer.

Explanation

The configuration shown contains 18 electrons, and the neutral atom must have three more, giving 21 electrons and therefore 21 protons. The element with atomic number 21 is scandium.

Question 43Reactivity

Which statement about the periodic trend in first ionisation energy across Period 3 (Na to Ar) is correct?

  1. It generally decreases because nuclear charge increases.
  2. It generally increases, but drops at Al and at S relative to their preceding elements. Correct answer.
  3. It increases steadily with no exceptions from Na to Ar.
  4. It increases, but drops only at S because of electron pairing.

Explanation

Across Period 3 the general trend is increasing first ionisation energy as nuclear charge rises and atomic radius falls. However, Al is lower than Mg because the outer electron removed from Al is in a 3p orbital, which is higher in energy and better shielded than the 3s electron of Mg; S is lower than P because removing one of the paired 3p electrons relieves electron-electron repulsion.

Question 44Reactivity

A reaction has an activation energy of 50 kJ mol⁻¹ without a catalyst and 30 kJ mol⁻¹ with a catalyst. By how much does the catalyst lower the activation energy, in kJ mol⁻¹?

  1. 50
  2. 20 Correct answer.
  3. 80
  4. 30

Explanation

The catalyst provides an alternative reaction pathway with a lower activation energy. The reduction is the difference between the uncatalysed and catalysed values: 50 − 30 = 20 kJ mol⁻¹.

Question 45Reactivity

Which species acts as a Lewis acid in the reaction BF₃ + NH₃ → F₃B–NH₃?

  1. BF₃, because it accepts an electron pair Correct answer.
  2. BF₃, because it donates an electron pair
  3. NH₃, because it accepts a proton
  4. NH₃, because it donates a lone pair

Explanation

A Lewis acid is an electron-pair acceptor. BF₃ has an incomplete octet on boron and accepts the lone pair from ammonia, so it is the Lewis acid, while NH₃ is the Lewis base (electron-pair donor).

Question 46Reactivity

In the reaction between aqueous silver nitrate and aqueous potassium chloride, which observation indicates that a precipitation reaction has occurred?

  1. The solution becomes strongly acidic.
  2. The temperature falls sharply.
  3. A white solid forms. Correct answer.
  4. A colourless gas is evolved.

Explanation

Silver ions and chloride ions combine to form silver chloride, which is insoluble and appears as a white precipitate. The formation of an insoluble solid from two aqueous solutions is the defining observation of precipitation.

Question 47Reactivity

Which change would shift the equilibrium of the exothermic reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) to the right?

  1. Adding a catalyst
  2. Increasing the temperature
  3. Decreasing the total pressure
  4. Increasing the total pressure Correct answer.

Explanation

There are four gas moles on the left and two on the right, so increasing pressure shifts equilibrium towards the side with fewer gas molecules, i.e. towards ammonia. A catalyst affects only the rate of reaching equilibrium, not the position.

Question 48Reactivity

Which statement correctly describes the role of the standard hydrogen electrode in electrochemical measurements?

  1. It is assigned E⦵ = 0.00 V and serves as the reference against which other electrode potentials are measured. Correct answer.
  2. It can only be used with acidic solutions of pH greater than 7.
  3. It measures the absolute potential of an electrode directly.
  4. It is assigned E⦵ = +1.00 V because hydrogen is oxidised readily.

Explanation

The standard hydrogen electrode is defined as having a potential of exactly 0.00 V under standard conditions, providing a reference point. Electrode potentials of other half-cells are measured relative to it.

Question 49Reactivity

Which factor explains why increasing the concentration of a reactant usually increases the rate of reaction?

  1. It lowers the activation energy of the reaction.
  2. It increases the enthalpy change of the reaction.
  3. It raises the temperature of the reaction mixture.
  4. It increases the frequency of collisions between reactant particles. Correct answer.

Explanation

Higher concentration means more particles per unit volume, so collisions between reactant particles occur more frequently, increasing the rate. Activation energy is unaffected by concentration.

Question 50Reactivity

Which oxide is amphoteric?

  1. Al₂O₃ Correct answer.
  2. SO₂
  3. MgO
  4. Na₂O

Explanation

Aluminium oxide reacts with both acids and alkalis to form salts, so it is amphoteric. It dissolves in acid forming Al³⁺ salts and in alkali forming aluminate ions.