Apply The Ionic Model - IB Chemistry

A student measures the electrical conductivity of aqueous solutions containing 0.1 M concentrations of $\text{NaCl}$ , $\text{MgCl}_2$ , and $\text{AlCl}_3$ . According to the ionic model, rank these solutions in order of increasing conductivity and identify the primary reason.
Answer: $\text{NaCl} < \text{MgCl}_2 < \text{AlCl}_3$ because the total number of ions increases with cation charge: 2, 3, and 4 ions respectively Conductivity depends on the total number of ions in solution. At 0.1 M: NaCl produces 0.2 M total ions (0.1 M Na⁺ + 0.1 M Cl⁻), MgCl₂ produces 0.3 M total ions (0.1 M Mg²⁺ + 0.2 M Cl⁻), and AlCl₃ produces 0.4 M total ions (0.1 M Al³⁺ + 0.3 M Cl⁻). Higher charged ions also carry more charge per ion. A mentions ion-dipole interactions, which don't directly determine conductivity ranking. B incorrectly suggests ion pairing dominates at these concentrations and charges. D ignores the different numbers of ions produced by each compound.
The compound ammonium phosphate contains both ionic and covalent bonds. Which statement correctly identifies the location of these bonds?
Answer: Ionic bonds are between ammonium cations and phosphate anions; covalent bonds are within each polyatomic ion. Ammonium phosphate is formed from the ammonium cation (NH₄⁺) and the phosphate anion (PO₄³⁻). The electrostatic attraction between the oppositely charged NH₄⁺ and PO₄³⁻ ions constitutes the ionic bonding that holds the crystal lattice together. Within each polyatomic ion, the atoms (N and H in ammonium; P and O in phosphate) are held together by the sharing of electrons, which are covalent bonds.
Which statement best explains the characteristic brittleness of ionic crystals?
Answer: Applying a force causes layers of ions to shift, bringing ions with like charges into alignment, leading to repulsion and fracture. Brittleness in ionic crystals is a direct consequence of the ordered lattice structure. When a shearing force is applied, it can cause one layer of ions to slip relative to another. This slippage can bring positive ions next to positive ions and negative ions next to negative ions. The resulting strong repulsive forces push the layers apart, causing the crystal to fracture along a clean plane.
How is the magnitude of the lattice enthalpy expected to change when moving down the Group 1 chlorides, from LiCl to CsCl?
Answer: It decreases, because the increasing radius of the cation weakens the electrostatic attraction to the chloride ion. Lattice enthalpy is a measure of the strength of the electrostatic forces in an ionic lattice. According to Coulomb's Law, this force is inversely proportional to the distance between the ions. Moving down Group 1 from Li⁺ to Cs⁺, the ionic radius of the cation increases due to the addition of electron shells. The charge on the cation (+1) and the anion (Cl⁻) remains constant. The increased distance between the ion centers leads to a weaker electrostatic attraction, and thus a less exothermic (smaller magnitude) lattice enthalpy.
Which description accurately represents the electron transfer process during the formation of barium nitride?
Answer: Each of three barium atoms transfers two electrons to each of two nitrogen atoms, forming Ba²⁺ and N³⁻ ions. Barium (Group 2) forms a Ba²⁺ ion by losing two electrons. Nitrogen (Group 15) forms a N³⁻ ion by gaining three electrons. The formula for barium nitride is Ba₃N₂. This means three barium atoms are required for every two nitrogen atoms. In total, the three Ba atoms lose 3 × 2 = 6 electrons. These six electrons are gained by the two N atoms, with each N atom gaining 2 × 3 = 6 electrons. Therefore, three Ba atoms each transfer two electrons to two N atoms.
What is the ratio of cations to anions in one formula unit of iron(III) sulfate?
Answer: 2:3 Iron(III) indicates the iron ion has a charge of +3, i.e., Fe³⁺. The sulfate ion is a polyatomic ion with the formula SO₄²⁻ and a charge of -2. To form a neutral compound, the total positive and negative charges must balance. The lowest common multiple of 3 and 2 is 6. We need two Fe³⁺ ions (2 × +3 = +6) and three SO₄²⁻ ions (3 × -2 = -6). The formula is Fe₂(SO₄)₃. Therefore, the ratio of cations (Fe³⁺) to anions (SO₄²⁻) is 2:3.
Which pair of gaseous ions, when forming a solid ionic lattice, would be expected to release the greatest amount of energy (i.e., have the most exothermic lattice enthalpy)?
Answer: Al³⁺(g) and N³⁻(g) Lattice enthalpy is most strongly influenced by the product of the ionic charges. A higher product of charges leads to a much stronger electrostatic attraction and a more exothermic lattice enthalpy. The charges for the pairs are: (A) +1, -1; (B) +2, -2; (C) +3, -3; (D) +1, -1. The pair Al³⁺ and N³⁻ has the highest product of charges (3 × 3 = 9), and therefore would release the most energy upon formation of the lattice.
An ionic compound $\text{X}_2\text{Y}$ has a melting point of 1850°C, while compound $\text{XY}_2$ has a melting point of 2540°C. Both compounds contain the same metal X. According to the ionic model, what can be concluded about the charges on the ions?
Answer: In $\text{X}_2\text{Y}$ , X has charge +1 and Y has charge -2; in $\text{XY}_2$ , X has charge +4 and Y has charge -2 The formulas indicate charge ratios: X₂Y means 2X + 1Y = 0, so if Y = -2, then X = +1. XY₂ means 1X + 2Y = 0, so if Y = -2, then X = +4. The much higher melting point of XY₂ (2540°C vs 1850°C) supports this because lattice energy depends on the product of charges: (+1)(-2) = -2 for X₂Y vs (+4)(-2) = -8 for XY₂. The higher charge product creates much stronger ionic attractions. A gives the wrong charges for XY₂. B has unrealistic -4 charge. D incorrectly dismisses the strong relationship between charge and melting point.
A compound has the formula M(NO₃)₂ and a molar mass of 148.3 g mol⁻¹. Given the molar masses N=14.01 g mol⁻¹ and O=16.00 g mol⁻¹, what is the identity of the metal M?
Answer: Mg First, deduce the charge on M. The nitrate ion (NO₃⁻) has a charge of -1. Since there are two nitrate ions, the metal M must have a charge of +2 to form a neutral compound, M²⁺. Next, calculate the mass of the two nitrate ions in the formula: 2 × (Mass of N + 3 × Mass of O) = 2 × (14.01 + 3 × 16.00) = 2 × (62.01) = 124.02 g mol⁻¹. Subtract this from the total molar mass to find the molar mass of M: 148.3 - 124.02 = 24.28 g mol⁻¹. Looking at the periodic table for a metal that forms a +2 ion and has a molar mass of approximately 24.3 g mol⁻¹ points to Magnesium (Mg).
Why are ionic compounds generally considered non-volatile, having very low vapor pressures at room temperature?
Answer: The strong, non-directional electrostatic forces throughout the crystal lattice require a large amount of energy to be overcome. Volatility is the tendency of a substance to vaporize. For an ionic compound to enter the gas phase, the very strong electrostatic forces of attraction holding the ions together in the three-dimensional lattice must be overcome. This requires a substantial amount of thermal energy, which is why ionic compounds have very high boiling points and are thus non-volatile at room temperature. The other options are incorrect: molar mass is a secondary factor, intermolecular forces are not the primary forces in an ionic lattice, and covalent character does not explain non-volatility.
Using the ionic model to predict solubility trends, which compound would be expected to be LEAST soluble in water, and what is the primary reason?
Answer: $\text{Al}_2\text{O}_3$ because the high charge density of $\text{Al}^{3+}$ and $\text{O}^{2-}$ ions creates very strong lattice energy that water cannot overcome According to the ionic model, solubility depends on the balance between lattice energy and hydration energy. Al₂O₃ has extremely high lattice energy due to the high charges on Al³⁺ and O²⁻ ions (charge product = 6), making it essentially insoluble because water cannot provide enough hydration energy to overcome the strong ionic attractions. A incorrectly suggests large ions create weak interactions (they actually create strong lattice energy). C incorrectly invokes d-electrons, which aren't part of the basic ionic model. D gives an incorrect geometric explanation for carbonate solubility.
The chemical formula for a compound is X₂(SO₄)₃. Based on the rules for ionic compounds, what is the charge on the metal ion X?
Answer: +3 The sulfate ion (SO₄) is a polyatomic ion with a charge of -2. The formula X₂(SO₄)₃ indicates that there are three sulfate ions for every two X ions. The total negative charge from the three sulfate ions is 3 × (-2) = -6. To maintain electrical neutrality in the compound, the total positive charge from the two X ions must be +6. Therefore, the charge on a single X ion must be +6 / 2 = +3.
Which pair of factors best explains why the lattice enthalpy of magnesium oxide (MgO) is significantly more exothermic than that of sodium fluoride (NaF)?
Answer: The charges on both ions in MgO are double those in NaF, and the Mg²⁺ ion is smaller than the Na⁺ ion. Lattice enthalpy is determined by Coulomb's law, which depends on the product of the ionic charges and the distance between them (ionic radii). MgO consists of Mg²⁺ and O²⁻ ions, while NaF consists of Na⁺ and F⁻ ions. The product of charges in MgO (2 × 2 = 4) is much greater than in NaF (1 × 1 = 1). Additionally, while O²⁻ is slightly larger than F⁻, the Mg²⁺ ion is smaller than the Na⁺ ion, contributing to a smaller internuclear distance. The dominant factor is the much larger ionic charge, leading to a significantly more exothermic lattice enthalpy for MgO.
Two ionic compounds, $\text{LiF}$ and $\text{CsI}$ , have the same crystal structure type. Using the ionic model, predict which compound should have the higher electrical conductivity when molten, and explain the primary factor.
Answer: $\text{CsI}$ will have higher conductivity because the larger $\text{Cs}^+$ and $\text{I}^-$ ions have weaker interionic forces and greater mobility in the melt In the molten state, electrical conductivity depends on ion mobility and the ease of breaking interionic attractions. CsI has much weaker interionic forces than LiF due to the larger ionic sizes, leading to lower viscosity and higher ion mobility in the melt. Although Li⁺ and F⁻ are individually smaller, the very strong electrostatic attractions in LiF (due to small size and high charge density) create a more viscous melt that impedes ion movement. A incorrectly ignores the effect of interionic forces. C incorrectly applies solution chemistry concepts to molten salts. D ignores the significant difference in interionic force strength.
Using the ionic model to analyze defects in crystal structures, predict what happens when a small amount of $\text{Ca}^{2+}$ is substituted for $\text{Na}^+$ in a $\text{NaCl}$ crystal lattice.
Answer: Each $\text{Ca}^{2+}$ substitution creates one cation vacancy to maintain charge neutrality, increasing ionic conductivity through vacancy migration When Ca²⁺ (charge +2) substitutes for Na⁺ (charge +1), the crystal gains +1 extra positive charge per substitution. To maintain overall electrical neutrality, one Na⁺ site must remain vacant for every Ca²⁺ substitution. These cation vacancies enable ionic conduction through vacancy migration mechanisms. B incorrectly suggests anion vacancies form instead. C incorrectly focuses on size effects rather than charge compensation (Ca²⁺ and Na⁺ are actually similar enough in size for substitution). D incorrectly invokes electronic conduction, which doesn't occur in ionic crystals.
Which statement best explains why many ionic compounds, such as sodium chloride, are soluble in water?
Answer: The energy released from the formation of ion-dipole forces during hydration is sufficient to overcome the lattice enthalpy. The process of dissolving an ionic compound involves two main energy changes: breaking the ionic lattice (which requires energy, the lattice enthalpy) and hydrating the separated ions (which releases energy, the hydration enthalpy). A compound is soluble if the energy released during hydration is comparable to or greater than the energy required to break the lattice. The attraction between the ions and the polar water molecules (ion-dipole forces) is key to this process.
While most ionic compounds are soluble in water, silver iodide (AgI) is considered insoluble. What can be deduced about the energetics of the dissolving process for AgI?
Answer: The hydration enthalpy of Ag⁺ and I⁻ ions is significantly less exothermic than the lattice enthalpy of AgI. For an ionic compound to dissolve, the energy released when the ions are hydrated by water must be sufficient to overcome the energy required to break apart the crystal lattice (the lattice enthalpy). If a compound is insoluble, it implies that the energy input needed to break the lattice is much larger than the energy output from hydrating the ions. Therefore, the hydration enthalpy is less exothermic (releases less energy) than the lattice enthalpy is endothermic (requires more energy).
What is the correct chemical formula for aluminium hydrogencarbonate?
Answer: Al(HCO₃)₃ Aluminium is in Group 13 and forms a stable ion with a +3 charge (Al³⁺). The hydrogencarbonate ion is a polyatomic ion with the formula HCO₃⁻ and a charge of -1. To create a neutral ionic compound, three hydrogencarbonate ions are needed to balance the +3 charge of one aluminium ion. Therefore, the correct formula is Al(HCO₃)₃.
Why does solid potassium chloride not conduct electricity, whereas molten potassium chloride is a good electrical conductor?
Answer: The K⁺ and Cl⁻ ions are held in fixed positions in the solid lattice but are free to move as mobile charge carriers when molten. Ionic compounds are composed of ions. For a substance to conduct electricity, it must have mobile charge carriers. In solid KCl, the K⁺ and Cl⁻ ions are the charge carriers, but they are locked in a rigid crystal lattice and cannot move. When KCl melts, the ions are no longer in fixed positions and become mobile, allowing them to move towards the electrodes and carry an electric current.
In the sodium chloride crystal lattice, what is the immediate environment of a single chloride ion (Cl⁻)?
Answer: It is surrounded by six sodium ions (Na⁺) in an octahedral geometry. The sodium chloride lattice has a face-centered cubic structure. In this arrangement, each ion is surrounded by six ions of the opposite charge. A central chloride ion will have one sodium ion above, one below, one to the left, one to the right, one in front, and one behind. This arrangement of six nearest neighbors corresponds to an octahedral geometry. The term 'coordination number' describes this, and for both Na⁺ and Cl⁻ in NaCl, it is 6.

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