2.1 Exam Practice
Exam-style practice questions on The Ionic Model
Section B: Data Analysis (Paper 1B Style)
Calculator and Data Booklet permitted. Show all working clearly.
Question 1: Comparing Ionic Compounds Explain
5 marksThe table shows the melting points and ionic radii of four ionic compounds.
| Compound | Cation radius / pm | Anion radius / pm | Melting point / °C |
|---|---|---|---|
| NaF | 102 | 133 | 993 |
| NaCl | 102 | 181 | 801 |
| MgO | 72 | 140 | 2852 |
| KCl | 138 | 181 | 770 |
(a) Compare the melting points of NaF and NaCl. Explain the difference in terms of ionic radii and lattice enthalpy. [2]
(b) Explain why MgO has a much higher melting point than NaCl. [2]
(c) Predict whether CaO (Ca²⁺ radius = 100 pm, O²⁻ radius = 140 pm) would have a melting point higher or lower than MgO. Justify your answer. [1]
Show Mark Scheme
(a) NaF has a higher melting point than NaCl because F⁻ has a smaller ionic radius than Cl⁻ [1]
Smaller ions lead to a shorter interionic distance and a stronger electrostatic attraction (higher lattice enthalpy), requiring more energy to break apart [1]
(b) MgO contains Mg²⁺ and O²⁻ ions with charges of 2+ and 2−, compared to 1+ and 1− in NaCl [1]
Higher ionic charges and smaller ionic radii result in a much greater lattice enthalpy / much stronger electrostatic attraction [1]
(c) CaO would have a lower melting point than MgO because Ca²⁺ (100 pm) is larger than Mg²⁺ (72 pm), leading to a greater interionic distance and weaker attraction [1]
Section C: Structured Questions (Paper 2 Style)
Show all working. State answers with appropriate significant figures and units.
Question 2: Ion Formation and Ionic Bonding State
5 marksPotassium (Z = 19) reacts with bromine (Z = 35) to form an ionic compound.
(a) Write the electron configurations of K and Br atoms and the ions they form. [2]
(b) Write the formula of potassium bromide using the criss-cross method. [1]
(c) Explain why potassium bromide has a high melting point but does not conduct electricity as a solid. [2]
Show Mark Scheme
(a) K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ → K⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ [1]
Br: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵ → Br⁻: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ [1]
(b) K⁺ has charge 1+, Br⁻ has charge 1−, so the formula is KBr [1]
(c) High melting point: strong electrostatic attraction between K⁺ and Br⁻ ions in a giant ionic lattice requires a large amount of energy to overcome [1]
Does not conduct as a solid: ions are fixed in position in the lattice and cannot move to carry charge [1]
Question 3: Lattice Enthalpy and Properties Explain
4 marksMagnesium chloride (MgCl₂) and sodium chloride (NaCl) are both ionic compounds with different physical properties.
(a) Explain why MgCl₂ has a higher melting point than NaCl. [2]
(b) Both compounds dissolve in water. Explain why ionic compounds are generally soluble in polar solvents but not in non-polar solvents. [2]
Show Mark Scheme
(a) Mg²⁺ has a higher charge than Na⁺ (2+ vs 1+) [1]
Mg²⁺ also has a smaller ionic radius than Na⁺, so the electrostatic attraction between cation and anion is stronger / greater lattice enthalpy [1]
(b) Polar water molecules can surround and stabilise the ions through ion-dipole interactions / hydration [1]
The energy released during hydration compensates for the energy needed to break apart the lattice. Non-polar solvents cannot form such interactions, so they cannot overcome the lattice enthalpy [1]
Question 4: Transition Metal Ions Deduce
4 marksIron can form two different ionic compounds with chlorine: FeCl₂ and FeCl₃.
(a) State the charge on the iron ion in each compound. [1]
(b) Write the electron configuration of the Fe²⁺ ion. Explain why transition metals lose their 4s electrons before 3d electrons. [2]
(c) Predict which compound, FeCl₂ or FeCl₃, has the higher lattice enthalpy. Justify your answer. [1]
Show Mark Scheme
(a) FeCl₂: Fe²⁺; FeCl₃: Fe³⁺ [1]
(b) Fe²⁺: [Ar] 3d⁶ (or 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶) [1]
Although 4s fills before 3d in neutral atoms, when transition metals form ions the 4s electrons are removed first because the 4s orbital is further from the nucleus and has a higher energy in the ion [1]
(c) FeCl₃ has a higher lattice enthalpy because Fe³⁺ has a greater charge and smaller radius than Fe²⁺, resulting in stronger electrostatic attraction [1]