Chemistry HL
Reactivity 1: entropy and spontaneity (HL) — Reactivity 1 HL
- 1.
Define lattice enthalpy, and state whether its value is always exothermic or endothermic when defined as the enthalpy change for forming a solid ionic lattice from its gaseous ions.
[2 marks] · no calculator - 2.
Marking analysis: A learner attempts the following task: “Define lattice enthalpy, and state whether its value is always exothermic or endothermic when defined as the enthalpy change for forming a solid ionic lattice from its gaseous ions.” Their response addresses only this point: “Defines lattice enthalpy as the enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gaseous state.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 3.
Construct a Born-Haber cycle for the formation of sodium chloride, NaCl(s), from its elements, listing the enthalpy terms needed: atomization of sodium, atomization of chlorine, first ionization energy of sodium, first electron affinity of chlorine, and lattice enthalpy.
[5 marks] · no calculator - 4.
Marking analysis: A learner attempts the following task: “Construct a Born-Haber cycle for the formation of sodium chloride, NaCl(s), from its elements, listing the enthalpy terms needed: atomization of sodium, atomization of chlorine, first ionization energy of sodium, first electron affinity of chlorine, and lattice enthalpy.” Their response addresses only this point: “Includes enthalpy of atomization of sodium: Na(s) → Na(g).” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks] · no calculator - 5.
Using the Born-Haber cycle for NaCl (ΔHf = −411, atomization Na = +107, atomization Cl = +122, IE1 Na = +496, EA1 Cl = −349, all in kJ mol⁻¹), calculate the lattice enthalpy of NaCl.
[4 marks] - 6.
Marking analysis: A learner attempts the following task: “Using the Born-Haber cycle for NaCl (ΔHf = −411, atomization Na = +107, atomization Cl = +122, IE1 Na = +496, EA1 Cl = −349, all in kJ mol⁻¹), calculate the lattice enthalpy of NaCl.” Their response addresses only this point: “States Hess's law relation: ΔHf = ΔHatom(Na) + ΔHatom(Cl) + IE1(Na) + EA1(Cl) + ΔHlattice.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 7.
State and explain the second law of thermodynamics in terms of the entropy of the universe.
[2 marks] · no calculator - 8.
Marking analysis: A learner attempts the following task: “State and explain the second law of thermodynamics in terms of the entropy of the universe.” Their response addresses only this point: “States that the entropy of the universe (system plus surroundings) increases in any spontaneous process.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 9.
Predict, with a reason, whether the entropy change is positive or negative for the reaction: CaCO₃(s) → CaO(s) + CO₂(g).
[2 marks] · no calculator - 10.
Marking analysis: A learner attempts the following task: “Predict, with a reason, whether the entropy change is positive or negative for the reaction: CaCO₃(s) → CaO(s) + CO₂(g).” Their response addresses only this point: “States that entropy increases (positive ΔS).” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 11.
State the Gibbs free energy equation and explain, using this equation, why a reaction with a positive ΔH and a positive ΔS can become spontaneous at high temperature.
[3 marks] · no calculator - 12.
Marking analysis: A learner attempts the following task: “State the Gibbs free energy equation and explain, using this equation, why a reaction with a positive ΔH and a positive ΔS can become spontaneous at high temperature.” Their response addresses only this point: “States ΔG = ΔH − TΔS.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 13.
A reaction has ΔH = +58 kJ mol⁻¹ and ΔS = +176 J K⁻¹ mol⁻¹. Calculate the minimum temperature (in K) at which the reaction becomes spontaneous.
[4 marks] - 14.
Marking analysis: A learner attempts the following task: “A reaction has ΔH = +58 kJ mol⁻¹ and ΔS = +176 J K⁻¹ mol⁻¹. Calculate the minimum temperature (in K) at which the reaction becomes spontaneous.” Their response addresses only this point: “States that the reaction becomes spontaneous when ΔG = 0, i.e. the boundary condition ΔH = TΔS.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 15.
Explain why lattice enthalpies calculated from a Born-Haber cycle (experimental) sometimes differ from those calculated theoretically assuming a purely ionic model.
[3 marks] · no calculator - 16.
Marking analysis: A learner attempts the following task: “Explain why lattice enthalpies calculated from a Born-Haber cycle (experimental) sometimes differ from those calculated theoretically assuming a purely ionic model.” Their response addresses only this point: “States that the theoretical (purely ionic) model assumes the ions are perfect spheres with no distortion of electron clouds.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 17.
Explain, in terms of entropy, why ice melts spontaneously above 0°C but not below 0°C, given that ΔH for melting is positive (endothermic) at all temperatures.
[3 marks] · no calculator - 18.
Marking analysis: A learner attempts the following task: “Explain, in terms of entropy, why ice melts spontaneously above 0°C but not below 0°C, given that ΔH for melting is positive (endothermic) at all temperatures.” Their response addresses only this point: “States that melting has a positive ΔS, since the liquid state is more disordered than the solid state.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 19.
Outline one limitation of using standard enthalpy of formation values alone (without considering entropy) to predict whether a reaction will occur spontaneously.
[2 marks] · no calculator - 20.
Marking analysis: A learner attempts the following task: “Outline one limitation of using standard enthalpy of formation values alone (without considering entropy) to predict whether a reaction will occur spontaneously.” Their response addresses only this point: “States that enthalpy alone does not account for the entropy change of the reaction.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 21.
Using standard entropy values S°(CaCO₃(s)) = 93 J K⁻¹ mol⁻¹, S°(CaO(s)) = 40 J K⁻¹ mol⁻¹ and S°(CO₂(g)) = 214 J K⁻¹ mol⁻¹, calculate ΔS° for the reaction CaCO₃(s) → CaO(s) + CO₂(g).
[3 marks] - 22.
Marking analysis: A learner attempts the following task: “Using standard entropy values S°(CaCO₃(s)) = 93 J K⁻¹ mol⁻¹, S°(CaO(s)) = 40 J K⁻¹ mol⁻¹ and S°(CO₂(g)) = 214 J K⁻¹ mol⁻¹, calculate ΔS° for the reaction CaCO₃(s) → CaO(s) + CO₂(g).” Their response addresses only this point: “Uses ΔS° = ΣS°(products) − ΣS°(reactants).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 23.
A reaction (the Haber process) has ΔH° = −92 kJ mol⁻¹ and ΔS° = −199 J K⁻¹ mol⁻¹ at 298 K. Calculate ΔG° for this reaction at 298 K, and state whether it is spontaneous under these conditions.
[4 marks] - 24.
Marking analysis: A learner attempts the following task: “A reaction (the Haber process) has ΔH° = −92 kJ mol⁻¹ and ΔS° = −199 J K⁻¹ mol⁻¹ at 298 K. Calculate ΔG° for this reaction at 298 K, and state whether it is spontaneous under these conditions.” Their response addresses only this point: “Converts ΔH° to consistent units: −92 000 J mol⁻¹.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 25.
State the equation relating standard Gibbs free energy change (ΔG°) to the equilibrium constant (K) of a reaction, and explain what a large negative ΔG° indicates about the value of K.
[3 marks] · no calculator - 26.
Marking analysis: A learner attempts the following task: “State the equation relating standard Gibbs free energy change (ΔG°) to the equilibrium constant (K) of a reaction, and explain what a large negative ΔG° indicates about the value of K.” Their response addresses only this point: “States ΔG° = −RT ln K.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 27.
State and explain the general trend in standard entropy (S°) for the same substance in its solid, liquid and gas states.
[3 marks] · no calculator - 28.
Marking analysis: A learner attempts the following task: “State and explain the general trend in standard entropy (S°) for the same substance in its solid, liquid and gas states.” Their response addresses only this point: “States that S°(solid) < S°(liquid) < S°(gas).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 29.
Construct an enthalpy cycle relating the enthalpy of solution of an ionic compound to its lattice enthalpy (of dissociation) and the enthalpies of hydration of its ions, and hence state the equation linking these three enthalpies.
[4 marks] · no calculator - 30.
Marking analysis: A learner attempts the following task: “Construct an enthalpy cycle relating the enthalpy of solution of an ionic compound to its lattice enthalpy (of dissociation) and the enthalpies of hydration of its ions, and hence state the equation linking these three enthalpies.” Their response addresses only this point: “States that dissolving an ionic solid in water can be considered in two steps: first, breaking apart the lattice into gaseous ions (lattice dissociation enthalpy, endothermic).” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator