5.1. Enthalpy change, \(\Delta H\)
A subsection of Chemistry, 9701, through 5. Chemical energetics
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Predict and explain the variation in enthalpy change of hydration for the ions F–, Cl –, Br – and I–. shows an incomplete energy cycle involving calcium fluoride, CaF2. process 1: enthalpy change of formation of Ca2+plus twice the enthalpy change of formation of F–, ΔHf Ca2++ 2ΔHf F–process 3: enthalpy change of formation of calcium fluoride, ΔHf CaF2process 2: enthalpy change of hydration of calcium ions plus twice the enthalpy change of hydration of fluoride ions, ΔHhyd Ca2++ 2ΔHhyd F–line A: Ca+ F2line B: Ca2++ 2F–line C: CaF2line D: process 4: enthalpy change of solution of calcium fluoride, ΔHsol CaF2Complete line D. Include state symbols. The value of the enthalpy change for process 1 can be calculated using the values of five other enthalpy changes which are not referred to in . process 1: Ca+ F2Ca2++ 2F–Identify these five other enthalpy changes, using either names or symbols. Define lattice energy, ΔHlatt. Complete the expression to give the mathematical relationship between ΔHlatt of calcium fluoride and the enthalpy changes for processes 1 and 3. ΔHlatt = Use data from Table 2.1 to calculate a value for the hydration energy, ΔHhyd, of fluoride ions, F–. Table 2.1 value / kJ mol–1 enthalpy change of solution of calcium fluoride, CaF2+13 overall enthalpy change of process 1 in +1395 enthalpy change of formation of calcium fluoride –1214 enthalpy change of hydration of Ca2+–1650 ΔHhyd F–= kJ mol–1 Define entropy. At 298 K, the Gibbs free energy change, ΔG, for the solution of compound T is +6.00 kJ mol–1. The enthalpy change of solution, ΔHsol, of compound T is +30.0 kJ mol–1 at 298 K. Calculate the value of the entropy change, ΔS, for the solution of compound T at 298 K. ΔS = J K–1 mol–1 Predict whether compound T becomes more or less soluble as the water is heated from 298 K to 360 K. Explain your answer.
9701_w24_qp_41
THEORY
2024
Paper 4, Variant 1
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Predict and explain the variation in enthalpy change of hydration for the ions Na+, Mg2+ and Al 3+. shows an incomplete energy cycle. change 1: lattice energy of magnesium chloride, ΔHlatt MgCl 2change 3: enthalpy change of solution of magnesium chloride, ΔHsol MgCl 2change 2: line A: MgCl 2line C: line B: Mg2++ 2Cl –Complete line C on . Include state symbols. Use both words and symbols to identify change 2 on . Use changes 1 and 3 as examples of how this should be done. Calculate a value for the lattice energy of magnesium chloride, ΔHlatt MgCl 2, by selecting and using appropriate data from Table 2.1. Table 2.1 energy change value / kJ mol–1 enthalpy change of solution of magnesium chloride –155 enthalpy change of formation of magnesium chloride –642 first ionisation energy of magnesium +736 second ionisation energy of magnesium +1450 electron affinity of chlorine –349 enthalpy change of hydration of Mg2+ –1920 enthalpy change of hydration of Cl – –364 ΔHlatt MgCl 2= kJ mol–1 Define entropy. At 25 °C the enthalpy change of solution of compound Z is +26 kJ mol–1. The entropy change of solution of Z at the same temperature is +52 J K–1 mol–1. Calculate the value of the Gibbs free energy change, ΔG, for the solution of Z at 25 °C. ΔG = kJ mol–1 Use your answer to to predict whether or not Z is soluble in water at 25 °C. Explain your answer. Predict whether Z becomes more or less soluble as the water is heated from 25 °C to 95 °C. Explain your answer.
9701_w24_qp_42
THEORY
2024
Paper 4, Variant 2
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Predict and explain the variation in enthalpy change of hydration for the ions F–, Cl –, Br – and I–. shows an incomplete energy cycle involving calcium fluoride, CaF2. process 1: enthalpy change of formation of Ca2+plus twice the enthalpy change of formation of F–, ΔHf Ca2++ 2ΔHf F–process 3: enthalpy change of formation of calcium fluoride, ΔHf CaF2process 2: enthalpy change of hydration of calcium ions plus twice the enthalpy change of hydration of fluoride ions, ΔHhyd Ca2++ 2ΔHhyd F–line A: Ca+ F2line B: Ca2++ 2F–line C: CaF2line D: process 4: enthalpy change of solution of calcium fluoride, ΔHsol CaF2Complete line D. Include state symbols. The value of the enthalpy change for process 1 can be calculated using the values of five other enthalpy changes which are not referred to in . process 1: Ca+ F2Ca2++ 2F–Identify these five other enthalpy changes, using either names or symbols. Define lattice energy, ΔHlatt. Complete the expression to give the mathematical relationship between ΔHlatt of calcium fluoride and the enthalpy changes for processes 1 and 3. ΔHlatt = Use data from Table 2.1 to calculate a value for the hydration energy, ΔHhyd, of fluoride ions, F–. Table 2.1 value / kJ mol–1 enthalpy change of solution of calcium fluoride, CaF2+13 overall enthalpy change of process 1 in +1395 enthalpy change of formation of calcium fluoride –1214 enthalpy change of hydration of Ca2+–1650 ΔHhyd F–= kJ mol–1 Define entropy. At 298 K, the Gibbs free energy change, ΔG, for the solution of compound T is +6.00 kJ mol–1. The enthalpy change of solution, ΔHsol, of compound T is +30.0 kJ mol–1 at 298 K. Calculate the value of the entropy change, ΔS, for the solution of compound T at 298 K. ΔS = J K–1 mol–1 Predict whether compound T becomes more or less soluble as the water is heated from 298 K to 360 K. Explain your answer.
9701_w24_qp_43
THEORY
2024
Paper 4, Variant 3
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