5.1. Enthalpy change, \(\Delta H\)
A subsection of Chemistry, 9701, through 5. Chemical energetics
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Calcium metal reacts with chlorine gas to form calcium chloride, CaCl 2. Write an equation, including state symbols, to represent the lattice energy of calciumchloride, CaCl 2. Complete a fully labelled Born-Haber cycle that could be used to calculate the lattice energy, ,for calcium chloride. Ca+ Cl 2CaCl 2 Use your answer to and the following data, together with relevant data from the DataBooklet, to calculate a value for for calcium chloride. standard enthalpy change of formation of CaCl 2, –796 kJ mol–1 standard enthalpy change of atomisation of Ca, +178 kJ mol–1 electron affinity of chlorine atoms –349 kJ mol–1 = kJ mol–1 Entropy is a measure of the disorder of a system. Describe and explain what happens to the entropy of a gas when the temperature is increased. The table shows four reactions. For each reaction, predict the sign of the entropy change, ∆S o. If you predict no entropy change, write ‘no change’ in the table below. The first one has been done for you. reaction sign of ∆S o CO+ O2→ CO2negative Mg+ 2 1 O2→ MgOCuSO4+ 5H2O→ CuSO4.5H5ONaHCO3+ H+→ Na++ CO2+ H2O Explain why the entropy change for the first process is negative. Calculate the standard entropy change, ∆S o, for this reaction. N2+ 3H2→ 2NH3Standard entropies, S o, in J K–1 mol–1 are given. N2H2NH3+192 +131 +193 ∆S o J K–1 mol–1 Whether or not a chemical reaction is spontaneous can be deduced by calculating the change in free energy, ∆G o, at a given temperature. MgCO3→ MgO+ CO2∆H o = +117 kJ mol–1 ∆S o = +175 J K–1 mol–1 Calculate the value of ∆G o at 298 K for the above reaction. Use your answer to to explain whether or not this reaction is spontaneous at 298 K.
9701_m16_qp_42
THEORY
2016
Paper 4, Variant 2
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Complete the table using ticks () to indicate whether the sign of each type of energy change, under standard conditions, is always positive, always negative or could be either positive or negative. energy change always positive always negative either positive or negative electron affinity enthalpy change of atomisation ionisation energy lattice energy The Born-Haber cycle for magnesium chloride is shown. Mg2++ 2Cl + 2e– Mg++ 2Cl + e– Mg2++ 2Cl –Mg+ 2Cl Mg+ Cl 2Mg+ Cl 2MgCl 2ΔH4 ΔH5 ΔH3 ΔH2 ΔH1 ΔH7 ∆H6 Explain why ΔH4 is greater than ΔH3. What names are given to the enthalpy changes ΔH6 and ΔH7? ΔH6 ΔH7 Chlorine is in Group 17. Suggest the trend in the first electron affinity of the elements in Group17. Explain your answer. The equation for the formation of magnesium oxide from its elements is shown. Mg+ 1 2O2MgOΔH o = – 602 kJ mol–1 substance S o / J K–1 mol–1 Mg32.7 O2205 MgO26.9 Use the equation and the data given in the table to calculate ΔG o for the reaction at 25 °C. ΔG o = units
9701_m17_qp_42
THEORY
2017
Paper 4, Variant 2
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