Q19.
(a) Start a clock when KCl is added to water
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Record the temperature every subsequent minute for about 5 minutes
Allow record the temperature at regular time intervals until
some time after all the solid has dissolved for M2
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Plot a graph of temperature vs time
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Extrapolate back to time of mixing = 0 and determine the temperature
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(b) Heat taken in = m × c × ΔT = 50 × 4.18 × 5.4 = 1128.6 J
Max 2 if 14.6 °C used as ΔT
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Moles of KCl = 5.00 / 74.6 = 0.0670
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Enthalpy change per mole = +1128.6 / 0.0670 = 16 839 J mol-1
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= +16.8 (kJ mol-1)
Answer must be given to this precision
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(c) ΔHsolution = ΔHlattice + ΔH(hydration of calcium ions) + 2 × ΔH(hydration of chloride ions)
ΔHlattice = ΔHsolution – ΔH(hydration of calcium ions) –2 ×ΔH(hydration of chloride ions)
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ΔHlattice = –82–9 – (–1650 + 2 × –364) = +2295 (kJ mol–1)
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(d) Magnesium ion is smaller than the calcium ion
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Therefore, it attracts the chloride ion more strongly / stronger ionic bonding
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