CHEMISTRY · Multi-part · A Level · Chemistry · Group 2 elements (alkaline earth metals) · Group 2 elements (alkaline earth metals)
Problem
For the series Mg, Ca, Sr, and Ba, the initial rate of reaction with 1.0 M HCl(aq) at 25 °C was measured. The natural logarithm of the observed rate (in mol L⁻¹ s⁻¹) was plotted versus the atomic number of the metal, producing a nearly straight line with negative slope.
Additional data:
• Hydration enthalpy of M²⁺ (kJ·mol⁻¹): Mg –1 920, Ca –1 570, Sr –1 410, Ba –1 350
• Metal–chloride lattice energy for MCl₂(s) (kJ·mol⁻¹): MgCl₂ –2 525, CaCl₂ –2 250, SrCl₂ –2 100, BaCl₂ –1 980
Assume that the overall rate is controlled by the energy change associated with forming hydrated M²⁺ and solid MCl₂ from the metal surface.
(a) Explain qualitatively why the plot of ln(rate) versus atomic number exhibits a negative slope.
(b) Using the hydration enthalpy and lattice-energy data, outline a thermodynamic argument that correlates the sign and magnitude of the slope with the trend in enthalpy change for the net process M(s) + 2 HCl(aq) → MCl₂(s) + H₂(g).
(c) Extrapolate the linear trend to predict the relative rate for radium (atomic number 88) under the same conditions. State any assumptions in your reasoning.
(d) Discuss two limitations of using such an extrapolation to estimate the reactivity of radium with hydrochloric acid.