Magnesium oxide reacts with liquid water to produce solid magnesium hydroxide according to the equation
MgO(s) + H2O(l) → Mg(OH)2(s) ΔH° = –81.2 kJ mol⁻¹
A second process in which the hydroxide reacts with carbon dioxide is described by
Mg(OH)2(s) + CO2(g) → MgCO3(s) + H2O(l) ΔH° = –49.5 kJ mol⁻¹
Answer the following questions about manipulating these reactions and their enthalpy changes.
(a) The first reaction is written in the reverse direction. Describe qualitatively how its enthalpy change is affected and give the numerical value that would be reported for the reversed process.
(b) The first reaction is instead written with every coefficient multiplied by three. Explain why the associated enthalpy change must also be multiplied by three, and report the new numerical value.
(c) The two reactions are combined to obtain the overall process
MgO(s) + CO2(g) → MgCO3(s)
Explain, in words, how the enthalpy change for this overall reaction can be obtained from the two individual enthalpy changes given above without explicitly performing the arithmetic addition. Your explanation should make specific reference to energy conservation.