Which of the equations shows 'twice' the enthalpy change of neutralization as the following equation:HCl + NaOH → NaCl + H2O
Correct answer: D. H2SO4 + Mg(OH)2 → MgSO4 + 2H2O
- A. MgCO3 + 2HCl → MgCl2 + CO2 + H2O
- B. KOH + HCl → KCl + H2P
- C. NH4Cl + NaOH → NaCl + H2O + NH3
- D. H2SO4 + Mg(OH)2 → MgSO4 + 2H2O
Explanation
The given reaction depicts a neutralization reaction, where one mole of acid is neutralized by one mole of alkali (as indicated by 1 mole of HCl reacting with 1 mole of NaOH). For a reaction to have twice the enthalpy change as this reaction, it should involve neutralization of TWO moles of acid with TWO moles of alkali. Such an option is D, which has two moles of acid (which are present in H2SO4, as one molecule of H2SO4 has two molecules of hydrogen ions) reacting with two moles of alkali (Mg(OH)2 has two moles of hydroxide ions).Option B has the same enthalpy change of neutralisation as the reaction in the question as both involve one mole of H+ reacting with one mole of OH-.Options A and C are not reactions that fit the description of enthalpy change of neutralization, as they form gases in the products aside from a salt and water (for enthalpy change of neutralisation, an acid and a base react to form a salt and water only) so, their enthalpies cannot be compared to the given reaction.
This question appeared on the UHS MDCAT 2019 paper, which you can sit online with every answer explained.
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About Enthalpy
Enthalpy is the heat content of a system measured at constant pressure, and its change indicates whether a reaction or physical process is endothermic or exothermic. The topic covers state functions, enthalpy diagrams, standard enthalpy of formation, combustion and neutralisation, and the relationship between ΔH and internal energy change.
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