The reaction of aqueous HCl with solid Mg produces and

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

The reaction of aqueous HCl with solid Mg produces

MgCl_2\,(aq)

and

H_2\,(g)

. Under the laboratory conditions used, the enthalpy change is

\Delta H = -462\,\text{kJ mol}^{-1}

of Mg consumed. Four metal samples, each of a different mass, are allowed to react completely with excess HCl. Sample 1:

1.20\,\text{g Mg}

Sample 2:

2.40\,\text{g Mg}

Sample 3:

0.800\,\text{g Mg}

Sample 4:

1.60\,\text{g Mg}

(The molar mass of Mg is

24.3\,\text{g mol}^{-1}

.) (a) For each sample, determine the heat

q

(in kJ) released during the reaction. (b) Rank the samples from the one that releases the most energy per gram of Mg present to the one that releases the least. Show all work necessary to support your ordering. (c) Describe how your ranking in part (b) would change, if at all, if the enthalpy change were reported per mole of HCl consumed instead of per mole of Mg. Provide a brief explanation.

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