Heather is measuring the enthalpy change associated with reaction of magnesium w

Question

Heather is measuring the enthalpy change associated with reaction of magnesium with hydrochloric acid. She records the following in her laboratory notebook: aqueous mass Mg (g) volume (mL) 1.00 M HCI 120.44 initial T of solution (oC) 19.7 final T of solution (C) 29.28 0.053 Assuming the density 1.00 M HCI is identical to that of pure water (1.00 g/mL) and that the specific heat capacity of the solution is identical to that of liquid water (4.184 J/g-K), what is the enthalpy change associated with the reaction, in units of J? Report your response to two digits after the decimal. Note: Do not use units (i.e.) in your answer and don't forget to use a negative sign (-) before type your answer in number.with no-space in between (i.e.-3125.20)

Explanation / Answer

Mg + 2HCl = H2 + MgCl2

mol of Mg = mass/MW = 0.053/24.3 = 0.00218 mol of Mg

mol of HCl = MV = 1*120.44 = 120.44 mmol = 0.12044 mol of HCl

clearly, HCl is in excess

Qsolution = m*C*(Tf-Ti)

Qsoln = 120.44*4.184*(29.28-19.7)

Qsoln = 4827.56 J

this is lost by reaction so

Qrxn = -Qsoln = -4827.56 J

HRxn = QRxn /moles = (-4827.56)/0.00218

HRxn = -2214477.06 J/mol

HRxn = -2214.48 kJ/mol

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