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 aqueous hydrochloric acid. She records the following in her laboratory notebook:

Assuming the density 1.00 M HCl 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.

mass Mg (g) 0.083 volume (mL) 1.00 M HCl 100.24 initial T of solution (oC) 19.92 final T of solution (oC) 29.29

Explanation / Answer

volume of HCl = volume of water = 100.24 mL

mass = density x volume

mass = 1 x 100.24

mass of water = 100.24 g

dT = 29.29 - 19.92 = 9.37 oC

Q = m Cp dT

Q   = 100.24 x 4.184 x 9.37

Q = 3930 J

moles of Mg = 0.082 / 24

                  = 3.42 x 10^-3

enthalpy change = - Q / n

                           = -3930 / 3.42 x 10^-3

                         = - 1.15 x 10^6 J / mol

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