Question

Heather is measuring the enthalpy change associated with reaction of magnesium with aqueous hydrochloric acid. She records the following in her laboratory notebook:

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

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.

Answer 1

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