Question

You used 50.0mL of hydrochloric acid with 0.500g of magnesium. If the temperature changed from 20.0oC to 24.0oC, what was the heat of reaction (ΔH) for your experiment? Report your answer in kJ without writing the units.

Answer 1

the reaction we have is

Mg(s) + 2HCl (aq) ---> MgCl2 (aq) + H2 (g)

Mg moles = mass / atomic mass of Mg

         = 0.5 g / ( 24.3 g/mol) = 0.020576

since density of HCl is not given we assume 1g/ml

HCl mass = vol x density = 50 ml x 1g/ml = 50 g

solution mass = HCl mass + Mg mass = 500 + 0.5 = 500.5 g

Temperature rise in solution = 24-20 = 4C

heat releaed by reaction is absorbed by solution

heat absorbed by solution = specific heat of solution x temperature change x mass of solution

             = 4.184 J/gC x 4C x 500.5 g

            = 845.17 J   = 0.845 KJ

Thus heat given in reaction = - 0.845 KJ

if we consider enthalphy per mol of Mg then dH = 0.845 KJ / 0.020576 mol

          = - 41 KJ/mol           ( -ve sign indicates heat is released)