Question

You drop 0.050 g of Mg chips into 100.0 mL of 1.00 M HCl and the temperature of the solution increases from 22.21 °C to 24.46 °C. Assume that c_s (solution) - 4.20 J/g °C and that the density of the solution is 1.00 g/mL. What is the enthalpy of the reaction per mole of Mg?

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

Answer 1

1) The heat liberated from the solution is given by

Q = m Cs T

Given that, m = mass of sample = 100mL*1g/mL = 100g

C = Specific heat capacity of solution = 4.20 J/g °C

T = 24.46 - 22.21 = 2.25 oC

Q = 100g*4.20J/g °C*2.25 oC = 945 J = 0.945 KJ

Thus, 0.945 KJ per 0.05 g of Mg

2) Moles of Mg solute =Weight of Mg/Atomic wt. of Mg

Moles of Mg = 0.050g/24.30g/mol = 0.00205 mole

3) Enthalpy change per moles of Mg

0.945 KJ/0.00205 mole = 460.97 KJ = 461 KJ (3 sig. figs)