Question

Manganese reacts with hydrochloric acid to produce manganese(II) chloride and hydrogen gas.
Mn(s)+2HCl(aq) → MnCl2(aq)+H2(g)

When 0.615 g Mn is combined with enough hydrochloric acid to make 100.0 mL of solution in a coffee-cup calorimeter, all of the Mn reacts, raising the temperature of the solution from 24.0 ∘C to 28.4 ∘C.

Find ΔHrxn = ? kJ for the reaction as written. (Assume that the specific heat capacity of the solution is 4.18 J/g∘C and the density is 1.00 g/mL.)

Answer 1

Answer – given, mass of Mn = 0.615 g , volume of solution = 100 mL

Initial temp, ti = 24.0 ^oC , final temp, tf = 28.4 ^oC ,

specific heat capacity of the solution = 4.18 J/g∘C , density = 1.00 g/mL

so, mass of solution = 100 mL * 1.00 g/mL

                             = 100 g

We know the formula

Heat, q = - m* C * ∆t

             = - 100 g * 4.184 J/g ^oC * (28.4-24.0)^oC

             = - 1841 J

Moles of Mn = 0.615 g / 54.94 g.mol^-1 = 0.0112 moles

So , 0.0112 moles of Mn = -1841 J

So, 1 mole of Mn = ?

= -164459 J

= -164.5 kJ

So, Mn(s)+2HCl(aq) -----> MnCl₂(aq)+H₂(g) ∆H rxn = -164.5 kJ