Question

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

1. When 0.630 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 28.1 ∘C.
Find ΔHrxn 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.) (express answer in 3 sig figs)

Answer 1

ΔHrxn = q/mol

Convert the volume of solution into grams of the solution.
100ml*(1.00g/ml)= 100g solution

Using 4.18G/g C for the Cs of the solution, plug it into the equation for q.
q = (100g)(4.18J/g C)(28.1 - 24.0)
q= 1713.8J

Also remember this is negative so q=-1713.8 J

Now calculate the number of moles Mn used in the reaction from the given 0.630g Mn. (You also multiply by the number of moles of the reactant required to produce the reaction)
0.630gMn(1mol Mn/54.9g Mn)(1) = 0.01147mol

Finally its ΔHrxn = 1713.8J/0.01147
=-149415.8J

=-149.4 kj