constant pressure calorimeter

Question

In a constant‑pressure calorimeter,  70 mL of 0.350 M Ba(OH)2 was added to 70 mL of 0.7 M HCl.

The reaction caused the temperature of the solution to rise from  21.35 degrees C to 26.12 degrees C. If the solution has the same density and specific heat as water (1 g/mL and  4.184 J/g * K respectively), what is delta H  for this reaction (per mole H2O produced)? Assume that the total volume is the sum of the individual volumes.

Answer 1

Write a balanced equation:

Ba(OH)₂ + 2HCl ==> 2 H₂O + BaCl₂

q = mCΔT

q = heat = ?

m = mass = 70 ml + 70 ml = 140 ml x 1g/ml = 140g

C = specific heat = 4.184 J/g/deg

ΔT = change in temperature = 26.12 - 21.35 = 4.77 deg

(note: the calculated mass does not include the mass of the reagents, products etc. In more advanced courses, this would be included)

q = (140g)(4.184 J/g/deg)(4.77 deg) = 2794 J

Now we need to find the moles of water produced.

Moles Ba(OH)₂ = 0.070 L x 0.350 mol/L = 0.0245 moles

Moles HCl = 0.070 L x 0.7 mol/L = 0.049 moles

Neither reactant is limiting so, moles H₂O formed = 0.049 moles

molar ΔH = 2794 J/0.049 mol = 57,020 J/mol = 57 kJ/mol

= 60 kJ/mole (to 1 sig. fig. based on 1 sig fig in 0.7 M HCl)