Question

The addition of hydrobromic acid to a silver nitrate solution precipitates silver bromide according to the reaction:

AgNO3(aq)+HBr(aq)→AgBr(s)+HNO3(aq)

When 50.0 mL of 4.00×10⁻²M  AgNO3 is combined with 50.0 mL of 4.00×10⁻²M HBr in a coffee-cup calorimeter, the temperature changes from 25.60 ∘C to 26.42 ∘C.

Calculate ΔHrxn for the reaction as written. Use 1.00 g/mL as the density of the solution and Cs=4.18J/(g⋅∘C) as the specific heat capacity of the solution.

Express the energy to two significant figures and include the appropriate units.

Answer 1

moles of AgNO3 = 50 x 4.00 x 10^-2

                            = 0.00200 mol

moles of HBr = 50 x 4.00 x 10^-2

                            = 0.00200 mol

mass of solution = 50 + 50 = 100 g

Cp = 4.18 J / g oC

Q = m Cp dT

   = 100 x 4.18 x (26.42 - 25.60)

Q = 342.76 J

AgNO3(aq)+HBr(aq)→AgBr(s)+HNO3(aq)

ΔHrxn = - Q / n

           = - 0.34276 / 0.002

ΔHrxn = - 1.7 x 10^2 kJ/mol