Question

51.5g of aqueous phosphoric acid is mixed with 52.0g of aqueous sodium hydroxide in a calorimeter. The initial temperature was 22.3 Celsius (C) and the final temperature was 31.5 Celsius (C). The calorimeter constant is 31.72 J/C and the heat capacity of the final system is 4.00 J/gC. Based on this data the enthalpy of the reaction (Hrxn) would be calculated as ________ kJ(report to 3 sig figs and include appropriate sign).

Answer 1

Answer:

4,10kJ

Explanation:

The reaction of phosphoric acid (H₃PO₄; MW: 97.994 g/mol ) with sodium hydroxide (NaOH; MW: 40 g/mol) is:

H₃PO₄ + NaOH ⇄ H₂PO₄⁻ + Na⁺ + OH⁻

Now, the energy produced for the change in temperature in a calorimeter is:

Q = -H×m×ΔT - C×ΔT

Where H×m×ΔT is the energy absorbed for the system and C×ΔT is the energy absorbed for the calorimeter.

Replacing:

Q = -4.00J/g°C × 103,5g × (31,5°C-22,3°C) - 31,72J/°C × (31,5°C-22,3°C)

H is heat capacity of the system, ΔT is change in temperature, m is total mass of the system: 52,0g + 51,5g = 103,5g and C is calorimeter constant.

Q = -4100J ≡ -4,10kJ

I hope it helps!