Question

Benzoic acid (C6H5COOH) is a common standard used in bomb calorimeters, which maintain a constant volume. If 1.20 g of benzoic acid gives off 31,723 J when burned in excess oxygen at a constant temperature of 24.6 ºC, calculate q, w, ∆H, and ∆U for the reaction.

Answer 1

q = -31,723 J, w = 0, ΔU = -31,723 J, ΔH = -31,723J.

Step-by-step explanation:

The combustion reaction is exothermic: the heat flows from the system to the surroundings. It means that the system is losing heat, then:

q = -31,723 J.

By the first law of thermodynamics, the energy in the system should be conservated, and the energy can occur in three forms: heat (q), work (w), and internal energy (ΔU). The heat is the energy that flows for the difference of temperature, the work, for expansion or compression in a gas (change in the volume), and the internal energy is the molecules vibration.

So, ΔU = q + w

How the volume doesn't change, w = 0, and ΔU = q

ΔU = -31,723 J.

The enthalpy, is the measure of the heat of the system, and it is the sum of the internal energy with the product of the pressure for the volume:

ΔH = ΔU + pΔV

The volume is constant, so ΔH = ΔU

ΔH = -31,723J.