Question

A bomb calorimeter is placed in a water bath, and a mixture of fueland oxygen is burned inside it.The temperature of the water isobserved to rise during the combustion reaction. Thecalorimeter ant the water remain at constant volume.

a. If the reaction products are the system, which thermodynamicsquantities - ΔU, Q or W- are positive and which arenegative?

b. If the water bath is the system, which thermodynamic quantities-- ΔU, Q -- are positive and which are negative?

Answer 1

Assume

Q - W = dU is valid

so

a)

fuel + oxygen = system

the system is breaking its bonds to release energy

so they give off heat, dU = must be negative, since it is losing energy

Q = negative, as well, releasing heat

W =the system expands, thereofre system doing work on surroundings, W = +

b)

if bath system is surroundings then

dU = increases, since it is absorbing heat

Q = positive, since it is absorbing heat

W = negative, since it is getting compressed, i.e. the system is doing work on the surrouindings