Q.A mixture of fuel and oxygen is burned in a constant-volume chamber surrounded by a water bath. It was noticed that the temperature of water is increased during the process. Treating the mixture of fuel and oxygen as the system,

Step 1. (a) Since the water bath's temperature rises, heat must be leaving the burning fuel-oxygen mixture (the system) and entering the water — so yes, heat IS transferred, and by the sign convention $Q$ is negative (energy leaving the system).

Step 2. (b) The chamber has constant volume ($\Delta V = 0$), so no work is done: $W = p\Delta V = 0$ (Section 4.7.3.4). …

Recognise the constant-volume chamber forces W = 0 (isochoric), then determine the sign of Q from the fact that the surrounding water bath hea …

Assuming ΔU is positive because a combustion reaction 'releases energy' — from the SYSTEM's own point of view (the burning mixture), that released chemical energy leaving …