Q.Explain sign convention of heat.

Thermodynamics uses the IUPAC (system-centred) sign convention: any energy (heat or work) that flows INTO the system is positive, and any energy that flows OUT of the system is negative.

In the study of thermochemistry, the system is the part of the universe under study (e.g., the reacting substances), and the surroundings is everything else. The sign convention for heat (q) is defined from the system's point of view:

• If heat flows from the surroundings INTO the system (the system absorbs heat, an endothermic process), q is taken as POSITIVE (+q).
• If heat flows from the system OUT to the surroundings (the system releases/evolves heat, an exothermic process), q is taken as NEGATIVE (-q). …