Final answer:
When heat is gained by a system, the heat sign is positive because energy is transferred into the system, increasing its internal energy. The sign convention aligns with the fundamentals of enthalpy changes and the second law of thermodynamics regarding entropy change.
Step-by-step explanation:
When heat is gained by a system, the sign on heat is expected to be positive. This sign convention is consistent across all enthalpy changes, where a negative sign indicates heat is released, and a positive sign indicates heat is absorbed. We can think of it in terms of energy transfer: if energy is entering a system, increasing its internal energy, then the heat value and resulting temperature change (ΔT) are positive. This is based on the principle that energy flowing into an object increases its total energy. Conversely, if energy is being lost by the system, the heat and ΔT values are negative, suggesting a decrease in the system's total energy. This aligns with the second law of thermodynamics and contributes to the overall entropy change (ΔS) of the universe, which reflects the direction of the heat flow.